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Types of Nervous System

The peripheral nervous system includes nerves which carry impulses to and from central nervous system. On the basis of their origin the nerves are divided into two types i.e., cranial nerves and spinal nerves.

1. Cranial Nerves

These are the nerves which originate from the brain and supply to the different parts of the body. In all there are twelve pairs of cranial nerves.

Some of these nerves are purely sensory like the optic nerve and the auditory nerve going to eyes and ears, respectively.

Some of these are motor going to the eye muscles and some of them are mixed like the nerves going to the face and tongue.

2. Spinal Nerves

In all 31 pairs of spinal nerves are present in a human being. Out of these eight pairs are found in the neck region, twelve pairs in the thorax, five pairs in the lumber region, five pairs in the sacral and one pair in coccygeal region.

Each spinal nerve originates from the spinal cord by two roots, a dorsal root and a ventral root. The former comes out from the dorsal surface of the spinal cord and the later from the ventral surface. Each dorsal root has an ovoid dorsal ganglion. The two roots then join to form one spinal nerve which comes out from the opening between the two adjacent vertebrae. The dorsal root carries only afferent sensory fibres and the ventral root carries only efferent sensory fibres. The fibres from both the roots terminate in the grey matter of spinal cord. From the above it would be clear that the spinal nerves are mixed having both sensory fibres and motor fibres.

Autonomic Nervous System

The autonomic nervous system consists of a pair of chains of nerves present on either side of the backbone (vertebral) column. Each nerve has many ganglia. Many of the ganglia or the nerve are situated near or embedded in the organ which they control. This nervous system performs many functions which are not under the control or will of an individual.

For example, the movements of the wall of the stomach and intestine, the rate of heartbeat, the accommodation of the pupil of the eye, secretions of sweat glands and other glands are under the control of this system. In fact the activities of all visceral organs are coordinated through autonomic nervous system. The autonomic nervous system has two parts :

(a) Sympathetic system and

(b) Parasympathetic system.

The working of these systems is opposite to each other in action. For example, if the nerves of sympathetic system dilates the pupil of the eyes, the parasympathetic nerves constrict it. It is through the combined working of the two that eyes adjust to the intensity of light. Same is true for many other functions.

Sympathetic Nervous System

It consists of two long chains of ganglionated nerve fibres lying one on each side of the vertebral column. This system originates from the thoracic chest and lumbar (abdominal) areas of the spinal cord. Each chain contains 18 ganglia distributed over it from neck to abdominal region. The nerve fibres of this system have their bodies located in the lateral portions of the grey matter of spinal cord extending from the first thoracic to the second lumbar segments.

Parasympathetic Nervous System

It consists of nerve fibres originating in the brain and emerging via 3rd, 7th, 9th and 10th nerves and of fibres originating in the pelvic region of the spinal cord and emerging by way of spinal nerves in that region. The tenth nerve arises from the medulla and supplies the heart, respiratory system, and digestive tract. The lower regions including the large intestine and reproductive organs are supplied by the parasympathetic nerves of the pelvic spinal nerves. The anterior regions, i.e., iris of the eye, salivary glands, etc. are supplied by the 3rd, 7th and 9th cranial nerves respectively.

The sympathetic nervous system prepares the body for abnormal conditions and violent actions. The parasympathetic system tries to bring back and reestablish the normal conditions in the body.

Table 6.1 —Action of the Autonomic System

Organs innervated

Action of sympathetic system

Action of parasympathetic system

Heart

Strengthens and increases

Weakens and lowers heart­

the heartbeat.

beat.

Arteries

Constrict the lumen of

Dilates the lumen of arteries

arteries and increases the

and lowers the blood

blood pressure.

pressure.

Digestive tract

Slows peristaltic movements

Increases peristaltic

and decreases the activity

movements and speeds up the

of digestive tract.

activity of digestive tract.

Urinary bladder

Relaxes bladder.

Constricts bladder.

Bronchi

Dilates the passage of bronchi

Constricts the passages and

and makes breathing easier.

makes breathing difficult.

Iris

Dilates pupil.

Constricts pupil.

Hair

Stimulates muscles attached

Causes hair to lie flat.

to the hair, and causes them

to stand erect.

Sweat glands

Increases their secretion.

Decreases their secretion.

PERIPHERAL NERVOUS SYSTEM

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