The peripheral nervous system (PNS) consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs.
Unlike the CNS, the PNS is not protected by the bones of the spine and skull, or by the blood–brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.
The peripheral nervous system includes 12 cranial nerves and 31 pairs of spinal nerves that provide communication from the CNS to the rest of the body by nerve impulses to regulate the functions of the human body. The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.
Spinal Nerve Correspondences
Each pair of spinal nerves roughly correspond to a segment of the vertebral column: 8 cervical spinal nerve pairs (C1–C8), 12 thoracic pairs (T1–T12), 5 lumbar pairs (L1–L5), 5 sacral pairs (S1–S5), and 1 coccygeal pair.
The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that subserve the neck and back of the head.
The spinal nerve C1 (suboccipital nerve) provides motor innervation to muscles at the base of the skull.
C2 and C3 form many of the nerves of the neck, and provides both sensory and motor control. These include the greater occipital nerve that provides sensation to the back of the head, the lesser occipital nerve that provides sensation to the area behind the ears, the greater auricular nerve, and the lesser auricular nerve.
The phrenic nerve arises from nerve roots C3, C4, and C5. It innervates the diaphragm to enable breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.
The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the brachial plexus, or plexus brachialis, a tangled array of nerves, splitting, combining and recombining to form the nerves that subserve the upper limb region and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable with little variation among people.
The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes, this plexus is usually divided into three parts: lumbar plexus, sacral plexus, and pudendal plexus.
Autonomic Nervous System Function (ANS)
The sympathetic division
typically functions in actions that need quick responses. The parasympathetic
division functions with actions that do not require immediate reaction.
The somatic nervous system consists of afferent and
efferent nerves and is associated with the voluntary control of skeletal
muscle movements. The afferent
nerves are responsible for relaying sensations from the body to the central
nervous system (CNS), while the efferent nerves are responsible for sending out
commands from the CNS to the body to stimulate muscle contraction.
Upper motor neurons release acetylcholine.
Acetylcholine is released from the axon terminal knobs of alpha motor neurons
and received by postsynaptic receptors (nicotinic
acetylcholine receptors) of muscles, thereby relaying the stimulus
to contract muscle fibers.