Central Nervous System (F.Y B Pharm Sem-II)

Unit: I
Nervous System
Class: F.Y B Pharm (Sem II)
Presented by: Prof.Mirza Anwar Baig
Anjuman-I-Islam's Kalsekar Technical Campus
School of Pharmacy,New Pavel,Navi Mumbai,Maharashtra
1P 1

Contents:
• Organization of nervous system, neuron, neuroglia,
classification and properties of nerve fibre, electrophysiology,
action potential, nerve impulse, receptors, synapse,
neurotransmitters.
• Central nervous system: Meninges, ventricles of brain and
cerebrospinal fluid.structure and functions of brain (cerebrum,
brain stem, cerebellum), spinal cord (gross structure, functions
of afferent and efferent nerve tracts,reflex activity)
2

The Nervous system has three major functions:
 Sensory – monitors internal & external
environment through presence of receptors
 Integration – interpretation of sensory information
(information processing); complex (higher order)
functions
 Motor – response to information processed
through stimulation of effectors
 muscle contraction
 glandular secretion
3

1.1: Nervous system
Contents:
• Organization of nervous system.
• Neuron, neuroglia.
• Classification and properties of nerve fibre.
• Electrophysiology, action potential,nerve impulse.
• Receptors, synapse, neurotransmitters
4

1.2: Nervous system
1. Central nervous system: Meninges, ventricles of
brain and cerebrospinal fluid.
2. Structure and functions of brain (cerebrum, brain
stem, and cerebellum),
3. Structure and functions of spinal cord (gross
structure, functions of afferent and efferent nerve
tracts, reflex activity).
5

Basic Organization
• Sensory Input triggered by
stimuli
– conduction of signals to
processing center
• Integration
– interpretation of sensory
signals within processing
centers
• Motor output
– conduction of signals to
effector cells (i.e. muscles,
gland cells)
sensory receptor (sensory input)  integration  (motor output)  effector
6

• Brain
•WHAT PARTS DO YOU KNOW THAT
ARE IN THE NERVOUS SYSTEM?
• Spinal Cord
• Peripheral
Nerves
7

Two Anatomical Divisions
 Central nervous system (CNS)
 Brain
 Spinal cord
 Peripheral nervous system (PNS)
 All the neural tissue outside CNS
 Afferent division (sensory input)
 Efferent division (motor output)
 Somatic nervous system
 Autonomic nervous system
General Organization of the nervous system
8

General Organization of the nervous system
Brain & spinal
cord
9

Histology of neural tissue
Two types of neural cells in the nervous system:
 Neurons - For processing, transfer, and storage
of information
 Neuroglia – For support, regulation & protection
of neurons
10

Neuroglia (glial cells)
CNS neuroglia:
Astrocytes: Induces blood-brain barrier
Oligodendrocytes: Produce myelin
Microglia : Phagocytes in CNS
Ependymal cells : Line brain ventricles and
central canal of spinal cord
also part of structure that
makes CSF
PNS neuroglia:
Schwann cells : Produce myelin
Satellite cells : Support neurons in PNS
11

Astrocytes
• create supportive
framework for neurons
• create “blood-brain
barrier”
• monitor & regulate
interstitial fluid surrounding
neurons
• secrete chemicals for
embryological neuron
formation
• stimulate the formation of
scar tissue secondary to
CNS injury
15

Oligodendrocytes
• create myelin sheath
around axons of neurons
in the CNS. Myelinated
axons transmit impulses
faster than unmyelinated
axons
Microglia
• “brain macrophages”
• phagocytize cellular
wastes & pathogens
16

Ependymal cells
• line ventricles of brain &
central canal of spinal cord
• produce, monitor & help
circulate CSF
(cerebrospinal fluid)
17

Schwann cells
• surround all axons of neurons in
the PNS creating a neurilemma
around them. Neurilemma allows
for potential regeneration of
damaged axons
• creates myelin sheath around
most axons of PNS
Satellite cells
• support groups of cell bodies
of neurons within ganglia of the
PNS
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•Most axons of the nervous system are
surrounded by a myelin sheath
(myelinated axons)
•The presence of myelin speeds up the
transmission of action potentials along
the axon
•Myelin will get laid down in segments
(internodes) along the axon, leaving
unmyelinated gaps known as “nodes
of Ranvier”
•Regions of the nervous system
containing groupings of myelinated
axons make up the “white matter”
•“gray matter” is mainly comprised of
groups of neuron cell bodies, dendrites
& synapses (connections between
neurons)
of Ranvier
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Classification of neurons
1. Structural classification based on number of processes
coming off of the cell body:
21

Anaxonic neurons
• no anatomical clues to
determine axons from
dendrites
• functions unknown
Multipolar neuron
multiple dendrites & single
axon
most common type
22

Bipolar neuron
• two processes coming off
cell body – one dendrite &
one axon
• only found in eye, ear &
nose
Unipolar (pseudounipolar)
neuron
• single process coming
off cell body, giving rise
to dendrites (at one end)
& axon (making up rest of
process)
23

2. Functional classification based on type of information &
direction of information transmission:
Sensory (afferent) neurons –
•transmit sensory information from receptors of PNS towards the CNS
•most sensory neurons are unipolar, a few are bipolar
Motor (efferent) neurons –
transmit motor information from the CNS to effectors (muscles/glands/adipose
tissue) in the periphery of the body
all are multipolar
Association (interneurons) –
transmit information between neurons within the CNS; analyze inputs,
coordinate outputs
are the most common type of neuron (20 billion)
are all multipolar
24

Conduction across synapses
Most synapses within the nervous system are chemical
synapses, & involve the release of a neurotransmitter
In order for neural control to occur, “information” must not
only be conducted along nerve cells, but must also be
transferred from one nerve cell to another across a synapse.
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Anatomical organization of neurons
Neurons of the nervous system tend to group together into
organized bundles
The axons of neurons are bundled together to form nerves in
the PNS & tracts/pathways in the CNS. Most axons are
myelinated so these structures will be part of “white matter”
The cell bodies of neurons are clustered together into
ganglia in the PNS & nuclei/centers in the CNS. These are
unmyelinated structures and will be part of “gray matter”
27

Anatomical structure of Nerves
Fig. 14.6
28

Generation - Conduction of Neural Impulses
• Dependent on concentration
gradients of Na+ & K+
– Na+ 14x greater outside
– K+ 28x greater inside
• Membrane permeability
– lipid bilayer bars passage of K+ &
Na+ ions
– protein channels and pumps regulate
passage of K+ & Na+
• at rest more K+ move out than Na+
move in
• K+ ions diffuse out leave behind
excess negative charge
• Sodium-potassium pump
– Na+ out - K+ in (more Na+ out than
K+ in
– contributes to loss of (+)
29

• Maintenance of negative charge within neuron
– resting membrane potential about -70 millivolts
• Dissolved organic molecules [negative charge] kept
inside
• Na+ - K+ balance 31

• Stimulus causes opening of Na+
gates & closing of K+ gates -
• Threshold [~ +30 mV]
– all - or - nothing response
• Action potential localized
electrical event
• Changes permeability of region
immediately ahead
– changes in K+ & Na+ gates
– domino effect
– propagation of signal
• Intensity of stimuli (i.e. pinch vs.
punch) = number of neurons
firing
• Speed on impulse based on
diameter of axon & amount of
myelination 34

Neurons Communicate at Synapses
Electrical [no synapse]
– common in heart & digestive tract - maintains steady, rhythmic
contraction
– All cells in effector contain receptor proteins for neurotransmitters
Chemical - skeletal muscles & CNS
– presence of gap (SYNAPTIC CLEFT) which prevents action
potential from moving directly to receiving neuron
– ACTION POTENTIAL (electrical) converted to CHEMICAL SIGNAL
at synapse (molecules of neurotransmitter) then generate ACTION
POTENTIAL (electrical) in receiving neuron
35

Overview of Transmission of Nerve Impulse
Action potential
1.synaptic knob
2.opening of Ca+ channels
3.neurotransmitter vesicles fuse with membrane
4.release of neurotransmitter into synaptic cleft
5.binding of neurotransmitter to protein receptor
molecules on receiving neuron membrane
6.opening of ion channels
7.triggering of new action potential
• Neurotransmitter is broken down by enzymes
& ion channels close -- effect brief and precise
36

Nerve Impulse
• Presynaptic neuron
• Vesicles
• [Calcium channels]
• Synaptic cleft
• Postsynaptic neuron
• Neurotransmitter receptor
37

Nerve Impulse
• Action potential
1.synaptic knob
2. opening of Ca+
channels
3.neurotransmitter
vesicles fuse with
membrane
4.release of
neurotransmitter into
synaptic cleft
Ca2+
38

Nerve Impulse
neurotransmitter
vesicles fuse with
membrane
release of
neurotransmitter into
synaptic cleft
39

binding of
neurotransmitter to
protein receptor
molecules on receiving
neuron membrane
opening of sodium
channels
triggering of new
action potential
40

Classification of Nerve Fibers
• Axons can be classified into three major groups based on the amount of
myelination, their diameters, and their propagation speeds:
1. A fibers :
The largest-diameter axons (5–20 mm) and are myelinated. A fibers have a
brief absolute refractory period. and conduct nerve impulses (action potentials)
at speeds of 12 to 130 m/sec (27–280 mi/hr). Example: axons of sensory
neurons for touch ,pressure etc
2. B fibers:
having axons with diameters of 2–3 mm. Like A fibers,B fibers are myelinated
and exhibit saltatory conduction at speeds up to 15 m/sec (32 mi/hr). B fibers
have a somewhat longer absolute refractory period than A fibers. B fibers con-
duct sensory nerve impulses from the viscera to the brain and spinal cord.
3. C fibres: Smallest-diameter axons (0.5–1.5 mm) and unmyelinated. Nerve
impulse propagation 0.5 to 2 m/sec (1– 4 mi/hr),longest absolute refractory
periods. These unmyelinated axons conduct some sensory impulses for pain,
touch,pressure, heat, and cold from the skin, and pain impulses from the
viscera.
41

Neurotransmitters
• Catecholamine Neurotransmitters
– Derived from amino acid tyrosine
• Dopamine [Parkinson’s], norepinephrine, epinephrine
• Amine Neurotransmitters
– acetylcholine, histamine, serotonin
• Amino Acids
– aspartic acid, GABA, glutamic acid, glycine
• Polypeptides
– Include many which also function as hormones
– endorphins
42

Neurotransmitters:
• About 100 substances are either known or suspected neurotrans-mitters.
• Some neurotransmitters bind to their receptors and act quickly ,Others act
more slowly via second-messenger systems to influence chemical
reactions inside cells.
• The result of either process can be excitation or inhibition of postsynaptic
neurons. Many neurotransmitters are also hormones released into the
bloodstream by endocrine cells in organs throughout the body.
• Within the brain, certain neurons, called neurosecretory cells, also secrete
hormones.
• Neurotransmitters can be divided into two classes based on size: small-
molecule neurotransmitters and neuropeptides
• The small-molecule neurotransmitters include acetylcholine, amino acids,
biogenic amines, ATP and other purines, and nitric oxide.
43

Ion channels for neurotransmitters
44

1. Acetylcholine:
• The acetylcholine (ACh) is released by many PNS
neurons and by some CNS neu-rons.
• ACh is an excitatory neurotransmitter at some
synapses,such as the neuromuscular junction, where the
binding of ACh to ionotropic receptors opens cation
channels.
• It is also an inhibitory neurotransmitter at other synapses,
where it binds to metabotropic receptors coupled to G
proteins that open K channels.
• For example, ACh slows heart rate at inhibitory synapses
made by parasympathetic neu-rons of the vagus (X)
nerve.
• The enzyme acetylcholinesterase (AChE) inactivates
ACh by splitting it into acetate and choline fragments.
45

2. Amino Acids (Glutamic & aspartic acid):
• These neurotransmitters are present in the CNS.
• Glutamate (glutamic acid) and aspartate (aspartic acid) have
powerful excitatory effects.
• Most excitatory neurons in the CNS and perhaps half of the
synapses in the brain communicate via glutamate.
• At some glutamate synapses, binding of the neuro-transmitter to
ionotropic receptors opens cation channels.
• The consequent inflow of cations (mainly Na ions) produces an
EPSP. Inactivation of glutamate occurs via reuptake.
• Glutamate transporters actively transport glutamate back into the
synaptic end bulbs and neighboring neuroglia
46

Excitotoxicity:
i. A high level of glutamate in the interstitial fluid of the CNS
causes excitotoxicity—destruction of neurons through
prolonged activation of excitatory synaptic transmission.
ii. The most common cause of excitotoxicity is oxygen
deprivation of the brain due to ischemia (inadequate blood
flow), as happens during a stroke.
iii. Lack of oxygen causes the glu-tamate transporters to fail, and
glutamate accumulates in the interstitial spaces between
neurons and glia, literally stimulating the neurons to death.
iv. Clinical trials are underway to see if antiglutamate drugs
administered after a stroke can offer some protection from
excitotoxicity.
47

3. Gamma aminobutyric acid & Glycine:
i. GABA and glycine are important inhibitory neurotransmitters.
ii. At many synapses, the binding of GABA to ionotropic receptors
opens Cl ion channels.
iii. GABA is found only in the CNS, where it is the most common
inhibitory neurotransmitter.
iv. As many as one-third of all brain synapses use GABA.
v. Antianxiety drugs such as diazepam enhance the action of
GABA.
vi. Like GABA, the binding of glycine to ionotropic receptors opens
Cl channels.
vii. About half of the inhibitory synapses in the spinal cord use the
amino acid glycine; the rest use GABA.
48

4. Catecholeamine (Biogenic Amines)
i. Norepinephrine, dopamine, and epinephrine are classified
catecholamines, are synthesized from the amino acid tyrosine.by
modification and decarboxylation.
ii. Inactivation of catecholamines occurs via reuptake into synaptic end
bulbs.
iii. Then they are either recycled back into the synaptic vesicles or
destroyed by the enzymes.
iv. The two enzymes that break down catecholamines are catechol-O-
methyltrans- ferase or COMT, and monoamine oxidase or MAO.
v. Most biogenic amines bind to metabotropic receptors
vi. Biogenic amines may cause either excitation or inhibition.
49

a. Norepinephrine (NE) :
NE plays roles in arousal (awakening from deep sleep), dreaming,
and regulating mood.
• A smaller number of neurons in the brain use epinephrine as a
neurotransmitter.
• Both epinephrine and norepinephrine also serve as hormones.
• Cells of the adrenal medulla, the inner portion of the adrenal
gland, release them into the blood.
50

b. Dopamine:
• Brain neurons containing the neurotransmitter dopamine (DA)
are active during emotional responses, addictive behaviors, and
pleasurable experiences.
• In addition, dopamine-releasing neurons help to regulate
skeletal muscle tone and some aspects of movement due to
contraction of skeletal muscles.
• The muscular stiffness that occurs in Parkinson disease is due to
degeneration of neurons that release dopamine. One form of
schizophrenia is due to accumulation of excess dopamine.
c.Serotonin:
• Also known as 5-hydroxytryptamine (5-HT).
• Concentrated in the neurons in a part of the brain called the
raphe nucleus.
• It is thought to be involved in sensory perception,temperature
regulation, control of mood, appetite, and the induc-tion of
sleep.
51

5. ATP and Other Purines:
•It is an excitatory neurotransmitter in both the CNS and the PNS.
•Most of the synaptic vesicles that contain ATP also contain another
neurotransmitter.
•In the PNS, ATP and norepinephrine are released together from some
sympathetic neurons; some parasympathetic neurons release ATP and
acetyl-choline in the same vesicles.
6. Nitric Oxide:
•The simple gas nitric oxide (NO) is an important neurotransmitter
that has widespread effects throughout the body.
•NO contains a single nitrogen atom, in contrast to nitrous oxide
(N2O), or laughing gas, which has two nitrogen atoms.
•N2O is sometimes used as an anesthetic during dental procedures.
52

Neuropeptides:
• Neurotransmitters consisting of 3 to 40 amino acids linked by
peptide bonds called neuropeptides , are numerous and
widespread in both the CNS and the PNS.
• Neuropeptides bind to metabotropic receptors and have excita-
tory or inhibitory actions, depending on the type of
metabotropic receptor at the synapse.
• Neuropeptides are formed in the neuron cell body, packaged
into vesicles, and transported to axon terminals.
• Besides their role as neurotransmitters, many neuropeptides
serve as hormones that regulate physiological responses.
53

ANATOMY OF CENTRAL
NERVOUS SYSTEM
55

Parts of Brain:
1. Cerebrum-largest
part of brain.
responsible for
reasoning, thought,
memory, speech,
sensation, etc.
• Divided into two
halves.
• Further divided into
lobes; occipital,
parietal, temporal
and frontal.
56P

2. Cerebellum-responsible
for muscle coordination
3. Brain stem- most basic
functions; respiration,
swallowing, blood
pressure.
4.Lower part (medulla
oblongata) is continuous
with spinal cord
57P

5. Spinal cord- begins
at foramen magnum
and ends at second
lumbar vertebrae
Contains both afferent
(to the brain) and
efferent (motor
neurons- away from
the brain)
58P

Coverings:
Both the brain and spinal cord are covered by a membrane
system called the meninges,lying between the skull and the
brain and between the vertebrae and the spinal cord.
• Named from outside inwards they are:
1) Dura mater
2) Arachnoid mater
3) Pia mater
The dura and arachnoid maters are separated by a potential
space, the subdural space.
The arachnoid and pia maters are separated by the
subarachnoid space, containing cerebrospinal fluid.
59

Coverings and Blood Brain Barrier:
60

•Ventricles
 Filled with CSF (cerebrospinal fluid)
 Lined by ependymal cells (these cells
lining the choroid plexus make the CSF)
 Continuous with each other and central
canal of spinal cord
61

 Lateral ventricles
 Paired, horseshoe shape
 In cerebral hemispheres
 Anterior are close, separated only by thin
Septum pellucidum
62

Brain:
The brain constitutes about one-
fiftieth of the body weight and lies
within the cranial cavity.
The parts are
1.cerebrum
2.cerebellum.
3.Diencephlon
4.midbrain
5.pons
6.medulla oblongata
63P

Blood supply to the brain:
• The circulus arteriosus and its
contributing arteries play a vital
role in maintaining a constant
supply of oxygen and glucose.
• The brain receives about 15% of
the cardiac output, approx 750 ml
of blood/minute.
• Autoregulation maintain blood
flow to the brain constant by
adjusting the diameter (about
65-140 mmHg) with changes
occurring only outside these
limits.
64P

Cerebrum
This is the largest part of the brain and
it occupies the anterior and middle
cranial fossae.
It is divided by a deep cleft, the
longitudinal cerebral fis-sure, into right
and left cerebral hemispheres, each
containing one of the lateral ventricles.
Deep within the brain the hemispheres
are connected by a mass of white
matter (nerve fibres) called the corpus
callosum.
The superficial (peripheral) part of the
cerebrum is composed of nerve cell
bodies or grey matter, forming the
cerebral cortex, and the deeper layers
consist of nerve fibres or white matter.
65P

 Surface anatomy of Brain
Gyri (plural of gyrus)
 Elevated ridges
 Entire surface
 Grooves separate gyri
 A sulcus is a shallow groove (plural, sulci)
 Deeper grooves are fissures
66P

 Lateral sulcus separates temporal lobe from parietal
lobe
 Parieto-occipital sulcus divides occipital and parietal
lobes (not seen from outside)
 Transverse cerebral fissure separates cerebral
hemispheres from cerebellum
67P

Cerberal cortex:
The cerebral cortex shows many infoldings or furrows of varying
depth. The exposed areas of the folds are the gyri or convolutions
and these are separated by sulci or fissures.
These convolutions greatly increase the surface area of the
cerebrum.
For descriptive purposes each hemisphere of the cerebrum is
divided into lobes which take the names of the bones of the
cranium under which they lie: frontal, parietal,temporal and
occipital.
The boundaries of the lobes are marked by deep sulci (fissures).
These are the central, lateral and parieto-occipital ci .
68P

Interior of the cerebrum:
The surface of the cerebral cortex is
composed of grey matter (nerve cell
bodies).
Within the cerebrum the lobes are
connected by masses of nerve fibres,
or tracts, which make up the white
matter of the brain.
The afferent and efferent fibres
linking the different parts of the
brain and spinal cord are as follows.
• Association (arcuate) fibres
• Commissural fibres
• Projection fibres
69P

Afferent and efferent fibres:
• Association (arcuate) fibres connect different parts of a
cerebral hemisphere by extending from one gyrus to another, some of
which are adjacent and some distant.
• Commissural fibres connect corresponding areas of the two cerebral
hemispheres; the largest and most important issure is the corpus
callosum.
• Projection fibres connect the cerebral cortex with grey matter of lower
parts of the brain and with the spinal cord, e.g. the internal capsule ( lies
deep within the brain between the basal nuclei (ganglia) and the
thalamus.
Many nerve impulses passing to and from the cerebral cortex are carried
by fibres that form the internal capsule.
Motor fibres within the internal capsule form the pyramidal tracts
(corticospinal tracts) that cross over (decussate) at the medulla
oblongata.
70P

•Functions of cerbrum (simplified)
 Back of brain: perception
 Top of brain: movement
 Front of brain: thinking
72

Functional areas of the cerebrum
1. Motor areas of the cerebrum
a.The premotor area. This lies in the frontal lobe
immediately anterior to the motor area.
Motor speech (Broca's) area which controls the
movements necessary for speech. It is dominant in the
left hemisphere in right-handed people and vice versa.
b.The frontal area. This extends anteriorly from the
premotor area to include the remainder of the frontal
lobe.
73

Sensory areas of the cerebrum
The postcentral (sensory) area.
 This is the area behind the central sulcus.
 Sensations of pain, temperature, pressure and touch,
knowledge of muscular movement and the position of
joints are perceived.
The sensory area of the right hemisphere receives
impulses from the left side of the body and vice versa.
74

The auditory (hearing) area. This lies immediately below the lateral
sulcus within the temporal lobe. (8th cranial nerves).
The olfactory (smell) area. This lies deep within the temporal lobe
where impulses from the nose via the olfactory nerves (1st cranial
nerves) are received and interpreted.
The taste area. This is thought to lie just above the lateral sulcus in
the deep layers of the sensory area. (8th cranial nerves)
The visual area. This lies behind the parieto-occipital sulcus and
includes the greater part of the occipital lobe. (2nd cranial nerves)
75

•Association Areas
 Tie together different kinds of sensory input
 Associate new input with memories
 Is to be renamed “higher-order processing“ areas
Different areas
1. Premotor area:
2. Prefrontal area:
3. Wernicke’s area (speech area):
4. Pareito-occipital temporal area:
76

Other areas of the cerebrum
Deep within the cerebral
hemispheres there are groups of
cell bodies called nuclei
(called ganglia) which act as
relay stations where impulses
are passed from one neurone to
the next in a chain.
Important masses of grey matter
include:
• basal nuclei
• thalamus
• hypothalamus.
78

Basal nuclei.
These are areas of grey matter, lying deep within the cerebral
hemispheres, with connections to the cerebral cortex and thalamus.
The basal nuclei form part of the extrapyramidal tracts and
are thought to be involved in initiating muscle tone in slow
and coordi-nated activities.
If control is inadequate or absent, move-ments are jerky,
clumsy and uncoordinated.
79

Thalamus.
The thalamus consists of two masses of nerve cells and fibres situated
within the cerebral hemispheres just below the corpus callosum, one on
each side of the third ventricle.
Sensory input from the skin, viscera and special sense organs is transmitted
to the thalamus before redistribution to the cerebrum.
Hypothalamus.
• The hypothalamus is composed of a number of groups of nerve cells. It is
situated below and in front of the thalamus, immediately above the pituitary
gland.
• The hypothalamus is linked to the posterior lobe of the pituitary gland by
nerve fibres and to the anterior lobe by a complex system of blood vessels.
• Through these connections, the hypothalamus controls the output of
hormones from both lobes of the gland.
80

Other functions of hypothalamus:
autonomic nervous system
appetite and satiety
thirst and water balance
body temperature
emotional reactions, e.g. pleasure, fear, rage
sexual behaviour including mating and child rearing
biological clocks or circadian rhythms, e.g. sleeping
and waking cycles, body temperature and secretion of
some hormones.
81

Brain stem
1. Midbrain:
The midbrain is the area of the brain
situated around the cerebral aqueduct
between the cerebrum above and the
pons below.
It consists of groups of cell bodies
and nerve fibres (tracts) which
connect the cerebrum with lower
parts of the brain and with the spinal
cord.
The cell bod-ies act as relay stations
for the ascending and descending
nerve fibres.
82

2. Pons
The pons is situated in front of the cerebellum, below the
midbrain and above the medulla oblongata.
It consists mainly of nerve fibres which form a bridge between the
two hemispheres of the cerebellum, and of fibres passing
between the higher levels of the brain and the spinal cord.
There are groups of cells within the pons which act as relay stations
and some of these are associated with the cranial nerves.
The anatomical structure of the pons differs from that
of the cerebrum in that the cell bodies (grey matter) lie
deeply and the nerve fibres are on the surface.
83

3.Medulla oblongata
The medulla oblongata extends from the pons above and is
continuous with the spinal cord below.
It is about 2.5 cm long and it lies just within the cranium above the
foramen magnum.
Its anterior and posterior surfaces are marked by central fissures.
The outer aspect is composed of white matter which passes
between the brain and the spinal cord, and grey matter lies centrally.
Some cells con-stitute relay stations for sensory nerves passing
from the spinal cord to the cerebrum.
The vital centres, consisting of groups of cells associated
with autonomic reflex activity, lie in its deeper structure.
These are the:
cardiac centre
respiratory centre
vasomotor centre
reflex centres of vomiting, coughing, sneezing and
swallowing.
84

Other functions of medulla oblongata:
1. Decussation (crossing) of the pyramids. In the medulla motor
nerves descending from the motor area in the cerebrum to the
spinal cord in the pyramidal (corticospinal) tracts cross from one
side to the other.
These tracts are the main pathway for impulses to skeletal
(voluntary) muscles.
2. Sensory decussation. Some of the sensory nerves ascending to the
cerebrum from the spinal cord cross from one side to the other in the
medulla.
3. Others decussate at lower levels, i.e. in the spinal cord.
85

4. The cardiovascular centre :
controls the rate and force of cardiac contraction. sympathetic
and parasympathetic nerve fibres originating in the medulla
pass to the heart. Sympathetic stimulation increases the rate
and force of the heartbeat and parasympathetic stimulation has
the opposite effect.
5. The respiratory centre:
controls the rate and depth of respiration. From this centre,
nerve impulses pass to the phrenic and intercostal nerves
which stimulate contraction of the diaphragm and intercostal
muscles,thus initiating inspiration.
The respiratory centre is stimulated by excess carbon dioxide
and, to a lesser extent, by deficiency of oxygen in its blood
supply and by nerve impulses 86

6. The vasomotor centre:
It controls the diameter of the blood vessels, especially the small
arteries and arterioles which have a large proportion of smooth
muscle fibres in their walls.
Vasomotor impulses reach the blood vessels through the autonomic
nervous system.
Stimulation may cause either constriction or dilatation of blood
vessels depending on the site.
The sources of stimulation of the vasomotor centre are the arterial
baroreceptors, body temperature and emotions such as sexual
excitement and anger.
Pain usually causes vasoconstriction although severe pain
may cause vasodilatation, a fall in blood pressure and
fainting.
87

7. Reflex centres:
• When irritating substances are present in the stomach or
respiratory tract, nerve impulses pass to the medulla
oblongata, stimulating the reflex
centres which initiate the reflex actions of vomiting,
coughing and sneezing to expel the irritant.
88

Reticular formation:
The reticular formation is a collection of neurones in the core of the
brain stem, surrounded by neural pathways which conduct ascending
and descending nerve impulses between the brain and the spinal cord.
The reticular formation is involved in:
Coordination of skeletal muscle activity associated with voluntary
motor movement and the maintenance of balance
Coordination of activity controlled by the autonomic nervous system,
e.g. cardiovascular, respiratory and gastrointestinal activity.
Selective awareness that functions through the reticular activating
system (RAS) which selectively blocks or passes sensory
information to the cerebral cortex, e.g. the slight sound made by a
sick child moving in bed may arouse his mother but the noise of
regularly passing trains may be suppressed.
89

Cerebellum:
• The cerebellum is situated behind the pons and
immediately below the posterior portion of the cerebrum
occupying the posterior cranial fossa.
• It is ovoid in shape and has two hemispheres, separated by a
narrow median strip called the vermis.
• Grey matter forms the surface of the cerebellum, and the white
matter lies deeply.
90

•CerebellumTwo major hemispheres: three lobes
each
Anterior
Posterior
Floculonodular
Vermis: midline lobe connecting
hemispheres
Inner branching white matter, called
“arbor vitae”
Separated from brain stem by 4th ventricle
91

Functions:
• The cerebellum is concerned with the coordination of voluntary
muscular movement, posture and balance.
• Cerebellar activities are not under voluntary control.
• The sensory input for these functions is derived from the muscles and
joints, the eyes and the ears.
• Proprioceptor impulses from the muscles and joints indicate their
position in relation to the body as a whole and those impulses from the
eyes and the semicircular canals in the ears provide information about
the position of the head in space.
• Impulses from the cerebellum influence the contraction of skeletal
muscle so that balance and posture are maintained.
• Damage to the cerebellum results in clumsy uncoordinated muscular
movement, staggering gait and inability to carry out smooth, steady,
precise movements.
92

SPINAL CORD:
It is the elongated, almost cylindrical part of the central nervous system,
which is suspended in the vertebral canal surrounded by the meninges
and cerebrospinal fluid.
It is continuous above with the medulla oblongata and extends from the
upper border of the atlas to the lower border of the 1st lumbar vertebra.
It is approximately 45 cm long in an adult Caucasian male, and is about
the thickness of the little finger.
When a specimen of cerebrospinal fluid is required it is taken from a
point below the end of the cord, i.e. below the level of the 2nd lumbar
vertebra (lumbar puncture).
Nerves conveying impulses from the brain to the various organs and
tissues descend through the spinal cord.
94

Some activities of the spinal cord are independent of the
brain, i.e. spinal reflexes (through extensive neurone
connections between sensory and motor neurones ).
The spinal cord is incompletely divided into two equal parts,
anteriorly by a short, shallow median fissure and posteriorly
by a deep narrow septum, the posterior median septum.
A cross-section of the spinal cord shows that it is composed of grey
matter in the centre surrounded by white matter
supported by neuroglia.
95

Grey matter:
• The arrangement of grey matter in the spinal cord resembles the shape
of the letter H, having two posterior, two anterior and two lateral
columns.
• The area of grey matter lying transversely is the transverse commissure
and it is pierced by the central canal, an extension from the fourth
ventricle, containing cerebrospinal fluid.
• The cell bodies may be:
• Sensory cells: which receive impulses from the periphery of the body
• Lower motor neurones: which transmit impulses to the skeletal
muscles
•Connector neurones: linking sensory and motor neurones, at the same
or different levels, which form spinal reflex arcs.
• At each point where nerve impulses are passed from one neurone to
another there is a synaptic cleft and a neurotransmitter
96

Posterior columns of grey matter:
These are composed of cell bodies which are stimulated by sensory impulses from
the periphery of the body.
The nerve fibres of these cells contribute to the formation of the white matter of the
cord and transmit the sensory impulses upwards to the brain.
Anterior columns of grey matter
These are composed of the cell bodies of the lower motor
neurones which are stimulated by the axons of the upper
motor neurones or by the cell bodies of connector neurones
linking the anterior and posterior columns to form reflex
arcs.
The posterior root (spinal) ganglia
are composed of cell bodies which lie just outside the spinal cord on the
pathway of the sensory nerves.
•All sensory nerve fibres pass through these ganglia. The only function of the cells is
to promote the onward movement of nerve impulses. 97

White matter:
It is arranged in three columns or tracts
1. anterior
2.posterior
3.lateral.
These tracts are formed by sensory nerve fibres ascending to
the brain, motor nerve fibres descending from the brain and
fibres of connector neurones.
Tracts are often named according to their points of origin and
destination, e.g. spinothalamic, corticospinal.
98

Sensory nerve tracts (afferent or ascending) & motor nerve
tracts (efferent or descending)
in the spinal cord
99

The Stretch Reflex
• A stretch reflex causes contraction of a skeletal muscle (the
effector) in response to stretching of the muscle.
• This type of reflex occurs via a monosynaptic reflex arc. The reflex can occur
by activation of a single sensory neuron that forms one synapse in the CNS
with a single motor neuron.
101

103
•The Peripheral Nervous
System
103P

104
____Cranial nerves attach to brain
___Spinal nerves attach to spinal cord
104

•Peripheral Nervous System
105

•The Peripheral Nervous System
• Nervous structures outside the brain
and spinal cord
• Nerves allow the CNS to receive
information and take action
• Functional components of the PNS
• Sensory inputs and motor outputs
categorized as somatic or visceral
106

•Sensory Input and Motor Output
• Sensory (afferent) signals picked up by sensor
receptors, carried by nerve fibers of PNS to the CNS
• Motor (efferent) signals are carried away from the
CNS, innervate muscles and glands
• Divided according to region they serve
• Somatic body region
• Visceral body region
• Results in four main subdivisions
• Somatic sensory
• Visceral sensory
• Somatic motor
• Visceral motor
107

•PNS Afferent Division
• Afferent (sensory) division – transmits impulses from
receptors to the CNS.
1. Somatic afferent fibers – carry impulses from skin, skeletal
muscles, and joints
2. Visceral afferent fibers – transmit impulses from visceral
organs
108

•PNS Efferent Division
• Motor (efferent) division – transmits impulses from the CNS to
effector organs. Two subdivisions:
• 1. Somatic nervous system – provides conscious control of
skeletal muscles
• 2. Autonomic nervous system – regulates smooth muscle,
cardiac muscle, and glands
109

•Basic Structural Components of the PNS
• Sensory receptors – pick up stimuli from inside or outside the
body
• Motor endings – axon terminals of motor neurons innervate
effectors (muscle fibers and glands)
• Nerves and ganglia
• Nerves – bundles of peripheral axons
• Ganglia – clusters of peripheral neuronal cell bodies
110

References:
• Principles of Anatomy and Physiology by
Tortora Grabowski. Palmetto, GA, U.S.A.
• Ross & Wilson Anatomy and Physiology in
Health and Illness 12th Ed
111

Central Nervous System (F.Y B Pharm Sem-II)

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