Peripheral Nervous System

 Chapter 7

Peripheral Nervous System

Introduction 

The peripheral nervous system consists of the following components:

• 31 pairs of spinal nerves

• 12 pairs of cranial nerves

• The autonomic part of the nervous system.

• Most of the nerves of the peripheral nervous system are composed of sensory nerve fibers conveying impulses from sensory organs to the brain. Motor nerve fibers convey impulses from the brain through the spinal cord to the effector organs like skeletal muscles, smooth muscles and glands. Each nerve consists of numerous nerve fibers formed into bundles. Each bundle has several coverings of protective connective tissue.
  

Spinal Nerves

There are 31 pairs of spinal nerves, that leave from the vertebral column. The nerves pass through the intervertebral foramina formed by adjacent vertebrae. They are named and grouped as follows:

•        8 Cervical • 12 Thoracic •  5 Lumbar • 5 Sacral • 1 Coccygeal.

• Although there are only 07 cervical vertebrae, there are 08 cervical nerves; because the first pair leaves the vertebral canal between the occipital bone and the atlas and the eighth pair leaves below the last cervical vertebra. The lumbar, sacral and coccygeal nerves leave the spinal cords near its termination at the level of the first lumbar vertebra, and extend downwards inside the vertebral canal in the subarachnoid space, forming a sheath of nerves that resembles a horse’s tail, the caudal equina. These nerves leave the vertebral canal at the appropriate lumbar, sacral or coccygeal level, depending on their destination.
• Each spinal nerve is formed by the union of a motor and a sensory nerve root and is, therefore, termed as mixed nerve. Each spinal nerve is associated with sympathetic part of the autonomic nervous system in the form of a preganglionic fiber.

Plexuses

• Immediately after emerging from the intervertebral foramina each spinal nerve divides into a ramus communicant, a posterior ramus and an anterior ramus. The rami communicants are part of preganglionic sympathetic neurons of the autonomic nervous systems. The posterior rami pass backwards and divide into medial and lateral branches to supply skin and muscles of small areas of posterior part of head, neck and trunk. The anterior rami supply the anterior and lateral parts of the neck, trunk and the upper and lower limbs.
• The axillary nerve supplies to the deltoid muscle, shoulder joint and overlying skin. The radial nerve supplies the triceps muscle behind the humerus, crosses in front of the elbow joint and further extends to the wrist and finger joints. It continues into the back of the hand to supply the skin of the thumb, the first two fingers and the lateral half of the third finger. The musculocutaneous nerve supplies muscles of the upper arm and the skin of the forearm. The median nerve supplies the muscles of the front of the forearm. It continues into the hand where it supplies small muscles and the skin of the front of the thumb, the first two fingers and the lateral half of the third finger. The ulnar nerve supplies the muscles on the ulnar aspect of the forearm. It continues to supply the muscles in the palm of the hand and the skin of the whole of the little finger and the medial half of the third finger.

1. Coccygeal Plexus:

• It is formed by a part of the 4th and St h sacral and coccygeal nerves. It supplies impulses to the skin in the area of the coccyx and coccygeus muscles of the pelvic floor and the external anal sphincter.

Autonomic Nervous System

• The autonomic nervous system primarily controls involuntary functions of the body which are carried out almost automatically. The stimulus for activities of the autonomic nervous system initiate in the brain below the level of the cerebrum. Although the actions are not voluntary in nature, the individual is aware of the actions exerted by the autonomic nervous system, e.g. variation in heart rate. The effects of autonomic nervous system are essential for homeostasis and include stimulation or depression of glandular secretions and contraction of cardiac and smooth muscle tissues.

• The efferent or motor nerves of the autonomic nervous system originate from nerve cell in the brain and emerge at various levels between the midbrain and the sacral region of the spinal cord. Many of them travel along the same nerve sheath, as that of the peripheral nerves of the central nervous system.

The autonomic nervous system is divided into the following two parts:

• Sympathetic nervous system (Thoracolumbar outflow)

• Parasympathetic nervous system (craniofacial outflow)

Most of the organs are supplied with both sympathetic and parasympathetic fibbers; however, there are few exceptions. Sweat glands, the skin and blood vessels of skeletal muscles are not supplied with parasympathetic nervous system. The effects of stimulation of various structures by the sympathetic nervous system and their consequent function are mentioned in the.

There are three prevertebral ganglia situated in the abdominal cavity close to the arteries. Their names are as follows:

 
1 Celiac gang

2 Superior mesenteric ganglia

3 Inferior mesenteric ganglions

Neurotransmitters :

• Noradrenaline is the chemical secreted at the postganglionic nerve endings of sympathetic nervous system; the only exception to this is neuron 02 of the sympathetic nervous system. Acetylcholine is the chemical secreted at the postganglionic nerve endings of the parasympathetic nervous system. Both the chemical messengers of the autonomic nervous system are termed as neurotransmitters. Ali ganglion transmissions, of the parasympathetic and sympathetic nervous system, are mediated by acetylcholine.