Peripheral Nervous System
There are twelve pairs of Cranial nerves which have their origin from various parts of the brain. Some are mixed nerves, i.e. both motor and sensory, some motor only, and some sensory nerves, e.g. the nerves of the special senses. The cranial nerves are;
Olfactory - the nerve of smell.
Optic - the nerve of sight.
Oculo motor - the nerve of the external muscles of the eye.
Trochlear- innervates one muscle of the eye-ball, the external Oblique.
Trigeminal -innervates most of the skin of the head and face; the membranes of the mouth, nose, teeth and the muscles for mastication.
Abducent -innervates the muscle of the eye-ball, the lateral rectus.
Facial - innervates the muscles of expression of the face.
Auditory or Acoustic (Vestibulocochlear) - the nerve of hearing.
Glossopharyngeal - innervates the constrictor muscle of the pharynx, parotid gland, tongue and the soft palate.
Vagus - innervates most of the organs on the thoracic and abdominal cavities.
Accessory - innervates muscles of the neck.
Hypoglossal - innervates the muscle of the tongue.
Thus the Cranial nerves do their functions in various parts of the body.
There are 31 pairs of spinal nerves which are attached to the spinal cord by two roots. viz., the dorsal root and the ventral root. Each spinal nerve is formed by the union of anterior and posterior roots. Anterior root contains motor fibers and posterior root consists of sensory fibers. The larger anterior branches interlace to form networks called plexus which then give off branches to the various parts.
The three main plexuses are
The cervical plexus - muscles of the neck and back of the head.
The brachial plexus - shoulders, arm, forearm, the wrist and the hand.The lumbosacral plexus - nerves to the lower extremities.