>>Dr. Ketchum: So next then following the cephalic phase, the gastric phase occurs.
Gastric phase, the duration for it is three to four hours. This is a lot more complicated.
So the stimuli include distension of the stomach. So remember, gastric phase—this phase originates
because of the stimulus in the stomach, all right? So distension in the stomach. Elevated
pH in the stomach, okay? So more basic, if you will. You’re also going to have proteins
that are in the stomach that are going to help stimulate those chemoreceptors. And you’ll
also have amino acids, because some of the proteins will be broken down into amino acids.
So we know that the receptors, then, are the stretch receptors that detect the distension,
chemoreceptors are going to detect the elevated pH, the proteins and the amino acids that—amino acids
that are in the lumen of the stomach. So then that’s going to generate short and long-term reflexes.
Remember if it’s a long one, it involves the central nervous system. If it’s short,
it directly stimulates the enteric nervous system. We’re going to send action potentials,
then, to the enteric nervous system, okay? So here we’re going to send action potentials
to the submucosal plexus. Remember, that’s in the submucosal gut wall, and we’re also
going to send a signal to the myenteric plexus. Remember that this—this is the nerve network
that’s part of the muscular–muscularis externa. Now, let’s be a little more specific
here. Because what the myenteric plexus is going to do—it’s responsible for the mixing.
It’s responsible for the contraction of the smooth muscles so that you can mix the
chyme that’s in the stomach. Remember, you don’t call it food anymore when it’s in
the stomach; you call it chyme. Now meanwhile then, you have the submucosal plexus,
and once again, that’s going to stimulate mucus cells so that you synthesize and secrete mucus.
It’s going to stimulate chief cells so you can synthesize and secrete pepsinogen. It’s
going to stimulate the parietal cells so you can synthesize and secrete hydrochloric acid,
and it’s going to stimulate the G cells so that you can synthesize and secrete gastrin.
So again, gastrin’s being produced, this hormone, by the stomach. Now just like in
the cephalic phase, gastrin can go into the bloodstream. And once it goes into the bloodstream,
it will target those parietal cells and it will target those chief cells, and
then you will also get synthesis and secretion of pepsinogen and HCl that way. So just like
you did in the cephalic phase, here you have reinforcement of the parasympathetic nervous
system once again. So the G cells aren’t just stimulated by the submucosal plexus;
they’re also stimulated by partly digested peptides. So those themselves will stimulate
the G cells, and the G cells then will synthesize and secrete gastrin. What’s going to turn
this system off? What turns off the gastric phase? One thing to think about is that the
chyme won’t stay forever in the stomach, right? The chyme's going to move into the
duodenum. And when the chyme moves into the duodenum, the duodenum, then, is going to distend.
There’s going to be some distention of the duodenum, so that can help trigger the system
and shut the system off.
What if you had a sympathetic response? Would a sympathetic response shut the system down?
Hopefully you answered yes. So if you have a sympathetic response that’ll shut the
system down. And then what specifically turns off gastrin? So acidity in the lumen of the
stomach. If the lumen of the stomach is acidic, that will inhibit gastrin. So now we need
to remember that the chyme that was in the stomach is going to move into the duodenum,
and then we’re going to have the intestinal phase take place.