The peripheral nervous system (PNS) contains the somatic and autonomic nervous systems. The somatic nervous system (SNS) is primarily concerned with reactions to outside stimuli, while the autonomic nervous system (ANS) is primarily concerned with the proper functioning of the internal organs to maintain homeostasis.
Somatic Nervous System
The somatic nervous system includes all of the nerves that serve the musculoskeletal system and the exterior sense organs, including those of the skin. Exterior sense organs have receptors that receive environmental stimuli and then initiate nerve impulses. On the other hand, muscle fibers are effectors that bring about a reaction to the stimulus.
Cranial Nerves
Humans have twelve pairs of cranial nerves attached to the brain. Some of these are sensory, some are motor, and still others are mixed. Although the brain is a part of the eNS, the cranial nerves are a part of the PNS. All cranial nerves, except the vagus, are concerned with the head, neck, and facial regions of the body. The vagus nerve has many branches to serve the internal organs .
Spinal Nerves
The 31 pairs of spinal nerves are a part of the peripheral nervous system.
Each spinal nerve emerges from the spinal cord by two short branches, called the dorsal and ventral roots. The dorsal root can be identified by the presence of an enlargement called the dorsal-root ganglion. This ganglion contains the cell bodies of the sensory neurons whose fibers conduct impulses toward the spinal cord. The ventral root of each spinal nerve contains the axons of motor neurons that conduct impulses away from the spinal cord. These two roots join just before the spinal nerve leaves the vertebral column. Therefore, all spinal nerves are mixed nerves with many sensory dendrites and motor axons.
The segments of skin supplied by a particular spinal nerve are called dermatomes. Abnormal skin sensations in a particular dermatome can be used to detect which spmal nerve is damaged.
Cranial nerves take impulses to and/or from the brain. Spinal nerves take impulses to and from the spinal cord.
