Slow Waves – Gastrointestinal Motility

00:01 What controls these GI movements? These come back to these interstitial cells of Cajal.

00:08 These small little cells use a change in membrane potential to elicit different types of muscle contractions.

00:17 The frequency, however, of these muscle contractions vary depending upon if you’re in the stomach, the small intestine or the large intestine.

00:26 In the stomach, there are about 3 to 5 of these contraction waves that happen per minute.

00:34 In the small intestine, it’s about 12 to 20.

00:37 And the large intestine, about 6 to 8.

00:42 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.

00:48 The influence of these particular slow waves are usually not in terms of the rate, but rather their amplitude.

00:56 The higher the amplitude, the greater the contraction.

01:00 The lower the amplitude, the less of a contraction.

01:04 And this is where smooth muscle differs from skeletal muscles.

01:08 In skeletal muscle, either it contracts or it doesn’t.

01:11 In smooth muscle, it’s gradated, meaning that it can contract harder or less hard with any particular muscle fiber.

01:21 Okay.

01:21 So how does this process work? Well, you have to develop tension and you have these waves occurring.

01:31 So let’s look at this particular example of four slow waves.

01:37 Each of the slow waves causes a little change in muscle tone.

01:41 If there’s an increase in muscle tone, that’s a muscle contraction.

01:46 What those spikes are at the top of membrane potential are action potentials.

01:50 So it’s possible for a slow wave to reach and action potential.

01:55 But it is not necessary.

01:58 There still will be some contraction that occurs.

02:01 This is in a stimulated condition such as if you were in a relaxed and digest condition of the parasympathetic nervous system.

02:11 You see that in this case you have more action potentials occurring at the tops of these slow waves causing greater muscle contractions.

02:22 This is an inhibited condition such as what might happen with the sympathetic nervous system.

02:28 So it’s generating a little bit of a slow wave here.

02:30 But it’s not enough to cause much of a change in muscle tone.

02:35 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.

02:51 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.

03:06 So it’s simply opening and closing the various channels done at different rates throughout the GI system.

03:13 Remember, it’s slower in the stomach, faster in the small intestine and again slower in the large intestine.