C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
683
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.
2
3
To prevertebral
ganglia, spinal cord,
and brain stem
(mainly
postganglionic)
(preganglionic)
Parasympathetic
Myenteric
plexus
Submucosal
plexus
Epithelium
Sensory
neurons
Sympathetic
0
–40
Slow-wave
activity
(mV)
Interstitial
cell network
Smooth
muscle cells
