PHASES OF GASTRIC SECRETION

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.