Peripheral Nervous System Part 1

There are two main parts of peripheral nervous system. They are,

                        Central nervous system

                        Peripheral nervous system

Peripheral nervous system consists of nerves and ganglia outside of the brain and spinal cord. The main functions of the peripheral nervous system are, to connect the central nervous system to the limbs and organs, serve as a relay between the brain and spinal cord and the rest of the body. The PNS extends from the CNS to the outermost areas of the body.

There are two parts of PNS,

1.    Motor division,

2.    Sensory division

Motor division transmit impulses from CNS to muscles or glands. There are two parts,

i.                 Somatic nervous system

Under voluntary control

Transmit signals from the brain to end organs (muscles)

ii.               Autonomic nervous system

Influences the function of organs outside of voluntary control (heart rate, functions of digestive system)

Sensory division transmits impulses from sense organs to central nervous system.

PNS consists of 31 pairs of spinal nerves and 12 pairs of cranial nerves. 31 pairs of spinal nerves that leaves the vertebral canal (formed by 33 vertebrae) by passing through the intervertebral foramina. It connects spinal cord and various body organs. They are named and grouped according to the vertebrae with which they are associated,

            8 cervical

            12 thoracic

            5 lumber

            5 sacral

            1 coccygeal

There are 7 cervical vertebrae but there are 8 nerves, the reason is 1^st pair leaves the vertebral canal between the occipital bone and the atlas. 8^th pair leaves below the last cervical vertebra. nerves are given the name and number of the vertebra immediately above.

The anterior nerve root consists of motor nerve fibers, which are the axons of the lower motor neurons from the anterior column of grey matter in the spinal cord and in the thoracic and lumber regions sympathetic nerve fibers, which are the axons of cells in the lateral columns of grey matter. For a very short distance after leaving the spinal cord the nerve roots have a covering of dura and arachnoid maters. These terminate before the two roots join to form the mixed spinal nerve. The nerve roots have no covering of pia mater.

The posterior nerve root consists of sensory nerve fibers. just outside the spinal cord there is a spinal ganglion (posterior, or dorsal, root ganglion), consisting of a little cluster of cell bodies. Sensory nerve fibers pass through these ganglia before entering the spinal cord. Area of skin whose sensory receptors contribute to each nerve is called as a dermatome.

 

Branches.

Immediately, after emerging from the intervertebral foramen, spinal nerves divide into branches or rami,

            A ramus communicans

            A posterior ramus

            An anterior ramus

Rami communicants are part of preganglionic sympathetic neurons of the autonomic nervous system.

Posterior rami, pass backwards, divide into smaller medial and lateral branches, to supply skin and muscles of relatively small areas of the posterior aspect of the head, neck, and trunk.

Anterior rami, supply the anterior and lateral aspects of the neck, trunk and the upper and lower limbs.