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Phys 63 - Propulsion and Mixing of Food in the Alimentary Tract
Monday, January 28, 2013
1:51 PM
Propulsion and Mixing of Food in the Alimentary Tract
 Ingestion of Food
o Amount of food eaten principally from intrinsic desire called hunger.
o Type of food sought after determined by appetite.
o Mastication
 Jaw muscles and teeth close with force about 55 pounds on incisors and 200 lbs on
molars.
 Most muscles innervated by motor branch of fifth cranial nerve and process
controlled by nuclei in brain stem.
 Much of chewing caused by chewing reflex
 Bolus of food initiates reflex inhibition of mastication muscles, allowing jaw to
drop
 Drop initiates stretch reflex so the jaw closes (rebound contraction).
 Chewing important to break up indigestible cellulose membrane of fruits and vegies.
 Also increases SA so enzymes can work. They only work on surfaces.
 Rate of digestion depends on total SA.
o Swallowing (Deglutition)
 Pharynx only converted into tract for food for a few seconds at a time.
 Swallowing divided into stages
 Voluntary stage: initiator
 When food is ready, it is squeezed or rolled back into pharynx by pressure
of the tongue upward and backward against the palate.
 Pharyngeal Stage
 Once in pharynx, epithelial swallowing receptor areas stimulated and
1. Soft palate pulled upward to close posterior nares
2. Palatopharyngeal folds on each side of pharynx approximate to
form a channel for food that has been masticated sufficiently.
(Keeps larger food back)
3. Vocal cords approximate (most essential) and larynx is pulled
upward and anterior.
a. Causes the epiglottis to swing back over the opening of the
larynx.
4. Upward movement of larynx also pulls up and enlarges opening of
esophagus.
a. Upper esophageal relaxes, allowing food to move into upper
esophagus
5. Entire muscular wall of pharynx contracts to push food down.
 Most sensitive area of posterior mouth and pharynx is around the
pharyngeal opening, especially on the tonsillar pillars.
 Trigeminal and glossopharyngeal sensory nerves transmit signal into
medulla oblongata and then to the tractus solitarius.
 Areas that control swallowing in medulla and lower pons are called
the deglutition or swallowing center.
 Principally a reflex.
Swallowing center inhibits respiration for a fraction of a second.
 Esophageal Stage
 Two types of peristaltic movements
 Primary
 Continuation of wave beginning in pharynx, assisted by gravity
when in upright position.
 Secondary
 Occurs when food is stuck and the esophagus is distended.
 Signals moved through myenteric plexus to vagal
afferent fibers to medulla and then back through
glossopharyngeal and vagal efferent nerve fibers.
 Big push to get food down.
 Muscle of pharyngeal wall and upper third of esophagus is striated and
controlled by glossopharyngeal and vagus nerves.
 Lower two thirds of esophagus is smooth muscle and controlled by the
vagus nerves.
 If the vagus is cut, the myenteric will adapt for a couple days to get
things moving well again.
 Receptive Relaxation of Stomach
 As peristaltic wave approaches, a wave of relaxation precedes it through
myenteric inhibition.
 The stomach and even duodenum relax and are prepared to receive the food
from esophagus.
 Lower Esophageal Sphincter (Gastroesophageal Sphincter)
 Normally remains tonic and closed.
 Receptive relaxation also occurs with food dropping down into the stomach.
 Achalasia: condition where the LES does not relax satisfactorily.
 Tonic constriction of LES helps to prevent reflux from stomach except under
abnormal conditions.
 Additional Prevention of Reflux
 Distal end of esophagus acts as a valve that closes with increased abdominal
pressure to keep stomach contents from re-entering the esophagus.
Motor Functions of the Stomach
o Three Functions
 Storage
 Normally, when food stretches the stomach, a "vagovagal reflex" from the brain
reduces the tone of the stomach body so the wall bulges outward to
accommodate more and more food.
 Mixing and Propulsion of Food in the Stomach - Basis of Electrical Rhythm
 Digestive juices secreted by gastric glands.
 As long as there is food in stomach, there are weak peristaltic constrictor waves,
called mixing waves that begin in the top of stomach and move down.
 Waves are initiated by Basic Electrical Rhythm
 As the waves progress to antrum of stomach they become more intense
and create peristaltic action potential-driven constrictor rings that force
the contents under higher and higher pressure toward the pylorus.
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o
The Pylorus only lets a small amount of food through at a time so
most of these rings push the food back UP into the body of
stomach.
 Hunger contractions occur when the stomach has been empty for several hours
or more and sometimes pain is felt called hunger pangs.
 Stomach Emptying
 Promoted by intense peristaltic contractions in the antrum of stomach.
 Peristaltic waves not only mix contents of the stomach but provide a pumping
action called the "pyloric pump" to pump food out into the duodenum.
 The pylorus remains slight contracted all the time, the circular muscle is called
the sphincter.
 Constriction usually prevents passage of food until it has become mixed in
the chyme to almost fluid consistency.
Regulation of Stomach Empyting
 Regulated by stomach and the duodenum.
 Gastric Factors that Promote Emptying
 Increased food volume promotes emptying from stomach.
 Stretching of the stomach wall elicits local myenteric reflexes in the wall
that greatly increase activity of the pyloric pump and at the same time
inhibit the pylorus.
 Gastrin Hormone
 Has stimulatory effects on motor functions of stomach, enhances the
pyloric pump, and promotes stomach emptying.
 Duodenal Factors that Inhibit Stomach Emptying
 When food enters the duodenum there are nervous reflexes mediate by
 Directly from duodenum to stomach through the enteric nervous system
 Through extrinsic nerves that go to prevertebral sympathetic ganglia and
then back through inhibitory sympathetic nerves to the stomach
3. Through the vagus nerves to the brain
 Factors monitored in the duodenum that can initiate inhibitory reflexes
 Degree of distention in duodenum
 Presence of irritation of duodenal mucosa
 Degree of acidity of duodenal chyme
 When pH drops below 3.5-4, the reflex blocks further release of
acidic stomach contents.
 Degree of osmolality of duodenal chyme
 Presence of certain breakdown products in the chyme
 Hormonal Feedback with Inhibition
 Stimulus for releasing these hormones is mainly fats entering the
duodenum.
 Hormones are carried by blood to the stomach where they inhibit the
pyloric pump and increase the strength of contraction by the pylorus.
 Most potent appears to be cholecystokinin (CCK), which is released from
the mucosa of the jejunum in response to fatty substances in the chyme.
 Secretin is released from duodenal mucosa in response to gastric acid.
 Gastric Inhibitory Peptide (GIP) decreases GI motility and is released from
upper GI in response to fat.
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Feedback inhibitory mechanisms work together to slow the rate of emptying
when too much chyme is already in the small intestine or when the chyme is
excessively acidic.
Movements of the Small Intestine
o Mixing Contractions (Segmentation Contractions)
 When the small intestine has food in it or is distended, local areas begin to contract
reflexively.
 These fractions are spaced out and last only a fraction of a minute.
 The contractions are evenly spaced. Each contraction is in a different spot so the
chyme gets chopped.
 Frequency is determined by electrical slow waves in the intestinal wall. Max is about
12 per minute.
 Segmentation becomes very weak when the ENS is blocked by the drug Atropine.
 Does have SMALL propulsion affects.
o Propulsive Movements
 Peristalsis
 Chyme is propelled by peristalsis, but this is very slow. 3-5 hours to move from
pylorus to ileocecal valve.
 Control
 Peristalsis greatly increased after a meal but is also controlled by stretch of the
duodenal wall that initiates the Gastroenteric Reflex which is conducted by the
myenteric plexus.
 Gastrin, CCK, Insulin, Motilin, and serotonin all increase peristalsis.
 Secretin and Glucagon inhibit motility.
 Peristaltic waves are supposed to propel the food but also spread it out.
 On reaching the ileocecal valve, the chyme is sometimes blocked for several
hours or until the next meal.
 At this time, the Gastroileal Reflex moves the chyme into the cecum.
 Peristaltic Rush
 Infectious diarrhea causes powerful and rapid peristalsis, called Peristaltic Rush.
 This sweeps the small intestine within minutes to relieve irritative chyme and
excessive distention.
 Mucosal Folds increase the SA exposed to chyme to increase absorption.
 Contraction of the villi "milk" the villi so that lymph flows freely from the central
lacteals of the villi into the lymphatic system.
o Function of the Ileocecal Valve
 Prevents backflow of fecal contents from colon into the small intestine.
 The Ileocecal sphincter normally remains mildly constricted and slows the empyting of
ileal contents into the cecum.
 Immediately after a meal the gastroileal reflex intensifies peristalsis and empties
ileal contents into the cecum.
 Degree of contraction of the ileocecal sphincter and the intensity of peristalsis in the
terminal ileum are controlled by reflexes in the cecum.
 When the cecum is full, the sphincter shuts tighter.
 Reflexes from the cecum to the ileocecal sphincter are mediated by the myenteric
plexus and the ENS.
Movements of the Colon
o

Principle functions of the colon are absorption of water and electrolytes and storage of fecal
matter.
o Proximal half is for absorption and the other half is for storage.
o Mixing Movments - Haustrations
 Just like the SI, there are circular contractions every so often that contract along with
the longitudinal muscle strips called teniae coli.
 This causes bulges outward into baglike sacs called haustrations.
 The fecal matter is slowly dug into and rolled over in the same way you turn over dirt
with a shovel.
 This allows greater absorption.
o Propulsive Movements - Mass Movements
 Mass Movements can take over propulsion for about 15 minutes, three times a day,
usually in the first hour after eating.
 Modified peristalsis
 A constrictor ring develops due to distention or irritation in the colon.
 Rapidly, the downstream colon lose their haustrations and contract as a unit to
propel the fecal mass down farther.
3. Contraction develops progressively and then rests.
 When feces have been forced into the rectum, the urge to defecate is felt.
 Initiation
 Mass movements facilitated by gastrocolic and duodenolic reflexes that result
from distention of the stomach and duodenum.
 Reflexes transmitted by ANS.
o Defecation
 When Mass Movements are forced into the rectum, the urge to defecate happens
immediately, including anal contraction of the rectum and relaxation of the anal
sphincters.
 The internal and external anal sphincters prevent dribble of feces through the anus.
 EAS is controlled by pudendal nerve, which is under somatic nervous system and
has voluntary, conscious (subconscious) control.
 Defecation Reflex
 Intrinsic Reflex
 Feces enters and distends the rectum, stimulating myenteric plexus to
initiate persistaltis waves in the colon.
 As the waves approach the anus, the internal anal sphincter is relaxed by
myenteric plexus.
 If the EAS is also relaxed voluntarily, defecation occurs.
 The intrinsic reflex is weak and needs to be fortified by a parasympathetic
defecation reflex.
 These greatly intensify the peristaltic waves as well as relax the IAS.
 The addition of the parasympathetics makes the intrinsic reflex strong
enough to empty from the splenic flexure to the anus.
 Defecation signals entering the spinal cord also initiate taking a deep breath, closure
of the glottis, and contraction of the abdominal wall to force feces down the colon.
Other Autonomic Reflexes
o Peritoneaointestinal Reflex
 Result from irritation of peritoneum and strongly inhibits the excitatory enteric nerves
causing intestinal paralysis.
o
Renointestinal and vesicointestinal reflexes inhibit intestinal activity as a result of kidney or
bladder irritation.