Lecture 31: GIT 1 Flashcards
Learn the shit out of the diagram on slide?
It’s the wall of the GIT
Do it
What are the basic digestive processes?
•Mechanical reduction of particle size
-Secretion of enzyme-containing digestive juices – enzymatic breakdown of organic nutrients
•Absorption from GI tract
Mechanical:
•Reduction in particle size increases surface area for enzymatic breakdown
•Mixing facilitates digestion and absorption
•Mouth – mastication (chewing)
•Stomach – mixing and grinding (trituration) by distal stomach
•Small intestine – mixing and ordered aboral (= away from the mouth) passage of digesta
•Large intestine – retention and mixing for optimum fermentation, nutrient absorption, and absorption of water and electrolytes
Secretion of digestive enzymes:
•Glandular cells in wall of GIT release enzymes into lumen – nutrient breakdown
•Secretions from salivary glands, pancreas and liver reach GIT via ducts
•Enzymes operate at optimum pH, equal to that in region of GIT into which released
•Mucus secreted into GIT lubricates contents and protects mucosal epithelium
Enzymatic breakdown of nutrients:
•Ingested nutrients contain complex carbohydrates, proteins and fats that require degradation before absorption can occur
•Step-wise process as nutrients pass down GIT
•Initial breakdown – digestive enzymes secreted into lumen
•Further breakdown by epithelial brush border enzymes – enhanced by large epithelial surface area
•Nutrients undergoing breakdown similar to chemical composition of host tissues
•Protective mechanisms in GIT therefore paramount
Absorption:
•Water, ions, vitamins, and small molecules formed by breakdown of organic nutrients move from GIT lumen to blood and lymph
•Optimised by large epithelial surface area
•Absorption occurs by both active and passive transport mechanisms
•Unabsorbed material excreted in faeces
Tell me about the enteric nervous system
•Intrinsic system, integrated with autonomic nervous system
•Myenteric (Auerbach’s) plexus
ØBetween circular and longitudinal muscle layers
ØPrimarily regulates motility
•Submucosal (Meissner’s) plexus
ØBetween submucosal circular muscle layers
ØPrimarily regulates secretion and absorption
Parasympathetic innervation to the GIT?
Sympathetic innervation to GIT?
- Preganglionic cholinergic fibres arise from medulla (vagus nerve) and sacral spinal cord (pelvic nerve)
- Synapse with postganglionic cholinergic fibres in ganglia within enteric nervous system
- Mediator at target cells is acetylcholine
- Stimulates intestinal motility, secretion and release of hormones
Sympathetic innervation:
•Preganglionic cholinergic fibres synapse in sympathetic ganglia outside GIT (coeliac, cranial and caudal mesenteric)
•From these ganglia, postganglionic adrenergic fibres innervate cells in myenteric and submucosal plexuses
•Direct postganglionic innervation to some blood vessels and muscularis mucosae
•Constriction of blood vessels
•Inhibition of intestinal motility, secretion and hormone release
GIT has many types of sensory receptors. What are they’d what do the pick up?
Mechanoreceptors detect:
Østretching of intestinal wall
Ømechanical stimulation of mucosa
Chemoreceptors detect:
Øvarious chemicals in intestinal contents
(eg, acid, amino acids, glucose and lipid)
Nociceptors detect
Øintense mechanical and chemical stimuli that are interpreted by the brain as pain
What is the sensory innervation of GIT?
Visceral afferent fibres travel with sympathetic (red) and parasympathetic (blue) nerves
Vagus nerve:
•Vagus is mixed nerve (>75% sensory (= afferent) fibres)
•Receptors in mucosa and smooth muscle relay information to CNS via vagal afferents
•Afferent signal triggers vagovagal reflex, in which efferent signal is also in vagus
•Reflex vagal signals not consciously perceived
Sympathetic nerves:
•≈50% of fibres in GIT sympathetic nerves are afferent
•Information from GIT receptors relayed to spinal cord
•Some reflex activity, but most signals travel to cerebral cortex, where they are consciously perceived
•Involved in transmission of signals from visceral nociceptors to brain, where they are perceived as pain
What are the gastrointestinal peptides?
•ENDOCRINE
Gastrin:
•Structurally, closely related to cholecystokinin (gastrin activates CCK-2 receptors)
•Released
Øfrom G cells in antral (and duodenal) mucosa
Øin response to meal proteins and protein digestion products contacting antral mucosa
Øby vagal stimulation Gastrin Releasing Peptide (GRP)
Øby distension of gastric wall
•Release inhibited by gastric pH < 3
•Actions
Østimulates gastric acid secretion (gastrin is potent stimulator)
Østimulates growth of gastric oxyntic glands
Cholecystokinin
•Structurally, closely related to gastrin
•Released
Øfrom I-cells in duodenal and jejunal mucosa
Øin response to fatty acids (> 8 carbons), peptides and amino acids in dudoenum
•Actions
Østimulates gall bladder contraction (relaxes sphincter)
Øpotent stimulator of pancreatic enzyme secretion
Øweak stimulator of pancreatic water and bicarbonate, but potentiates secretin-mediated bicarbonate secretion
Øpotent inhibitor of gastric emptying
Secretin:
•Structurally related to glucagon
•Released
Øfrom S cells in duodenal mucosa
Øin response to acid (duodenal contents pH < 4.5) and (to lesser extent) by fatty acids in duodenum
•Actions
Øpotent stimulator of pancreatic bicarbonate and water secretion
Øsimilar effect in liver
Østimulates growth of exocrine pancreas
Øinhibits gastric acid secretion
Motilin:
Paracrine:
Histamine
Somatostatin:
•Originally identified as inhibitor of growth hormone secretion
•Released
Øfrom D cells in gastric antrum (and duodenal mucosa)
Øgastric D cells directly exposed to acid
Øacid stimulates D cells to produce somatostatin
Øprotein, fat and glucose in gastric contents and vagus nerve activity also stimulate release
•Actions
Øin gastric antrum, D cells are very close to G cells
Øsomatostatin acts as paracrine, inhibiting gastrin release, and gastric acid secretion
Øalso acts on parietal and ECL cells to inhibit acid secretion
Regulation of digestion occurs in 3 phases 1. Cephalon 2. Gastric 3. Intestinal Describe each
CEPHALIC
•Changes in secretion and motility that occur in response to sight, smell, taste and mastication of food
•Think of your dog drooling saliva whilst watching you eating
•Effected by input from CNS
•Regulated mainly via vagus nerve reflexes
GASTRIC
•Changes in GIT secretion and motility initiated in stomach
•Stimuli include gastric distension and release of peptides from protein digestion
•Release of gastrin and activation of neural reflexes (mainly via vagus nerve)
INTESTINAL
•Changes in GI secretion and motility (including pancreatic secretion)
•Initiated by alterations in volume and composition of digesta (particularly in duodenum)
•Release of secretin, CCK, and activation of activation of neural reflexes (mainly via vagus nerve)
What is the role of Leptin
- Hormone secreted by adipose cells
- When intake of calories in nutrients exceeds body’s need, fat is deposited in adipose tissue, which responds by secreting leptin
- Leptin acts on hypothalamus to reduce sensation of hunger
- Also increases metabolic rate, which reduces fat storage
What is the role of insulin?
- Insulin sensitivity decreases as fat storage increases
- Higher concentrations of insulin are required to maintain blood glucose within normal limits
- Thus, insulin level in blood increase or decrease in parallel with changes in body fat
- Via hypothalamic receptors, insulin produces leptin-like effects
- Insulin also stimulates leptin synthesis by adipocyte
Role of GI hormones
CCK, secreted by intestinal mucosa in response to ingestion of protein and fat
- activates receptors on vague nerve which sends satiety signal to brain (hypothalamus) to limit meal size
- Leptin and insulin may increase hypothalamic sensitivity to this signal
