34.4 Digestive System Regulation

The nervous system plays a crucial role in regulating digestion. The autonomic nervous system controls digestive processes, with the parasympathetic division increasing activities and the sympathetic division decreasing them. The enteric nervous system, embedded in the digestive tract, manages local functions independently.

Hormones also play a vital part in digestive control. Gastrin, secretin, and cholecystokinin regulate various aspects of digestion, from acid secretion to enzyme release. The digestive process is further divided into cephalic, gastric, and intestinal phases, each with specific triggers and responses.

Nervous System Regulation of Digestion

Nervous system's digestive regulation

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• Autonomic nervous system controls digestive processes
  • Parasympathetic division (rest and digest) increases digestive activities by stimulating salivation, gastric juice secretion (hydrochloric acid and pepsinogen), and peristalsis (muscular contractions that move food along the digestive tract)
  • Sympathetic division (fight or flight) decreases digestive activities by diverting blood flow away from digestive organs to skeletal muscles, allowing the body to respond to stress or danger
• Enteric nervous system (intrinsic) is a network of neurons embedded in the walls of the digestive tract that controls local digestive functions independently and communicates with the autonomic nervous system for overall regulation (e.g., coordinating peristalsis and secretion)
• Neural reflexes play a crucial role in regulating gastrointestinal motility and secretion

Hormonal Control of Digestion

Hormonal control of digestion

• Gastrin is secreted by G cells in the stomach and stimulates gastric acid secretion (lowers pH for protein denaturation) and gastric motility (mixing and propulsion of food)
• Secretin is secreted by S cells in the duodenum in response to acidic chyme (partially digested food) and stimulates pancreatic secretion of bicarbonate (neutralizes chyme) while inhibiting gastric acid secretion
• Cholecystokinin (CCK) is secreted by I cells in the duodenum in response to fatty acids and amino acids, stimulating gallbladder contraction and bile release (emulsifies fats), pancreatic enzyme secretion (digests proteins, carbohydrates, and fats), and slowing gastric emptying (allows more time for digestion)
• Hormones play a crucial role in regulating various digestive processes and maintaining homeostasis

Phases of gastric control

1. Cephalic phase is initiated by sight, smell, taste, or thought of food and mediated by the vagus nerve (parasympathetic), stimulating gastric acid and pepsinogen secretion to prepare the stomach for food arrival (e.g., salivation at the sight of a delicious meal)
2. Gastric phase is initiated by the presence of food in the stomach, where stretching of stomach walls activates long reflexes (involving the vagus nerve and brain) and short reflexes (mediated by the enteric nervous system) to stimulate gastric acid and pepsinogen secretion and increase gastric motility for mixing and digestion
3. Intestinal phase is initiated by the presence of chyme in the duodenum, where hormones (secretin and CCK) are released to regulate digestion by inhibiting gastric acid secretion and gastric emptying while stimulating pancreatic and biliary secretions (enzymes and bile) for further digestion

Feedback Mechanisms in Digestive Regulation

• Negative feedback mechanisms help maintain homeostasis in the digestive system by counteracting changes and returning the system to its normal state
• Positive feedback mechanisms amplify changes in the digestive system, often leading to a rapid response
• These feedback mechanisms work together to regulate various aspects of digestion, including gastrointestinal motility and gastric emptying