Nervous System

• Nervous system effects

  • Control of muscular and glandular activity by local nerves in alimentary canal

  • Long-distance regulation by the brain

Hormones

• Secreted mainly by cells scattered throughout the epithelium of stomach and small intestine

• Target cells in pancreas and gallbladder

• Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis.

• Glucagon: increases blood glucose levels by stimulating conversion of glycogen to glucose in the liver and the breakdown of fat and protein into glucose

• Insulin: reduces blood glucose levels by promoting the cellular uptake of glucose, slowing glycogen breakdown in the liver, promoting fat storage

• Leptin: produced by adipose (fat) tissue; suppresses appetite as its level increases.

  • When body fat decreases, leptin levels fall, and appetite increases.

• PYY: secreted by the small intestine after meals; acts as an appetite suppressant that counters the appetite stimulant ghrelin.

• Ghrelin: secreted by the stomach wall; triggers feelings of hunger.

  • In dieters who lose weight, ghrelin levels increase, which may be one reason it’s so hard to stay on a diet.

• Insulin: secreted by the pancreas when there’s a rise in blood sugar level after a meal; suppresses appetite by acting on the brain.

Homeostasis

• The human body imposes a limit in the amount of fat it stores or uses up.

• A complex feedback mechanism regulates the storage and use of fat.

  • The hormone leptin is produced by the adipose tissue.

  • High leptin level cues the brain to depress appetite and to increase energy-consuming muscular activity and body-heat production.

  • Loss of body fat decreases the leptin levels in the blood, and the brain is signaled to increase appetite and weight gain.

  • Genes are involved in fat homeostasis.