The document provides information about the nervous system. It states that the nervous system is composed of specialized cells called neurons that conduct stimuli through neural networks from sensory receptors to sites of response, such as glands or muscles. It notes that the human nervous system consists of the central nervous system (brain and spinal cord) and peripheral nervous system (nerves connecting to the central nervous system). Some key disorders of the nervous system are also mentioned.

2. System of specialized cells (neurons, or nerve cells) that conduct
stimuli from a sensory receptor through a neuron network to
the site (e.g., a gland or muscle) where the response occurs.
In humans, it consists of the central and peripheral nervous
systems, the former consisting of the brain and spinal cord
and the latter of the nerves, which carry impulses to and from
the central nervous system. The cranial nerves handle head
and neck sensory and motor activities, except the vagus
nerve, which conducts signals to visceral organs. Each spinal
nerve is attached to the spinal cord by a sensory and a motor
root. These exit between the vertebrae and merge to form a
large mixed nerve, which branches to supply a defined area of
the body. Disorders include amyotrophic lateral sclerosis,
chorea, epilepsy, myasthenia gravis, neural tube defect,
parkinsonism, and poliomyelitis. Effects of disorders range
from transient tics and minor personality changes to major
personality disruptions, seizures, paralysis, and death.

3. Part of the nervous system that is not under conscious control and that
regulates the internal organs.
It includes the sympathetic, parasympathetic, and enteric nervous
systems. The first, which connects the internal organs to the brain
via spinal nerves, responds to stress by increasing heart rate and
blood flow to the muscles and decreasing blood flow to the skin. The
second comprises the cranial nerves and the lower spinal nerves,
which increase digestive secretions and slow the heartbeat. Both
have sensory fibres that send feedback on the condition of internal
organs to the central nervous system, information that helps
maintain homeostasis. The third division, embedded in the walls of
the stomach and intestines, controls digestive movement and
secretions.

4. Pathways of the autonomic nervous
system. Nerve impulses begin in motor
neurons in the brain or spinal cord. Each
motor neuron connects with a second motor
neuron outside the central nervous system
to carry the impulse to the glands and
smooth muscles. In the sympathetic
nervous system, these second motor
neurons are found in ganglia (masses of
neurons) along either side of the spinal
cord. In the parasympathetic nervous
system, ganglia are located closer to, or
sometimes within, the target organs. Some
ganglia form large clusters called plexuses.
Preganglionic fibres of the sympathetic
system emerge along the thoracic (T) and
first three lumbar (L) segments of the spinal
cord. Fibres of the parasympathetic system
originating in the brainstem arise from the
third, seventh, ninth, and tenth cranial
nerves; other parasympathetic fibres
emerge from the second, third, and fourth
sacral (S) segments of the spinal cord.

5. Concentration of nerve tissue in the front or upper end
of a body.
It handles sensory information, controls motion, is vital
to instinctive acts, and in higher vertebrates is the
centre of learning. Vertebrate brains consist of the
hindbrain (rhombencephalon), midbrain
(mesencephalon), and forebrain (prosencephalon).
The hindbrain comprises the medulla oblongata and
the pons, which connects the spinal cord with higher
brain levels and transfers information from the cerebral
cortex to the cerebellum. The midbrain, a major
sensory integration centre in other vertebrates, serves
primarily to link the hindbrain and forebrain in
mammals. Large nerve bundles connect the
cerebellum to the medulla, pons, and midbrain. In the
forebrain the two cerebral hemispheres are connected
by a thick bundle of nerve fibres (corpus callosum) and
are divided by two deep grooves into four lobes
(frontal, parietal, temporal, and occipital). The
cerebrum, the largest part of the human brain, is
involved with its more complex functions. Motor and
sensory nerve fibres from each hemisphere cross over
in the medulla to control the opposite side of the body.

6. Side view of the brain showing its major
structures. The large cerebrum is divided into two
halves, or hemispheres, connected by the corpus
callosum, a band of nerve fibres. Two grooves
divide the hemispheres into four lobes: frontal,
temporal, parietal, and occipital. Many nerve cells
are found in the convoluted cerebrum's outer
surface, or cerebral cortex, which controls sensory
and motor activities. The thalamus relays
incoming sensory impulses from the spinal cord to
the cortex. The hypothalamus's many functions
include control of breathing, blood flow,
temperature regulation, and emotions. The
pituitary gland is attached to and regulated by the
hypothalamus. The midbrain relays signals
between the forebrain and hindbrain. The
cerebellum, along with the cerebrum, plays a role
in voluntary movement as well as balance. The
pons serves as a relay point linking the medulla
oblongata, midbrain, cerebellum, and cerebrum.
The medulla, lying between the pons and the
spinal cord and continuous with both, plays a role
in essential involuntary regulatory and reflexive
responses (including breathing, swallowing, and
heartbeat) and relays signals between the spinal
cord and other brain regions

7. Largest part of the brain.
The two cerebral hemispheres consist of an inner core of
myelinated nerve fibres, the white matter, and a heavily
convoluted outer cortex of gray matter (see cerebral cortex).
Nerve fibres in the white matter connect functional areas of
the cortex in the same hemispheres, connect functional areas
of the cortex in opposite hemispheres, and connect the
cerebral cortex to lower centres (e.g., the spinal cord). A front-
to-back fissure divides the cerebrum's two hemispheres. Each
hemisphere controls the opposite side of the body. The
corpus callosum, a thick band of white matter, connects them,
allowing integration of sensory data and responses from both
sides of the body. Other important cerebral structures include
the hypothalamus and the thalamus.

8. Part of the brain that integrates sensory input from the
inner ear and from proprioceptors in muscle with nerve
impulses from the cerebrum (see cerebral cortex),
coordinating muscle responses to maintain balance and
produce smooth, coordinated movements.
Located below the cerebral hemispheres and behind the
upper medulla oblongata and pons, each of its two
connected hemispheres has a core of white matter within
a cortex of gray matter. Disorders usually produce
neuromuscular disturbances, in particular ataxia.

9. Sensory neurons relay information from sense organs, motor
neurons carry impulses to muscles and glands, and
interneurons transmit impulses between sensory and motor
neurons. A typical neuron consists of dendrites (fibres that
receive stimuli and conduct them inward), a cell body (a
nucleated body that receives input from dendrites), and an
axon (a fibre that conducts the nerve impulse from the cell
body outward to the axon terminals). Both axons and
dendrites may be referred to as nerve fibres. Impulses are
relayed by neurotransmitter chemicals released by the axon
terminals across the synapses (junctions between neurons or
between a neuron and an effector cell, such as a muscle cell)
or, in some cases, pass directly from one neuron to the next.
Large axons are insulated by a myelin sheath formed by fatty
cells called Schwann cells. Bundles of fibres from neurons
held together by connective tissue form nerves.

10. Structure of a neuron. Dendrites,
usually branching fibres, receive
and conduct impulses to the cell
body, where inputs arriving from
various dendrites are integrated.
Nerve impulses are conducted
along the axon. When an impulse
reaches the axon terminals,
neurotransmitter chemicals are
released into a gap (synapse)
between the neuron and a
neighbouring cell, and the impulse
is transmitted to an adjacent neuron
or effector cell (as of a muscle or
gland). Schwann cells surround
large axons, forming an insulating
sheath. Spaces (nodes of Ranvier)
between Schwann cells serve to
conduct the nerve impulse quickly
along the axon.

11. The spinal cord is cylinder of nerves,
that is as thick as your finger. It starts
from below the medulla and goes
down to the middle of the back till the
waist. It is protected by the backbone
or spine. All messages travell from the
brain to the spinal cord and then to
different parts of the body. Message
from different parts of the body enter
the spinal cord and travel up to the
brain

12. Mechanism by which information is received about
one's external or internal environment.
Stimuli received by nerves, in some cases through
specialized organs with receptor cells sensitive to
one type of stimulus, are converted into impulses
that travel to specialized areas of the brain, where
they are analyzed. In addition to the “five
senses”—sight, hearing, smell, taste, and touch—
humans have senses of motion (kinesthetic
sense), heat, cold, pressure, pain, and balance.
Temperature, pressure, and pain are cutaneous
(skin) senses; different points on the skin are
particularly sensitive to each

13. Organ that receives light and visual images.
Non-image forming, or direction, eyes are found among worms, mollusks, cnidarians, echinoderms,
and other invertebrates; image-forming eyes are found in certain mollusks, most arthropods, and
nearly all vertebrates. Arthropods are unique in possessing a compound eye, which results in their
seeing a multiple image that is partially integrated in the brain. Lower vertebrates such as fish
have eyes on either side of the head, allowing a maximum view of the surroundings but producing
two separate fields of vision. In predatory birds and mammals, binocular vision became more
important. Evolutionary changes in the placement of the eyes permitted a larger overlap of the two
visual fields, resulting in the higher mammals in a parallel line of direct sight. The human eye is
roughly spherical. Light passes through its transparent front and stimulates receptor cells on the
retina (cones for colour vision, rods for black-and-white vision in faint light), which in turn send
impulses through the optic nerve to the brain. Vision disorders include near- and farsightedness
and astigmatism (correctable with eyeglasses or contact lenses), colour blindness, and night
blindness. Other eye disorders (including detached retina and glaucoma) can cause visual-field
defects or blindness

14. Structure of the human eye. The outer portion
consists of the white protective sclera and
transparent cornea, through which light enters.
The middle layer includes the blood-supplying
choroid and pigmented iris. Light passing into
the interior through the pupil is regulated by
muscles that control the pupil's size. The
retina comprises the third layer and contains
receptor cells (rods and cones) that transform
light waves into nervous impulses. The lens,
lying directly behind the iris, focuses light onto
the retina. The macula lutea, in the centre of
the retina, is a region of high visual acuity and
colour discrimination. Nerve fibres pass out
through the optic nerve to the brain's visual
centre. The eye's anterior and posterior
chambers contain a watery fluid that nourishes
the cornea and lens. The vitreous humour
helps maintain the eye's shape. A thin layer of
mucous membrane (conjunctiva) protects the
eye's exposed surface. External muscles,
including the medial rectus and lateral rectus
muscles, connect and move the eye in its
socket.

15. Organ of hearing and balance.
The outer ear directs sound vibrations through the auditory canal to the
eardrum, which is stretched across the end of the auditory canal and which
transmits sound vibrations to the middle ear. There a chain of three tiny
bones conducts the vibrations to the inner ear. Fluid inside the cochlea of
the inner ear stimulates sensory hairs; these in turn initiate the nerve
impulses that travel along the auditory nerve to the brain. The inner ear is
also an organ of balance: the sensation of dizziness that is felt after
spinning is caused when fluid inside the inner ear's semicircular canals
continues to move and stimulate sensory hairs after the body has come to
rest. The eustachian tube connects the middle ear with the nasal passages;
that connection allows the common cold to spread from the nasal passages
to the middle ear, especially in infants and small children. The most
common cause of hearing loss is otosclerosis, a surgically correctable
disease in which one of the bones of the middle ear loses its capacity to
vibrate.

16. Structures of the human ear. The
cartilaginous auricle and the auditory
canal of the outer ear direct sound
waves to the middle ear. The eardrum,
stretched across the end of the canal,
vibrates as sound waves reach it.
Vibrations are transmitted via three
small bones (hammer, anvil, stirrup) to
the membranous oval window, which
links the middle ear to the inner ear. The
cochlea is a coiled, fluid-filled tube lined
with sensory hairs. Vibrations in the oval
window cause movement of the
cochlear fluid, stimulating the hairs to
initiate impulses that travel along a
branch of the auditory nerve to the
brain. The eustachian tube, running
from the middle ear to the nasopharynx,
equalizes pressure between the middle
and outer ear. The fluid-filled
semicircular canals play a role in
balance, as hairs in the canals respond
to movement-induced changes in the
fluid by initiating impulses that travel to
the brain.

17. Prominent structure between and below the eyes.
With the complex nasal cavity behind it, it functions for
breathing and smelling. Behind the front section
(vestibule), which includes the nostrils, it is divided
vertically by three convoluted ridges (conchae) into air
passages. In the highest one, the olfactory region, a
small segment of mucous membrane lining contains
neurons covered by a moisture layer, in which
microscopic particles in inhaled air dissolve and
stimulate the neurons. The rest of the cavity warms
and moistens inhaled air and filters particles and
bacteria out of it. Sinus cavities in the bone on both
sides of the nose drain into the air passages. During
swallowing, the soft palate closes off the back of the
nose against food.

18. Muscular organ on the floor of the mouth.
It is important in motions of eating, drinking, and swallowing, and its complex movements shape the
sounds of speech. Its top surface consists of thousands of raised projections (papillae). The
receptors of taste (taste buds) are embedded in the papillae and are sensitive to four basic
flavours: sweet, salty, sour, and bitter. More specific flavours are influenced by the sense of smell.
The tongue's appearance (e.g., coated or red) can give clues to disease elsewhere. Disorders of
the tongue include cancer (often caused by smokeless tobacco), leukoplakia (white patches),
fungal infection, and congenital disorders. Different animals use the tongue to serve varied
functions; for example, frogs have an elongated tongue adapted to capturing prey, the snake's
tongue collects and transfers odours to a specialized sensory structure to help locate prey, and
cats use their tongues for grooming and cleaning.
Special sense for perceiving and distinguishing the sweet, sour, bitter, or salty
quality of a dissolved substance, mediated by taste buds on the tongue.
More than 9,000 taste buds on the tongue are responsible for the
chemoreception of taste. Some taste buds are also found on the roof of the
mouth and throat.

19. A. Taste centres on the tongue's surface. Taste buds on the tip of the tongue are most
sensitive to sweet tastes, those on the sides to sour, those at the back to bitter, and
those on the tip and sides to salty. B. Close-up of a papilla showing location of the taste
buds. C. Structure of a taste bud. Each is composed of narrow modified epithelial taste
cells with specialized hairs (microvilli) that project into a pore opening onto the tongue's
surface and of broader supporting cells. Impulses from the taste cells are carried by
nerve pathways to the brain.

20. Surface covering of the body that protects it and receives external sensory
stimuli, consisting of an epidermis over a thicker dermis.
The epidermis contains cells involved in immune defenses, sensory receptors,
pigment cells, and keratin-producing cells. The last harden and migrate to
the surface to form a dead, relatively dry outer layer of horny tissue that
constantly sloughs away. The dermis contains sensory nerves and blood
vessels within connective tissue. Collagen and elastin fibres give skin its
tough, elastic quality. Cells scattered through it produce its components and
take part in immune and other skin responses. A fat layer under the dermis
provides nutritional storage, cushioning, and insulation. Skin disorders
range from dermatitis and acne to skin cancer. Changes in skin colour (e.g.,
jaundice) or texture may be clues to systemic disorders

21. A section through the skin. The
tough, dead cells of the outer
epidermal surface (corneal layer)
serve as a physical barrier and are
continually replaced by cells
produced in the basal layer. The thick
supportive layer of dermis contains
nerve endings, blood vessels, sweat
glands, hair follicles, and oil glands.
The hair follicle encloses the root of
the hair. Oil glands associated with
hair follicles secrete an oily
substance (sebum) which lubricates
the skin surface. The watery
secretions of the tubular sweat
glands are released onto the skin's
surface through small pores. A layer
of fat cells lies below the dermis.