PHASES OF GASTRIC SECRETION
Secretion of gastric juice is a
continuous process. But the quantity varies, depending upon time and stimulus phases:
I. Cephalic phase
II. Gastric phase
III. Intestinal phase.
In human beings, a fourth phase called
interdigestive
phase exists.
Each phase is regulated by neural mechanism or hormonal mechanism or both.
CEPHALIC PHASE
Secretion of gastric juice by the
stimuli arising from head region (cephalus) is called cephalic phase. This
phase of gastric secretion is regulated by nervous mechanism. The gastric juice
secreted during
this phase is called appetite juice. During
this phase, gastric secretion occurs even without the presence of food in
stomach. The quantity of the juice is less but it is rich in enzymes and
hydrochloric
acid. Nervous mechanism regulates
cephalic phase through reflex action. Two types of reflexes
occur:
1. Unconditioned
reflex
2. Conditioned reflex.
1. Unconditioned Reflex
Unconditioned reflex is
the inborn reflex. When food is placed in the mouth, salivary secretion is induced.
Simultaneously, gastric secretion also occurs.
Stages
of reflex action:
i. Presence of food in the mouth
stimulates the taste buds and other receptors in the mouth
ii. Sensory (afferent) impulses from
mouth pass via afferent nerve fibers of glossopharyngeal and
facial nerves to amygdala and appetite center present in
hypothalamus
iii. From here, the efferent impulses
pass through dorsal nucleus of vagus and vagal efferent nerve
fibers to the wall of
the stomach
iv. Vagal efferent nerve endings
secrete acetylcholine, which stimulates gastric secretion.
2. Conditioned Reflex
Conditioned reflex is the
reflex response acquired by previous experience. Presence of food in
the mouth is not
necessary to elicit this reflex. The sight, smell, hearing or thought of food, which
induce salivary secretion induce gastric secretion also.
Stages
of reflex action:
i. Impulses from the special sensory
organs (eye, ear and nose) pass through afferent fibers of
neural circuits to the cerebral
cortex. Thinking of food stimulates the cerebral cortex directly
ii. From cerebral cortex, the impulses
pass through dorsal nucleus of vagus and vagal efferents and
reach the stomach wall
iii. Vagal nerve endings secrete
acetylcholine, which stimulates the gastric secretion.
Experimental
evidences to prove cephalic phase
i. Unconditioned
reflex of gastric secretion is proved by sham feeding along with Pavlov pouch.
After vagotomy, sham feeding does not cause gastric secretion. It proves the importance
of vagus nerve in this phase.
ii. Conditioned reflex
of gastric secretion is proved by Pavlov pouch and belldog experiment.
GASTRIC PHASE
Secretion of gastric juice when food
enters the stomach is called gastric phase. This phase is regulated by both nervous
and hormonal control. Gastric juice secreted during this phase is rich in
pepsinogen and hydrochloric acid.
Mechanisms involved in gastric phase
are:
1. Nervous mechanism
through local myenteric reflex and vagovagal reflex
2. Hormonal mechanism through gastrin Stimuli,
which initiate these two mechanisms are:
1. Distention of stomach
2. Mechanical stimulation of gastric
mucosa by bulk of food
3. Chemical stimulation of gastric
mucosa by the food contents.
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