Porth's Essentials of Pathophysiology, 4e

683

Structure and Function of the Gastrointestinal System

C h a p t e r 2 8

2

Sympathetic

(preganglionic) Parasympathetic

To prevertebral ganglia, spinal cord, and brain stem

ANS Innervation. The intestine is also innervated by the parasym- pathetic and sympathetic branches of the ANS (see Chapter 35, Fig. 35-23). Parasympathetic innervation is supplied mainly by the vagus nerve with postganglionic neurons located primarily in the myenteric and sub- mucosal plexuses. Stimulation of these parasympathetic nerves causes a general increase in both intesti- nal motility and secretory activity. Sympathetic innervation is supplied by nerves that run between the spinal cord and the prevertebral ganglia and between these ganglia and the intes- tine. Stimulation of the sympathetic nervous system is largely inhibitory, producing a decrease in intestinal motility and secretory activity. Intestinal Smooth Muscle. Intestinal smooth muscle has its own intrinsic slow-wave activity, which varies from about 12 waves minute in the duodenum to 8 or 9 waves per minute in the ileum. This slow- wave activity is thought to reside in a network of specialized pacemaker cells that are interposed between the smooth muscle cells. Slow waves are not action potentials and they do not directly induce muscle contraction; instead, they are rhythmic, wavelike fluctuations in the membrane poten- tial that cyclically bring the mem- brane closer to threshold. If the peak voltage of the slow wave exceeds the cell’s threshold potential, one or more action potentials may be triggered. Because action potentials occur at the peak of a slow wave, slow-wave frequency determines the rate of smooth muscle contractions. Stretching the intestinal smooth muscle and parasympathetic nervous system stimulation increase excit- ability of the smooth muscle cells, whereas sympathetic stimulation decreases excitability.

(mainly postganglionic)

Myenteric plexus

Submucosal plexus

Sensory neurons

Epithelium

3

Smooth muscle cells

Interstitial cell network

0

Slow-wave activity (mV)

–40