Human Physiology – Regulation of HCl and Pepsinogen Synthesis and Secretion: Cephalic Phase
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Human Physiology – Regulation of HCl and Pepsinogen Synthesis and Secretion: Cephalic Phase


>>Dr. Ketchum: So now that you know how hydrochloric acid and pepsinogen are synthesized and secreted, we’re going to look at what regulates their
synthesis and secretion, because you can’t just randomly synthesize and secrete hydrochloric
acid and pepsinogen. So this regulation of hydrochloric acid and pepsinogen involves
the cephalic phase, the gastric phase, and the intestinal phase. Here we go; we’re
going to go through each of these phases. Okay, so we’re going to start with the cephalic
phase. So remember the stimulus for the cephalic phase originates in the head. All right, so
we’re going to be thinking in terms of the brain, right? Remember the goal. What regulates
secretion of hydrochloric acid and pepsinogen? So we’re going to start off with the stimuli.
So the stimuli are the sight, the smell, the taste, and the thoughts of food. So that’s
going to stimulate the central nervous system. Now once the central nervous system is stimulated,
that’s going to stimulate the parasympathetic fibers, remember that? That’s the Vagus
nerve because we’re dealing with the parasympathetic system. We’ve stimulated the Vagus nerve.
So action potentials will travel down that Vagus nerve, and then we’ll synapse here
with the neurons in the submucosal plexus. Remember the submucosal plexus is in the gut
wall, right? It’s the submucosal layer of the gut wall. Remember too that the submucosal
plexus, the whole goal of it is to secrete. So here you have the parasympathetic nervous
system communicating to the submucosal plexus, which then stimulates the mucus cells to synthesize
and secrete mucus. The submucosal plexus will stimulate the chief cells to synthesize and
secrete pepsinogen. The submucosal plexus will stimulate the parietal cells to synthesize
and secrete hydrochloric acid. So all of these now are in the lumen. Now, the submucosal
plexus also stimulates the G cells that are in the gut. The G cells will then synthesize
and secrete gastrin, which is your hormone. So gastrin is synthesized and secreted by
the stomach, and it’s the submucosal plexus that’s responsible for gastrin being synthesized
and secreted. So before we continue, I want to ask you a
question. The submucosal plexus, remember we said that they—these contain adrenergic
and cholinergic neurons. What type of neurons are we dealing with here, cholinergic or adrenergic?
These should be cholinergic neurons, right? We’re stimulating digestion. Okay, now once
gastrin has been produced, gastrin’s going to go out into the blood, because hormones
travel out into the blood stream. And then what gastrin’s going to do is through the
blood—not through the submucosal plexus, but through the blood—it’s going to stimulate
the parietal cells and it’s going to stimulate the chief cells. So when you stimulate the
parietal cells, then that’s going to cause more synthesis and secretion of hydrochloric
acid, and then the chief cells are going to synthesize and secrete more pepsinogen. So
by gastrin going out into the blood, here we’re reinforcing the parasympathetic nervous
system. The actions of the parasympathetic nervous system are reinforced by gastrin going
into the blood, traveling through the blood, and then targeting those parietal cells and
those chief cells. This phase does not last very long—literally minutes, okay? Remember,
it’s the cephalic phase; the stimulus originates in the head. It’s originating with the site
and the smell and the taste or the thought of eating. That’s what’s originating or
causing this cephalic phase. So what’s going to turn it off? So to turn off the cephalic
phase, remove the stimulus. Because if the stimulus is gone, there’s no action potential
traveling down the Vagus nerve, and therefore, you’re not going to create a slow-wave potential
in the submucosal plexus.

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