Glossary of 1 - GI control mechanisms
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- What are the two basic structural subdivisions found throughout the GI tract? What are the functions of each?
- *mucosal layer: absorption and secretion
*muscular layer: movement of contents
- What are the two layers of the muscular layer? What action does each have on the size of the GI tract?
- *outer longitudinal layer - shortens the length of the tube
*inner circular layer - constricts the diameter of the lumen
- What is the primary blood supply of the GI system? What percentage of cardiac output is consummed by the GI tract during a meal? During flight-or-fight?
- *splanchnic circulation
*65% during a meal; 0% during flight-or-fight
- What is the role of parasympathetic innervation in control of GI function? Sympathetic innervation? How is different from the innervation of the CV system?
- *parasympathetic is stimulatory
*sympathetic is inhibitory
*this is the opposite of innervation of the CV system
- Describe the effect of parasympathetic and sympathetic innervation of motility, secretion, and sphincter and vasculature tone in the GI tract.
- *parasympathetic innervation increases motility, increases secretion, relaxes sphincters, and dilates vasculature
*sympathetic inervation has the opposite effects
- What are two reflexes controlled by motor programs.
- The swallowing and peristaltic reflexes.
- Where are the myenteric and submucosal plexuses located?
- The myenteric plexus is between the layers of the muscular layer and the submucosal plexus is found between the submucosa and the inner muscular layer.
- The movement of what ion controls the contraction of smooth muscle cells in the GI system?
- What are interstitial cells of Cajal? What is their function?
- They are the pacemaker cells of the GI tract that resemble a cross between a smooth muscle cell and a fibroblast. They generate driving potentials which stimulate slow wave potentials in smooth muscle cells.
- How are ICC connected to smooth muscle cells?
- Through gap junctions.
- How do GI smooth muscle cells repolarize (i.e. return to RMP)?
- Ca++ stimulates K+ channels to open which allows an efflux of this ion and repolarization of the cell.
- Is the rate of basal oscillation of GI smooth muscle cells constant throughout the tract? Why or why not?
- It is not - there is a gradient from 14/min in the duodenum to 7/min in the ileum. It is thought that this gradient propels lumenal contents in one direction.
- What is the action of ACh on smooth muscle cells?
- *it depolarizes them, bringing them above threshold potential and allowing voltage-gated Ca++ channels to open and cause "fast" action potentials
*inhibits K+ channels to prevent repolarization
*stimulates synthesis of IP3
- What is the action of VIP on smooth muscle cells?
- *hyperpolarizes cell
*activates K+ channels which induces repolarization
*induces cAMP production which lowers [Ca++]
*decreases pacemaker frequency
- What initiates the peristaltic reflex?
- Food in the lumen stimulates mechanoreceptors which release serotonin, which in turn stimulates interneurons in the submucosal plexus.
- What are the end actions of the peristaltic reflex?
- *mucus release is stimulated in front of the bolus
*ACh stimulates muscular contraction behind the bolus
*VIP and NO induce muscular relaxation in front of the bolus
- Describe the smooth muscle action seen in W/Wv mice.
- These mice lack ICC and their smooth muscle cells show erratic Ca++ dependent potentials. There are random contractions, but no organized waves. As a result, food moves much more slowly.
- What type of symptoms are seen in humans who have defective ICC or lack them altogether?
- Low motility resulting in constipative disease.
- What are two specific mechanisms of mucosal immunity in the GI tract?
- *M cells pick up and present antigen
*production of secretory IgA by B-cells
- How is the pacemaking seen in the GI system different from that of the cardiovascular system?
- In the cardiovascular system, pacemaking is focal, rapid, and nearly synchronous. In the gut it is distributed, slow, and variable in time and space.
- Are driver potentials produced by ICC due to the movement of Ca++, Na+, or K+?
- They are initiated by the opening of a channel that doesn't discriminate between Na+ or K+. Ca++ does not seem to be involved as the cells continue to generate potentials even when a Ca++ channel blocker is applied.
- How do excitatory motor neurons in the myenteric plexus stimulate smooth muscle cells of the muscular layer?
- They release ACh which has a variety of stimulatory effects on the smooth muscle cells.
- How do signals from excitatory motor neurons affect the slow wave potentials generated by smooth muscle cells in the gut?
- They increase the amplitude and duration of the slow wave potentials which causes the muscle to contract rhythmically. Also, ACh allows the generation of fast potentials.
- How do inhibitory motor neurons affect the slow wave potentials of smooth muscle cells in the gut?
- They decrease the amplitude, duration, and frequency of the slow wave potentials which causes the muscle to relax.
- How do inhibitory motor neurons exert their effect on smooth muscle cells?
- They release VIP which has a variety of inhibitory effects.
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