Coordination and Regulation of the Digestive Process

Neural Regulation

Efferent neurons:

• To skeletal muscles = somatic division
• To internal organs = autonomic division
• Parasympathetic
• stimulates digestive tract
• primarily through the vagus nerve
• Sympathetic
• decreases or slows down digestive motility and secretions

Enteric nervous system

• Embedded in layers of GIT from esophagus to anus
• Sensory information from receptors in GIT layers:
• Mechanoreceptors
• Chemoreceptors
• Osmoreceptors
• Response to sensory information:
• Change in smooth muscles (motility)
• Change in cells/glands (release of enzymes/hormones)
• Neural reflexes
• Ileogastric reflex
• gastric motility decreased when ileum becomes distended
• Gastroileal reflex
• ileal motility stimulated when gastric motility and secretions increase
• Colonileal reflex 
• colon inhibits emptying of contents from ileum into colon
• 2 neuronal networks:
• MYENTERIC (AUERBACH) PLEXUS
• lies between circular and longitudinal smooth muscle
• controls motility
• SUBMUCOSAL (MEISSNER) PLEXUS
• controls release of secretions and affects local blood flow
• Neurotransmitters of enteric system:
• Acetylcholine
• 5-hydroxytypamine (serotonin)
• norepinephrine
• gamma aminobutryic acid (GABA)
• vasoactive intestinal polpeptide
• nitric oxide

Regulatory Peptides - factors influencing digestion and absorption are coordinated by a group of GI molecules called regulatory peptides (of gastrointestinal hormones and neuropeptides).

- Endocrine - released into blood to affect distant tissue

• Gastrin (G cells in antrum of stomach and small intestine) 
• stimulates gastric acid secretion and pepsinogen secretion.
• stimulates release of Histamine
• + gastric distension, presence of protein, vagus nerve
• - acid, somatastatin

• Cholecystokinin or CCK (I cells in proximal small intestines and enteric nerves in distal ileum and colon)
• stimulates gallbladder contraction & sphincter of Oddi relaxation to facilitate release of bile 
• stimulates pancreatic acinar enzyme secretion into duodenum
• + protein digestion and fat in duodenum

• Secretin (S cells in proximal small intestine)
• stimulates pancreatic duct cells to release juice secretion rich in bicarbonate
• diminishes gastric emptying
• diminishes gastric acid secretion
• + unneutralized acidic chyme, protein products in the duodenum

• Peptide YY or PYY (L cells of the ileum and colon)
• diminishes appetite and gastric acid secretion
• diminishes gastric emptying
• + fat in small intestines

• Motilin (M cells in the stomach and intestines)
• stimulates gastric and intestinal motility between meals
• + serotonin and acetylcholine

- Paracrine - released locally to impact local tissue

• Somatostatin (pancreatic D cells, cell in the antrum of stomach and duodenum)
• diminishes gastric acid secretion through effect on parietal and enterochromaffin like cells
• diminishes gastric emptying
• diminishes pancreatic enzyme secretions
• inhibits gallbladder contractions
• suppresses actions of gastrin, secretin, VIP, motilin
• + drop of 2 units of pH in gastric juice

• Histamine (Mast cells and enterochromaffin like cells in stomach)
• stimulates parietal cells to secrete HCl acid
• trophic effect of gastric and intestinal mucosa
• + gastrin, AchCholine, nutrients in the small intestines

• Glucagon like peptides (L cells of the distal small and colon)
• stimulates pancreas to release insulin and inhibit glucagon
• decrease appetite and diminish gastric emptying, gastric secretions, and intestinal motility
• + nutrients in the small intestines

• Insulin like growth factor-1 (also endocrine mechanism)
• increases proliferation of the GIT
• + nutrients in the digestive tract

Neurocrine Peptides involved with GIT function

• Vasoactive Intestinal Peptide or VIP (produced by GIT nerves and CNS)
• stimulates intestinal and pancreatic secretions
• relax intestinal smooth muscle including sphincters
• inhibit gastric acid secretion

• Neurotensin (neurons, N cells of the ileum)
• unknown

• Glucose-dependent insulinotropic peptide or Gastric Inhibitory Peptide (peptide produced by K cells of the duodenum and jejunum)
• stimulates insulin release by the pancreatic beta cells
• inhibit gastric emptying

• Amylin 
• Cosecreted with insulin by pancreatic beta cells
• inhibits glucagon secretion
• inhibits gastric emptying

• Insulin
• promoting glucose uptake

• Glucagon

• Gherelin (peptide from endocrine cells of the stomach)
• acts on hypothalamus to stimulate food intake
• plasma levels rise before food intake, decrease immediately after eating

• Leptin (white adipose tissue)
• secreted in proportion to fat stores and suppress food intake