BIOL 1840U Lecture Notes - External Anal Sphincter, Migrating Motor Complex, Constipation

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29 Jan 2013
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Outline of Lecture 28 (05-13 B; Ravich)
GI Motility
- The GI tract can be seen as a series of pumps, conduits, reservoirs, and gates. This allows the
tract to perform several functions such as propulsion, grinding, mixing, and compartmentalization.
- There are 2 types of enteric smooth muscle contractions. Phasic contractions result from
momentary stimulation and are used for pumping. Tonic contractions result in continuous pressure such as
in sphincters, and must be relaxed to allow gating.
- Neural input for most of the GI tract is mostly autonomic. The parasympathetic side stimulates
the GI tract in general, and its nerves enter through the vagus and sacral plexus. The sympathethic side is
inhibitory in general, and nerves enter through the sympathetic ganglion.
- The myenteric plexus is the intrinsic nervous system of the gut and while it is modulated by the
CNS it can operate independently.
- Some GI muscle is directly innervated and others are indirectly innervated.
- The pattern of contraction determines the result. A smooth contraction wave gives propulsion; a
smooth but partial contraction wave gives mixing and propulsion; uncoordinated contractions give mixing
and grinding.
- Slow waves are rhythmic undulations in the membrane potential of myocytes. Spike potentials,
and hence contractions, only occur at the peaks of the slow waves, and this helps control the timing of
contractions.
- Sphincters act as one-way valves. Hence a proximal distention causes relaxation to allow
forward movement of material, but a distal distention causes increased contraction to prevent retrograde
flow.
- The oropharynx is common to both the GI and respiratory system. In swallowing, muscle
movements are coordinated to prevent food from entering the airway.
- The initiation of swallowing is voluntary; the vagus nerve controls the upper third of the
esophagus. Peristalsis down the esophagus is independent of CNS control and is a result of a latency
gradient down the esophagus (i.e. time between stimulus and contraction). Peristalsis can be can be
initiated by a voluntary swallow (primary peristalsis) or by distention of the esophagus (secondary
peristalsis); in primary peristalsis, there is deglutitive inhibition to a rapid sequence of swallows.
- The head pressure in the stomach is kept constant by expansion of the stomach.
- Antral contractions cause mixing and grinding in the antrum as well as propulsion of processed
material into the duodenum. To prevent retrograde flow, duodenal contractions are inhibited during antral
contraction.
- Contractions are propagated if myocytes are in close contact (tight gap junctions) and if the
contraction is not interrupted by external neural stimulation.
- The Migrating Motor Complex (MMC) is composed of four cycles
1) quiescent period
2) irregular contractions, for mixing
3) rapid contractions that start in stomach and propagate through the small intestine, for clearing
4) transition period, for slowing of contractions
- Motility in the colon is markedly segmental. Colonic motility also determines the extent of
water absorption, and hence constipation or diarrhea.
- As the rectum fills, the internal anal sphincter relaxes and the external anal sphincter tightens to
compensate for increased pressure without allowing incontinence. With time the rectal pressure subsides
and the need to defecate passes.
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