Human Physiology, 14th edition (2016)

632 Chapter 18

postganglionic axons and acting through muscarinic receptors,

increases the amplitude and duration of the slow waves. Thus,

it increases the production of action potentials and promotes

contractions and motility of the intestine.

cells are neither neurons nor smooth muscle cells; they are the
cells identified histologically as the interstitial cells of Cajal.
These compose about 5% of the cells in the muscularis layer,
and have long processes that join different interstitial cells of
Cajal to each other and to smooth muscle cells by gap junc-
tions. The gap junctions conduct depolarization from one cell
to the next, so that the entire muscularis functions electrically
as a single unit, or functional syncytium ( fig. 18.15 ).
Slow waves spread by way of gap junctions between the
interconnected cells of Cajal in the stomach and intestines. How-
ever, the slow waves can spread only a short distance (a few cen-
timeters) and thus must be regenerated by the next pacemaker
region. This produces the segmentation contractions of the intes-
tine (see fig. 18.13 ). The production of slow waves, and resulting
contractions, are faster at the proximal end of the intestine than
at the distal end, so that there is a pressure head that pushes the
intestinal contents along the GI tract.
The slow waves produced and conducted by the interstitial
cells of Cajal serve to depolarize the adjacent smooth muscle cells.
When the slow-wave depolarization exceeds a threshold value, it
triggers action potentials in the smooth muscle cells by opening
voltage-gated Ca^2 1 channels. This produces Ca^2 1 action potentials
on the tops of the slow-wave depolarizations (see fig. 8.14). The
inward flow of Ca^2 1 has two effects: (1) it produces the upward
depolarization phase of the action potential (repolarization is pro-
duced by outward flow of K^1 ); and (2) it stimulates contraction (as
described in chapter 12; see fig. 12.36). Contraction may then be
aided by additional calcium released from the sarcoplasmic reticu-
lum through calcium-induced calcium release.
Autonomic nerves modify the automatic contractions of
the intestine largely by influencing the enteric nervous system
(section 18.6; see fig. 18.31 ), which in turn stimulates or inhib-
its the interstitial cells of Cajal. Acetylcholine, released by

Figure 18.14 Slow waves in the intestine. The slow
waves are produced by the interstitial cells of Cajal (ICC), not by
smooth muscle cells, and are apparently conducted by networks
of ICC that are electrically joined together within the intestinal
wall. Smooth muscle cells respond to this depolarization by
producing action potentials and contracting. Note that the slow
waves occur much slower (with a rate measured in seconds) than
do the pacemaker potentials in the heart.
See the Test Your Quantitative Ability section of the Review
Activities at the end of this chapter.

5 sec

mV

–28

–62

Slow waves

Interstitial

cells of

Cajal

Smooth

muscle

cells

Autonomic

axon

Production

of slow waves

Conduction of

slow waves to

smooth muscle

Depolarization

and opening of

Ca2+ channels,

production of

action potentials

Neural input to ICC

and smooth muscle

Structures Functions

Figure 18.15 Cells responsible for the electrical

events within the muscularis. The interstitial cells of Cajal

(ICC) generate the slow waves, which pace the contractions

of the intestine. Slow waves are conducted into the smooth

muscle cells where they can stimulate opening of Ca^2 1

channels. This produces action potentials and stimulates

contraction. Autonomic axons have varicosities that release

neurotransmitters, which modify the inherent electrical activity of

the interstitial cells of Cajal and smooth muscle cells.

| CHECKPOINT

5. Describe the adaptations of the small intestine that
   increase its surface area and explain their functional
   significance; also explain the function of the intestinal
   crypts.


6. Identify the nature and significance of the brush
   border enzymes and explain why many adults cannot
   tolerate milk.


7. Describe the smooth muscle contractions of the
   small intestine and explain how they are regulated.

18.4 LARGE INTESTINE

The large intestine absorbs water, electrolytes, and certain

vitamins from the chyme it receives from the small intes-

tine. The large intestine then passes waste products out of

the body through the rectum and anal canal.