Human Physiology, 14th edition (2016)

The Digestive System 649

Figure 18.31 The enteric nervous system coordinates peristalsis. Peristalsis is produced by local reflexes involving the
enteric nervous system. Notice that the enteric nervous system consists of motor neurons, interneurons, and sensory neurons. The
neurotransmitters that stimulate smooth muscle contraction are indicated with a ⊕ , while those that produce smooth muscle relaxation
are indicated with a . (NO  5  nitric oxide; VIP  5  vasoactive intestinal peptide.)

Contraction

Smooth

muscle

Substance P

ACh

ACh

Excitation

Distention

Mucosal

stimulus

Bolus

Inhibition

Lumen Mucosa and

submucosa

Circular

muscle

Longitudinal

muscle

Relaxation

ACh

Motor neurons

Interneurons

Sensory

2.

Motor neurons

stimulate contraction

behind bolus

1.

Bolus of food

stimulates sensory

neurons

2.

Motor neurons

produce relaxation

in front of bolus

Interneurons activate

motor neurons

Interneurons activate

motor neurons

+

NO

VIP

additional chyme into the duodenum is thereby retarded by the
inhibitory effects of neural reflexes and enterogastrone, allow-
ing time for the previous load of chyme in the duodenum to
be digested with the aid of pancreatic juice enzymes and bile.
The secretion of pancreatic juice and bile is stimulated both by
neural reflexes initiated in the duodenum and by secretion of
the duodenal hormones cholecystokinin and secretin.

Secretion of Pancreatic Juice

The secretion of pancreatic enzymes (including trypsin, lipase,
and amylase) from the acinar cells is stimulated by ACh released
by the vagus nerves, and by the hormone CCK secreted by the
duodenum. However, the ability of CCK to directly stimulate
the acinar cells has been questioned, suggesting that the stimu-
latory effect of CCK on pancreatic enzyme secretion may be
indirect, perhaps mediated by vagus nerve release of ACh.
Secretin and CCK are secreted by the duodenum in
response to different stimuli and have different primary effects.

Secretion of CCK is stimulated by the protein and fat content

of the chyme; CCK then stimulates the secretion of the pan-

creatic juice enzymes that help digest these molecules. Par-

tially digested protein and fat are the most potent stimulators

of CCK secretion, which diminishes as the chyme passes out

of the duodenum into the jejunum.

By contrast, the secretion of secretin by the duodenum

is stimulated by a fall in duodenal pH below 4.5, caused by

the arrival of acidic chyme from the stomach. This low pH is

maintained for only a short time until the arrival of bicarbon-

ate from pancreatic juice. After a meal, when acidic chyme

reaches the duodenum, secretin stimulates bicarbonate and

water secretion from the pancreatic ductules. Secretin uses

cAMP as a second messenger in this process, which is poten-

tiated by ACh (released by postganglionic vagus nerve fibers)

using Ca^2 1 as a second messenger. The bicarbonate entering

the chyme helps protect the duodenal mucosa, and it provides

an optimal pH environment for the pancreatic juice digestive

enzymes.