Page 926 - Fundamentals of anatomy physiology

smooth muscle cells undergo spontaneous depolarization, trig- Chapter 24 The Digestive System 913
gering a wave of contraction that spreads throughout the entire
muscular sheet. Pacesetter cells are located in the muscularis Figure 24–4 Peristalsis. Peristalsis propels materials along the
mucosae and muscularis externa, which surround the lumen
of the digestive tract. The coordinated contractions of the mus- length of the digestive tract.
cularis externa play a vital role in moving materials along the
tract, through peristalsis, and in mechanical processing, through Initial State
segmentation.
Longitudinal From To
Peristalsis muscle mouth anus

The muscularis externa propels materials from one portion Circular
of the digestive tract to another by contractions known as muscle
peristalsis (per-i-STAL-sis). Peristalsis consists of waves of
muscular contractions that move a bolus (BO. -lus), or soft 1 Contraction of circular muscles behind bolus
rounded ball of digestive contents, along the length of the
digestive tract (Figure 24–4). During a peristaltic movement, Contraction
the circular muscles contract behind the bolus while circular
muscles ahead of the bolus relax. Longitudinal muscles ahead 2 Contraction of longitudinal muscles ahead of
of the bolus then contract, shortening adjacent segments. A
wave of contraction in the circular muscles then forces the bolus
bolus forward.
Contraction
&T i p s T r i c k s
Squeezing toothpaste out of a tube is similar to peristalsis:
Your squeezing hand (contracting circular muscles)
forces toothpaste (the bolus) along and out of the tube
(the d­ igestive tract).

Segmentation Contraction

Most areas of the small intestine and some portions of the large 3 Contraction in circular muscle layer forces
intestine undergo cycles of contraction that churn and fragment
the bolus, mixing the contents with intestinal secretions. This bolus forward
activity, called segmentation, does not follow a set pattern. For
this reason, segmentation does not push materials along the 24
tract in any one direction.
Prostaglandins, histamine, and other chemicals released into
Control of Digestive Functions interstitial fluid may affect adjacent cells within a small seg-
ment of the tract. For example, the release of histamine in the
Local factors interact with neural and hormonal mechanisms to lamina propria of the stomach stimulates the secretion of acid
regulate the activities of the digestive system (Figure 24–5, p. 914). by cells in the adjacent epithelium.

Local Factors

The initial regulation of digestive function occurs at the local
level ( 1 ). Local environmental factors such as the pH, vol-
ume, or chemical composition of the intestinal contents can
have a d2irect effect on digestive activity in that segment of the
digestive tract. Some of these local factors have a direct effect on
local dig3estive activities; for example, stretching of the intestinal
wall can stimulate localized contractions of smooth muscles. In
other cases, the local factors stimulate the release of chemicals.