This document provides information about a neuroanatomy review book written by Ahmed A. E. Eljack. It lists the contributors to the book and acknowledges their assistance. It also provides contact information for the graphic designer who designed the book cover and information on how to obtain physical copies of the book. The purpose of the book is to combine neuroanatomy and CNS physiology topics in a concise, bullet-pointed format with illustrations to make these subjects easier for medical students to understand and master.

2. 2
AUTHOR:
Ahmed A. E. Eljack
Medical student at Alneelain University
ahmed.adel3119@gmail.com
CONTRIBUTORS:
Adil Abbas, MSc (revision of the neuroanatomy part)
Lecturer of human anatomy- Alneelain University
Hafsa Alfadil (observations and drawings)
Medical student at Alneelain University
hafsaalfadil22@gmail.com
Anas H. Z. Elshiekh
Medical student at Alneelain University
Fatima Saad
Medical student at Alneelain University
BOOK COVER DESIGN:
Abdelmalik Graphic Designer

3. 1
‫البوستة‬ ‫جنوب‬ ‫الطبية‬ ‫العلمية‬ ‫المكتبة‬ ‫في‬ ‫متوفرة‬ ‫الكتاب‬ ‫هذا‬ ‫من‬ ‫الورقية‬ ‫النسخة‬
‫على‬ ‫اتصل‬ ‫الكميات‬ ‫وطلب‬ ‫(لالستفسار‬ ‫بأمدرمان‬0912665045‫ومتوفرة‬ )
‫النيلين‬ ‫بجامعة‬ ‫الطب‬ ‫كلية‬ ‫في‬ ‫الخدمات‬ ‫مكتبة‬ ‫في‬ ً‫ا‬‫أيض‬
Thank you for choosing this book!
After you finish reading it, please take few minutes and scan
this QR Code to answer the online survey about the book.
Your opinions are of great importance and will certainly help
us in the next editions.
Ahmed Eljack

4. 2
Dedication:
To my family, friends, and teachers.

5. 3
PREFACE
Neuroanatomy along with CNS physiology are considered by many
medical students as tough subjects. Although being closely related, most
universities in Sudan teach them separately. The idea of this review book is to
bring those subjects together in bullet points fashion with figures and
illustrations in order to make understanding and mastering these topics easier
for medical students. This book is also beneficial to those who want to revise
neuroscience quickly before tests and examinations.
Please feel free to contact me if you have any feedback or suggestions.
Acknowledgement:
I would like to thank my parents for their support and help throughout
my life. I would also like to thank my colleagues Anas H. Z. Elshiekh and Fatima
Saad for their help and suggestions.
My sincere thanks to my colleague Hafsa Alfadil for the nice drawings
and illustrations. Special thanks to my mentor Dr. Hosam Eldeen Elsadig for his
great help and guidance.

6. 4
Contents
1. INTRODUCTION..........................................................................................................5
2. GENERAL HISTOLOGY OF THE NERVOUS SYSTEM......................................................9
3. SPINAL CORD............................................................................................................13
4. BRAIN STEM..............................................................................................................21
5. CEREBELLUM............................................................................................................32
6. DIENCEPHALON........................................................................................................38
7. TELENCEPHALON (CEREBRAL HEMISPHERES)..........................................................44
8. THE SOMATOSENSORY SYSTEM...............................................................................60
9. THE VISUAL SYSTEM.................................................................................................72
10.AUDITORY AND VESTIBULAR SYSTEMS ....................................................................77
11.SMELL (OLFACTION) & TASTE (GUSTATION)............................................................82
12.MOTOR SYSTEM .......................................................................................................85
13.ELECTRICAL ACTIVITY OF THE BRAIN & SLEEP..........................................................92
14.HIGHER FUNCTIONS & THE LIMBIC SYSTEM ............................................................94
15.THE AUTONOMIC NERVOUS SYSTEM.......................................................................97
16.CRANIAL NERVES....................................................................................................103
17.IMPORTANT CLINICAL CONSIDERATIONS OF THE NERVOUS SYSTEM ...................114
References........................................................................................................................134

7. 5
INTRODUCTION
- The nervous system is one of the most important systems in body (if not
the most important). It contains more than 100 billion neurons. It
controls the whole body and it’s responsible for sensory, motor, and
cognitive and higher functions.
- The human nervous system is really what distinguish it from other
animals. It’s capable of thinking, memorizing, feeling, and doing a lot of
other tasks. Animals can do that but in much less quality than humans.
The brain can perform millions of calculations and processes in one
second, from receiving sensation from your hands while you are holding
the book to thinking about how marvelous is the brain while you read
these lines.
 Applications of Neuroscience:
1- Clinical applications: To diagnose and treat neurological and
psychiatric disorders which affect large populations of the world.
2- To discover how our wonderful brains perform their function and use
this to get the maximum output of it.
3- Many uses in artificial intelligence and computing neuroscience.
4- Many other uses in economics, politics, marketing…etc.
 Embryological Origin of the Central Nervous System (figure 1-1):
 The central nervous system appears at the beginning of the third week
as a thickening of the ectoderm which is called the neural plate. Its
lateral edges soon elevate to form the neural folds which develop and
further and fuse to form the neural tube.
 The neural tube has two ends, cranial end (which forms the brain) and a
caudal end (which forms the spinal cord).

8. 6
 Matrix cells develop and migrate to form the intermediate zone. This
intermediate zone forms the gray matter of the spinal cord.
 The matrix cells also form the astrocytes and the oligodendrocytes.
 The Neuroblasts form the marginal zone which give rise to nerve fibers
that become myelinated and form the white matter of the spinal cord.
 The cranial end of the neural tube gives three vesicles:
i) Forebrain vesicle (which forms the telencephalon and diencephalon
by the fifth week).
ii) Midbrain vesicle.
iii)Hindbrain vesicle (which forms the metencephalon and
myelencephalon by the fifth week).
 Further information will be mentioned in each part.
 General Organization of the Nervous System (figure 1-2):
 The nervous system is divided into anatomical and physiological
divisions, each one with subdivisions.
 Anatomical subdivisions:
i) Central nervous system (CNS): It includes the brain and the spinal
cord and they are protected by cerebrospinal fluid, meninges, and the
skull and vertebral column.
ii) Peripheral nervous system: It includes the spinal and cranial nerves.
 Physiological subdivisions:
i) Somatic nervous system: It innervates structures originating from the
somites (muscles, skin and mucous membranes).
ii) Autonomic (visceral) nervous system: It control smooth muscles and
glands of the internal organs and blood vessels and return sensory
inputs to the brain.
 Some physiologists divide the nervous system into:
i) The motor system: it controls body activity by inducing the
contraction of skeletal and smooth muscles and stimulating the
release of important chemicals.

9. 7
ii) The sensory system: it tells us about the outside and even the inside
of our bodies.
 The brain consists of cerebrum, brain stem and cerebellum. The
cerebrum is divided into Telencephalon (cerebral cortex, subcortical
white matter and the basal ganglia) and Diencephalon (thalamus,
subthalamus, epithalamus and hypothalamus).
 The brainstem consists of midbrain (mesencephalon), pons and medulla
oblongata.
 General Information about the Blood Supply of the CNS (figure 1-3):
 All the arteries that supply the brain originate from the two internal
carotid arteries and the two vertebral arteries. Their branches run in the
subarachnoid space and anastomose to form the circle of Willis.
 Each internal carotid artery divides into the anterior and middle cerebral
arteries. The two anterior cerebral arteries are connected together by
the anterior communicating artery.
 The vertebral arteries form many branches and the posterior cerebral
arteries are among them. The posterior cerebral arteries communicate
with branches of the carotid arteries by the posterior communicating
arteries.
 The blood supply of the spinal cord consists of the two posterior spinal
arteries and anterior spinal artery along with small segmental arteries
that arise from outside the vertebral column.
 Further information will be mentioned following each part.

10. 8
Figure 1-1: Development of the neural
tube
Figure 1-2: General organization of the CNS
Figure 1-3: General blood supply of the CNS

11. 9
GENERAL HISTOLOGY OF THE NERVOUS SYSTEM
 Neurons:
 They are the functional unit in the entire nervous system.
 The neuron usually consists of three parts (figure 2-1):
i) The cell body (perikaryon): contains the nucleus and most organelles.
ii) The dendrites: elongated processes extending from the perikaryon
and specialized in receiving impulses from other neurons via
synapses.
iii) The axon: single long process specialized to generate and conduct
impulses to other cells. They may also receive stimuli from other
neurons.
 Neurons can be divided into (figure 2-2):
i) Multipolar neurons: have one axon and two or more dendrites.
ii) Bipolar neurons: have one dendrite and one axon.
iii)Unipolar (or pseudounipolar) neurons: have one process that
bifurcates and the longer one extends to the periphery unlike the
short one which extends to the CNS.
iv)Anaxonic neurons: many dendrites with no axon.
 Most perikarya are in the gray matter of the CNS while the axons are in
the white matter of the CNS and the peripheral nervous system.
 The white matter is called so because of the lipid-rich myelin sheath that
encloses the axons which gives the relatively white appearance.
 Synapses are sites where the action potential is conducted from a neuron
to another or to other cells. In the synapse the electrical impulse is
converted to chemical signal to affect the postsynaptic cell by a
neurotransmitter. The signal passes in the forward direction. Synapses
can be excitatory or inhibitory.

12. 10
i) Excitatory synapses: neurotransmitters cause postsynaptic Na+
channels to open which initiate depolarization.
ii) Inhibitory synapses: neurotransmitters open anion channels (usually
Cl-) causing hyperpolarization.
 Neurotransmitters are removed quickly after their release by enzymes,
endocytosis, or diffusion.
 Morphological types of synapses (figure 2-3):
i) Axosomatic: axon with perikaryon.
ii) Axodendritic: axon with dendrites.
iii)Axoaxonic: axon with another axon.
 The most important neurotransmitters are: glutamine, serotonin,
dopamine, substance P, and catecholamines.
 Glial Cells (figure 2-4):
 Glial cells have an important role in supporting the neurons in many
ways. They provide neurons with nutrition, support, and protection and
help in regulating the neural activity.
 They are the most abundant cells in the brain and most of them develop
from the progenitor cells of the neural plate.
 There are six types of the glial cells:
i) Oligodendrocytes: they are the predominant glial cells in CNS white
matter. They produce myelin sheaths around the axons in the CNS.
ii) Schwann cells (neurolemmocytes): they are responsible for the
myelination of PNS. They are similar to the oligodendrocytes.
iii)Astrocytes: they regulate the extracellular ionic concentration,
physically support the neurons, contribute to the blood-brain barrier,
Clinical note: Selective Serotonin Reuptake Inhibitors are used to treat
depression and anxiety. They inhibit the uptake of serotonin and
augment their levels in the postsynaptic membrane.

13. 11
and control vasodilation within the CNS. They are the most common
source of brain tumors.
iv)Ependymal cells: they are columnar or cuboidal cells that line the
brain ventricles and the central canal in the spinal cord. They help in
cerebrospinal fluid movement and absorption.
v) Microglia: they originate from the blood monocytes and migrate
through the neuropils to attack microorganisms and remove damaged
cells. They are the major immune defense in the CNS.
vi)Satellite cells of ganglia: they form a layer around the perikarya of the
ganglia and have supporting effect on these neurons.
Clinical note: in multiple sclerosis the myelin sheaths are damaged by T
lymphocytes and microglia due to autoimmune disease.
Figure 2-1: Neuron
Figure 2-2: Morphological
types of neurons

14. 12
Figure 2-3: Types of synapses
Figure 2-4: Some types of glial cells

15. 13
SPINAL CORD
 Development of the Spinal Cord:
 The cells in the neural tube (which is derived from the ectoderm)
differentiate to form an ependymal layer that encircles the central canal
and is surrounded by the mantle and marginal zones of primitive
neurons and glial cells.
 The mantle zone gives the alar plate which contains neuroblasts that will
form sensory neurons and a basal plate which contains neuroblasts that
will form motor neurons. Sulcus limitans demarcates the two regions.
 External Features of the Spinal Cord:
 The spinal cord is about 42-45 cm in length. Conus medullaris is the
terminal end of the spinal cord and it ends about the level of L1 or L2
vertebrae. Filum terminale is a continuation of the pia mater and is
attached to the distal dural sac in the back of the first segment of the
coccyx.
 The central canal is continuous above with the fourth ventricle and it is
filled with CSF.
 Lateral enlargements of the spinal cord are in the cervical and lumbar
segments. They contain more lower motor neurons and provide origins
of nerves for the upper and lower limbs
 The spinal cord is divided into about 31 segments (figure 3-1):
i) 8 cervical segments.
Clinical note: failure of the caudal end of the neural tube to fuse causes
spina bifida. Meningocele occurs when part of the meninges balloon out.
If it contains nervous tissue, it’s called meningomyelocele which causes
severe malfunction.

16. 14
ii) 12 thoracic segments.
iii)5 lumbar segments.
iv)5 sacral segments
v) Few coccygeal segments.
 The spinal cord is shorter than the vertebral column. That is why the
lower segments of the spinal cord are located above the similarly
numbered vertebra.
 The spinal cord is divided to right and left halves by the anterior median
fissure and the posterior median sulcus (figures 3-3 and 3-4). The dorsal nerve
roots are attached to the posterolateral sulcus while the ventral nerve
roots exit in the anterolateral sulcus.
 Cauda Equina is formed by the lower lumbosacral segments of the spinal
cord. Its name is derived from its horse tail-like appearance.
 The ventral roots give mainly the motor part in a spinal nerve. They carry
the alpha motor neurons (to extrafusal muscle fibers), gamma motor
neurons (to intrafusal muscle fibers), some autonomic preganglionic
fibers and few afferent axons that convey sensation from the thoracic
and abdominal viscera.
 The intrafusal fibers are specialized muscle fibers that control the basic
muscle tone. The extrafusal fibers are large muscle fibers that perform
the voluntary muscle contraction.
 The dorsal roots are mainly sensory and contain fibers from
subcutaneous and deep structures.
 The physioanatomic classification of the nerve fibers:
i) Somatic Efferent Fibers: form the ventral roots and innervate the
skeletal muscles.
ii) Somatic Afferent Fibers: they carry sensations from the skin, muscles,
and joints. Their cell bodies are unipolar.
iii)Visceral Efferent Fibers: they form autonomic motor fibers to viscera,
including sympathetic and parasympathetic fibers.

17. 15
iv)Visceral Afferent Fibers: they carry sensation from the viscera.
 Internal Divisions of the Spinal Cord:
 The spinal cord consists of gray matter and white matter. They are
furtherly divided into columns and laminae.
 The gray matter consists of two portions connected by a commissure of
gray matter. The anterior gray column (horn) is in the front of the central
canal and contains the cells of the ventral roots.
 The intermediolateral column (horn) lies between the anterior and
posterior gray columns. It is prominent in the thoracic and upper lumbar
regions. In thoracolumbar region (T1-L2) it contains the sympathetic
preganglionic neurons which give rise to sympathetic axons that travel
via ventral roots to the sympathetic ganglia. In the sacral region (S2, 3
and 4) it contains the preganglionic parasympathetic neurons which give
rise to axons that leave within the ventral roots.
 The dorsal column (horn) extends to the posterolateral sulcus.
 The shape of the gray matter differs at different segments of the spinal
cord. The proportion of the gray matter to white matter is greatest in the
cervical and lumbar enlargements.
 Laminas (or laminae) are layers of nerve cells. In the gray matter of the
spinal cord they are termed Rexed’s laminae (figure 3-5).
i) Lamina I: thin layer. Respond to noxious stimuli. Has high
concentration of Substance P.
ii) Lamina II (Subistantia Gelatinosa): some of its neurons respond to
pain. Has high concentration of Substance P.
iii)Laminas III and IV (nucleus proprius): their main input is fibers that
carry position and light touch sensations.
iv)Lamina V: cells respond to noxious and visceral afferent stimuli.
v) Lamina VI: cells respond to mechanical signals from joints and skin.
vi)Lamina VII: contains cells of the dorsal nucleus (Clarke’s column).

18. 16
vii) Laminas VIII and IX: they represent motor neurons in the ventral
gray column.
viii) Lamina X: small cells around the central canal.
 The white matter of the spinal cord is divided into dorsal, lateral and
ventral columns (funiculi). In the cervical and upper thoracic region the
dorsal column is divided into fasciculus gracilis and fasciculus cuneatus.
 Tracts in the Spinal Cord (figure 3-6):
 The spinal cord contains ascending and descending tracts. You can tell
whether a tract is ascending or descending by checking the order of the
words in its name. For example, the descending tracts are named
______spinal tract while the ascending tracts are usually named
spino____ tract.
 The descending tracts are the following:
i) The corticospinal tract.
ii) The vestibulospinal tract.
iii)The rubrospinal tract.
iv)The reticulospinal tract.
v) The tectospinal tract.
vi)The descending autonomic system.
 The ascending tracts are the following:
i) The medial longitudinal fasciculus.
ii) The dorsal column tract.
iii)The spinothalamic tracts.
iv)The spinoreticular tract.
v) The spinocerebellar tracts.
 The corticospinal tract arises from the cerebral cortex and descends
through the brainstem then decussates downward into the lateral white
column. It contains the axons of the upper motor neurons. It controls
voluntary and highly skilled movements and some fibers function as
modifiers of sensory information.

19. 17
 The vestibulospinal tract arises from the medial and lateral vestibular
nuclei. Fibers of this tract provide excitatory input to the lower motor
neurons for extensor muscles. The vestibulospinal system reacts to
sudden changes in body position.
 The rubrospinal tract arises from the contralateral red nucleus and
travels in the lateral white column. It plays a role in motor function.
 The reticulospinal tract arises from the reticular formation and descends
in the ventral and lateral white columns. It modifies the transmission of
sensations and various reflexes.
 The descending autonomic system arises from the hypothalamus and
brainstem and modulates autonomic functions.
 The tectospinal tract arises from the superior colliculus and descends in
the contralateral anterior white column. It transmits the response to
sudden visual stimuli.
 The medial longitudinal fasciculus arises from vestibular nuclei and
coordinates head and eye movements.
 The ascending dorsal column tract (part of the medial lemniscal system)
carries localized fine touch, vibration, two-point discrimination, and
proprioception from the skin and joints. The fasciculus gracilis carries
input from the lower half of the body and the fibers that arise from a
lower segment course more medially. The fasciculus cuneatus carries
input from the upper half of the body and the fibers from a lower
Clinical note: the second order neurons of the dorsal column tracts
decussate at the level of the lower medulla while those of the
spinothalamic tract decussate at the level of the same segment of the
spinal cord. This fact helps in determining the site of a lesion in the CNS.
For example, if there is a lesion in the spinal cord proprioception will be
lost ipsilaterlly but if the lesion is in the medulla or higher
proprioception will be lost contralaterlly.

20. 18
segment are more medial than the upper segments. Fibers of these
tracts decussate at the lower part of the medulla oblongata.
 The spinothalamic tracts (ventrolateral system) carry sharp pain,
temperature and poorly localized touch. They decussate at the level of
the spinal cord. Sensations from a lower part of the body are carried
more laterally than sensations of an upper part. The anterior
spinothalamic tract conveys light touch sensation, while the lateral
spinothalamic tract conveys pain and temperature sensations.
 The spinoreticular tract travels within the ventrolateral portion of the
spinal cord. It terminates in the reticular formation of the brainstem. It
plays an important role in the sensation of pain (especially the chronic
pain).
 The spinocerebellar tracts provide inputs to the cerebellum, they are
two types:
i) Dorsal spinocerebellar tract: afferent fibers from the muscles and
skin enter the spinal cord and synapse on nucleus dorsalis (Clarke’s
column). Second order neurons form the cuneocerebellar tract which
remain ipsilateral then enter the cerebellum via the inferior
cerebellar peduncle.
ii) Ventral spinocerebellar tract: it is involved in movement control.
 Blood Supply of the Spinal Cord:
 The spinal cord is supplied mainly by one anterior and two posterior
(right and left) spinal arteries.
 The anterior spinal artery lies in the anterior median fissure. It is formed
by the union of the two anterior spinal branches of the two vertebral
arteries above foramen magnum. It supplies the whole cord anterior to
the posterior gray columns (the anterior two thirds of the spinal cord).
 A posterior spinal artery arises from the posterior inferior cerebellar or
vertebral artery above foramen magnum. It supplies the gray and white
posterior columns of its own site (the posterior third of the spinal cord).

21. 19
 Reticular arteries are important in supplying the spinal cord. They are
derived from various vessels depending on the level. They are variable in
number and position and blood from them may flow up or down the
cord.
 The spinal veins form plexuses that contain an anterior and posterior
midline longitudinal vein and a pair of longitudinal veins near the nerve
roots. These veins drain to the internal vertebral venous plexus then to
the segmental veins via the external vertebral venous plexus or veins of
the medulla at foramen magnum.
 The segmental veins are:
i) Vertebral vein (in the neck).
ii) Azygos vein (in the thorax).
iii)Lumbar veins (in the lumbar region).
iv)Lateral sacral vein (in the sacral region).
Figure 3-1: The spinal cord as a continuation
of the brainstem and its divisions
Figure 3-2: Cross section of the spinal cord in
different segments showing the variation in
the gray and white matters: (A) cervical, (B)
thoracic, (C) lumbar, and (D) sacral

22. 20
Figure 3-3: Cross section of the spinal cord with the spinal roots
Figure 3-4: Another cross section of the
spinal cord showing the central canal and
the median fissure
Figure 3-5: The laminae in the spinal cord
Figure 3-6: Tracts of the spinal cord. The blue color indicates ascending
tracts while the red color indicates descending tracts

23. 21
BRAIN STEM
 Development of the Brainstem:
 The lower cranial portion of the neural tube gives rise to the brainstem,
which is divided to the mesencephalon and rhombencephalon.
 The primitive central canal widens to form the forth ventricle which
extends over the pons and the medulla.
 The mesencephalon forms the quadrigeminal plate, midbrain
tegmentum, and the cerebral peduncles. While the rhombencephalon
forms the metencephalon and myelencephalon.
 The metencephalon forms the cerebellum and pons. While the
myelencephalon forms the medulla oblongata.
Main Divisions and External Features (figure 4-1):
 The brainstem consists of the midbrain, pons and medulla oblongata
(sometimes the cerebellum is considered as a part).
 There are three internal longitudinal divisions which are the tectum (in
the midbrain), tegmentum and basis.
 The superior, middle and inferior cerebellar peduncles form connections
with the cerebellum.
 There are four hillocks in the posterior aspect of the midbrain: two
superior and two inferior colliculi.
 Internal Components:
 Descending tracts that originate in the brain stem or terminate in the
spinal cord pass through the brain stem. All ascending tracts that reach
cerebral cortex pass through the brain stem.
 All cranial nerves nuclei are located in the brain stem except the first two.
Some of the cranial nerves pass through the brain stem.

24. 22
 The reticular formation lies in the tegmentum of the brain stem and
involved in many important functions like respiration,
consciousness….etc.
 Cranial Nerves Nuclei:
 General somatic efferent (GSE) components innervate striated muscles
that are derived from the somites and are involved in the movement of
the tongue, eyes, and some muscles in the neck.
 Special visceral efferent (SVE) components innervate muscles that are
involved in chewing, facial expressions, swallowing, producing vocal
sounds and turning the head.
 General visceral efferent (GVE) components are parasympathetic
preganglionic components that innervate the head and neck.
 General somatic afferent (GSA) components receive sensory stimuli
from the skin and mucosa of the head.
 General visceral afferent (GVA) components receive sensations and
taste stimuli.
 The special sensory (SS) nuclei are the four vestibular and two cochlear
nuclei.
 Further information are mentioned in chapter 16.
 Medulla Oblongata:
 The medulla (myelencephalon) is located between the spinal cord and
the pons.
 It is divided into caudal part (which is called closed medulla and contains
the pyramidal decussation (figure 4-2) and the decussation of the medial
lemniscus) and rostral part (called open medulla) depending on the
absence or presence of the lower part of the fourth ventricle.
 There are two swelling on the dorsal surface of the medulla. They are
referred to as the tuberculum gracilis and tuberculum cuneatus. They
reflect the nucleus gracilis and nucleus cuneatus, respectively. These
nuclei receive inputs from fasciculus gracilis and fasciculus cuneatus.

25. 23
 The inferior cerebellar peduncle is an expansion of the dorsolateral
surface of the open medulla which contains proprioceptive and
vestibular fibers projecting to the cerebellum (all of them are afferent to
the cerebellum) which are:
i) The cuneocerebellar and dorsal spinocerebellar tracts.
ii) Fibers from the lateral reticular nucleus.
iii)Olivocerebellar tract (from the contralateral inferior olivary nucleus)
iv)Fibers from the vestibular part of the nerve VIII and fibers from the
vestibular nuclei.
 The pyramid is a swelling in the medial aspect of the ventral surface. It is
composed of nerve fibers that arise from the precentral, postcentral, and
premotor areas of the cerebral cortex.
 The decussation of the pyramids is a bundle of fibers that cross to the
opposite side (about 90% of the corticospinal tracts). The decussated
fibers continue as the lateral corticospinal tract while the uncrossed
fibers continue ipsilaterally as the anterior (ventral) corticospinal tract.
 The olives are swellings lateral to the pyramids due to the underlying
inferior olivary nucleus (figure 4-3).
 In the caudal part of the medulla, the relay nuclei of the dorsal column
pathway give rise to the medial lemniscus, in which the lower part of the
body is represented in the anterior portion while the upper part is
represented in the posterior portion.
 The spinothalamic, spinoreticular, and the ventral spinocerebellar
tracts continue upward through the medulla. While the dorsal
spinocerebellar and the cuneocerebellar tracts continue into the
inferior cerebellar peduncle.
 Most of the axons of the corticospinal tract arise from the motor
cerebral cortex. Some fibers arise from the sensory cortex and end in the
dorsal column nuclei to modify their function. Thus modifying incoming
sensory information.

26. 24
Figure 4-1: Main divisions of the brainstem
Figure 4-2: Transverse section of the
medulla at the level of the decussation
of the pyramids

27. 25
Figure 4-3: Transverse section of the medulla at the level of the olives
Figure 4-4: Cross section of the medulla a level above the olives

28. 26
 The medial longitudinal fasciculus is involved in head movements. It
arises in vestibular nuclei and projects to the abducens, trochlear, and
oculomotor nuclei.
 The tectospinal tract passes through the medulla and it is involved in
controlling neck and trunk movements in response to visual stimuli.
 The medulla contains many cranial nerves nuclei, here is a summary:
i) The hypoglossal nucleus sends its fibers anteriorly between the
pyramid and the olives to innervate the tongue.
ii) The dorsal motor nucleus of the vagus sends its fibers to the vagus
and accessory nerves. It is a preganglionic parasympathetic nucleus
that controls parasympathetic tone of the heart, lung, and abdominal
viscera.
iii)The ambiguus nucleus gives rise to the efferent axons in the
glossopharyngeal and vagus nerves. It controls swallowing and
speech.
iv)The solitary nucleus is a sensory nucleus that receives axons from the
facial, glossopharyngeal, and vagus nerves. It conveys taste and
visceral sensations. Secondary fibers ascend to the
ventroposteromedial nucleus of the thalamus then project to the
taste cortical area (area 43). The gustatory nucleus lies in the rostral
part of the solitary nucleus.
v) There are four vestibular nuclei (superior, inferior, medial, and
lateral). They are found under the floor of the fourth ventricle.
vi)The two cochlear nuclei (ventral and dorsal) are relay nuclei for fibers
that arise in the spiral ganglion in the cochlea.
 Pons:
 The pons serves as a connection between the medulla and the midbrain.
The word pons means “bridge”.
 The pons is divided into the basilar pons (basis pontis) ventrally and the
tegmentum dorsally.

29. 27
 The basis pontis contains three components:
i) Fibers of the corticospinal tract.
ii) Pontine nuclei (receive input from the cerebral cortex by the
corticopontine pathway.
iii)Pontocerebellar fibers from the pontine nuclei (project to the
cerebellum by the middle cerebellar peduncle).
 The raphe nuclei lie along the midline of the pons and the upper medulla.
They are important in controlling the level of arousal and modulating
pain sensation.
 The lower part of the tegmentum contains the abducens nucleus (of the
abducent nerve) and the nuclei of the nerve VII (the facial, superior
salivatory and gustatory nuclei).
 The motor component of the facial nerve loops around the abducens
nucleus forming a bulge. This is called the facial colliculus (figure 4-5).
 In the pons the medial lemniscus assumes a different position in which
the lower body is represented medially while the upper body is
represented laterally.
 The tegmentum also contains the tectospinal tract and the medial
longitudinal fasciculus.
 The middle cerebellar peduncle is the largest peduncle. It contains fibers
from the contralateral basis pontis.
 The trigeminal system in the brainstem is formed by the following nuclei:
i) The main sensory nucleus (for fine touch).
ii) The descending spinal tract of the trigeminal (for pain and
temperature).
iii)The mesencephalic tract and nucleus (for proprioception).
iv)The masticatory nucleus (sends efferent fibers to innervate the
muscles of mastication and tensor tympani muscle).
 Midbrain:
 The midbrain lies between the pons and the cerebrum.

30. 28
 It is divided into three components:
i) The tectum in the most dorsal part. Which is sometimes referred to
as the corpora quadrigeminal.
ii) The crus cerebri in the ventral part. It contains the fibers that pass
from the cerebrum to the brainstem and spinal cord.
iii)The tegmentum in the central part, which is continuous with the
tegmentum of the pons.
 Substantia nigra separates the tegmentum from the crus cerebri. The
cerebral aqueduct separates the tectum from the tegmentum.
 The substantia nigra cells contain neuromelanin. It receives afferent
fibers from the cerebral cortex and the striatum and sends dopaminergic
fibers to the striatum. It plays an important role in movement.
 The cerebral peduncle is the external aspect of the crus cerebri which
connects it to the cerebrum.
 The tegmentum of the midbrain contains all the ascending and many
descending tracts.
 The red nucleus receives efferent fibers from the cerebellum and sends
fibers to the thalamus and the contralateral spinal cord via the
rubrospinal tract. It is important in motor coordination.
 The trochlear nucleus and the oculomotor nuclei lie in the upper
tegmentum. The oculomotor nerve cells are divided into subgroups. The
subgroup for the superior rectus muscle in the eye is contralateral while
the others are ipsilateral to the innervated muscle.
 The preganglionic parasympathetic system of the eye has its origin near
or in the Edinger-Westphal nucleus.
 The locus ceruleus nuclei lie close to the periventricular gray matter.
Neurons in these nuclei regulate the sleep-wake cycle and arousal and
may modulate sensitivity of the sensory nuclei.

31. 29
 The superior colliculi (figure 4-7) receive visual input mainly and serve
ocular reflexes. They are linked to the lateral geniculate body by the
superior quadrigeminal brachium.
 The inferior colliculi (figure 4-8) are involved in determining the side on
which a sound originates and auditory reflexes. They receive input from
both ears and project to the medial geniculate body by the inferior
quadrigeminal brachium.
 The periaqueductal gray matter contains endorphin-producing cells
(suppress pain) and descending autonomic tracts.
 Efferent fibers from the dentate nucleus of the cerebellum exit to the
contralateral red nucleus through the superior cerebellar peduncle,
which also carries the afferent ventral spinocerebellar tract. The
cerebellar fibers decussate below the red nuclei.
 Blood Supply of the Brainstem:
 There are three groups of vessels that supply the brainstem:
i) Circumferential vessels.
ii) Median (paramedian) perforators (they penetrate the brainstem
from the basilar artery).
iii)Small medullary and spinal branches of the vertebral artery.
 The circumferential vessels are:
i) Posterior inferior cerebellar artery.
ii) Anterior inferior cerebellar artery.
iii)Superior cerebellar artery.
iv)Posterior cerebral artery.
v) Pontine artery.

32. 30
Figure 4-5: Transverse section of the caudal part of the pons showing the facial colliculus
Figure 4-6: Transverse section of the rostral part of the pons

33. 31
Figure 4-7: Transverse section of the midbrain at the level of the superior colliculi
Figure 4-8: Transverse section of the midbrain at the level of the inferior colliculi

34. 32
CEREBELLUM
 Development, External Features, and Divisions of the
Cerebellum:
 The cerebellum originates from the rhombic lips (which are formed by
the bending of the dorsolateral parts of the alar plate).
 The cerebellum is located posterior to the brainstem in the posterior
cranial fossa. It is separated from the occipital lobe of the cerebrum by
the tentorium cerebelli. The vermis is a midline portion that separates
the cerebellum into two lateral lobes. The external surface of the
cerebellum has a large number of narrow folds which are termed folia.
 The cerebellum consists of cerebellar cortex and cerebellar white
matter. The deep cerebellar nuclei are four in number and they are
located in the cerebellar white matter and sometimes they are referred
to as roof nuclei because they lie in the roof of the forth ventricle.
 The deep cerebellar nuclei are (from medial to lateral):
i) Fastigial nucleus.
ii) Globose nucleus.
iii)Emboliform nucleus.
iv)Dentate nucleus.
 The horizontal fissure (figure 5-1) extends around the posterolateral border
of each hemisphere and divides the cerebellum into superior and
inferior halves. The primary fissure divides each hemisphere into a small
anterior lobe in front of the fissure and a large posterior lobe behind it.
Clinical note: tumors or edema of the cerebellum can cause
obstructive hydrocephalus because of the location of the fourth
ventricle in relation to the cerebellum.

35. 33
 The flocculonodular system is connected to the vestibular system and it
is concerned with equilibrium. It consists of the flocculus, the nodulus,
and interconnections.
 The paleocerebellum is involved in propulsive movements. It consists of
the anterior portions of the hemispheres and the anterior and posterior
vermis.
 The neocerebellum is the remainder of the cerebellum and is involved in
coordination of fine movements.
 Functions of the Cerebellum:
 The main functions of the cerebellum are:
i) Coordinating skilled voluntary movements.
ii) Controlling equilibrium.
iii)Controlling muscle tone.
 The cerebellar cortex has a somatotopic arrangement of the body parts.
 The cerebellum receives collateral input from the sensory and special
sensory systems.
 The vermis affects the muscles of the trunk while each cerebellar
hemisphere has the same function on the same side of the body.
 Peduncles:
 There are three pairs of peduncles that attach the cerebellum to the
brainstem and contain pathways to and from the brainstem.
 The inferior cerebellar peduncle contains many fibers from the spinal
cord and the medulla oblongata. It also contains fibers to and from the
vestibular nuclei.
 The middle cerebellar peduncle contains fibers from the contralateral
pontine nuclei which receive inputs from the cerebral cortex.
 The superior cerebellar peduncle contains axons that send impulses to
the thalamus and the spinal cord with relay in the red nuclei.

36. 34
 Mossy and Climbing Fibers:
 Mossy and climbing fibers are afferent fibers to the cerebellar cortex.
 Mossy fibers course through the granular layer (will be described
shortly) and give many branches in this layer which terminate by forming
mossy fibers rosettes which are held by dendrites of the granule cells.
 The cerebellar glomerulus consists of:
i) A mossy fiber rosette.
ii) Dendrites of many granule cells.
iii)The proximal aspect of Golgi cells nuclei.
iv)Terminals of Golgi cells axons.
 Mossy fibers are excitatory and arise from all the regions of the CNS that
project to the cerebellar cortex (except the inferior olivary nucleus).
Glutamate is believed to be their neurotransmitter.
 Climbing fibers arise from the inferior olivary nucleus and reach the
molecular layer of the cerebellar cortex. They make synapses by
climbing up the branches of the dendrites of Purkinje cells (one climbing
fiber excite one Purkinje cell in contrast to mossy fibers). Their
neurotransmitter is believed to be aspartate.
 Histology of the Cerebellar Cortex:
 The cerebellar cortex consists of three layers of cells (from outside to
inside):
i) The outer molecular layer (contains basket, Golgi, and stellate cells).
ii) The Purkinje cell layer (contains cell bodies of Purkinje cells).
iii)The inner granular layer (contains cell bodies of granule cells).
 Granule cells have their cell bodies located in the granular layer and they
send their axons to the molecular layer. They are the only excitatory
neurons in the cerebellar cortex. The parallel fiber are T-like bifurcation
of the granule cells axons. The nonmyelinated parallel fibers synapse on
the Purkinje cells dendrites (excitatory). The neurotransmitter is believed
to be glutamate.

37. 35
 Purkinje cells have their cell bodies in the Purkinje cell layer. They are
the primary output from the cerebellar cortex. Their axons project to the
deep cerebellar nuclei (especially the dentate nucleus) and form
inhibitory synapses.
 Basket cells are located in the molecular layer. They receive excitatory
inputs from the parallel fibers but they inhibit the Purkinje cells
 Golgi cells are located in the molecular layer. They receive inputs from
parallel fibers and mossy fibers and they inhibit the granule cells.
 Stellate cells are similar to the basket cells.
 Deep Cerebellar Nuclei:
 They are four nuclei that are embedded in the white matter of the
cerebellum. They represent the major efferent pathway from the
cerebellum.
 Purkinje cells send inhibitory impulses to the deep cerebellar nuclei cells
(the neurotransmitter is GABA).
 Sites that send excitatory outputs to the deep cerebellar nuclei:
i) Pontine nuclei.
ii) Inferior olivary nucleus.
iii)Reticular formation.
iv)Locus ceruleus.
v) Raphe nuclei.
vi)Inputs that give climbing and mossy fibers.
 Efferents from the Cerebellum:
 The dentatorubrothalamucortical pathway (from the dentate nucleus
through the red nucleus and the thalamus to the motor cortex) pass
contralaterally through the superior cerebellar peduncle. Via this
pathway, the deep cerebellar nuclei modulate the activity of the
contralateral motor cortex.
 Each cerebellar hemisphere coordinates the ipsilateral side of the body
due to this cross connection.

38. 36
 Neurons of the fastigial nucleus project to the vestibular nuclei
bilaterally and the contralateral reticular formation, pons, and spinal
cord via the inferior cerebellar peduncle.
 Blood Supply of the Cerebellum:
 Two arteries supply the posteroinferior surface and one artery supplies
the upper surface of each hemisphere.
 The posterior inferior cerebellar artery arises from the vertebral artery.
It supplies the inferior vermis and the back of the cerebellar
hemispheres.
 The anterior inferior cerebellar artery arises from the basilar artery. It
supplies the inferior surface of the cerebellar hemispheres and the
adjacent flocculus.
 The superior cerebellar artery arises near the termination of the basilar
artery. It supplies the superior surface of the cerebellum.
 The venous drainage is from the cerebellar surface to the nearest venous
sinus.
Figure 5-1: Superior view of the cerebellum

39. 37
Figure 5-2: Sagittal section of the cerebellum

40. 38
DIENCEPHALON
 Organization and Divisions of the Diencephalon:
 The diencephalon includes the following (figure6-1):
i) The thalamus (including its geniculate bodies).
ii) The hypothalamus.
iii)The subthalamus.
iv)The epithalamus.
 The two halves of the diencephalon lie in either sides of the third
ventricle.
 The hypothalamic sulcus separates the thalamus from the
hypothalamus and subthalamus.
 Thalamus (figure 6-2):
 The thalamus is a large, ovoid, gray mass of nuclei in both hemispheres.
The pulvinar (posterior end) extends over the medial and lateral
geniculate bodies. The interthalamic adhesion (found in some
individuals) connects the two thalami across the third ventricle.
 There are some white matter fibers that emerge through or close to the
thalamus. The thalamic radiations emerge from the lateral surface of the
thalamus and terminates in the cerebral cortex. The external medullary
lamina emerge near the internal capsule. The internal medullary lamina
bifurcates in its anterior end and divide the thalamus into lateral, medial,
and anterior portions.
 The thalamus is divided into groups of nuclei. There are five major groups
of nuclei and each one has specific connections and functions. These are:
i) The anterior nuclear group.
ii) Nuclei of the midline.
iii)The medial nuclei.

41. 39
iv)The lateral nuclear mass.
v) The posterior nuclei.
 The anterior nuclear group is bordered by the limbs of the internal
lamina. It receives fibers from the mammillary bodies and projects to
the cingulate cortex.
 There are nuclei in the midline that are located beneath the lining of the
third ventricle. They connect with the hypothalamus and the
periaqueductal gray matter. Also, there is a centromedian nucleus that
connects with the cerebellum and corpus striatum.
 The medial nuclei includes:
i) Intralaminar nuclei.
ii) Dorsomedial nucleus (projects to the frontal cortex).
 The lateral nuclear mass lies anterior to the pulvinar between the
internal and external medullary laminas. It includes the following:
i) Reticular nucleus.
ii) Ventral anterior nucleus (connects with corpus striatum).
iii)Ventral lateral nucleus (projects to the motor cortex).
iv)Dorsolateral nucleus (projects to the parietal cortex).
v) Ventral posterior (or ventral basal) group (projects to the postcentral
gyrus via the internal capsule), which is divided into the ventral
posterolateral nucleus (relays sensory inputs from the body) and the
ventral posteromedial nucleus (relays sensory input from the face).
 The posterior nuclei include the following:
i) The pulvinar nucleus (connects with the parietal and temporal
cortices).
ii) The medial geniculate nucleus, which lies lateral to the midbrain
(receives acoustic fibers from the inferior colliculus and projects
fibers to the temporal cortex via the acoustic radiation).
iii)The lateral geniculate nucleus (receives fibers from the optic tract
and projects to visual cortex via the geniculocalcarine radiation).

42. 40
 Beside the anatomical divisions, the thalamus is also divided functionally
to the following:
i) The sensory nuclei (ventral posterior group and geniculate bodies).
The thalamus is a very important structure in the perception of some
types of sensations (especially pain).
ii) The motor nuclei (ventral anterior and lateral). They carry
information from the cerebellum and globus pallidus to the motor
cortex.
iii)The anterior limbic nuclei.
iv)The multimodal nuclei (pulvinar, posterolateral, and dorsolateral).
They have connections with the parietal lobes.
v) Nonspecific thalamic nuclei (intralaminar and reticular nuclei and
centrum medianum).
 Hypothalamus:
 The hypothalamus extends from the level of the optic chiasm to the
posterior commissure. It forms the floor of the third ventricle.
 The hypothalamus is divided into:
i) The anterior portion (chiasmatic region).
ii) The central hypothalamus (includes the tuber cinereum and the
infundibulum).
iii)The posterior portion (mammillary area).
 The medial hypothalamic area contains many nuclei and the lateral
hypothalamic area contains fiber systems and diffuse lateral nuclei
 The medial hypothalamus is divided into three portions:
i) The supraoptic portion (contains the supraoptic, suprachiasmatic,
and paraventricular nuclei).
ii) The tuberal portion (contains the ventromedial, dorsomedial, and
arcuate nuclei).
iii)The mammillary portion (contains the posterior nucleus and several
mammillary nuclei).

43. 41
 The lateral hypothalamus is associated with a number of behavioral
processes.
 The preoptic area lies anterior to the hypothalamus.
 The hypothalamus receives inputs from the following:
i) Limbic system structures.
ii) Thalamus.
iii)Cerebral cortex.
iv)Visceral and somatic afferents.
v) Sensors (like osmoreceptors).
 Efferent connections from the hypothalamus include the following tracts
and systems:
i) The hypothalamohypophyseal tract (from supraoptic and
paraventricular nuclei to the posterior pituitary).
ii) Mamillotegmental tract (to the tegmentum).
iii)The mamillothalamic tract (to the anterior thalamic nuclei).
iv)The periventricular system.
v) The tuberhypophyseal tract (from the tuberal portion to the
posterior pituitary).
vi)Fibers from the septal region to the hippocampus.
 The hypothalamus has important regulatory functions, some of them will
be discussed in brief:
i) The lateral hypothalamus contains the feeding center which evokes
eating behavior, whereas the satiety center lies in the ventromedial
nucleus and it inhibits the feeding center.
ii) Some parts of the hypothalamus function as sympathetic activators
whereas others function as parasympathetic activators.
iii)The hypothalamus contains the temperature regulation center.
iv)The thirst center in the hypothalamus contains the osmoreceptors
that detect changes in the blood osmolarity which is important in
water balance and cardiovascular processes.

44. 42
v) The hypothalamus regulates many endocrine functions.
vi)Circadian rhythm is controlled by some nuclei in the hypothalamus.
Thus affecting sleep and other biological rhythms.
vii) The hypothalamus is involved in expression of emotions.
viii) The hypothalamus is involved in sexual behaviors.
 Subthalamus:
 The subthalamus lies in the posterior diencephalon between the dorsal
thalamus and the tegmentum of the midbrain.
 The subthalamus receives and projects fibers to the globus pallidus.
 The subthalamus contains two nuclear groups:
i) The subthalamic nucleus.
ii) Zona incerta.
 The subthalamic nucleus plays an important role in regulation of motor
functions.
 Epithalamus:
 The epithalamus forms the roof of the diencephalon. It consists of the
following structures:
i) The habenular complex.
ii) The stria medullaris.
iii)The pineal gland.
 The pineal gland has no direct connections with the CNS but it receives
neural inputs from the sympathetic nervous system via the superior
cervical ganglia.
 The pineal gland displays a circadian rhythm which results in releasing of
several hormones from specialized secretory cells called pinealocytes
and hypothalamic releasing hormones.
 Blood Supply of the Diencephalon:
 See the blood supply of the cerebrum and brainstem.

45. 43
Figure 6-1: Cross section of the brain showing the diencephalon
Figure 6-2: Thalamus

46. 44
TELENCEPHALON (CEREBRAL HEMISPHERES)
 Development of the Telencephalon:
 The telencephalon is a derivative of the upper end of the neural tube.
The basal ganglia arise from the primitive telencephalic vesicles.
 Anatomy and External Features of the Cerebral Hemispheres:
 The cerebral hemispheres are the largest portion of the human brain.
They occupy the anterior and middle cranial fossae and the posterior
cranial fossa above the tentorium cerebelli.
 The cortex is folded into gyri (plural of gyrus). The sulci (plural of sulcus)
are the grooves between the gyri and fissures are deeper sulci (figures 7-1
and 7-2).
 The cerebral hemispheres are divided into lobes in each side (figures 7-3 and
7-4):
i) Frontal lobe.
ii) Parietal lobe.
iii)Temporal lobe.
iv)Occipital lobe.
v) Insula.
 The lateral cerebral fissure (Sylvian fissure) separates the frontal and
parietal lobes from the temporal lobe.
 The circular sulcus (circuminsular fissure) separates the insula (part of
the cortex) from other lobes.
 The longitudinal cerebral fissure separates the two hemispheres.
 The central sulcus (fissure of Rolando) begins near the longitudinal
cerebral fissure about the middle of the hemisphere and extends
downward and forward. It separates the frontal lobe from the parietal
lobe.

47. 45
 The parieto-occiptal fissure passes along the medial side of the
hemisphere in the posterior portion. It separate the parietal lobe from
the occipital lobe.
 The medial surfaces of the cerebral hemispheres are flat. They are
connected to each other by a large bundle of myelinated and
nonmyelinated fibers that cross the longitudinal cerebral fissure which is
called corpus callosum. The anterior and posterior commissures also
connect the two hemispheres.
 The corpus callosum has two curved portions: the genu (anteriorly) and
the splenium (over the midbrain). The other two parts are the rostrum
and body. It serves to integrate the activity of the two hemispheres and
permits them to communicate.
 Lobes of the Cerebral Hemisphere:
 The frontal lobe includes the motor cortex and the areas responsible for
creativity, judgement, abstract reasoning, behavior, and other higher
functions.
 The superior and inferior frontal sulci divide the lateral surface of the
frontal lobe into the superior, middle, and inferior gyri.
 The parietal lobe contains the sensory cortex and the angular gyrus
which has an important role in language perception.
 The occipital lobe contains the visual cortex. The medial surface of the
occipital lobe is divided by the calcrine fissure into the cuneus and the
lingual gyrus.
 The temporal lobe contains the auditory cortex. It is divided by the
superior and middle temporal sulci into the superior, middle, and
inferior temporal gyri.
 The insula can be exposed by separating the upper and lower lips of the
lateral fissure.

48. 46
 White Matter of the Cerebral Hemispheres:
 The white contains myelinated fibers of many sizes as well as neuroglia
(mostly oligodendrocytes).
Figure 7-1: Lateral view of the cerebrum showing some important gyri and sulci
Figure 7-2: Medial view of the cerebrum showing some important gyri and sulci

49. 47
Figure 7-3: Lobes of the cerebrum from the lateral side
Figure 7-4: Lobes of the cerebrum from the medial side

50. 48
 There are three main groups of fibers in the white matter:
i) Commissural (transverse) fibers.
ii) Projection fibers.
iii)Association (arcuate) fibers.
 The commissural fibers connect the cortices of the two cerebral
hemispheres. Most of them are gathered in the corpus callosum and a
few lie in the anterior and posterior commissures. The anterior
commissure interconnects the two temporal lobes.
 Projection fibers connect the cerebral cortex with the subcortical nuclei
and nuclei of the brainstem and spinal cord.
 The association fibers connects various portions of the cerebral cortex
within the same cerebral hemisphere.
 Internal Capsule:
 The internal capsule is a band of myelinated fibers that run through the
basal ganglia and separate the lentiform nucleus (lateral) from the
caudate nucleus and thalamus (medial).
 The internal capsule has anterior and posterior limbs and the genu
between them (figure 7-8). It consists of afferent fibers (from the thalamus
to the cortex) and efferent fibers (from the cortex to the cerebral
peduncle of the midbrain).
 The anterior limb lies between the lentiform nucleus and the caudate
nucleus. It contains the following tracts and fibers:
i) Thalamocortical and corticothalamic fibers (connects the lateral
thalamic nucleus and the frontal lobe).
ii) Frontopontine tracts (from the frontal lobe to the pontine nuclei).
iii)Fibers from the caudate nucleus to the putamen.
 The genu is the region of the bend in the internal capsule. It contains
mainly the corticonuclear fibers (from the cortex to the motor nuclei of
the cranial nerves).

51. 49
 The posterior limb lies between the thalamus and the lentiform nucleus.
The corticospinal and corticobulbar fibers occupy the anterior two
thirds of the posterior limb. The head fibers lie most anteriorly and then
the corticospinal fibers for the arm, hand, trunk, leg, and perineum. The
posterior limb also contains some thalamocortical fibers which include
sensory fibers from the opposite side of the body. There are also some
frontopontine fibers. It’s a common site of stroke due to thrombosis or
hemorrhage of the striate arteries
 Histology of the Cerebral Cortex:
 There are three main types of neurons in the cerebral cortex:
i) Pyramidal cells.
ii) Stellate neurons.
iii)Fusiform neurons.
 The projection and association fibers are formed mainly by the axons of
pyramidal and fusiform cells. Whereas stellate neurons form
interneurons whose axons remain in the cortex.
 There are two types of cerebral cortices:
i) The allocortex (archicortex- found in the limbic system and has fewer
layers).
ii) The isocortex (neocortex- found in most of the cerebral hemisphere
and has six layers).
 Cytoarchitecture is the organization of the neocortex layers of cells. The
six layers are (form outermost to innermost):
i) The molecular layer.
ii) The external granular layer (dense layer of small cells).
iii)The external pyramidal layer (contains pyramidal cells).
iv)The internal granular layer (thin layer of small cells).
v) The internal pyramidal layer (contains fewer and larger pyramidal
cells than those of the external pyramidal layer).
vi)The fusiform (multiform) layer (consists of fusiform cells).

52. 50
 Some neurons with similar functions are interconnected in vertically
oriented columns. This feature contribute in giving the brain its complex
functions.
 Principal Areas of the Cerebral Hemispheres:
 The most common classification of the cerebral hemispheres is
Brodmann’s classification which is based on the cytoarchitectonics and
uses numbers to label different areas of the cortex that Brodmann
believed differ from others (figures 7-5 and 7-6).
 The area 4 is the primary motor area and it lies in the precentral gyrus
(frontal lobe). The motor cortex shows somatotopical organization (the
lips, tongue, and face are represented on the lower part of the convexity
of the hemisphere whereas the legs and foot are represented on the
upper part of the convexity).
 The area 6 (the premotor area) also lies in the frontal lobe and several
motor zones like supplementary motor area are clustered nearby.
 The area 8 (frontal eye field) is also in the frontal lobe and it is concerned
with eye movements.
 The Broca’s area (areas 44 and 45) (figure 7-7) is located in the inferior
frontal gyrus (frontal lobe). It is an important area for speech.
 The prefrontal cortex lies anterior to the areas mentioned before. It
serves a set of executive functions (like planning, sequencing actions,
judgement….etc.).
 Areas 3, 1, and 2 in the postcentral gyrus (parietal lobe) are the primary
sensory area. They are also somatotopically organized.
 The area 17 in the occipital lobe is the striate (primary visual) cortex.
Area 18 and 19 are visual association area also in the occipital lobe.
 The Heschl’s gyrus (temporal lobe) contains the area 41 (primary
auditory cortex) and the area 42 (secondary auditory cortex). The area
22 (temporal lobe) is the auditory association cortex.

53. 51
 Wernicke’s area lies in the posterior third of the superior temporal
gyrus. It plays an important role in comprehension of language (figure 7-
7).
 Basal Ganglia (Nuclei):
 The basal ganglia are masses of gray matter embedded in the white
matter of the cerebral hemispheres. They play a key role in regulation of
motor function and believed to have a role in some cognitive processes.
They participate in the extrapyramidal system.
 The main components of the basal ganglia are (figures 7-8 and 7-9):
i) Caudate nucleus.
ii) Putamen.
iii)Globus pallidus.
 The substantia nigra and the subthalamic nucleus are included as parts
of the basal ganglia.
 The corpus striatum is composed of the caudate nucleus, putamen, and
globus pallidus.
 The caudate nucleus and putamen are termed the striatum. It constitute
the major site of input to the basal ganglia.
 The lenticular nuclei contain the putamen and globus pallidus.
 The caudate nucleus follows the lateral ventricle for its entire length. Its
largest part is the head and the narrowest part is the tail.
 The globus pallidus consists of a lateral segment and a medial segment
which are separated by the medial medullary lamina. The globus
pallidus is the primary for the output from the basal ganglia.
 The claustrum is a thin layer of gray matter beneath the insula. The
external capsule (thin layer of white matter) separates it from the
putamen.
 The caudate nucleus sends fibers to the putamen which sends short
fibers to the globus pallidus.

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 The putamen and globus pallidus receive fibers from the substantia nigra
and the caudate nucleus receives fibers from the thalamus.
Figure 7-5: Lateral side of the cerebrum with Brodmann’s numbers
Figure 7-6: Medial side of the cerebrum with Brodmann’s numbers

55. 53
Figure 7-8: Horizontal section of the cerebrum showing the internal capsule and the basal ganglia
Figure 7-9: Frontal section of the cerebrum showing the basal ganglia
Figure 7-7: lateral view of the cerebrum showing the Wernicke’s and Broca’s areas

56. 54
 Some fibers that leave the basal ganglia via the globus pallidus pass
through the internal capsule and form the fasciculus lenticularis on the
medial side. The ansa lenticularis is the other fibers from the globus
pallidus that form a loop. The two groups of fibers have terminals in the
thalamus and subthalamic and red nuclei.
 Arterial Supply of the Cerebral Hemispheres:
 The three main arteries that supply the cerebral hemisphere are (figures 7-
10 and 7-11):
i) The anterior cerebral artery.
ii) The middle cerebral artery.
iii)The posterior cerebral artery.
 The circle of Willis is a confluence of arteries that gives rise to the major
cerebral arteries. It is composed of the following:
i) The anterior cerebral arteries.
ii) The anterior communicating artery.
iii)The internal carotid arteries.
iv)The posterior cerebral arteries.
v) The posterior communicating artery.
vi)The basilar artery.
 The circle of Willis shows many variations among individuals.
 The cortical arteries travel deep in the sulci.
 The anterior choroidal artery contributes in supplying the hemispheres
but it does not supply the cerebral cortex.
 The anterior and middle cerebral and the anterior choroidal arteries are
branches of the internal carotid artery. The posterior cerebral artery is
the terminal branch of the basilar artery. The perforating branches of
the three cerebral arteries are end arteries.
 Capillaries in the brain have abundant tight junctions which are the
structural component of the blood-brain barrier.

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 The internal carotid artery also gives the striate arteries (supply the
internal capsule, thalamus, and basal ganglia) and the posterior
communicating artery.
 The anterior cerebral artery of one side is connected to its fellow of the
opposite side by the anterior communicating artery. It supplies the
orbital surface of the frontal lobe and most of the medial aspect of the
hemisphere. It supplies the parts of sensory and motor areas that are
not supplied by the middle cerebral artery (read below). Sometimes both
anterior cerebral arteries arise from one internal carotid artery.
 The middle cerebral artery is the largest branch of the internal carotid
artery (it is the most subject to embolism). It supplies most of the lateral
aspect of the hemisphere including the sensory and motor areas of the
opposite side of the body (except leg, foot and perineum, and the
auditory and speech areas).
 The posterior cerebral artery supplies the cerebral peduncle, the optic
tract, the inferior temporal gyrus, and the inferomedial surface of the
temporal and occipital lobes.
 The anterior choroidal artery supplies the choroid plexus (also supplied
by the posterior cerebral artery), optic tract and radiation, lateral
geniculate body, posterior part of the internal capsule, basal ganglia
and limbic system.
 Venous Drainage of the Cerebral Hemisphere (figure 7-12):
 The venous drainage of the brain and meninges include the following:
i) Veins of the brain.
ii) Dural venous sinuses.
iii)Meningeal veins.
iv)Diploic veins.
 Cerebral veins have no valves and they ultimately drain into the internal
jugular vein or the pterygoid plexus.

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 The cortical veins tend to be adherent to the deep surface of the
arachnoid mater in the sulci. They usually drain to the nearest venous
sinus.
 The superior surface of the hemisphere drain into the superior sagittal
sinus by several superior cerebral veins.
 Few inferior cerebral veins drain into the transverse sinus.
 The superficial middle cerebral vein (lies in the lateral sulcus) drains the
adjacent cortex and empties into the cavernous sinus.
 The inferior and inferomedial surfaces of the hemisphere drain by the
inferior cerebral veins into the nearest venous sinus.
 The deep middle cerebral vein drains the depth of the lateral sulcus and
the surface of the insula.
 The anterior cerebral vein drains the orbital surface of the frontal lobe.
 The striate veins drain the corpus striatum and join the deep middle
cerebral vein and the anterior cerebral vein to form the basal vein.
Then, the two basal veins join the great cerebral vein.
 The internal cerebral vein is formed by the choroidal vein (which drains
the choroidal plexus) and the thalamostriate vein (which drains the
thalamus and caudate nucleus). The two internal cerebral veins join
together to form the great cerebral vein (vein of Galen). The great
cerebral vein enters the straight sinus with the inferior sagittal sinus.
 The Ventricular System of the Brain (figure 7-13):
 The ventricular system is a communicating system of cavities in the brain
that are filled with cerebrospinal fluid.
 The ventricular system contains the following:
i) The two lateral ventricles.
ii) The interventricular foramens.
iii)The third ventricle.
iv)The cerebral aqueduct.
v) The fourth ventricle.

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 The choroid plexus in the ventricular system produces the cerebrospinal
fluid which is protective to the brain. It remove waste products from
neuronal activity, controls brain excitability, and provide protection
from pressure change.
 The lateral ventricles are the largest. Each one contains two central
portions and three extensions.
 The anterior horn of the lateral ventricle lies anterior to the
interventricular foramen. The body of the lateral ventricle extends from
the interventricular foramen to a point opposite the splenium of the
corpus callosum. The atrium (trigone) is an area of the body that
connects with the posterior and inferior horns. The posterior horn
extends into the occipital lobe and the inferior horn traverses the
temporal lobe.
 The two lateral ventricles communicate with the third ventricle through
the two interventricular foramens (foramens of Monro).
 The third ventricle lies between the two halves of the diencephalon. The
optic recess and the suprapineal recess are extensions of the third
ventricle.
 The cerebral aqueduct is a channel that runs from the posterior third
ventricle to the fourth ventricle. It contains no choroid plexus.
 The fourth ventricle is a cavity in the brainstem. It extends under the
obex into the central canal of the medulla.
 The foramen of Majendie is an opening in the roof of the fourth
ventricle. Most of the outflow of the CSF passes through this foramen.
 The foramen of Luschka is an opening of the lateral recess.

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Figure 7-10: Arterial supply the brain
Figure 7-11: Areas that are supplied by the three cerebral arteries

61. 59
Figure 7-12: Main veins and sinuses that drain the brain
Figure 7-13: The ventricular system of the brain

62. 60
THE SOMATOSENSORY SYSTEM
 Sensory Receptors:
 The sensory receptors are specialized neural structures that tell us about
the external and internal environments.
 The basic types of sensory receptors are:
i) Mechanoreceptors.
ii) Thermoreceptors.
iii)Nociceptors.
iv)Electromagnetic receptors.
v) Chemoreceptors.
 Stretch or compression of the tissue is detected by mechanoreceptors.
There are many types of mechanoreceptors.
 Increase or decrease in environment temperature is detected by
Thermoreceptors.
 Nociceptors detect damage to the tissue (physical or chemical).
 Light on the retina of the eye is detected by electromagnetic receptors.
 Chemoreceptors detect various chemical events that occur in the body
from oxygen levels in arterial blood to the osmolality of the body fluids.
They also detect taste and olfaction (smelling) sensations.
 Each type of receptors is responsive to one type of stimulus and not to
the other stimuli.
 Characteristics of stimuli:
i) Modality (refers to the type of stimulus).
ii) Intensity (determined by the strength of the stimulus).
iii)Duration.
iv)Location.
v) Stimulus transduction.

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vi)Receptive field (the area that when stimulated the receptor produces
the transduction of the stimuli).
 The labeled line principle is the specificity of each nerve fiber for
transmitting one type of sensation. Each nerve tract terminates at a
specific point in the central nervous system and a type of sensation is felt
when the nerve fiber is stimulated. The type of sensation felt is
determined by the point in the nervous system to which the fiber leads
and the type of its receptor.
 Receptor Potential:
 The receptor potential is the change in the membrane electrical
potential of the receptor when a stimulus excites the receptor.
 There are several mechanisms that cause receptor potential, the main
mechanisms are:
i) Mechanical deformation of the receptor (opens ion channels in the
receptor’s membrane).
ii) Chemicals (opens ion channels).
iii)Change in the temperature of the receptor (changes the permeability
of the membrane).
iv)Electromagnetic radiation (changes the membrane characteristics).
 Action potential is generated in the nerve fiber attached to the receptor
when the receptor potential reaches the threshold.
 Sensory receptors adapt (partially or completely) to constant stimuli
after a period of time. Some receptors adapt slowly while others adapt
very quickly. The nonadapting receptors take hours or days to adapt.
Chemoreceptors and nociceptors never adapt completely. Each type of
receptors adapt in different ways.
 The slowly adapting receptors (tonic receptors) keep the brain informed
of the status of the body.
 Rapidly adapting receptors (phasic receptors) detect stimulus strength
change and help in the predictive function of the brain.

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 Types of Nerve Fibers:
 Nerve fibers are divided mainly into types A and C.
 Type A fibers are furtherly divided into α, β, γ, and δ fibers.
 Type A fibers are myelinated fibers of spinal nerves. Some of them are
large and the others are medium in size.
 Type C fibers are small unmyelinated fibers. They conduct impulses at
low velocities.
 As a general rule, the larger the fiber the greater velocity it conducts the
impulses.
 There is an alternative classification in which nerve fibers are furtherly
divided into:
i) Group Ia (from muscle spindles).
ii) Group Ib (from Golgi tendon organs).
iii)Group II (from cutaneous tactile receptors and some muscle spindles).
iv)Group III (carry temperature, crude touch, and pricking pain).
v) Group IV (unmyelinated - carry pain, temperature, itch, and crude
touch).
 Spatial and Temporal Summation:
 In spatial summation, the increase in signal strength is transmitted using
a larger number of fibers.
 In temporal summation, the increase in signal strength is transmitted by
increasing the frequency of nerve impulses in each fiber.
 Some Aspects of Somatosensory Function:
 It is assumed that the thalamus has a slight ability to discriminate tactile
sensations but a major role in perception of pain and a moderate effect
on the perception of temperature.
 The thalamus contains relay nuclei which transmit the signals to the
different areas in the somatosensory cortex

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 Corticofugal signals are sent from the cerebral cortex to the thalamus,
medulla, and spinal cord to control the intensity of the sensory input.
They are almost entirely inhibitory.
 A dermatome is a segmental field of the skin which is innervated by a
single spinal nerve. There is much overlap from segment to segment.
 Classification of Somatic Senses:
 The somatic senses are classified into:
i) Mechanoreceptive somatic senses (stimulated by mechanical
displacement of the tissue).
ii) Thermoreceptive senses (for heat and cold).
iii)Pain sense (detects tissue damage).
 There is another classification in which somatic sensations are classified
into:
i) Exteroreceptive sensations (from the surface of the body).
ii) Proprioceptive sensations (detect the physical state of the body).
iii)Visceral sensations (from the internal organs).
iv)Deep sensations (from deep tissue).
 Tactile Sensations:
 The main tactile sensations are touch, pressure, and vibration. They are
mechanical sensations.
 The main tactile receptors are (figures 8-1 and 8-2):
i) Some free nerve endings.
ii) Meissner’s corpuscle.
iii)Expanded tip tactile receptors (like Merkel’s disc).
iv)Hair end-organ.
v) Ruffini’s endings.
vi)Pacinian corpuscle.
 The tactile free nerve endings are distributed everywhere in the skin and
some other tissues. They can detect touch and pressure.

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 The Meissner’s corpuscle is an encapsulated nerve ending. Its nerve fiber
is large (Aβ) myelinated fiber. It is mainly found in the nonhairy part of
the skin especially fingertips and lips. It adapts rapidly and has low
threshold.
 The expanded tip tactile receptors are found mainly in the fingertips,
lips, and other areas of the skin. They are slowly adapting receptors. The
Merkel’s disk is innervated by a large (Aβ) myelinated fiber.
 The hair end-organ detects slight movement of hair on the body.
 Ruffini’s endings are found in the deeper layers of the skin and joint
capsules. They are multibranched encapsulated endings and adapt very
slowly.
 Pacinian corpuscles lie beneath the skin and deep in the fascial tissue.
They adapt rapidly.
 All tactile receptors detect vibration but in different frequencies. The
best receptor to detect vibration is pacinian corpuscles.
 Tickle and itch are detected by the free nerve endings and transmitted
by the small (type C) unmyelinated fibers.
 Sensory Pathways for Somatic Signals:
 The somatic sensations enter the spinal cord through the posterior roots
of the spinal nerves.
 There are two sensory pathways to carry sensory signals:
i) The dorsal column-medial lemniscus system (figure 8-3).
ii) The anterolateral system (spinothalamic tracts) (figure 8-4).
 The dorsal column-medial lemniscus system carry signals in the dorsal
column of the spinal cord. Then the signals synapse and decussate at the
medulla and synapse in the gracilus and cuneate nuclei. Then they
continue through the brainstem by way of the medial lemniscus.
 The dorsal column-medial lemniscus system is composed of large,
myelinated nerve fibers. It also has a high degree of spatial orientation.

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That is why it transmit sensory information that should be transmitted
rapidly with spatial fidelity. These sensations are:
i) Discriminative touch.
ii) Vibration.
iii)Pressure.
iv)Proprioception.
 Two-point discrimination is an important method used in clinical
practice. In this test two needles are pressed on the skin at the same time
and the person is asked whether he feels it as one or two needles. Tips
of the fingers can discriminate the two points easily even if the two
needles are very close to each other unlike the back which requires more
space between the two needles. This difference is mainly due to
increased number of receptors in the fingers.
 Signals in the anterolateral system synapse in the dorsal horns of the
gray matter after entering the spinal cord. Then they decussate to the
opposite side of the spinal cord and travel through the anterior and
lateral white matter.
 The anterolateral system is composed of small myelinated fibers. It
transmits sensory information that doesn’t need to be transmitted
rapidly or with great spatial fidelity but it has the ability to transmit a
broad spectrum of sensory modalities. These sensations are:
i) Pain.
ii) Thermal sensations (heat and cold).
iii)Crude touch and pressure.
iv)Tickle and itch.
v) Sexual sensations.
 There are some differences between the two pathways; the dorsal
column is faster, transfer more accurately localized sensations, and able
to transmit rapidly repetitive signals.

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Figure 8-1: The tactile receptors
Figure 8-2: Section of the skin showing some tactile receptors and their different
distributions in the skin

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Figure 8-3: The dorsal column-medial lemniscus system
(the proprioception is an example of a sensation
transmitted by this pathway)
Figure 8-4: The anterolateral system (the pain is an example of a sensation transmitted
by this pathway)

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 The Somatosensory Cortex:
 The somatosensory areas lie in the anterior parietal lobe. They are called
somatosensory area I and somatosensory area II. The somatosensory
cortex shows somatotopic organization of the body parts (figure 8-5).
 Somatosensory area I is more important the area II. The term
somatosensory cortex usually refers to the somatosensory area I.
 The somatosensory area I serves many functions, here are the most
important:
i) Precise localization of different sensations in different body parts.
ii) Judges critical degrees of pressure against the body.
iii)Enables the person to judge weights and shapes or forms of objects
(the inability to judge shapes of objects is called astereognosis).
iv)Enables the person to judge texture of materials
 Some parts of the body (like the face and lips) are represented by large
areas in the somatosensory cortex while other parts (like the trunk) are
represented in small areas. The size of the represented area is directly
proportional to the number of sensory receptors in that part of the
body.
 Neurons in the somatosensory area are organized in vertical columns
extending through the six layers of the cortex. Each column serves a
specific sensory modality.
 Areas 5 and 7 are called somatosensory association areas because they
play an important role in “deciphering” meanings of the sensory
information.
 People with removed sensory association area experience a complex of
sensory deficit called amorphosynthesis. In this condition the patient
loses the ability to recognize complex objects and most of the sensations
of his body in the opposite side. This condition has many other effects.
 Proprioception:
 Proprioception is the sense of position. It can be divided into two types:

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i) Static proprioception (the perception of orientation of different body
parts).
ii) Dynamic proprioception (rate of movement).
 Proprioception can also be divided into:
i) Conscious proprioception (provides information to the cerebral
cortex which are used to generate conscious awareness of
kinesthesia).
ii) Nonconscious proprioception (provides information to the
cerebellum).
 Here are the proprioceptors. We will discuss them in more details when
we get to the motor system:
i) Free nerve endings.
ii) Encapsulated receptors.
iii)Muscle spindles.
iv)Golgi tendon organs.
 Pain:
 Pain is a protective mechanism. It tells the person about tissue damage
so that the person react to remove the pain stimulus.
 Pain is classified into two types:
i) Fast (acute or sharp) pain (not felt in deeper tissues).
ii) Slow (chronic) pain (associated with tissue destruction).
 All pain receptors are free nerve endings. They adapt very little and
sometimes not at all.
 Hyperalgesia is the increase in the sensitivity of pain receptors.
 Multiple types of stimuli can elicit pain. They are:
i) Mechanical stimuli (like knife cuts).
ii) Thermal stimuli (like burns).
iii)Chemical stimuli (like bradykinin and acids).
 The intensity of pain depends on the rate of tissue destruction more
than the total damage that occurred.

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 The fast pain signals are transmitted by type Aδ fibers. Then the signals
travel through the neospinothalamic tract and pass to the brain through
the anterolateral system. Glutamate is believed to be the
neurotransmitter.
 The slow pain signals are transmitted by type C fibers then through the
paleospinothalamic tract and the anterolateral system to the brain.
Substance P is believed to be the major neurotransmitter.
 The fast pain can be localized more exactly than the slow pain.
 Few fibers in the paleospinothalamic tract reach the thalamus. The
majority of the fibers terminate in one of these areas in the brainstem:
i) The reticular formation.
ii) The tectum of the midbrain.
iii)The periaqueductal gray matter.
 The brain has an analgesia system that suppress the input of pain signals
to nervous system. It consists of three major components:
i) The periaqueductal gray matter and periventricular areas in the
midbrain.
ii) The raphe magnus nucleus and the nucleus reticularis
paragigantocelluraris.
iii)Pain inhibitory complex in the dorsal horn of the spinal cord.
 Enkephalin and serotonin are the main neurotransmitters involved in
the analgesia system.
 The brain also has opiate-like substances which are involved in
analgesia. These are:
i) β-endorphin.
ii) Met-enkephalin.
iii)Leu-enkephalin.
iv)Dynorphin.
 Referred pain is the pain that is felt in an area remote from the tissue
causing the pain. When pain is referred, it is usually referred to a

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structure that developed from the same embryonic segment or
dermatome.
 Visceral pain originates from the deep viscera. It is poorly localized and
associated with nausea and other autonomic symptoms (like
hypotension).
 Thermal Sensations:
 Thermal gradations are discriminated by three receptors:
i) Cold receptors.
ii) Warmth receptors.
iii)Pain receptors (stimulated by extreme degrees of heat and cold).
 Cold receptors are more than warmth receptors (3 to 10 times).
 Warmth receptors are assumed to be free nerve endings because their
signals are transmitted mainly in type C nerve fibers. But cold receptors
are myelinated nerve endings; their signals are transmitted in type Aδ
nerve fibers.
 Temperature sensations are mediated through the same pathways of
pain.
Figure 8-5: The somatotopic organization of the somatosensory
cortex

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THE VISUAL SYSTEM
 The Anatomy and Function of the Retina:
 The retina is light sensitive part of the eye. It consists of:
i) The cones (responsible for colored vision).
ii) The rods (responsible for black and white vision and vision in the
dark).
 The retina is composed of many layers. These layers are (from outside to
inside) (figure 9-1):
i) Pigment epithelium layer.
ii) Layer of rods and cons (contains the photosensitive part of the rods
and cones).
iii)External limiting membrane.
iv)Outer nuclear layer (contains cell bodies of rods and cones).
v) Outer plexiform layer.
vi)Inner nuclear layer.
vii) Inner plexiform layer.
viii) Layer of ganglion cells.
ix) Optic nerve layer.
 Light enters the retina from inside the eye (figure 9-2).
 The macula is a yellowish pigmented spot near the posterior pole of the
eye. The fovea (which lies in the center of the macula) is a rod-free
portion that has a high visual acuity due to increased number of cones.
 The melanin in the pigment epithelium layer prevents light reflection
throughout the eyeball. The pigment layer also stores large quantities of
vitamin A which is very important in vision.

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 Photoreception:
 Chemical reactions occur in both rods and cones on exposure to light and
excite nerve fibers.
 Rhodopsin and color pigments are the light-sensitive chemicals of the
rods and cones, respectively.
 Rhodopsin is composed of the protein scotopsin and the vitamin A
derivative retinal. Steps of photoreception include the following:
i) When light strikes the retina, a series of conformational changes occur
in the scotopsin which result in the formation of activated rhodopsin.
ii) Activated rhodopsin activates transducin (a G protein). Activated
transducin increase breakdown of cGMP to GMP by stimulating a
phosphodiesterase.
iii)The decrease of cGMP in the light closes Na+ channels in the receptor
membrane which reduces Na+ influx and produces hyperpolarization.
iv)Hyperpolarization decreases the release of glutamate from synaptic
terminals of the photoreceptor. Decreased release of glutamate that
interacts with ionotropic receptors causes hyperpolarization and
inhibition of the bipolar or horizontal cells. But decreased release of
glutamate that interacts with metabotropic receptors will result in
depolarization and excitation of bipolar or horizontal cells
 Vitamin A is important in reformation of rhodopsin. When severely
deficient, night blindness occurs.
 The color pigments are composed of photopsin and retinal. There are
different types of color pigments:
i) Blue-sensitive pigment.
ii) Green-sensitive pigment.
iii)Red-sensitive pigment.
 One type of the color pigments is present in each of the different cones.

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 Optic pathways (figure 9-3):
 Axons from the ganglion cells in the retina form the optic nerves and
then the optic tracts (which decussate partially to form the optic
chiasm). Then they synapse in the dorsal lateral geniculate nucleus (in
the thalamus) and continue to the visual cortex by way of the optic
radiation in the geniculocalcarine tract.
 At the optic chiasm, fibers from the nasal part of the retinas (which
transmit light signals from the temporal fields) cross to the opposite
sides and join fibers from the temporal part of the retina (which transmit
light signals from the nasal fields) of the opposite side.
 Several areas of the brain also receive visual fibers. These are:
i) Suprachiasmatic nucleus of the hypothalamus (related to the
circadian rhythm).
ii) Pretectal nuclei in the midbrain (related to eye and pupillary
reflexes).
iii)Superior colliculi (rapid directional movement of the eyes).
iv)Ventral lateral geniculate nucleus (related to body’s behavioral
function).
 The visual cortex:
 The visual cortex is divided into:
i) Primary visual cortex (Brodmann’s area 17)
ii) Visual association areas (Brodmann’s areas 18 and 19).
 The primary visual cortex is the terminus of direct visual signals from the
eyes. Visual association areas receive secondary signals from the primary
visual cortex for analysis of visual meanings.
 Visual reflexes:
 The two main visual reflexes are:
i) Pupillary light reflex.
ii) Accommodation reflex (focusing from a distant object to a near
object).

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 In pupillary light reflex, postganglionic sympathetic fibers from the
superior cervical ganglia innervate the dilator pupillae muscle which
causes pupillary dilation when it contracts. The preganglionic
parasympathetic neurons are located in the Edinger-Westphal nucleus.
Their axons leave the brainstem through the oculomotor nerve and
synapse in the ciliary ganglion. The postganglionic fibers innervate the
constrictor pupillae muscle which constricts the pupil when it contracts.
 Bright light excites neurons in the Edinger-Westphal nucleus which leads
to constriction of the pupil and reduction of the light entering the eyes.
When the bright light strikes one eye this reflex occur in both eyes.
 The response in the eye that was struck by the bright light is called direct
pupillary light reflex. Whereas the response in the contralateral eye is
called consensual pupillary light reflex.
Figure 9-1: Layers of the retina

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Figure 9-2: Normal eye with normal refraction
Figure 9-3: Optic pathways

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AUDITORY AND VESTIBULAR SYSTEMS
 Structure of the ear:
 The ear consists of three parts:
i) Inner ear.
ii) Middle ear.
iii)External ear.
 The external ear directs sound waves into the auditory canal.
 The middle ear consists of the tympanic membrane and the auditory
ossicles (malleus, incus, and stapes).
 The inner ear consists of membranous labyrinth and bony labyrinth. The
semicircular canal and cochlea lie in the bony labyrinth.
 The cochlea contains the organ of Corti (in the basilar membrane of the
cochlea) which is the site of auditory transduction and contains receptor
hair cells.
 The cochlea contains three ducts:
i) Scala vestibuli.
ii) Scala tympani.
iii)Scala media.
 Scala vestibuli and scala tympani contain a fluid that is similar to the
extracellular fluid which is called perilymph. The scala media contains a
fluid that has high potassium concentration and low sodium
concentration and similar to the intracellular fluid which is called
endolymph.
 Auditory transduction:
 Auditory transduction is the transformation of the sound waves into
electrical changes and impulses.

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 Sound waves strike the tympanic membrane causing the movement of
the ossicles. Movement of the ossicles pushes the stapes into the oval
window resulting in fluid displacement in the cochlea and cause vibration
of the organ of Corti.
 Bending of the cilia on the hair cells occurs due to vibration in the organ
of Corti and produces a change in the K+ conductance of the hair cell
membrane. In one direction it causes an increase in K+ conductance and
hyperpolarization and in the other direction it causes a decrease in K+
conductance and depolarization.
 The depolarization opens the voltage-gated Ca2+ channels. Ca2+ enters
the presynaptic terminal in the hair cells and release glutamate which
causes action potentials in the cochlear nerve and transmit the
information to CNS. Hyperpolarization decreases the release of
glutamate.
 Auditory pathways (figure 10-1):
 Fibers from the spiral ganglion in the organ of Corti synapse in the dorsal
and ventral dorsal nuclei in the upper medulla. Most second-order
neurons decussate to terminate in the contralateral superior olivary
nucleus but some fibers ascend to the ipsilateral superior auditory
nucleus.
 After the superior olivary nucleus, auditory fibers pass through the
lateral lemniscus (some of them synapse in the nucleus of the lateral
lemniscus) and then synapse in the inferior colliculus. Then fibers pass
and synapse in the medial geniculate nucleus in the thalamus. From
there, fibers proceed to the auditory cortex by way of the auditory
radiation.
 Many collateral fibers from the auditory tracts pass to the reticular
activating system in the brainstem which activates the nervous system
in response to loud noise. Other collateral fibers go to the cerebellum
which is also activated in response to loud noise.

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 Auditory Cortex:
 The auditory cortex lies in the superior temporal gyrus but extends to
the lateral side of the temporal lobe and the insula.
 The auditory cortex is divided into the primary auditory cortex and the
auditory association cortex (secondary auditory cortex).
 The primary auditory cortex receives impulses from the medial
geniculate body whereas the auditory association cortex receives
impulses from the primary association area and some projections from
the thalamic association area.
 Vestibular Organ:
 The vestibular system is the system responsible for balance and
equilibrium by detecting linear and angular accelerations of the head.
 The vestibular organ (or apparatus) lies in a system of bony tubes and
chambers in the temporal bone (bony labyrinth). The vestibular organ
lies in the membranous labyrinth within the bony labyrinth.
 The three perpendicular semicircular canals (horizontal, superior, and
posterior or in some books lateral, anterior, and posterior) and the
utricle and saccule (otolith organs) lie in the membranous labyrinth.
They are filled with endolymph and surrounded with perilymph.
 The semicircular canals are arranged perpendicularly so that they
represent all three planes. They detect angular acceleration of the head.
Each semicircular canal has an enlargement in one end called ampulla
which contains vestibular hair cells. These hair cells are covered with
cupula (gelatinous mass).
 The cupula is displaced in angular acceleration of the head. Bending the
cupula in one side causes depolarization of the hair cells whereas
bending the cupula in the other side causes hyperpolarization.
 The utricle and saccule detect linear acceleration. The macula of the
utricle lies in the horizontal plane and helps in determining orientation
of the head when it is upright. The macula of the saccule lies in the

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vertical plane and helps in determining the orientation of the head when
it is lying down (like lying in bed). Each of the hair cells is oriented in a
different direction in each macula. This orientation causes stimulation of
the hair cells in different directions of linear movement.
 Vestibular Pathways:
 The vestibular nerve connects the vestibular organ with the vestibular
nuclei in the junction of the medulla and pons.
 Some fibers pass directly to the reticular nuclei and the cerebellum.
 Second order neurons from the vestibular nuclei send fibers to the
following:
i) The cerebellum.
ii) The vestibulospinal tract.
iii)The medial longitudinal fasciculus.
iv)Areas in the brainstem (especially reticular nuclei).
 The vestibulospinal and reticulospinal tracts control equilibrium by
controlling the interplay between the facilitation and inhibition of the
antigravity muscles. The lateral vestibulospinal tract is especially
important in maintaining postural reflexes.
 The flocculonodular lobes of the cerebellum are important in dynamic
equilibrium. Whereas the uvula plays an important role in the static
equilibrium.
 The eyes can be fixed on a specific visual object because of signals from
the medial longitudinal fasciculus which cause corrective movements of
the eyes when the head rotates. This is called the vestibulo-ocular reflex.
 Nystagmus is a reflex to angular and rotational acceleration of the head.
In the slow component of nystagmus, the eyes move in the opposite
direction of the rotation. In the rapid component of nystagmus, the eyes
move in the same direction of movement.

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Figure 10-1: Auditory pathways

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SMELL (OLFACTION) & TASTE (GUSTATION)
 Olfactory Cells:
 The olfactory cells are bipolar neurons which are responsible for smell
sensation. Olfactory nerve axons are small unmyelinated fibers (very
slow).
 The olfactory cells are slowly adapting but there is a psychological
adaptation that causes the person to “get used” to the smell quickly.
 The olfactory cilia are part of the olfactory receptor cell that respond to
the chemical stimuli. The odorant binds with receptor proteins in the
membrane of the cilium. This binding activates a G protein system which
results in forming cAMP that activates a gated sodium ion channels
causing transmission of action potential to CNS.
 Olfactory Pathways:
 Axons from the olfactory cells leave the olfactory epithelium, pass
through the cribriform plate, and synapse on mitral cells in the olfactory
bulb. About 1000 olfactory cell axons synapse on one mitral cell in
clusters called glomeruli (figure 11-1).
 Mitral cells project into the higher centers by way of the olfactory tract
(first cranial nerve). The olfactory tract divides into medial and lateral
tracts.
 The medial olfactory tract projects into the contralateral olfactory bulb.
Whereas the lateral olfactory tract terminates in the regions of the
olfactory cortex which are:
i) Anterior olfactory nucleus.
ii) Olfactory tubercle.
iii)Piriform cortex.
iv)Amygdala.

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v) Entorhinal cortex.
 Information from the olfactory cortex travel to the orbitofrontal cortex
for conscious discrimination of smell. Amygdala is involved with
emotion in response to olfactory stimuli. Whereas the entorhinal cortex
is involved with olfactory memories.
 Taste Buds:
 Taste receptors are located in taste buds on the tongue, palate,
pharynx, and larynx. They are chemoreceptors (not neurons! Just
specialized epithelial cells) that detect tastants (chemicals responsible
for taste sensations).
 There are five elementary taste qualities:
i) Salty (caused by ionized salts especially sodium ions).
ii) Sweet (caused by different types of chemicals like sugars and alcohol).
iii)Sour (caused by acids especially the organic acids).
iv)Bitter (caused by different types of chemicals).
v) Umami (means pleasant, caused mainly by monosodium glutamate).
 Taste buds on the tongue are organized in three types of papillae:
i) Circumvallate papillae.
ii) Foliate papillae.
iii)Fungiform papillae.
 Each of the taste buds usually responds to one of the five elementary
taste qualities.
 Taste receptors adapt quickly (adaptation here means to a lower steady
level and not necessary to zero level). Part of the adaptation occurs in
the CNS.
 Taste Transduction:
 Taste qualities have different mechanisms to initiate transduction.
 In bitter taste, the tastants binds to a G protein-coupled receptor and
open the transient receptor potential channels by IP3/Ca2+ mechanism
which results in depolarization and action potential.

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 In sweet and umami tastes, the tastants bind to a different class of G
protein-coupled receptor but cause depolarization in the same way like
the bitter taste.
 In sour taste, H+ ions enter taste receptor through Na+ channel which
leads to depolarization and thus action potential.
 In salty taste, Na+ enters the taste receptor through Na+ channels and
causes depolarization and thus action potential.
 Taste Pathways:
 Taste sensations are carried by three cranial nerves:
i) The facial nerve (anterior two thirds of the tongue).
ii) The glossopharyngeal nerve (posterior one third of the tongue).
iii)The vagus nerve (back of the throat and epiglottis).
 Taste fibers in the three nerves enter the brainstem, ascend in the
solitary tract (tractus solitarius), and terminate in the solitary nucleus
(in the medulla).
 Second order neurons ascend ipsilaterally to the ventral posteromedial
nucleus of the thalamus.
 Third order neurons from the thalamus terminate in the taste cortex in
the anterior insulafrontal operculum.
Figure 11-1: The cribriform plate and the olfactory epithelium