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Regulation of GI function (Introduction (CNS role: Sight & smell food,…
Regulation of GI function
Introduction
CNS role: Sight & smell food, regulation of appetite
Digestion mainly regulated by negative feedback mechanisms
Stimulation initiates reflexes co-ordinate activity of smooth muscle and hormones
Digestion mainly regulated by negative feedback mechanisms
Sensory cells in GI tract (wall stretch, pH, osmolarity, nutrient conc, metabolite conc, irritation of mucous membrane)
Digestive processes co-ordinated by: Neural and hormonal regulation
Reflex arcs
Short reflex arcs - Enteric NS, sensory and motor nerve cells within wall of GI tract
Long reflex arcs = Autonomic NS
Enteric nervous system (ENS)
Short reflex arcs located within wall of GI tract
Can operate independently of rest of body
Sensory cells - Respond to contents of lumen and degree of wall stretch
Motor cells - Stimulate smooth muscle cells (motility) and epithelial cells (secretory)
Short arcs allow for extensive control
Autonomic nervous system (ANS)
Parasympathetic promotes digestion
Preganglionic fibres in vagus, cholinergic
Post ganglionic - embedded in walls of GI tract connect to ENS, cholinergic
Sympathetic
Preganglionic fibres run in splanchnic nerve, cholinergic
Post ganglionic run alongside arteries to organ of inntervation or hypogastric nerves, adrenergic
Connection to CNS (sight/smell/taste)
Entero-enteric reflexes (co-ordinate activity)
Hormonal regulation
Gastrin
- Caudal stomach - Stimulated by peptides and amino acids - Stimulates growth of gastric/intestinal mucosa, increase HCl
Secretin
- Mainly duodenum - Stimulated by HCl - Stimulates pancreatic HCO3
Choleystokinin (CCK)
- Mainly duodenum - Stimulated by fatty acids, monoglycerides, amino acids, peptides - Release of pancreatic enzymes and contraction of gall bladder
Gastric inhibitory peptide (GIP)
- Cranial small intestine - Stimulated by fat, glucose, amino acids - Inhibits HCl, stimulates insulin production
Phases of regulation
Gastric phase - Stomach, e.g. stomach distension/presence of peptides. Coordinated by ANS, ENS and hormones
Intestinal phase - Pertains to intestines. Coordinated by ANS, ENS and hormones
Cephalic phase - Pertains to head, anticipation of food, coordinated by ANS
Motility
Co-ordinated contraction of smooth muscle in GI tract
4 types of contraction
Peristalsis
- Movement of food in aboral direction
Anti-peristalsis
Moves contents in oral direction
Segmental
- Mixing/mechanical breakdown
Mass movement
- Empties entire sections of GI tract (e.g. colon)
Pacemaker cells
Repetitive and spontaneous oscillations (approx 5 per minute)
Located between circulate and longitudinal smooth muscle
If stimulus absent depolarisation is too weak to reach threshold potential = no muscle contraction
If stimulus present depolarisation reached threshold potential = smooth muscle contraction
Action potential due to opening of Ca2+ channels
Frequency of action potentials determines strength of contraction
Regulation of appetite
Satiety centre - (ventro-medial hypothalamus), refusal of food
Exact mechanism unknown. 3 theories, level of glucose (glucostat), levels of CCK, levels of fat (lipostat)
Appetite centre - (ventro-lateral hypothalamus), food searching and voracious eating