在本课程中，学生将学习认识和应用说明人体九个器官系统中整体人体机能（作为完整有机体）的基本概念。
能动性
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人体生理学导论（中文版）
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消化系统
在此模块中，我们将探讨消化道的内部运作。我们重点讨论胃肠道特定区域的功能，复杂食物在这些区域中被处理为身体可吸收以用作能源物质的溶质和养分。 此“加工厂”以从嘴到肛门的单向方式工作，需要胃酸、酶、碱和液体协调分泌来正常运作。对胃肠 (GI) 道及其附属器官（唾液腺、肝脏和胰腺）来说不平常的是，它们无需来自大脑的意识输入即可及时协调运作。 取而代之，消化道通过局部产生化学物质（激素和旁分泌）以及肠道神经系统（自主神经系统的组成部分）的协调运作来整合其多样化的动作。在此模块的最后一课中，我们将介绍消化道的正常能动性，以及导致胃肠不适（如呕吐和腹泻）的扰动。
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Jennifer CarbreyAssistant Research Professor
Department of Cell Biology
Emma JakoiAssociate Research Professor
Department of Cell Biology
Welcome. This is our last lecture on the GI tract.
Today, we want to talk about motility and how we move materials through the GI
tract. So if you recall, we've been talking
about the, the gastrointestinal tract or GI tract as a processing plant where we
start in the mouth and we digest the materials in the next compartment, which
is in the stomach and in the small intestines.
And then, move materials into the lower portion of the intestine where we're
absorbing most of our fluids and our nutrients.
And then, move into the, to the colon where the large intestinal region, where
we are absorbing a lot of the fluids. And so, this, this is a unidirectional
movement through the tract. And what's important is we have a timed
event. So that there are sufficient amounts of
time for digestion to occur, and for reabsorb, for absorption to occur before
the materials then exit from the body as the feces, fecal material.
A motility then is going to govern this time, this timing as we're moving along
the, the tube. So let's consider the first, the general
anatomy of the tube. As you recall from our first lecture that
in, of this particular series that we have, on the outer most aspect of the
gastrointestinal tract or tube, we have two layers of smooth muscle and that's
what's shown here. The smooth muscle is said to be in the
muscularis externa. The muscularis externa has these two
layers the inner most layer is circumferential around the lumen of the
tube and the outermost layer is longitudinally-oriented along the long
axis of the tube. When we contract this, the circular
muscle, the circular layer or the inner layer, if that contacts, it will make the
lumen of the tubes smaller, and if it relaxes, then the lumen of the tube
increases. In contrast, when we contract, when we
contract the outermost layer, as we as we contract the outermost layer, the tube
will shorten, and it will shorten in regions or segments along the tube.
The from the time that we enter into the esophagus to the time that we leave the
tube, most of the, of this muscle is going to be smooth muscle.
It starts in the in the lower 1 3rd of the esophagus and moves all the way
through to to the end of the large intestine.
This is all smooth muscle, and the smooth muscle is the signal, signal unit type
smooth muscle, where all of the cells are going to be connected to one another
through gap junctions, so they're electrically coupled cells.
[COUGH] Excuse me. The activity of the two layers of muscle
have to be coordinated and this coordination is done by a local nervous
system which is called the enteric system enteric nervous system, and that is what
shown here. So, the enteric nervous system is located
between the two muscle layers. The enteric nervous system effectively
governs the mortality along the tract independent of the central nervous
system. The central nervous system can modulate
this activity through other branches of the autonomic nervous system and that is
the parasympathetic and the sympathetic nervous system.
Now, the, there's another factor that we have to consider when we talking about
motility of the smooth muscle through this tract.
And that is, is that there is located within the tract, within the, within the
fundic region of the stomach, within the small intestine and within the large
intestine specialized smooth muscle cells which are called pacemakers.
And like all pacemakers, they have an unstable resting membrane potential and
that's what's shown here. So in our top image, we have the cells
are slowly depolarizing. And then they repolarize, and then again,
slowly repolarize. As the cells are slowly repolarizing,
depolarizing and repolarizing, they're generating what is called the electrical
slow-waves. These electrical slow-waves then bring
the resting membrane potential of the smooth muscle cells of these pacemaker
cells close to threshold, and at that point, we can initiate an action
potential. And if the action potential is initiated,
it means we've reached the voltage at which the voltage gated calcium channels
open. The voltage gated calcium channels open,
calcium enters the cells, and action potential is generated.
To repolarize the cells, we then close the voltage-gated calcium channels and
open a voltage-gated potassium channel, and that then repolarizes the cells.
It, within this, the intestines, that is, within the small intestine and the large
intestine, every every one of these slow-waves, electrical slow-waves is not
associated with an action potential. Only those that reach threshold and will
generate will generate the action potential.
But those that generate the action potential will be followed by a
contraction. So, wherever we have an action potential
generated, there would be a contraction of the smooth muscle.
Within the stomach, this is not true. So within the stomach, as we have these
rhythmic slow-waves, the rhythmic slow-waves actually can give rise to a
contraction. So within the stomach, you don't have to
generate an action potential, but the threshold of the slow-wave is associated
with, with contraction. There's one other thing about these
slow-waves, and that is that the slow-wave determines the frequency of the
action potential, and therefore, the frequencies of the contraction, but then
the slow-waves can be modulated. That is the pacemaker cells.
Can, their timing can be modulated by the sympathetic system and by the
parasympathetic, parasympathetic system. The sympathetic nervous system will cause
a hyperpolarization of these cells and move this and move this resting membrane
potential of the pacemaker cell further from threshold.
This simply means that it takes a longer time for the cells to be able to reach
threshold, and eventually, to fire off an action potential.
So the sympathetic nervous system then delays the, the ability of the cell to
reach, to reach a threshold and to generate an action potential, and
therefore, delays contraction. The converse is true of the
parasympathetic system. The parasympathetic system moves the
resting membrane potential towards threshold, so it's fast, the cells reach
threshold at a faster pace. And by reaching threshold at a faster
pace, they can then generate an action potential and, and contraction, so we can
speed up contractions by having input from the parasympathetic system.
Now, what kinds of motility are we actually talking about?
So, in the fed, in the fed state, you have two types of motility.
The motility that we're going to talk about first is called segmentation or
this is a mixing type of motility. In segmentation, we have a neighboring
two neighboring regions of the, of the tract, where the first is relaxed, and we
have a food bolus within that region. And the second is contracted and has no
food within the region. The first will contract and the second
relaxes, and that moves the food bolus, then to the second contract, second
chamber. And then, as the, as the second chamber
contracts, the first then open, relaxes again, and so we have this sloshing of
material back and forth. This kind of movement or segmentation or
mixing movement is very critical for mixing the materials the food substances
with the enzymes and with the buffers, and all of the secretions that we have
added to the lumens of the tract. In addition to that, it's also mixing the
material with the surfaces of the epithelial cells that are lining this
track and, and enhancing the absorption of the nutrients that we, that we have
generated. So, the smooth, the small amino acids,
the small sugars, and so forth are now being delivered to the surfaces of these
cells, and they can then be absorbed by these cells.
The sloshing back and forth between segments or segmentation, is using the
inner the inner muscle mass of this, of the tract.
So it's the inner muscle mass that constricts, closing the first, the lumen
of the first chamber, and then its the inner muscle mass that relaxes and allows
the second chamber to recieve the food bolus.
And so the sloshing back and forth then is due to the inner muscle masses changes
in inner muscle masses. The second type of movement is called
peristalsis, and that's where we're moving from one region of the, of the
tube to the second region of the tube. And then, to a third region of the tube,
and so forth. In order to do this, again, we're going
to have a contraction and a relaxation event occurring.
But we're, now, we're going to contract the outermost muscle mass.
The outermost muscle mass shortens, and the innermost muscle mass has to relax,
so that, as we shorten the tube, the tube will get fatter.
This will occur and in segments as we're going as we precede from from the
beginning towards the anus. So from the mouth towards the anus, we
will have peristalsis or this segments segmental movement of the food bolus.
It's a progressive movement as we go along the entire, of the entire tract.
And again, the waves of, of motility for both the segments segmentation and
peristalsis is going to be set not only by the interact, activity of the enteric
nervous system, but also by the pacemakers.
And so, it's the combination of those two signals which will generate our timing
for the movement of the material as we move along the tract.
Now, in the fasting state, we also have a type of peristalsis, but it's different
in timing from that which we see when food, is present within, within, the
lumen of the gastrointestinal tube. This particular type of movement is
propulsive movement. It's going to be moving materials from
one region that is from the stomach towards the anus.
and it's called the migratory motor complex.
Migratory motor complex is sort of a good housekeeping type of movement.
And in this case, what it's doing is, is sweeping clean the entire tract during
the fasting period. So, it's sweeping clean any food food
debris or nutrients that may have been left behind.
This is a very important type of activity, because if these materials are
left within the tract, you can have bacterial growth or overgrowth within the
tract, and that can lead to painful, to a painful situation.
It could lead to diarrhea, it could lead to nausea, it could lead to vomiting.
These migratory motor complexes are initiated, not by our pacers, pacemaker
cells, but by the hormone motilin. This hormone is secreted in the absence
of food within the lumen of the stomach. It's also secreted by the duodenum and by
the intestine the jejunum of the small intestines in the absence of food.
Motilin then will initiate the contract, the contractal events, both within the
stomach, as well as within the intestines.
The migratory motor complex usually starts in the stomach, but it can
initiate in the duodenum and in the and or in the jejunum.
And the frequency of contractions, as I said, is going to be different from this
frequency of contractions that we saw with the peristalsis that, that was
present when we had food present within the lumen.
Now, there are regions which are, are isolated from one another and, as you
know, we isolate the stomach, which has a very caustic acidic acid hydrochloric
acid in it from the duodenum and from the esophogus, because these latter two
regions are not ca-, are not protected to the caustic actions of the acid.
These regions that are isolated by sphincters.
These sphincters are not under voluntary control.
These are sphincters which are under involuntary control.
And it's due to the tonic inhibition of the smooth muscle.
And is, again, the the inner circular layer of the smooth muscle, the
muscularis externis. So when we contracted that smooth muscle
layer, we can include the lumen of the tube.
We open it at very small amounts between the the pylorus region of the stomach,
and the duodenum, to allow very small amounts of our acidic chyme to enter into
the duodenum. So we have a regulated opening of that
region. And so, that means that we have to, for
relaxation of the sphincter, we have to release the inhibition that the sphincter
is contracted under an inhibitory activity.
We release that inhibition and allow the sphincter to relax, so that this, so that
the there's is a small lumen through which the acidic chyme can exit.
This is a regulated relaxation. And it's, by turning off this tonic
inhibition that we're then able to then open the sphincter.
Very small amounts are ejected from the stomach into the duodenum.
And this is really important, because if we have a very large amount of acidic
chyme or, or, or for instance, a hype a hypotonic solution coming from the
stomach into the duodenum. You can then get water moving into the
duodenum, from osmotic action, and what that will do is stretch the duodenum, and
that's going to be very painful. So this is a regulated exiting of
material at this point. We also have regulated openings of these
sphincters. There's a second sphincter, not only at
the pipe, pipe, there's a sphincter between the esophagus and the stomach,
and between the stomach and the duodenum, but there's also one between the ileum
and the colon. and these, these regulated openings then
are allowing the receiving section region of the tube to be able to handle the
material that's being that's being delivered.
We have regulated openings in respond to the migrating motor complex as well.
And that is between meals, so that when we're sweeping clean the these, the
intestine of any debris or any nutrients, then, at that time, we will open
periodically the sphincters and to allow the materials to pass, and then the
sphincters will close again. There is no time in which the sphincters
are all patently open along, along the tract.
So what about mass movements? Now, mass movements is where we have
waves of contractions that are occurring within this, the large intestine.
And here, we're now propelling the the fecal material through the large
intestine toward the, toward the rectum and the anus.
The, this waves of contraction are not two peristaltic waves in the sense they
are not using those outer the outer layer of smooth muscle that we were talking
about in the muscularis externis. But instead, it's using the inner layer
of the circumferentially-oriented muscle layer of the muscularis externis.
So we have contraction, then we're essentially contracting a part of the
chamber. We've contracted, and then, we relax to
receive it. And again, it's sort of almost like the
segmentation type of movement, but it's propulsive.
This is, has a special name. It's called mass movements.
It occurs about 1 to 3 times per day under normal circumstances.
and that it's then moving the materials to the anal canal.
The anal canal has an, has an involuntary sphincter.
And we have an external, an external sphincter at the anus, the which is
voluntary. The mass movement, you are all of, are
familiar with is, is that you have the sensation that you need to go to the
bathroom, but but it may not be appropriate.
And so you don't nothing happens and you're able to retain the fecal material
until you can get to the bathroom, and then eliminate it, and you're eliminating
it under voluntary control. The elimination of the fecal material
from the body is called defecation, and defecation there's an A in there.
It's simply expelling the fecal material from the body.
And it occurs with relaxation of the voluntary anal sphincter.
That's the external sphincter and then, also an increase in the intra-abdominal
pressure to push the material out from the body.
Alright, so one thing that we haven't talked about is vomiting.
We talked about diarrhea in the last lecture.
And here, I wanted to just to spend some time talking about vomiting.
I'm pretty sure everybody has experienced vomiting at some point on time.
Vomiting is a protective function where if you're bringing in some type of toxic
material into the body. Then the body expels the, the material
from the stomach, the stomach content being expelled back out through the
mouth. This can also be triggered by either
delayed empyting of the stomach or by rapid emptying of the stomach, where in
the delayed emptying of the stomach we have material that sort of stays in the
stomach, let's say for instance from a very fatty meal.
And it's retained and retained and retained for several hours, and
eventually the body just expels through the mouth.
And this, the other one is where we have this very rapid dump emptying of the, of
the stomach content into the duodenal and again as I said, then you can have a, you
can have a very rapid swelling. Of the duodenal region due to the
movement of water coming into this region to titrate the hypertonic material that
is coming from the stomach. This actually has a name and that's
called the Dumping Syndrome of the stomach.
The vomiting of course, is the expulsion of the chyme from your, from the stomach,
but it can be that you can are eliminating material from the beginning
of the duodenal. And, and under those conditions, then the
vomit it will have a color sort of a greenish color and that's due to the
bowel which has been which has been delivered from the gallbladder to, to the
duodenal. Vomiting is controlled by the vomiting
center, and it's, that center is located in the medulla at the brain stem.
The and it is actually controlling the respiratory muscles and the abdominal
muscles. It is not causing a reversed peristalsis
of the smooth muscles of the muscularis externus, but instead you are changing
the pressure within the inter, the inter-thoracic cavity.
And then help to to move the, the material, then out from the stomach and
out, out of the mouth. Alright, so what's our general concepts?
So, the first of the general concepts is that we have movement which involves
coordinated activities of the two outer muscle masses of the wall of the
gastrointestinal tract. And this move, the second general concept
is that movement and activities are coordinated by this local nervous system
which is called the enteric nervous system.
And that this, this can be modulated from input from both the sympathetic nervous
system and the parasympathetic nervous system.
The third is that there's pacemakers cells which generate a spontaneous
electrical activity and these are called electrical slow waves.
And that these can lead to an action potential which fires at the peak of the
depolarization, as they're coming up to threshold.
And at that point, if you're an action potential contraction can follow.
And this is true throughout the intestine but in the stomach itself.
That the, that the slow waves, the electrical slow waves can can be
associated with contractions directly. The fourth concept is that in the fed
state we have two types of motility. We have segmentation or mixing.
and this is allowing the material to slosh back and forth in order to have
optimal absorption of the materials. And then we also have peristalsis.
And peristalsis is the propulsive movement of the, of the food bolus along
the tract. In the fasted state, the migratory, the
migrating motor complex sweeps the tract clean and this is the propulsive type of
movement along the tract. It has very different timing from the
peristalsis movement that you, that is associated with feeding.
The fifth general concept is that we have tonic contractions can occur at the
sphincters and this closes the sphincters.
And in order to have material moving from one segment of the tract to a second, we
have to have a relaxation of the sphincter, a slight opening of the
sphincter to allow the material to to, to exit from ss-, segment one to segment
two. The tonic contractions of the, of the
sphincters are going to occur when there's no motility within the tract.
And so the, they will open when there's food within the tract at designated
times. and they will also open during the
migratory the migratory motor complex, as we're sweeping the debris the extra
nutrient from the tract. But that they will be closed at all the
other times, and in particular when there is no motility within the GI tract.
So this, then, ends our discussion of the GI tract.
And the next time we meet, we're going to talk about the urinary system.
So see you then. Bye-bye.
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