7.3 Regulation of Digestive Processes

The digestive system is a complex network of organs and processes that break down food into nutrients. Regulation of these processes involves the enteric nervous system and various hormones, working together to ensure efficient digestion and absorption.

From the cephalic phase triggered by the sight of food to the intestinal phase when chyme enters the duodenum, each step is carefully controlled. Hormones like gastrin and cholecystokinin stimulate or inhibit digestive processes, while the enteric nervous system coordinates local responses.

Regulation of Digestive Processes by the Enteric Nervous System

Structure and Function of the Enteric Nervous System

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• The enteric nervous system consists of the submucosal plexus and the myenteric plexus, which are networks of neurons embedded in the walls of the gastrointestinal tract
• The enteric nervous system can function independently of the central nervous system, allowing for local control and coordination of digestive processes
• Communicates with the central nervous system through the vagus nerve and the splanchnic nerves, allowing for higher-level control and modulation of digestive functions (parasympathetic and sympathetic input)

Sensory and Motor Neurons in the Enteric Nervous System

• Sensory neurons in the enteric nervous system detect mechanical and chemical stimuli within the gastrointestinal tract
  • Respond to stretch (distension of the gut wall), pH changes, and nutrient presence (glucose, amino acids, fatty acids)
• Motor neurons in the enteric nervous system control the contraction and relaxation of smooth muscles in the gastrointestinal tract
  • Regulate peristalsis (coordinated wave-like contractions) and segmentation (mixing contractions)
• Secretomotor neurons in the enteric nervous system stimulate the release of digestive enzymes (pepsin, pancreatic enzymes) and mucus from glands in the gastrointestinal tract
• Interneurons within the enteric nervous system integrate sensory information and coordinate the activity of motor and secretomotor neurons

Role of Hormones in the Regulation of Digestive Processes

Hormones Stimulating Digestive Processes

• Gastrin is released by G cells in the stomach in response to the presence of food
  • Stimulates the release of hydrochloric acid and promotes gastric motility (churning and emptying)
• Cholecystokinin (CCK) is secreted by I cells in the duodenum in response to the presence of fatty acids and amino acids
  • Stimulates the release of pancreatic enzymes (lipase, trypsin, chymotrypsin) and bile from the gallbladder
• Secretin is released by S cells in the duodenum in response to acidic chyme
  • Stimulates the release of bicarbonate-rich pancreatic juice to neutralize the acid

Hormones Inhibiting Digestive Processes

• Gastric inhibitory peptide (GIP) is secreted by K cells in the duodenum in response to glucose and fat
  • Inhibits gastric acid secretion and motility while stimulating insulin release from the pancreas
• Somatostatin is released by D cells in the pancreas and intestine
  • Inhibits the release of other digestive hormones (gastrin, CCK, secretin) and reduces gastrointestinal motility and secretion

Hormones Regulating Gastrointestinal Motility

• Motilin is released by M cells in the duodenum and jejunum during the fasting state
  • Stimulates the migrating motor complex (MMC) to clear the digestive tract of remaining contents between meals

Phases of Digestive Regulation

Cephalic Phase

• Initiated by the sight, smell, or thought of food, triggering a parasympathetic response through the vagus nerve
• Vagus nerve stimulates the release of gastric juices (hydrochloric acid, pepsinogen) and increases gastric motility in preparation for food intake
• Salivation is increased during the cephalic phase, facilitating the initial digestion of food in the mouth (amylase breaks down starch)

Gastric Phase

• Begins when food enters the stomach, triggering local reflexes and the release of gastrin
• Gastrin stimulates the release of hydrochloric acid and pepsinogen in the stomach, creating an acidic environment for protein digestion
• Mechanical stretching of the stomach wall stimulates the release of gastrin and promotes gastric motility (churning and mixing)

Intestinal Phase

• Initiated when chyme enters the duodenum, triggering the release of intestinal hormones and stimulating local reflexes
• The presence of fatty acids, amino acids, and acidic chyme in the duodenum stimulates the release of CCK, secretin, and GIP
  • These hormones regulate the release of pancreatic enzymes, bicarbonate, and bile, as well as modulate gastric emptying and intestinal motility
• Local reflexes, such as the enterogastric reflex, inhibit gastric emptying when the duodenum is distended or contains high concentrations of nutrients (prevents overloading the small intestine)