central nervous system and peripheral nervous system

8/2/2019 Central Nervous System and Peripheral Nervous System

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CENRAL NERVOUS SYSTEM AND THE PERIPHERAL NERVOUS SYSTEM

The Central nervous system (CNS) consists of the brain and spinal cord. The

Brain is that part of the CNS housed within the braincase. The spinal cord is in the

vertebral column. The peripheral nervous system (PNS) lies outside the CNS and consist

of nerves and ganglia. The PNS collects information from numerous sources both inside

and on the surface of the body and relays it by way of sensory fibers to the CNS, where the

information is ignored, triggers a reflex, or is evaluated more extensively. Motor fibers in

the PNS relay information from the CNS to muscles and glands in various parts of the

body, regulating activity in those structures. Nerves of the PNS can be divided into two

groups: 12 pairs of cranial nerves and 31 pairs of spinal nerves.


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SPINAL CORD

Spinal Cord Extends from the foramen magnum at the base of the skull to the second

lumbar vertebra. Spinal nerves communicate between the spinal cord and the body. The inferior

end of the spinal cord and the spinal nerves existing there resemble a horses tail and are called the

cauda equine.

A cross section of the spinal cord reveals that the cord consists of a peripheral white matter

portion and a central gray matter portion. The white matter consists of myelinated axons, and the

gray matter is mainly a collection of neuron cell bodies. The white matter in each half of the spinal

cord is organized into three columns called dorsal(posterior), ventral (anterior) and lateral columns.Each columns of the spinal cord contains nerve tract. Ascending tract consist of axons that

conduct action potential towards the brain, and descending tracts consist of axons that conduct

action potentials away from the brain.

The gray matter of the spinal cord is shaped like the letter H, with posterior and anterior

horns. Small lateral horns exist in levels of the cord associated with autonomic nervous system.

The central canal is a fluid-filled space in the center of the cord.


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Spinal nerves arise from numerous rootlets along the dorasal and ventral surfaces of the

spinal cord. The ventral rootlets combine to form a ventral root on the ventral(anterior) side of the

spinal cord, and the dorsal (posterior) side of the cord at each segement. The ventral and dorsal

roots unite just lateral to the spinal cord to form a spinal nerve. The dorsal root contains a ganglion,

called the dorsal root ganglion.


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The spinal nerves arise along the spinal cord from the union of the dorsal roots and ventral

roots. All the spinal nerves are mixed nerves because they contain axons of both sensory and

somatic motor neurons. Some spinal nerves also contain parasympathetic or sympathetic axons.

Most of the spinal nerves axit the vertebral column between adjacent vertebrae. Spinal nerves are

categorized by the region of the vertebral column from which they emerge cervica (C ), thoracic

(T), lumbar (L), sacral (S) and coccygeal (Cx). The spinal nerves are also numbered (starting

superiorly) according to their order within the region. The 31 pairs of spinal nerves are therefore c1

through c8, T1 through T12, L1 through L5, S1 through S% and Co.

Most of the spinal nerves are organized into three plexuses where nerves come together

and then separate: the cervical plexus, the brachial plexus and the lumbosachral plexus. The major

nerves of the neck and limbs are branches of these plexuses. Spinal nerves T2 through T11 do not

join a plexus. Instead these nerves extend around the thorax betweeen the ribs, giving off branches

to muscles and skin.


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BRAIN

The major region of the brain are the brainstem,the diencephalon, the cerebrum and the

cerebellum.

Braimstem connects the spinal cord to the remainder of the brain. It consist of the medulla

oblongata, pons and midbrain and contains several nuclei involved in the vital body functions

such as the control of heart rate, blood pressure and breathing. Damage to small areas of the brain

sten can cause death, whereas damage to relatively large areas of the cerebrum or cerebellum often

do not cause death. Medulla Oblongata the most inferior portion of the brain stem and is

continuous with the spinal cord. It extends from the level of the foramen magnum to the pons.

Superior to the medulla oblongata is the pons. It contains ascending and dscending nerve tracts, as

well as several nuclei. Some nuclei in the pons relay information between the berebrum and the

cerebellum. Several nuclei in medulla oblongata extend into the lower part of the pons, so that

function such as breathing, swallowing and balance are controlled in the lower pons as well as in

the medulla oblongata. Other nuclei in the pons control functions such as chewing and salivation.

The epithalamus is a small area superior and posterior to the thalamus. It consist of a few small

nuclei that are involved in the emotional and visceral response to odors, and the pineal body. The

pineal body is an endocrine gland that may influence the onset of puberty. The hypothalamus is

the most inferior part of the diencephalon and contains several small nuclei which are very

important in maintaining homeostasis. Hypothalamus plays the role in the control of body

temperature, hunger and thirst. The cerecrum is the largest part of the brain. Each cerebralhemisphere is divided into lobes. The frontal lobe important in control of voluntary motor function,

motivation, aggresion, mood, and olfactory(Smell). The parietal lobe is the principal center for the

reception and conscious perception of most sensory information such as touch, pain, teperature,

balance, and taste. The occipital lobe functions in the reception and perception of visual input and

is not distinctly separate from the other lobes. The temporal lobes is involved in olfactory (smell)

and auditory (hearing) and plays important role in memory.


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FLOW OF CEREBROSPINAL FLUID

Cerebrospinal fluid bathes the brain and spinal cord providing a protective cushion around

the CNS. It os produced by the choroid and plexuses. CSF fills the brain ventricles, the central

canal of the spinal cord, and the subarachnoid space.

The CSF flow from: The lateral ventricles into the third ventricle and then through thecerebral aqueduct into the fourth ventricle. A small amount CSF enters the central canal of the

spinal cord. The CSF exits from the fourth ventricle through small opening in its walls and roof

and enters the subarachnoid space

central nervous system and peripheral nervous system

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