Ganong's Review of Medical Physiology, 23rd Edition

432
SECTION V
Gastrointestinal Physiology

mucus secreted by the cells in the necks of the glands. Several
of the glands open on a common chamber
(gastric pit)
that
opens in turn on the surface of the mucosa. Mucus is also se-
creted along with HCO
3

• by mucus cells on the surface of the
  epithelium between glands.
  The stomach has a very rich blood and lymphatic supply. Its
  parasympathetic nerve supply comes from the vagi and its
  sympathetic supply from the celiac plexus.

ORIGIN & REGULATION

OF GASTRIC SECRETION

The stomach also adds a significant volume of digestive juices
to the meal. Like salivary secretion, the stomach actually read-
ies itself to receive the meal before it is actually taken in, dur-
ing the so-called cephalic phase that can be influenced by food
preferences. Subsequently, there is a gastric phase of secretion
that is quantitatively the most significant, and finally an intes-
tinal phase once the meal has left the stomach. Each phase is
closely regulated by both local and distant triggers.
The gastric secretions (Table 26–1) arise from glands in the
wall of the stomach that drain into its lumen, and also from
the surface cells that secrete primarily mucus and bicarbonate
to protect the stomach from digesting itself, as well as sub-
stances known as trefoil peptides that stabilize the mucus-
bicarbonate layer. The glandular secretions of the stomach
differ in different regions of the organ. The most characteris-
tic secretions derive from the glands in the fundus or body of
the stomach. These contain two distinctive cell types from

which the gastric secretions arise: the parietal cells, which

secrete hydrochloric acid and intrinsic factor; and the chief

cells, which produce pepsinogens and gastric lipase (Figure

26–5). The acid secreted by parietal cells serves to sterilize the

meal and also to begin the hydrolysis of dietary macromole-

cules. Intrinsic factor is important for the later absorption of

vitamin B

12

, or cobalamin (Figure 26–6). Pepsinogen is the

precursor of pepsin, which initiates protein digestion. Lipase

similarly begins the digestion of dietary fats.

There are three primary stimuli of gastric secretion, each

with a specific role to play in matching the rate of secretion to

functional requirements (Figure 26–7). Gastrin is a hormone

that is released by G cells in the antrum of the stomach both

in response to a specific neurotransmitter released from

enteric nerve endings, known as gastrin releasing peptide

(GRP, or bombesin), and also in response to the presence of

oligopeptides in the gastric lumen. Gastrin is then carried

through the bloodstream to the fundic glands, where it binds

to receptors not only on parietal (and likely, chief cells) to

activate secretion, but also on so-called enterochromaffin-like

cells (ECL cells) that are located in the gland, and release his-

tamine. Histamine is also a trigger of parietal cell secretion,

via binding to H

2

histamine receptors. Finally, parietal and

chief cells can also be stimulated by acetylcholine, released

from enteric nerve endings in the fundus.

During the cephalic phase of gastric secretion, secretion is

predominantly activated by vagal input that originates from the

brain region known as the dorsal vagal complex, which coordi-

nates input from higher centers. Vagal outflow to the stomach

then releases GRP and acetylcholine, thereby initiating secretory

FIGURE 26–3
Regulation of salivary secretion by the parasympathetic nervous system.
ACh, acetylcholine.
(Adapted from Barrett KE:
Gas-
trointestinal Physiology

. McGraw-Hill, 2006.)

Smell

Taste

Sound

Sight

Pressure

in mouth

ACh

ACh

Parasympathetics

Sleep

Fatigue

Increased Fear

salivary

secretion

via effects on

Salivatory

nucleus of

medulla

Higher

centers

Otic

ganglion

• Acinar secretion


• Vasodilatation

Parotid

gland

Submandibular

gland

Submandibular

ganglion

−