The Enteric Nervous System

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The digestive tract has a nervous system of its own called the enteric10 nervous system. Unlike the ANS proper, it does not arise from the brainstem or spinal cord, but like the ANS, it innervates smooth muscle and glands. Thus, opinions differ on whether it should be considered part of the ANS. It consists of about 100 million neurons embedded in the wall of the digestive tract—perhaps more neurons than there are in the spinal cord—and it has its own reflex arcs. The enteric nervous system regulates the motility of the esophagus, stomach, and intestines and the secretion of digestive enzymes and acid. To function normally, however, these digestive activities also require regulation by the sympathetic and parasympathetic systems. The enteric nervous system is discussed in more detail in chapter 25.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

3. Explain why the sympathetic division is also called the thoracolumbar division even though its paravertebral ganglia extend all the way from the cervical to the sacral region.

4. Describe or diagram the structural relationships among the following: preganglionic fiber, postganglionic fiber, ventral ramus, gray ramus, white ramus, and paravertebral ganglion.

5. Explain in anatomical terms why the parasympathetic division affects target organs more selectively than the sympathetic division does.

6. Trace the pathway of a parasympathetic fiber of the vagus nerve from the medulla oblongata to the small intestine.

enter = intestines + ic = pertaining to ot = ear + ic = pertaining to

Table 15.3

Comparison of the Sympathetic and Parasympathetic Divisions




Origin in CNS



Location of ganglia

Paravertebral ganglia adjacent to spinal column and prevertebral ganglia

Terminal ganglia near or within target organs

anterior to it

Fiber lengths

Short preganglionic

Long preganglionic

Long postganglionic

Short postganglionic

Neuronal divergence

Extensive (about 1:17)

Minimal (about 1:2)

Effects of system

Often widespread and general

More specific and local

Saladin: Anatomy & I 15. The Autonomic Nervous I Text I © The McGraw-Hill

Physiology: The Unity of and Visceral Reflexes Companies, 2003 Form and Function, Third Edition

574 Part Three Integration and Control

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