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Physiology 3120 Lecture Notes - Silly Putty, Stress Relaxation, Skeletal Muscle

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Tom Stavraky

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Human Physiology
Monday, March 29, 2010
“Gastro V”
Basic principles of enteric smooth muscle (ESM); designed to move food along the GI tract and can
accommodate a large meal
Can deal with a great degree of stretch; has an intrinsic (biophysical property, not influenced by
nerves or hormones) response to stretch; skeletal muscle that is stretched for a period of time
will increase tension for a period of time, and the tension will remain high as long as the muscle
is stretched; in enteric smooth muscle, get an increase in tension initially, but relaxes over time
if the stretch stimulus remains; think of it as a piece of Silly Putty; this property allows you to eat
a large meal without feeling satiety until you’ve had enough; this phenomenon is called stress
Electrical properties; in skeletal muscle, voltage is negative inside the cell, and it doesn’t
fluctuate; in ESM, in most parts of the GI tract, membrane potential oscillates (sinusoidal) over
time; this membrane potential is called the slow wave; to initiate contraction, must induce an
action potential across the cell membrane; APs typically occur at the peak of the slow wave
(closer to threshold); contraction measured in mmHg (increases pressure); pressure increases
during an AP; slow wave does not cause contraction, but does seem to dictate the number of
contractions you can have per unit time; frequency of slow wave dictates maximum frequency of
contraction in any portion of the GI tract; different parts of GI tract will have their own intrinsic
frequency; more APs cause the ESM to contract harder and for longer; adrenergic/sympathetic
activation drives the peak of the slow wave down
Gastric motility
Functional distinction in motility between the proximal and distal stomach
Proximal stomach
Not all the biophysical properties apply (i.e. there is NO slow wave); doesn’t actually contract
very much
Relaxes frequently (NOT stress relaxation); active relaxation that occurs before food ever enters
the stomach receptive relaxation; therefore, must be influenced in some chemical way;
initiated in response to swallowing, vagus innervation (doesn’t ONLY release ACh, some
branches produce neurotransmitters that cause relaxation [candidates are purines, nitric oxide, or
Distal stomach
Contractions are orientated such that you get a proximal-to-distal movement of food; short
peristaltic contraction (not true peristalsis); progressive contractions at more and more distal
areas; related to slow wave (frequency of slow wave at the various regions is the same, but they
are staggered/out-of-phase); phase “lag” (time between blips) decreases/blips occur closer
together, which increases the velocity of contraction as you move down the stomach
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