Muscles that move the shoulder joint are innervated mainly by spinal nerves from spinal cord levels C5 and C6.
Muscles that move the elbow are innervated mainly by spinal nerves from spinal cord levels C6 and C7.
Muscles in the hand are innervated mainly by spinal nerves from spinal cord levels C8 and T1.
Visceral part of the nervous system
The visceral part of the nervous system, as in the somatic part, consists of motor and sensory components:
Sensory nerves monitor changes in the viscera.
Motor nerves mainly innervate smooth muscle, cardiac muscle, and glands.
The visceral motor component is commonly referred to as the autonomic division of the PNS and is subdivided into sympathetic and parasympathetic parts.
Like the somatic part of the nervous system, the visceral part is segmentally arranged and develops in a parallel fashion (Fig. 1.39).
Visceral sensory neurons that arise from neural crest cells send processes medially into the adjacent neural tube and laterally into regions associated with the developing body. These sensory neurons and their processes, referred to as general visceral afferent fibers (GVAs), are associated primarily with chemoreception, mechanoreception, and stretch reception.
Visceral motor neurons that arise from cells in lateral regions of the neural tube send processes out of the anterior aspect of the tube. Unlike in the somatic part, these processes, containing general visceral efferent fibers (GVEs), synapse with other cells, usually other visceral motor neurons, that develop outside the CNS from neural crest cells that migrate away from their original positions close to the developing neural tube.
The visceral motor neurons located in the spinal cord are referred to as preganglionic motor neurons and their axons are called preganglionic fibers; the visceral motor neurons located outside the CNS are referred to as postganglionic motor neurons and their axons are called postganglionic fibers.
The cell bodies of the visceral motor neurons outside the CNS often associate with each other in a discrete mass called a ganglion.
Visceral sensory and motor fibers enter and leave the CNS with their somatic equivalents (Fig. 1.40). Visceral sensory fibers enter the spinal cord together with somatic sensory fibers through posterior roots of spinal nerves. Preganglionic fibers of visceral motor neurons exit the spinal cord in the anterior roots of spinal nerves, along with fibers from somatic motor neurons.
Postganglionic fibers traveling to visceral elements in the periphery are found in the posterior and anterior rami (branches) of spinal nerves.
Visceral motor and sensory fibers that travel to and from viscera form named visceral branches that are separate from the somatic branches. These nerves generally form plexuses from which arise branches to the viscera.
Visceral motor and sensory fibers do not enter and leave the CNS at all levels (Fig. 1.41):
In the cranial region, visceral components are associated with four of the twelve cranial nerves (CN III, VII, IX, and X).
In the spinal cord, visceral components are associated mainly with spinal cord levels T1 to L2 and S2 to S4.
Visceral motor components associated with spinal levels T1 to L2 are termed sympathetic. Those visceral motor components in cranial and sacral regions, on either side of the sympathetic region, are termed parasympathetic:
The sympathetic system innervates structures in peripheral regions of the body and viscera.
The parasympathetic system is more restricted to innervation of the viscera only.