Nervous Mechanism

Nervous Mechanism

Local myenteric reflex

Local myenteric reflex is the reflex elicited by stimulation of myenteric nerve plexus in stomach wall. After entering stomach, the food particles stimulate the local nerve plexus present in the wall of the stomach. These nerve fibers release acetylcholine, which stimulates the gastric glands to secrete a large quantity of gastric juice. Simultaneously, acetylcholine stimulates G cells to secrete gastrin .

Vagovagal reflex

Vagovagal reflex is the reflex which involves both afferent and efferent vagal fibers. Entrance of bolus into the stomach stimulates the sensory (afferent) nerve endings of vagus and generates sensory impulses. These sensory impulses are transmitted by sensory fibers of vagus to dorsal nucleus of vagus, located in medulla of brainstem. This nucleus in turn, sends efferent impulses through the motor (efferent) fibers of vagus, back to stomach and cause secretion of gastric juice. Since, both afferent and efferent impulses pass through vagus, this reflex is called vagovagal reflex.

2. Hormonal Mechanism – Gastrin

Gastrin is a gastrointestinal hormone secreted by the G cells which are present in the pyloric glands of stomach. Small amount of gastrin is also secreted in mucosa of upper small intestine. In fetus, it is also secreted by islets of Langerhans in pancreas. Gastrin is a polypeptide containing G14, G17 or G34 amino acids. Gastrin is released when food enters stomach. Mechanism involved in the release of gastrin may be the local nervous reflex or vagovagal reflex. Nerve endings release the neurotransmitter called gastrinreleasing peptide, which stimulates the G cells to secrete gastrin.

Actions of gastrin on gastric secretion

Gastrin stimulates the secretion of pepsinogen and hydrochloric acid by the gastric glands. Refer Chapter

44 for other actions of gastrin. Experimental evidences of gastric phase Nervous mechanism of gastric secretion during gastric phase is proved by Pavlov pouch. Hormonal mechanism of gastric secretion is proved by Heidenhain pouch, Bickel pouch and Farrel and Ivy pouch.

INTESTINAL PHASE

Intestinal phase is the secretion of gastric juice when chyme enters the intestine. When chyme enters the

intestine, initially, the gastric secretion increases but later it stops. Intestinal phase of gastric secretion is

regulated by nervous and hormonal control.

Initial Stage of Intestinal Phase

Chyme that enters the intestine stimulates the duodenal mucosa to release gastrin, which is transported to stomach by blood. There it increases gastric secretion.

Later Stage of Intestinal Phase

After the initial increase, there is a decrease or complete stoppage of gastric secretion. Gastric secretion is inhibited by two factors:

1. Enterogastric reflex

2. Gastrointestinal (GI) hormones.

1. Enterogastric reflex

Enterogastric reflex inhibits the gastric secretion and motility. It is due to the distention of intestinal mucosa by chyme or chemical or osmotic irritation of intestinal mucosa by chemical substances in the chyme. It is mediated by myenteric intestine, initially, the gastric secretion increases but later it stops. Intestinal phase of gastric secretion is regulated by nervous and hormonal control.

Initial Stage of Intestinal Phase

Chyme that enters the intestine stimulates the duodenal mucosa to release gastrin, which is transported to stomach by blood. There it increases gastric secretion.

Later Stage of Intestinal Phase

After the initial increase, there is a decrease or complete stoppage of gastric secretion. Gastric secretion is inhibited by two factors:

1. Enterogastric reflex

2. Gastrointestinal (GI) hormones.

1. Enterogastric reflex

Enterogastric reflex inhibits the gastric secretion and motility. It is due to the distention of intestinal mucosa by chyme or chemical or osmotic irritation of intestinal mucosa by chemical substances in the chyme. It is mediated by myenteric nerve (Auerbach) plexus and vagus.

2. Gastrointestinal hormones

Presence of chyme in the intestine stimulates the secretion of many GI hormones from intestinal mucosa and other structures. All these hormones inhibit the gastric secretion. Some of these hormones inhibit the gastric motility also.

GI hormones which inhibit gastric secretion:

i. Secretin: Secreted by the presence of acid chyme in the intestine

ii. Cholecystokinin: Secreted by the presence of chyme containing fats and amino acids in

intestine

iii. Gastric inhibitory peptide (GIP): Secreted by the presence of chyme containing glucose and fats

in the intestine

iv. Vasoactive intestinal polypeptide (VIP): Secreted by the presence of acidic chyme in intestine

v. Peptide YY: Secreted by the presence of fatty chyme in intestine.

In addition to these hormones, pancreas also secretes a hormone called somatostatin during intestinal phase. It also inhibits gastric secretion Thus, enterogastric reflex and intestinal hormones collectively apply a strong brake on the secretion and motility of stomach during intestinal phase.

Experimental evidences for intestinal phase

Intestinal phase of gastric secretion is demonstrated by Bickel pouch and Farrel and Ivy pouch.

INTERDIGESTIVE PHASE

Secretion of small amount of gastric juice in between meals (or during period of fasting) is called interdigestive phase. Gastric secretion during this phase is mainly due to the hormones like gastrin. This phase of gastric secretion is demonstrated by Farrel and Ivy pouch.

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