GI 4

Regulation of GI function Control mechanisms Long reflexes integrated in CNS Some originate outside GI tract but others originate in enteric nervous system Cephalic reflexes ? original in cephalic brain Feedforward reflexes- begin w/ stimuli such as sight, smell, sound, or thought of food; prepare digestive system for food that brain is anticipating (i.e. mouth watering) Influence of emotions on GI tract travelers? constipation, butteries in stomach, etc. Smooth muscle and glands under autonomic control Short reflexes integrated in enteric nervous system Are carried out by neurons in wall of gut Reflexes involving GI peptides Excite/inhibit motility and secretion i.e. cholecystokinin (CCK) enhance satiety but also manufactured by neurons and functions as NT in brain i.e. ghrelin, secreted by stomach and acts on brain to increase food intake Reflex pathway: Cephalic phase of digestion (feedforward).Distension.Acid.Presence of food. Secretory cells of the stomach and small intestine. GI peptides. GI smooth muscle.Exocrine cells of stomach, pancreas, intestineOther endocrine cellsNervous system Changes in GI motility.Release of bile and pancreatic secretions.Enzyme, acid, and bicarbonate synthesis/releaseHunger/satiety. The enteric nervous system can act independently of CNS Features ENS shares w/ CNS: Intrinsic neurons of ENS similar to interneurons of CNS (extrinsic neurons = autonomic neurons that bring signals from the CNS to digestive system) Neurotransmitters and neuromodulators ? i.e. 5-HT, vasoactive intestinal peptide, nitric oxide Support cells ? w/in ENS; similar to astroglia of brain Diffusion barrier ? capillaries surrounding ganglia in ENS are not very permeable and create diffuse barrier similar to blood-brain barrier of cerebral blood vessels Integrating center ? reflexes in GI tract can be integrated and acted on w/o neural signals leaving ENS (ENS neuron network has own integrating center) ENS have no discrete command center GI peptides include hormones, neuropeptides, and cytokines Families of gastrointestinal hormones: Gastrin family ? bind to CCKb receptor found in parietal cells; Gastrin Cholescystokinin (CCK) Secretin family ? Secretin Vasoactive intestinal peptide (VIP) ? neurocrine molecule GIP (glucose-dependent insulinotropic peptide) ? stimulates insulin release in response to glucose in lumen ; at higher doses, inhibits gastric acid secretion (used to be called gastric inhibitory peptide) Glucagon-like peptide 1 (GLP-1) ? plays important role in glucose homeostasis The rest? Motilin