TheAutonomic Nervous System Differs from the Somatic Motor System in at Least Two ImportantWays

The ANS differs from the somatic motor system in its target organs and in the number of neurons in its peripheral circuit. Thesomatic motor system innervates skeletal muscle, which is the muscle responsible for all movements of the body, as described in Chapters 5 and 6.

Box 13-1

Two Subdivisions of Motor Neurons in the Peripheral Nervous System

Somatic motor nerves

Skeletal muscle

Autonomic motor nerves

Cardiac muscle

Smooth muscle

Exocrine glands

gastrointestinal tract, in the bladder, and in other hollow visceral structures.

The ANS also differs in the number of neurons it has in the peripheral nervous system (Figure 13-1). The somatic nervous system has one neuron whose cell body is located in the central nervous system (CNS) and whose axon extends, uninterrupted, to the skeletal muscle, where the peripheral chemical synapse occurs. In contrast, the ANS has two peri­pheral neurons. The first, called a preganglionic neuron, also has its cell body in the CNS, but its axon innervates a second neuron in the chain, called the postganglionic neuron. Its cell body is in a peripheral structure called a ganglion, a collection of nerve cell bodies outside the CNS (recall the example of the dorsal root ganglia of sensory nerves leading to the spinal cord). There are chemically mediated synapses both between the preganglionic and postganglionic neurons and between the postganglionic neuron and its target organ.

The ANS also differs from the somatic motor system in the amount of myelin along the peripheral axons; postganglionic neurons are usually unmyelinated. In addition, somatic motor neurons always excite their skeletal muscle targets, whereas the autonomic postganglionic neurons can either excite or inhibit their targets.