Question

For each of polypeptides secreted by the enteric nervous system in the alimentary tract A–E, select the most appropriate effect on the gut from the list below. 

1. Release of gastrin. 

2. Relaxation of sphincters. 

3. Increase in small intestinal motility. 

4. Inhibition of gut motility. 

5. Inhibition of gastric emptying. 

A. VIP. 

B. CCK-PZ. 

C. Somatostatin. 

D. Substance P. 

E. GRP.

Answer 1

A. Option 2 Relaxation of sphincters. Vasoactive intestinal polypeptide (VIP) also causes vasodilatation by relaxing vascular smooth muscle. 

B. Option 5 Inhibition of gastric emptying. Besides its main action of stimulating the gall bladder to contract, cholecystokinin (CCK) decreases the rate of emptying of the stomach. 

C. Option 4 Inhibition of gut motility. Somatostatin, the growth hormone-inhibiting hormone, originally isolated in the hypothalamus, is secreted by D cells in the intestinal mucosa. It inhibits the release of several gut hormones including motilin and so inhibits gut motility. 

D. Option 3 Increase in small intestinal motility. Substance P, a polypeptide found in intestinal mucosal endocrine cells, acts locally to increase gut motility. 

E. Option 1 Release of gastrin. Gastrin-releasing polypeptide (GRP) is released by the postganglionic vagal fibres that innervate the G cells in the lateral walls of the glands in the pyloric antrum. Activity in these nerves causes release of the gastrin granules in the G cells.