Gastrointestinal movements
Ingestion
of Food
The time that food remains in each part of the
alimentary tract is very important for optimal processing and absorption of
nutrients. Also, appropriate mixing must be provided. Because the requirements
for mixing and propulsion are quite different at each stage of processing,
multiple automatic nervous and hormonal mechanisms control the timing of each
of these so that they will occur optimally, not too rapidly, not too slowly.
The amount of food that a person ingests is
determined principally by intrinsic desire for food called hunger. The type of
food that a person preferentially seeks is determined by appetite. These
mechanisms are extremely important for maintaining an adequate nutritional
supply for the body.
Mastication
(Chewing)
The teeth are admirably designed for chewing. The
anterior teeth (incisors) provide a strong cutting action and the posterior
teeth (molars) a grinding action. All the jaw muscles working together can
close the teeth with a force as great as 25 kg on the incisors and 90 kg on the
molars.
Most of the muscles of chewing are innervated by the
motor branch of the fifth cranial nerve, and the chewing process is controlled
by nuclei in the brain stem. Stimulation of specific reticular areas in the
brain stem taste centers will cause rhythmical chewing movements. Also,
stimulation of areas in the hypothalamus, amygdala, and even the cerebral
cortex near the sensory areas for taste and smell can often cause chewing.
Much of the chewing process is caused by a chewing
reflex. The presence of a bolus of food in the mouth at first initiates reflex
inhibition of the muscles of mastication, which allows the lower jaw to drop.
The drop in turn initiates a stretch reflex of the jaw muscles that leads to
rebound contraction. This automatically raises the jaw to cause closure of the
teeth, but it also compresses the bolus again against the linings of the mouth,
which inhibits the jaw muscles once again, allowing the jaw to drop and rebound
another time; this is repeated again and again.
Chewing is important for digestion of all foods, but
especially important for most fruits and raw vegetables because these have
indigestible cellulose membranes around their nutrient portions that must be
broken before the food can be digested. Also, chewing aids the digestion of
food for still another simple reason: Digestive enzymes act only on the
surfaces of food particles; therefore, the rate of digestion is absolutely
dependent on the total surface area exposed to the digestive secretions.
In addition, grinding the food to a very fine
particulate consistency prevents excoriation of the gastrointestinal tract and
increases the ease with which food is emptied from the stomach into the small
intestine, then into all succeeding segments of the gut.
Swallowing
(Deglutition)
Swallowing is a complicated mechanism, principally
because the pharynx takes part in both respiration and swallowing. The pharynx
is converted for only a few seconds at a time into a tract for propulsion of
food. It is especially important that respiration not be compromised because of
swallowing.
In general, swallowing can be divided into
(1) A voluntary stage, which
initiates the swallowing process
(2) A pharyngeal stage, which is
involuntary and constitutes passage of food through the pharynx into the
esophagus
(3) An esophageal stage, another
involuntary phase that transports food from the pharynx to the stomach.
Voluntary
Stage of Swallowing
When the food is ready for swallowing, it is
voluntarily squeezed or rolled posteriorly into the pharynx by pressure of the
tongue upward and backward against the palate. As the bolus of food enters the
posterior mouth and pharynx, it stimulates epithelial swallowing receptor areas
all around the opening of the pharynx, especially on the tonsillar pillars, and
impulses from these pass to the brain stem to initiate a series of automatic
pharyngeal muscle contractions as follows:
1. The soft palate is pulled upward
to close the posterior nares, to prevent reflux of food into the nasal
cavities.
2. The palatopharyngeal folds on
each side of the pharynx are pulled medially to approximate each other. In this
way, these folds form a sagittal slit through which the food must pass into the
posterior pharynx. This slit performs a selective action, allowing food that
has been masticated sufficiently to pass with ease. Because this stage of
swallowing lasts less than 1 second, any large object is usually impeded too
much to pass into the esophagus.
3. The vocal cords of the larynx
are strongly approximated, and the larynx is pulled upward and anteriorly by
the neck muscles. These actions, combined with the presence of ligaments that
prevent upward movement of the epiglottis, cause the epiglottis to swing
backward over the opening of the larynx. All these effects acting together
prevent passage of food into the nose and trachea. Most essential is the tight
approximation of the vocal cords, but the epiglottis helps to prevent food from
ever getting as far as the vocal cords. Destruction of the vocal cords or of
the muscles that approximate them can cause strangulation.
4. The upward movement of the
larynx also pulls up and enlarges the opening to the esophagus. At the same
time, the upper 3 to 4 centimeters of the esophageal muscular wall, called the
upper esophageal sphincter also called the pharyngoesophageal sphincter,
relaxes. Thus, food moves easily and freely from the posterior pharynx into the
upper esophagus. Between swallows, this sphincter remains strongly contracted,
thereby preventing air from going into the esophagus during respiration. The
upward movement of the larynx also lifts the glottis out of the main stream of
food flow, so the food mainly passes on each side of the epiglottis rather than
over its surface; this adds still another protection against entry of food into
the trachea.
5. Once the larynx is raised and
the pharyngo esophageal sphincter becomes relaxed, the entire muscular wall of
the pharynx contracts, beginning in the superior part of the pharynx, then
spreading downward over the middle and inferior pharyngeal areas, which propels
the food by peristalsis into the esophagus.
To summarize the mechanics of the pharyngeal stage
of swallowing-
The trachea is closed, the esophagus is opened, and
a fast peristaltic wave initiated by the nervous system of the pharynx forces
the bolus of food into the upper esophagus, the entire process occurring in
less than 2 seconds.
Nervous
Initiation of the Pharyngeal Stage of Swallowing
The most sensitive tactile areas of the posterior
mouth and pharynx for initiating the pharyngeal stage of swallowing lie in a
ring around the pharyngeal opening, with greatest sensitivity on the tonsillar
pillars. Impulses are transmitted from these areas through the sensory portions
of the trigeminal and Glossopharyngeal nerves into the medulla oblongata,
either into or closely associated with the tractus solitarius, which receives
essentially all sensory impulses from the mouth.
The successive stages of the swallowing process are
then automatically initiated in orderly sequence by neuronal areas of the
reticular substance of the medulla and lower portion of the Pons. The sequence
of the swallowing reflex is the same from one swallow to the next, and the
timing of the entire cycle also remains constant from one swallow to the next.
The areas in the medulla and lower Pons that control swallowing are
collectively called the deglutition or swallowing center.
The motor impulses from the swallowing center to the
pharynx and upper esophagus that cause swallowing are transmitted successively
by the fifth, ninth, tenth, and twelfth cranial nerves and even a few of the
superior cervical nerves.
It may be said that the pharyngeal stage of
swallowing is principally a reflex act. It is almost always initiated by voluntary
movement of food into the back of the mouth, which in turn excites involuntary
pharyngeal sensory receptors to elicit the swallowing reflex.
Effect
of the Pharyngeal Stage of Swallowing on Respiration
The entire pharyngeal stage of swallowing usually
occurs in less than 6 seconds, thereby interrupting respiration for only a
fraction of a usual respiratory cycle. The swallowing center specifically
inhibits the respiratory center of the medulla during this time, halting
respiration at any point in its cycle to allow swallowing to proceed. Yet even
while a person is talking, swallowing interrupts respiration for such a short
time that it is hardly noticeable.
Esophageal
Stage of Swallowing
The esophagus functions primarily to conduct food
rapidly from the pharynx to the stomach, and its movements are organized
specifically for this function. The esophagus normally exhibits two types of
peristaltic movements-
1. Primary peristalsis
2. Secondary peristalsis.
Primary
peristalsis
It is simply the continuation of the peristaltic
wave that begins in the pharynx and spreads into the esophagus during the
pharyngeal stage of swallowing. This wave passes all the way from the pharynx
to the stomach in about 8 to 10 seconds. Food swallowed by a person who is in
the upright position is usually transmitted to the lower end of the esophagus
even more rapidly than the peristaltic wave itself, in about 5 to 8 seconds,
because of the additional effect of gravity pulling the food downward.
Secondary
Peristalsis
If the primary peristaltic wave fails to move into
the stomach all the food that has entered the esophagus, secondary peristaltic
waves result from distention of the esophagus itself by the retained food;
these waves continue until all the food has emptied into the stomach.
Secondary peristaltic waves are initiated partly by
intrinsic neural circuits in the myenteric nervous system and partly by
reflexes that begin in the pharynx and are then transmitted upward through
vagal afferent fibers to the medulla and back again to the esophagus through Glossopharyngeal
and vagal efferent nerve fibers. The musculature of the pharyngeal wall and
upper third of the esophagus is striated muscle. Therefore, the peristaltic
waves in these regions are controlled by skeletal nerve impulses from the Glossopharyngeal
and vagus nerves.
In the lower two thirds of the esophagus, the
musculature is smooth muscle, but this portion of the esophagus is also
strongly controlled by the vagus nerves acting through connections with the
esophageal myenteric nervous system.
When the vagus nerves to the esophagus are cut, the
myenteric nerve plexus of the esophagus becomes excitable enough after several
days to cause strong secondary peristaltic waves even without support from the
vagal reflexes.
Therefore, even after paralysis of the brain stem
swallowing reflex, food fed by tube or in some other way into the esophagus
still passes readily into the stomach.
Receptive
Relaxation of the Stomach
When the esophageal peristaltic wave approaches
toward the stomach, a wave of relaxation, transmitted through myenteric
inhibitory neurons, precedes the peristalsis. Furthermore, the entire stomach
and, to a lesser extent, even the duodenum become relaxed as this wave reaches
the lower end of the esophagus and thus are prepared ahead of time to receive
the food propelled into the esophagus during the swallowing act.
Function
of the Lower Esophageal Sphincter (Gastroesophageal Sphincter)
At the lower end of the esophagus extending upward
about 3 centimeters above its juncture with the stomach the esophageal circular
muscle functions as a broad lower esophageal sphincter. It is called the gastro
esophageal sphincter.
This sphincter normally remains tonically
constricted with an intra luminal pressure at this point in the esophagus of
about 30 mm Hg, in contrast to the mid portion of the esophagus, which normally
remains relaxed.
When a peristaltic swallowing wave passes down the
esophagus, there is receptive relaxation of the lower esophageal sphincter
ahead of the peristaltic wave, which allows easy propulsion of the swallowed
food into the stomach.
The stomach secretions are highly acidic and contain
many proteolytic enzymes. The esophageal mucosa, except in the lower one eighth
of the esophagus, is not capable of resisting for long the digestive action of
gastric secretions. The tonic constriction of the lower esophageal sphincter
helps to prevent significant reflux of stomach contents into the esophagus
except under abnormal conditions.
Another factor that helps to prevent reflux is a
valve like mechanism of a short portion of the esophagus that extends slightly
into the stomach. Increased intra-abdominal pressure caves the esophagus inward
at this point.
Thus, this valve like closure of the lower esophagus
helps to prevent high intra-abdominal pressure from forcing stomach contents
backward into the esophagus.
Motor
Functions of the Stomach
The motor functions of the stomach are threefold:
(1) Storage of large quantities of
food until the food can be processed in the stomach, duodenum, and lower
intestinal tract;
(2) Mixing of this food with
gastric secretions until it forms a semi fluid mixture called chyme;
(3) Slow emptying of the chyme from
the stomach into the small intestine at a rate suitable for proper digestion
and absorption by the small intestine.
Anatomically,
the stomach is usually divided into two major parts:
·
Body
·
Antrum.
Physiologically, it is divided into-
·
The oral portion, comprising about the
first two thirds of the body,
·
The caudal or aboral portion, comprising
the remainder of the body plus the antrum.
As food enters the stomach, it forms concentric
circles of the food in the oral portion of the stomach, the newest food lying
closest to the esophageal opening and the oldest food lying nearest the outer
wall of the stomach. Normally, when food stretches the stomach, a vasovagal
reflex from the stomach to the brain stem and then back to the stomach reduces
the tone in the muscular wall of the body of the stomach so that the wall
bulges progressively outward, accommodating greater and greater quantities of
food up to a limit in the completely relaxed stomach of 0.8 to 1.5 liters. The
pressure in the stomach remains low until this limit is approached.
Mixing
and Propulsion of Food in the Stomach
Basic
Electrical Rhythm of the Stomach Wall
The digestive juices of the stomach are secreted by
gastric glands, which are present in almost the entire wall of the body of the
stomach except along a narrow strip on the lesser curvature of the stomach.
These secretions come immediately into contact with that portion of the stored
food lying against the mucosal surface of the stomach.
As long as food is in the stomach, weak peristaltic
constrictor waves, called mixing waves, begin in the mid to upper portions of
the stomach wall and move toward the antrum about once every 15 to 20 seconds.
These waves are initiated by the gut wall basic electrical rhythm, consisting
of electrical “slow waves” that occur spontaneously in the stomach wall.
As the constrictor waves progress from the body of
the stomach into the antrum, they become more intense, some becoming extremely
intense and providing powerful peristaltic action potential–driven constrictor
rings that force the antral contents under higher and higher pressure toward
the pylorus. These constrictor rings also play an important role in mixing the
stomach contents in the following way-
Each time a peristaltic wave passes down the antral
wall toward the pylorus, it digs deeply into the food contents in the antrum.
Yet the opening of the pylorus is still small enough that only a few
milliliters or less of antral contents are expelled into the duodenum with each
peristaltic wave.
Also, as each peristaltic wave approaches the
pylorus, the pyloric muscle itself often contracts, which further impedes
emptying through the pylorus. Therefore, most of the antral contents are
squeezed upstream through the peristaltic ring toward the body of the stomach,
not through the pylorus.
Thus, the moving peristaltic constrictive ring,
combined with this upstream squeezing action, called retropulsion, is an
exceedingly important mixing mechanism in the stomach.
Chyme
After food in the stomach has become thoroughly
mixed with the stomach secretions, the resulting mixture that passes down the
gut is called chyme. The degree of fluidity of the chyme leaving the stomach
depends on the relative amounts of food, water, and stomach secretions and on
the degree of digestion that has occurred.
The appearance of chyme is that of a murky semi
fluid or paste.
Besides the peristaltic contractions that occur when
food is present in the stomach, another type of intense contractions, called
hunger contractions, occur when the stomach has been empty for several hours.
They are rhythmical peristaltic contractions in the body of the stomach. When
the successive contractions become extremely strong, they often fuse to cause a
continuing tetanic contraction that sometimes lasts for 2 to 3 minutes.
Hunger contractions are most intense in young,
healthy people who have high degrees of gastrointestinal tonus; they are also
greatly increased by the person’s having lower than normal levels of blood
sugar.
When hunger contractions occur in the stomach, the
person sometimes experiences mild pain in the pit of the stomach, called hunger
pangs. Hunger pangs usually do not begin until 12 to 24 hours after the last
ingestion of food; in starvation, they reach their greatest intensity in 3 to 4
days and gradually weaken in succeeding days.
Stomach
Emptying
Stomach emptying is promoted by intense peristaltic
contractions in the stomach antrum. At the same time, emptying is opposed by
varying degrees of resistance to passage of chyme at the pylorus.
Intense
Antral Peristaltic Contractions during Stomach Emptying-Pyloric Pump
The rhythmical stomach contractions are mostly weak
and function mainly to cause mixing of food and gastric secretions. However,
for about 20 percent of the time while food is in the stomach, the contractions
become intense, beginning in mid-stomach and spreading through the caudal part
of stomach; these contractions are strong peristaltic, very tight ring like
constrictions that can cause stomach emptying.
As the stomach becomes progressively more and more
empty, these constrictions begin farther and farther up the body of the
stomach, gradually pinching off the food in the body of the stomach and adding
this food to the chyme in the antrum. These intense peristaltic contractions
often create 50 to 70 centimeters of water pressure, which is about six times
as powerful as the usual mixing type of peristaltic waves.
When pyloric tone is normal, each strong peristaltic
wave forces up to several milliliters of chyme into the duodenum. Thus, the
peristaltic waves, in addition to causing mixing in the stomach, also provide a
pumping action called the pyloric pump.
Role
of the Pylorus in Controlling Stomach Emptying
The distal opening of the stomach is the pylorus.
Here the thickness of the circular wall muscle becomes 50 to 100 percent
greater than in the earlier portions of the stomach antrum, and it remains
slightly tonically contracted almost all the time.
Therefore, the pyloric circular muscle is called the
pyloric sphincter. Despite normal tonic contraction of the pyloric sphincter,
the pylorus usually is open enough for water and other fluids to empty from the
stomach into the duodenum with ease.
Conversely, the constriction usually prevents
passage of food particles until they have become mixed in the chyme to almost
fluid consistency. The degree of constriction of the pylorus is increased or
decreased under the influence of nervous and humoral reflex signals from both
the stomach and the duodenum.
Regulation
of Stomach Emptying
The rate at which the stomach empties is regulated
by signals from both the stomach and the duodenum. However, the duodenum
provides by far the more potent of the signals, controlling the emptying of
chyme into the duodenum at a rate no greater than the rate at which the chyme
can be digested and absorbed in the small intestine.
Gastric
Factors That Promote Emptying
Effect
of Gastric Food Volume on Rate of Emptying
Increased food volume in the stomach promotes
increased emptying from the stomach. It is not increased storage pressure of
the food in the stomach that causes the increased emptying because, in the
usual normal range of volume, the increase in volume does not increase the
pressure much. However, stretching of the stomach wall does elicit local
myenteric reflexes in the wall that greatly accentuate activity of the pyloric
pump and at the same time inhibit the pylorus.
Effect
of the Hormone Gastrin on Stomach Emptying
Stomach wall stretch and the presence of certain
types of foods in the stomach particularly digestive products of meat elicit
release of the hormone gastrin from the antral mucosa. This has potent effects
to cause secretion of highly acidic gastric juice by the stomach glands.
Gastrin also has mild to moderate stimulatory effects on motor functions in the
body of the stomach. Most important, it seems to enhance the activity of the
pyloric pump. Thus, gastrin likely promotes stomach emptying.
Duodenal
Factors that Inhibit Stomach Emptying
Inhibitory
Effect of Enterogastric Nervous Reflexes from the Duodenum
When food enters the duodenum, multiple nervous
reflexes are initiated from the duodenal wall. They pass back to the stomach to
slow or even stop stomach emptying if the volume of chyme in the duodenum
becomes too much. These reflexes are mediated by three routes:
(1) Directly from the duodenum to the stomach
through the enteric nervous system in the gut wall
(2) Through extrinsic nerves that
go to the prevertebral sympathetic ganglia and then back through inhibitory
sympathetic nerve fibers to the stomach
(3) Probably to a slight extent through the
vagus nerves all the way to the brain stem, where they inhibit the normal
excitatory signals transmitted to the stomach through the vagi.
All these parallel reflexes have two effects on
stomach emptying-
A. they strongly inhibit the
pyloric pump propulsive contractions
B. they increase the tone of the
pyloric sphincter.
The types of factors that can initiate enterogastric
inhibitory reflexes include the following-
1. The degree of distention of the
duodenum
2. The presence of any degree of
irritation of the duodenal mucosa
3. The degree of acidity of the
duodenal chyme
4. The degree of osmolality of the chyme
5. The presence of certain
breakdown products in the chyme, especially breakdown products of proteins and,
perhaps to a lesser extent, of fats.
The enterogastric inhibitory reflexes are especially
sensitive to the presence of irritants and acids in the duodenal chyme, and
they often become strongly activated within as little as 30 seconds. For
instance, whenever the pH of the chyme in the duodenum falls below about 3.5 to
4, the reflexes frequently block further release of acidic stomach contents
into the duodenum until the duodenal chyme can be neutralized by pancreatic and
other secretions.
Breakdown products of protein digestion also elicit
inhibitory enterogastric reflexes; by slowing the rate of stomach emptying,
sufficient time is ensured for adequate protein digestion in the duodenum and
small intestine.
Finally, either hypotonic or hypertonic fluids
(especially hypertonic) elicit the inhibitory reflexes. Thus, too rapid flow of
non -isotonic fluids into the small intestine is prevented, thereby also
preventing rapid changes in electrolyte concentrations in the whole-body
extracellular fluid during absorption of the intestinal contents.
Hormonal
Feedback from the Duodenum Inhibits Gastric Emptying
Role
of Fats and the Hormone Cholecystokinin
Not only do
nervous reflexes from the duodenum to the stomach inhibit stomach emptying, but
hormones released from the upper intestine do so as well. The stimulus for
releasing these inhibitory hormones is mainly fats entering the duodenum,
although other types of foods can increase the hormones to a lesser degree.
On entering the duodenum, the fats extract several
different hormones from the duodenal and jejunal epithelium, either by binding
with “receptors” on the epithelial cells or in some other way. In turn, the
hormones are carried by way of the blood to the stomach, where they inhibit the
pyloric pump and at the same time increase the strength of contraction of the
pyloric sphincter.
These effects are important because fats are much
slower to be digested than most other foods. Precisely which hormones cause the
hormonal feedback inhibition of the stomach is not fully clear. The most potent
appears to be cholecystokinin (CCK), which is released from the mucosa of the
jejunum in response to fatty substances in the chyme. This hormone acts as an
inhibitor to block increased stomach motility caused by gastrin.
Other possible inhibitors of stomach emptying are
the hormones secretin and gastric inhibitory peptide (GIP), also called
glucose-dependent insulinotropic peptide. Secretin is released mainly from the
duodenal mucosa in response to gastric acid passed from the stomach through the
pylorus. GIP has a general but weak effect of decreasing gastrointestinal
motility.
GIP is released from the upper small intestine in
response mainly to fat in the chyme, but to a lesser extent to carbohydrates as
well. Although GIP inhibits gastric motility under some conditions, its main
effect at physiologic concentrations is probably mainly to stimulate secretion
of insulin by the pancreas.
Summary
of the Control of Stomach Emptying
Emptying of the stomach is controlled only to a
moderate degree by stomach factors such as the degree of filling in the stomach
and the excitatory effect of gastrin on stomach peristalsis. Probably the more
important control of stomach emptying resides in inhibitory feedback signals
from the duodenum, including both enterogastric inhibitory nervous feedback
reflexes and hormonal feedback by CCK.
The feedback inhibitory mechanisms work together to
slow the rate of emptying when-
A. Too much chyme is already in the
small intestine or
B. The chyme is excessively acidic,
contains too much unprocessed protein or fat, is hypotonic or hypertonic, or is
irritating.
In this way, the rate of stomach emptying is limited
to that amount of chyme that the small intestine can process.
Movements
of the Small Intestine
The movements of the small intestine, like those
elsewhere in the gastrointestinal tract, can be divided into following-
·
Mixing contractions
·
Propulsive contractions.
To a great extent, this separation is artificial
because essentially all movements of the small intestine cause at least some
degree of both mixing and propulsion. The detailed discussion of these
processes is the following-
Mixing
Contractions (Segmentation Contractions)
When a portion of the small intestine becomes distended
with chyme, stretching of the intestinal wall elicits localized concentric
contractions spaced at intervals along the intestine and lasting a fraction of
a minute. The contractions cause segmentation of the small intestine. These
divide the intestine into spaced segments. As one set of segmentation
contractions relaxes, a new set often begins, but the contractions this time
occur mainly at new points between the previous contractions.
Therefore, the segmentation contractions chop the
chyme two to three times per minute, in this way promoting progressive mixing
of the food with secretions of the small intestine.
The maximum frequency of the segmentation
contractions in the small intestine is determined by the frequency of
electrical slow waves in the intestinal wall, which is the basic electrical
rhythm.
As this frequency normally is not over 12 per minute
in the duodenum and proximal jejunum, the maximum frequency of the segmentation
contractions in these areas is also about 12 per minute, but this occurs only
under extreme conditions of stimulation. In the terminal ileum, the maximum
frequency is usually eight to nine contractions per minute.
The segmentation contractions become exceedingly
weak when the excitatory activity of the enteric nervous system is blocked by
the drug atropine. Therefore, even though it is the slow waves in the smooth
muscle that cause the segmentation contractions, these contractions are not
effective without background excitation mainly from the myenteric nerve plexus.
Propulsive
Movements
Peristalsis
in the Small Intestine
Chyme is propelled through the small intestine by
peristaltic waves. These can occur in any part of the small intestine, and they
move toward the anus at a velocity of 0.5 to 2.0 cm/ sec, faster in the proximal
intestine and slower in the terminal intestine.
They are normally weak and usually die out after
traveling only 3 to 5 centimeters, rarely farther than 10 centimeters, so
forward movement of the chyme is very slow, so slow that net movement along the
small intestine normally averages only 1 cm/min.
This means that 3 to 5 hours are required for
passage of chyme from the pylorus to the ileocecal valve.
Control
of Peristalsis
Peristaltic activity of the small intestine is
greatly increased after a meal. This is caused partly by the beginning entry of
chyme into the duodenum causing stretch of the duodenal wall. Also, peristaltic
activity is increased by the so-called gastro enteric reflex that is initiated
by distention of the stomach and conducted principally through the myenteric
plexus from the stomach down along the wall of the small intestine.
In addition to the nervous signals that may affect
small intestinal peristalsis, several hormonal factors also affect peristalsis.
They include gastrin, CCK, insulin, motilin, and serotonin, all of which
enhance intestinal motility and are secreted during various phases of food
processing. Conversely, secretin and glucagon inhibit small intestinal
motility.
The function of the peristaltic waves in the small
intestine is not only to cause progression of chyme toward the ileocecal valve
but also to spread out the chyme along the intestinal mucosa. As the chyme
enters the intestines from the stomach and elicits peristalsis, this
immediately spreads the chyme along the intestine; and this process intensifies
as additional chyme enters the duodenum.
On reaching the ileocecal valve, the chyme is
sometimes blocked for several hours until the person eats another meal; at that
time, a gastroileal reflex intensifies peristalsis in the ileum and forces the
remaining chyme through the ileocecal valve into the cecum of the large
intestine.
Propulsive
Effect of the Segmentation Movements
The segmentation movements, although lasting for
only a few seconds at a time, often also travel 1 centimeter or so in the anal
direction and during that time help propel the food down the intestine.
Peristaltic
Rush
Although peristalsis in the small intestine is
normally weak, intense irritation of the intestinal mucosa, as occurs in some
severe cases of infectious diarrhea, can cause both powerful and rapid
peristalsis, called the peristaltic rush. This is initiated partly by nervous
reflexes that involve the autonomic nervous system and brain stem and partly by
intrinsic enhancement of the myenteric plexus reflexes within the gut wall
itself.
The powerful peristaltic contractions travel long
distances in the small intestine within minutes, sweeping the contents of the
intestine into the colon and thereby relieving the small intestine of
irritative chyme and excessive distention.
The muscularis mucosae can cause short folds to
appear in the intestinal mucosa. In addition, individual fibers from this
muscle extend into the intestinal villi and cause them to contract
intermittently. The mucosal folds increase the surface area exposed to the chyme,
thereby increasing absorption. Also, contractions of the villi, shortening,
elongating, and shortening again milk the villi so that lymph flows freely from
the central lacteals of the villi into the lymphatic system. These mucosal and
villous contractions are initiated mainly by local nervous reflexes in the
submucosal nerve plexus that occur in response to chyme in the small intestine.
Function
of the Ileocecal Valve
A principal
function of the Ileocecal valve is to prevent backflow of fecal contents from
the colon into the small intestine. The Ileocecal valve itself protrudes into
the lumen of the cecum and therefore is forcefully closed when excess pressure
builds up in the cecum and tries to push cecal contents backward against the
valve lips. The valve usually can resist reverse pressure of at least 50 to 60
centimeters of water.
In addition, the wall of the ileum for several
centimeters immediately upstream from the Ileocecal valve has a thickened circular
muscle called the ileocecal sphincter.
This sphincter normally remains mildly constricted
and slows emptying of ileal contents into the cecum. However, immediately after
a meal, a gastroileal reflex intensifies peristalsis in the ileum, and emptying
of ileal contents into the cecum proceeds. Resistance to emptying at the Ileocecal
valve prolongs the stay of chyme in the ileum and thereby facilitates
absorption. Normally, only 1500 to 2000 milliliters of chyme empty into the
cecum each day.
Feedback Control of the Ileocecal Sphincter
The degree of contraction of the ileocecal sphincter
and the intensity of peristalsis in the terminal ileum are controlled
significantly by reflexes from the cecum. When the cecum is distended,
contraction of the ileocecal sphincter becomes intensified and ileal
peristalsis is inhibited, both of which greatly delay emptying of additional
chyme into the cecum from the ileum. Also, any irritant in the cecum delays
emptying. The reflexes from the cecum to the ileocecal sphincter and ileum are
mediated both by way of the myenteric plexus in the gut wall itself and of the
extrinsic autonomic nerves, especially by way of the prevertebral sympathetic
ganglia.
Movements
of the Colon
The principal functions of the colon are
A. Absorption of water and
electrolytes from the chyme to form solid feces
B. Storage of fecal matter until it
can be expelled.
The proximal half of the colon is concerned
principally with absorption, and the distal half with storage. Because intense
colon wall movements are not required for these functions, the movements of the
colon are normally sluggish. Yet in a sluggish manner, the movements still have
characteristics similar to those of the small intestine and can be divided once
again into mixing movements and propulsive movements.
Mixing
Movements
Haustrations
In the same manner that segmentation movements occur
in the small intestine, large circular constrictions occur in the large
intestine. At each of these constrictions, about 2.5 centimeters of the
circular muscle contract, sometimes constrict the lumen of the colon almost to
occlusion.
At the same time, the longitudinal muscle of the
colon, which is aggregated into three longitudinal strips called the teniae
coli, contracts. These combined contractions of the circular and longitudinal
strips of muscle cause the un stimulated portion of the large intestine to
bulge outward into baglike sacs called haustrations. Each haustration usually
reaches peak intensity in about 30 seconds and then disappears during the next
60 seconds.
They also at times move slowly toward the anus
during contraction, especially in the cecum and ascending colon, and thereby
provide a minor amount of forward propulsion of the colonic contents.
After another few minutes, new haustral contractions
occur in other areas nearby. Therefore, the fecal material in the large
intestine is slowly dug into and rolled over in much the same manner that one
spades the earth.
In this way, all the fecal material is gradually
exposed to the mucosal surface of the large intestine, and fluid and dissolved
substances are progressively absorbed until only 80 to 200 milliliters of feces
are expelled each day.
Mass
Movements
Much of the
propulsion in the cecum and ascending colon results from the slow but
persistent haustral contractions, requiring as many as 8 to 15 hours to move
the chyme from the ileocecal valve through the colon, while the chyme itself
becomes fecal in quality, a semisolid slush instead of semi fluid From the
cecum to the sigmoid, mass movements can, for many minutes at a time, take over
the propulsive role.
These movements usually occur only one to three
times each day, in many people especially for about 15 minutes during the first
hour after eating breakfast. A mass movement is a modified type of peristalsis
characterized by the following sequence of events:
First, a constrictive ring occurs in response to a
distended or irritated point in the colon, usually in the transverse colon.
Then, rapidly, the 20 or more centimeters of colon distal to the constrictive
ring lose their haustrations and instead contract as a unit, propelling the
fecal material in this segment en masse further down the colon. The contraction
develops progressively more force for about 30 seconds, and relaxation occurs
during the next 2 to 3 minutes.
Then, another mass movement occurs, this time
farther along the colon. A series of mass movements usually persists for 10 to
30 minutes. Then they cease but return a half day later. When they have forced
a mass of feces into the rectum, the desire for defecation is felt.
Appearance of mass movements after meals are facilitated
by gastrocolic and duodenocolic reflexes. These reflexes result from distention
of the stomach and duodenum. They occur either not at all or hardly at all when
the extrinsic autonomic nerves to the colon have been removed; therefore, the
reflexes almost certainly are transmitted by way of the autonomic nervous
system.
Irritation in the colon can also initiate intense
mass movements. For instance, a person who has an ulcerated condition of the
colon mucosa (ulcerative colitis) frequently has mass movements that persist
almost all the time.
Defecation
Most of the
time, the rectum is empty of feces. This results partly from the fact that a
weak functional sphincter exists about 20 centimeters from the anus at the
juncture between the sigmoid colon and the rectum. There is also a sharp
angulation here that contributes additional resistance to filling of the
rectum. When a mass movement forces feces into the rectum, the desire for
defecation occurs immediately, including reflex contraction of the rectum and
relaxation of the anal sphincters.
Continual dribble of fecal matter through the anus
is prevented by tonic constriction of the following.
A. An internal anal sphincter, a
several-centimeters-long thickening of the circular smooth muscle that lies immediately
inside the anus, and
B. An external anal sphincter
composed of striated voluntary muscle that both surrounds the internal
sphincter and extends distal to it.
The external sphincter is controlled by nerve fibers
in the pudendal nerve, which is part of the somatic nervous system and
therefore is under voluntary, conscious, or at least subconscious control;
subconsciously, the external sphincter is usually kept continuously constricted
unless conscious signals inhibit the constriction.
Defecation
Reflexes
Ordinarily, defecation is initiated by defecation
reflexes. One of these reflexes is an intrinsic reflex mediated by the local
enteric nervous system in the rectal wall. When feces enter the rectum,
distention of the rectal wall initiates afferent signals that spread through
the myenteric plexus to initiate peristaltic waves in the descending colon,
sigmoid, and rectum, forcing feces toward the anus.
As the peristaltic wave approaches the anus, the
internal anal sphincter is relaxed by inhibitory signals from the myenteric
plexus; if the external anal sphincter is also consciously, voluntarily relaxed
at the same time, defecation occurs.
The intrinsic myenteric defecation reflex
functioning by itself normally is relatively weak. When the nerve endings in
the rectum are stimulated, signals are transmitted first into the spinal cord
and then reflexly back to the descending colon, sigmoid, rectum, and anus by
way of parasympathetic nerve fibers in the pelvic nerves.
These parasympathetic signals greatly intensify the
peristaltic waves and relax the internal anal sphincter, thus converting the
intrinsic myenteric defecation reflex from a weak effort into a powerful
process of defecation that is sometimes effective in emptying the large bowel
all the way from the splenic flexure of the colon to the anus.
Defecation signals entering the spinal cord initiate
other effects, such as taking a deep breath, closure of the glottis, and
contraction of the abdominal wall muscles to force the fecal contents of the
colon downward and at the same time cause the pelvic floor to relax downward
and pull outward on the anal ring to evacuate the feces.
When it becomes convenient for the person to
defecate, the defecation reflexes can purposely be activated by taking a deep
breath to move the diaphragm downward and then contracting the abdominal
muscles to increase the pressure in the abdomen, thus forcing fecal contents
into the rectum to cause new reflexes.
Reflexes initiated in this way are almost never as
effective as those that arise naturally, for which reason people who too often
inhibit their natural reflexes are likely to become severely constipated.
In new born babies and in some people with
transected spinal cords, the defecation reflexes cause automatic emptying of
the lower bowel at inconvenient times during the day because of lack of
conscious control exercised through voluntary contraction or relaxation of the
external anal sphincter.
Other
Autonomic Reflexes That Affect Bowel Activity
Several other important nervous reflexes also can
affect the overall degree of bowel activity. They are the following-
·
Peritoneointestinal reflex
·
Renointestinal reflex
·
Vesicointestinal reflex
The peritoneointestinal reflex results from
irritation of the peritoneum; it strongly inhibits the excitatory enteric
nerves and thereby can cause intestinal paralysis, especially in patients with
peritonitis. The renointestinal and vesicointestinal reflexes inhibit
intestinal activity as a result of kidney or bladder irritation, respectively.
Throughout the gastrointestinal tract, secretory
glands perform two primary functions:
A. digestive enzymes are secreted
in most areas of the alimentary tract, from the mouth to the distal end of the
ileum.
B. mucous glands, from the mouth to
the anus, provide mucus for lubrication and protection of all parts of the
alimentary tract.
Most digestive secretions are formed only in
response to the presence of food in the alimentary tract, and the quantity
secreted in each segment of the tract is usually the precise amount needed for
proper digestion.
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