Organization of the Autonomic Motor System

General Features

Motor systems require an effector. The output of the nervous system is either contraction of a muscle or changing the secretion of a gland.

The somatic motor system contracts skeletal muscle. The autonomic motor system (visceral motor system) contracts smooth muscle, contracts cardiac muscle, or changes the secretion of glands.

Remember that the sympathetic and parasympathetic nervous systems have different anatomy. We will learn in this objective that they have different signaling molecules as well. Both pathways consist of two neurons: a preganglionic neuron, with its cell body in the central nervous system and its axon terminals in an autonomic ganglion; and a postganglionic neuron, with its cell body in an autonomic ganglion and its axon terminals sending a chemical signal to the effector organ (cardiac muscle, smooth muscle, or a gland).

Motor (Efferent) Pathways

Efferent information flows out to effectors. Effectors are grouped into two categories: those that are controlled consciously, and those that are controlled automatically. The first is the somatic motor system, also called the voluntary motor system. The second of these is the autonomic nervous system (ANS).

The voluntary muscle system is familiar to us. When we were babies, our parents gave us mobiles and windup thingies in our cribs; we developed eye-hand coordination as we discovered how our skeletal muscles worked. Now, we merely think “touch my nose with my right index finger” and it happens.

The somatic motor system controls voluntary movement. Signals that initiate movement begin in the cerebral cortex and cerebellar cortex. These cortices are the gray matter located on the outer portions of the brain (cerebrum) and little brain (cerebellum).

From there, signals pass through nerve cables (axons) to the brainstem centers controlling movement or to the spinal cord. The brainstem centers control the face while the spinal cord has control centers for the rest of the body.

In these control centers, contacts between nerve cells (synapses) relay the information to another set of neurons, called alpha motor neurons (α motor neurons). It is these motor neurons which make contact with skeletal muscle at the neuromuscular junction, as we saw in the muscular system.

In practice, we use the terms somatic motor system, voluntary muscle, and skeletal muscle more or less interchangeably.

We don’t spend much time thinking about the “other” motor system, called the autonomic or visceral nervous system, but it’s critically important for maintaining homeostasis in the human body.

The autonomic motor system (ANS), comprises those effectors which are not under conscious control. (In other words, they do not involve the cerebral cortex, which is where consciousness resides.)

This information outflow originates in parts of the brainstem or spinal cord. Then, it travels to autonomic ganglia and nerves. Then, the information passes through one or several neurons of the ANS to the effectors: smooth muscle, cardiac muscle, or glands.

Many neuroscientists believe there are three divisions to the autonomic nervous system: the enteric nervous system; the sympathetic nervous system; and the parasympathetic nervous system.

The enteric nervous system controls the motility of the gut. It is influenced by, but not controlled by, the central nervous system. Even in the absence of central nervous system input, it carries out the essential functions: controlling secretion from glands of the digestive organs and moving substances within the digestive tract.