REGULATION OF PANCREATIC SECRETION

REGULATION OF PANCREATIC SECRETION

Secretion of pancreatic juice is regulated by both nervous and hormonal factors.

STAGES OF PANCREATIC SECRETION

Pancreatic juice is secreted in three stages like the gastric juice:

1. Cephalic phase

2. Gastric phase

3. Intestinal phase.

These three phases of pancreatic secretion correspond with the three phases of gastric secretion.

1. CEPHALIC PHASE

As in case of gastric secretion, cephalic phase is regulated by nervous mechanism through reflex action.

Two types of reflexes occur:

1. Unconditioned reflex

2. Conditioned reflex.

Unconditioned Reflex

Unconditioned reflex is the inborn reflex. When food is placed in the mouth, salivary secretion and gastric secretion are induced. Simultaneously, pancreatic 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 reach dorsal nucleus of vagus and efferent impulses

reach pancreatic acini via vagal efferent nerve fibers

iii. Vagal efferent nerve endings secrete acetylcholine, which stimulates pancreatic secretion.

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 and gastric secretion induce pancreatic 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 pancreatic acini

iii. Vagal nerve endings secrete acetylcholine, which stimulates pancreatic secretion.

2. GASTRIC PHASE

Secretion of pancreatic juice when food enters the stomach is known as gastric phase. This phase of

pancreatic secretion is under hormonal control. The hormone involved is gastrin. When food enters the stomach, gastrin is secreted from stomach. When gastrin is transported to pancreas through blood, it stimulates the pancreatic secretion. The pancreatic juice secreted during gastric phase is rich in enzymes.

3. INTESTINAL PHASE

Intestinal phase is the secretion of pancreatic juice when the chyme enters the intestine. This phase is also under hormonal control. When chyme enters the intestine, many hormones are released. Some hormones stimulate the pancreatic secretion and some hormones inhibit the pancreatic secretion.

Hormones Stimulating Pancreatic Secretion

i. Secretin

ii. Cholecystokinin.

Secretin

Secretin is produced by S cells of mucous membrane in duodenum and jejunum. It is secreted as inactive

prosecretin, which is activated into secretin by acid chyme. The stimulant for the release and activation of

prosecretin is the acid chyme entering intestine. Products of protein digestion also stimulate the hormonal

secretion.

Action of secretin

Secretin stimulates the secretion of watery juice which is rich in of bicarbonate ion and high in volume. It increases the pancreactic secretion by acting on pancreatic ductules via cyclic AMP (messenger)..

Cholecystokinin

Cholecystokinin (CCK) is also called cholecystokininpancreozymin (CCKPZ).

It is secreted by I cells in duodenal and jejunal mucosa. The stimulant for the

release of this hormone is the chyme containing digestive products such as fatty acids, peptides and

amino acids.

Action of cholecystokinin

Cholecystokinin stimulates the secretion of pancreatic juice which is rich in enzyme and low in volume,

by acting on pancreatic acinar cells via inosine triphosphate (second messenger).

Hormones Inhibiting Pancreatic Secretion

i. Pancreatic polypeptide (PP) secreted by PP cells in islets of Langerhans of pancreas

ii. Somatostatin secreted by D cells in islets of Langerhans of pancreas

iii. Peptide YY secreted by intestinal mucosa

iv. Peptides like ghrelin and leptin

 

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