What controls these GI movements?
These come back to these
interstitial cells of Cajal.
These small little cells use a
change in membrane potential
to elicit different types
of muscle contractions.
The frequency, however, of these muscle
contractions vary depending upon
if you’re in the stomach, the small
intestine or the large intestine.
In the stomach, there are
about 3 to 5 of these
contraction waves that
happen per minute.
In the small intestine,
it’s about 12 to 20.
And the large intestine,
about 6 to 8.
Now, slow waves are very
hard to kind of visualize,
so I’m going to show you a few
in the next couple of slides.
The influence of these particular
slow waves are usually
not in terms of the rate,
but rather their amplitude.
The higher the amplitude, the
greater the contraction.
The lower the amplitude,
the less of a contraction.
And this is where smooth muscle
differs from skeletal muscles.
In skeletal muscle, either
it contracts or it doesn’t.
In smooth muscle, it’s gradated,
meaning that it can contract harder or less
hard with any particular muscle fiber.
So how does this process work?
Well, you have to develop tension
and you have these
So let’s look at this particular
example of four slow waves.
Each of the slow waves causes a
little change in muscle tone.
If there’s an increase in muscle
tone, that’s a muscle contraction.
What those spikes are at the top of
membrane potential are action potentials.
So it’s possible for a slow wave
to reach and action potential.
But it is not necessary.
There still will be some
contraction that occurs.
This is in a stimulated condition
such as if you were in a relaxed
and digest condition of the
parasympathetic nervous system.
You see that in this case you have more
action potentials occurring at the tops
of these slow waves causing
greater muscle contractions.
This is an inhibited
condition such as what might
happen with the sympathetic
So it’s generating a little
bit of a slow wave here.
But it’s not enough to cause
much of a change in muscle tone.
So muscle tone is very much
related to this slow wave,
but also how many action
potentials that are engaged
once the slow wave reaches a certain
amount of membrane potential.
And so you might ask, “Well, what
controls these slow waves?”
Well, the upward slope of the slow
wave is usually done by a calcium channel
and the return of that slow
wave back down to more of a
resting membrane potential is
done by a potassium channel.
So it’s simply opening and
closing the various channels
done at different rates
throughout the GI system.
Remember, it’s slower in the stomach,
faster in the small intestine and
again slower in the large intestine.