The sensory functions of the somatic nervous system give rise to conscious awareness and its motor functions are voluntary. The sensory functions of the autonomic nervous system are largely not subject to conscious perception and its motor functions are involuntary. The somatic division of the peripheral nervous system consists of 31 pairs of spinal nerves and 12 pairs of cranial nerves. Spinal nerves arise from the spaces between adjoining vertebrae of the spinal cord. The two roots join to form a mixed nerve.
Cranial nerves arise mainly from the ventral brainstem.
The autonomic division of the peripheral nervous system consists of two anatomically and functionally distinct components. Secondary motor neurons in these ganglia send noradrenergic axon terminals to the organs to be enervated. They send axons to individual ganglia near the organ to be enervated by cholinergic post-ganglionic motor neuron. It is surrounded by white matter consisting of myelinated ascending and descending axons. Simple reflexes may be organized by neurons within the segment of entry and exit of related sensory and motor nerves.