Lecture 16 Flashcards
What are the layers of the gut track from outer to inner?
Serosa, longitudinal smooth muscle layer, myenteric plexus (of Auerbach), circular smooth muscle layer, meissner’s plexus (submucosal plexus), submucosa, mucosa
Describe the serosa
Outer layer of connective tissue and simple squamous epithelium that is continuous with the mesentery; missing in some parts where it is replaced by adventitia connecting to abdominal wall
What is the function of the mesentery?
Pathway for blood vessels to gut tube
Describe the submucosa
Incorporates blood vessels and submucosal plexus
Describe the mucosa
Forms a continuous sheet lining entire GI tract; made of loose CT with sensory nerves, blood vessels, and some galnds; includes muscularis mucosa (thin layer of smooth muscle that creates ridges and folds)
Describe propulsive movements
Stimulation at any point in the gut can cause contractile ring to occur, with the usual stimulus being distension; requires presence of functional myenteric plexus; can occur in any direction but usually dies out when traveling in the oral direction
Describe mixing movements
May be caused by perstaltic contractions themselves; at other times, local intermittent constrictive contractions occur every few centimeters in the gut wall
Describe slow waves
Complex interactions among the smooth muscle cells and the interstitial cells of Cajal (smooth muscle cell electrical pacemakers); set baseline for intermittent spike potentials; occur spontaneously and intensity varies between 5-15mV with a frequency range of 3-12 per minute; set the maximum frequency of contraction for each part of GI tract
Describe spike potentials
Action potentials that excite muscle contraction; occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40mV; last 10-40 times as long as typical action potential in a large nerve fiber; responsible for opening calcium-sodium channels in GI smooth muscle cells
What is the realtionship between slow wave potentials and spike potentials?
Higher the slow wave potential, greater the frequency of the spike potentials
What are factors that cause depolarization of the GI tract?
Stretching of muscle, stimulation by acetycholine (parasympathetic) or lack of sympathetic stimulation, and stimulation by specific GI hormones
What are factors that cause hyperpolarization of the GI tract?
Norepinephrine and epinephrine (sympathetic)
What causes tonic contractions?
Continuous repetitive spike potentials, hormones, or continuous entry of calcium ions
Describe the pattern of motility
Local stimulus like stretch from bolus that opens sodium channels; slow waves pass locally through gap junctions and spikes occur causing action potentials
What are the 3 types of GI reflexes?
Reflexes entirely within the gut wall enteric nervous system
Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract
Reflexes from the gut to the spinal cord/brain stem and then back to the GI tract
Describe reflexes that are entirely within the gut wall enteric nervous system
Account for much of the GI secretion, peristalsis, mixing contractions, and local inhibitory effects
Describe reflexes from the gut to the prevertebral ganglia and back to the GI tract
Transmit signals long distance to other areas of the tract; cause evacuation of the colon (gastrocolic reflex); inhibit stomach motility and secretion (enterogastric reflex); empty ileal contents into the colon (colonoileal reflex)
What are the 4 “official” GI hormones?
Gastrin, CCK (cholecystokinin), secretin, and gastric inhibitory peptide (GIP)
What do we call other hormones, including motilin, that aren’t “official” GI hormones?
“Candidate” hormones
In terms of gastrin, what is the stimulus for secretion, how is the secretion inhibited, where is it secreted from, and what are its actions?
Secretion: small peptides and AA in stomach lumen, especially F and Y, distension of stomach, vagal stimulation mediated by GRP (gastrin-releasing peptide)
Secretion Inhibited: acid from stomach and somatostatin
Source: secreted from G cells of small intestines in response to stimuli associated with ingestion of a meal
Actions: stimulates gastric acid secretion by parietal cells and stimulates mucosal growth by stimulating synthesis of RNA and new protein
In terms of CCK, what is the stimulus for secretion, where is it secreted from, and what are its actions?
Secretion: small peptides and AA, fatty acids and monoglycerides
Source: I cells of the small intestines
Actions: stimulates pancreatic enzyme secretion, pancreatic bicarbonate secretion, growth of exocrine pancreas, and contraction of gallbladder and relaxation of sphincter of Oddi for secretion of bile; inhibits gastric emptying and appetite
In terms of secretin, what is the stimulus for secretion, where is it secreted from, and what are its actions?
Secretion: acid and fat in lumen of duodenum
Source: S cells of the small intestines
Actions: stimulates pepsin secretion, pancreatic bicarbonate secretion, biliary bicarbonate secretion, and growth of exocrine pancreas; inhibits gastric acid secretion by parietal cells
In terms of GIP, what is the stimulus for secretion, where is it secreted from, and what are its actions?
Secretion: ONLY GI hormone released in response to protein, fat, and carbohydrate; released in response to orally administered glucose
Source: K cells of duodenum and jejunum
Actions: stimulates insulin release to bring blood glucose down; inhibits gastric acid secretion by parietal cells
In terms of motilin, what is the stimulus for secretion, where is it secreted from, and what are its actions?
Secretion: fat, acid, and nerve action
Source: M cells of duodenum and jejunum
Actions: stimulates gastric and intestinal motility; secreted during fasting
What is receptive relaxation?
Initiated by stomach distension; proximal region of the stomach relaxes to accommodate ingested meal; CCK increases distensibility of proximal end of stomach
What is the gastroileal reflex?
Presence of food in stomach triggers peristalsis in ileum
What are segmentation contractions in the small intestine?
Mix intestinal contents by sending chyme in both directions without a net forward movement
What is the gastrocolic reflex?
Food in stomach increases frequency of mass movements; when stomach is stretched with food, there is a rapid parasympathetic component; slower CCK and gastrin component is also involved
What are the principal functions of the colon?
Absorption of water and electrolytes from fecal matter; storage of fecal matter until it can be expelled
What are haustrations?
Produced by large circular constrictions plus contractions of the teniae coli
What is the function of saliva?
Initial starch digestion and triglyceride digestion; lubrication of food and protection of mouth and esophagus
What are some factors that increase/decrease saliva production?
Increase: food in mouth, smells, conditioned reflexes, nausea
Decrease: sleep, dehydration, fear, anticholinergic drugs
What types of secretory cells are found in gastric glands?
Mucous neck cells, chief (peptic) cells, parietal (oxyntic) cells
Describe chief cells
Secrete pepsinogen (inactive form of pepsin) which is a proteolytic enzyme with a pH range of activity between 1.8-3.5; release is stimulated by Ach from vagus nerve or gastric nervous plexus in response to acid in stomach
What do parietal cells secrete?
HCl
What inhibits gastric secretions?
Somatostatin and prostaglandins
