Phys Ch 63
Ingestion of Food
 Amount of food a person ingests determined principally by intrinsic desire for food (hunger)
 Type of food a person preferentially seeks determined by appetite
 Most of the muscles of chewing innervated by CN V, and chewing process controlled by nuclei in brain stem
o Stimulation of specific reticular areas in brain stem taste centers will cause rhythmical chewing
movements
o Stimulation of areas in hypothalamus, amygdala, and cerebral cortex near sensory areas for taste and
smell can often cause chewing
 Much of chewing process caused by chewing reflex; presence of bolus of food in mouth first initiates reflex
inhibition of muscles of mastication, which allows lower jaw to drop
o Drop initiates stretch reflex of jaw muscles that leads to rebound contraction, raising jaw to cause
closure of teeth, which compresses the bolus, which inhibits the jaw muscles again, etc.
 Chewing especially important for fruits and raw vegetables because they have indigestible cellulose membranes
around their nutrient portions that must be broken before food can be digested
 Chewing aids digestion of food because digestive enzymes act only on surfaces of food particles, so rate of
digestion dependent on total surface area exposed to digestive secretions
o Grinding food to fine particulate consistency prevents excoriation of GI tract and increases ease with
which food emptied from stomach into small intestine, then into all succeeding segments of gut
 Swallowing – divided into voluntary stage (initiates swallowing process), pharyngeal stage (involuntary;
constitutes passage of food through pharynx into esophagus), and esophageal stage (involuntary; transports
food from pharynx to stomach)
o Voluntary stage – food voluntarily squeezed or rolled posteriorly into pharynx by pressure of tongue
upward and backward against palate
o Pharyngeal stage – as bolus enters posterior mouth and pharynx, it stimulates epithelial swallowing
receptor areas all around opening of pharynx, especially on tonsillar pillars, and impulses from these
pass to brain stem to initiate automatic pharyngeal muscle contractions
 Soft palate pulled upward to close posterior nares
 Palatopharyngeal folds on each side of pharynx pulled medially to approximate each other; folds
form sagittal slit through which food must pass into posterior pharynx, allowing food that has
been masticated sufficiently to pass with ease and impeding large objects
 Vocal cords of larynx strongly approximated and larynx pulled upward and anteriorly by neck
muscles; because of this motion and ligaments that prevent upward movement of epiglottis,
epiglottis swings backward over opening of larynx
 Destruction of vocal cords or muscles that approximate them can cause strangulation
 Upward movement of larynx also pulls up and enlarges opening to esophagus
 Upper 3-4 cm of esophageal muscular wall (upper esophageal sphincter or
pharyngoesophageal spincter) relaxes, so food moves easily and freely from posterior
pharynx into upper esophagus; between swallows, sphincter remains strongly
contracted, preventing air from going into esophagus during respiration
 Upward movement of larynx also lifts glottis out of main stream of food flow, so food
mainly passes on each side of epiglottis rather than over its surface
 Entire muscular wall of pharynx contracts, from superior to inferior
o Esophageal stage – exhibits primary and secondary peristalsis
 Primary peristalsis – continuation of peristaltic wave that begins in pharynx and spreads into
esophagus during pharyngeal stage of swallowing; wave passes from pharynx to stomach in 8-10
sec (person in upright position has gravity aiding so it takes shorter time)
 Secondary peristalsis – if primary peristaltic wave fails to move into stomach all the food that
entered the esophagus, secondary peristalsis waves result from distention of esophagus
 Initiated partly by intrinsic neural circuits in myenteric nervous system and partly by
reflexes that begin in pharynx and are transmitted upward through vagal afferent fibers
to medulla and back again to esophagus through glossopharyngeal and vagal efferent
nerve fibers
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Most sensitive tactile areas of posterior mouth and pharynx for initiating pharyngeal stage of swallowing lie in
ring around pharyngeal opening, with greatest sensitivity on tonsillar pillars
o Impulses transmitted from these areas through sensory portions of trigeminal and glossopharyngeal
nerves into medulla oblongata, either into or closely associated with tractus solitarius, which receives
essentially all sensory impulses from mouth
o Successive stages of swallowing process automatically initiated in orderly sequence by neuronal areas of
reticular substance of medulla and lower portion of pons
o Sequence of swallowing reflex and timing of entire cycle remain exactly the same
o Areas in medulla and lower pons that control swallowing collectively called deglutition center
(swallowing center)
 Motor impulses from swallowing center to pharynx and upper esophagus that cause swallowing transmitted
successively by CN V, CN IX, CN X, and CN XII with a few of superior cervical nerves
 Swallowing center specifically inhibits respiratory center of medulla during entire pharyngeal stage of
swallowing, halting respiration at any point in its cycle to allow swallowing to proceed
 Musculature of pharyngeal wall and upper 1/3 of esophagus is striated muscle, so peristaltic waves in these
regions controlled by skeletal nerve impulses from glossopharyngeal and vagal nerves
o Lower 2/3 of esophagus is smooth muscle, strongly controlled by vagus nerves acting through
connections with esophageal myenteric nervous system
o When vagus nerves to esophagus cut, myenteric nerve plexus of esophagus becomes excitable enough
after several days to cause strong secondary peristaltic waves even without support from vagal reflexes
 Reason that even when brain stem swallowing reflex paralyzed, food fed by tube or in some
other way into esophagus still passes readily into stomach
 When esophageal peristaltic wave approaches toward stomach, wave of relaxation, transmitted through
myenteric inhibitory neurons, precedes peristalsis
o Entire stomach, and to lesser extent duodenum, become relaxed as wave reaches lower end of
esophagus and thus are prepared to receive food propelled into esophagus during swallowing act
 At lower end of esophagus, extending upward 3 cm above juncture with stomach, esophageal circular muscle
functions as broad lower esophageal sphincter (gastroesophageal sphincter); normally remains tonically
constricted; when peristaltic swallowing wave passes down esophagus, there is receptive relaxation of lower
esophageal sphincter ahead of peristaltic wave
o Rarely, sphincter doesn’t relax satisfactorily (achalasia)
 Esophageal mucosa, except in lower 1/8 of esophagus, not capable of resisting digestive action of gastric
secretions for long; tonic constriction of lower esophageal sphincter helps prevent significant reflux
 Valve-like mechanism of short portion of esophagus that extends slightly into stomach – increased intraabdominal pressure caves esophagus inward at this point
o Valve-like closure of lower esophagus helps prevent high intra-abdominal pressure from forcing stomach
contents backward into esophagus
Motor Functions of Stomach
 Motor functions of stomach include
o Storage of large quantities of food until food can be processed in stomach, duodenum, and lower
intestinal tract
o Mixing of food with gastric secretions until it forms semifluid mixture (chyme)
o Slow emptying of chyme into small intestine at rate suitable for proper digestion and absorption
 Antrum of stomach is waiting area to leave it
 As food enters stomach, the stomach forms concentric circles of the food in the orad portion of the stomach,
newest food lying closest to esophageal opening and oldest food lying nearest outer wall of stomach
o Normally, when food stretches stomach, vagovagal reflex from stomach to brain stem and then back to
stomach reduces tone in muscular wall of body of stomach so that wall bulges progressively outward,
accommodating greater and greater quantities of food; pressure in stomach remains low until physical
limit approached
 Digestive juices of stomach secreted by gastric glands in wall of body of stomach (except along narrow strip on
lesser curvature of stomach); secretions come immediately into contact with portion of stored food lying against
mucosal surface of stomach
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As long as food is in stomach, weak peristaltic constrictor waves (mixing waves) begin in mid to upper
portions of stomach wall and move toward antrum once every 15-20 sec; initiated by gut wall basic
electrical rhythm, consisting of electrical slow waves that occur spontaneously in stomach wall
o As constrictor waves progress from body of stomach into antrum, they become more intense, some
becoming extremely intense and providing powerful peristaltic AP-driven constrictor rings that force
antral contents under higher and higher pressure toward pylorus
o Each time peristaltic wave passes down antral wall toward pylorus, it digs deeply into food contents in
antrum; opening of pylorus is still small enough that only a few mL or less of antral contents expelled
into duodenum with each peristaltic wave
 As each peristaltic wave approaches pylorus, pyloric muscle often contracts, further impeding
emptying through pylorus, so most of antral contents squeezed upstream through peristaltic
ring toward body of stomach, not through pylorus
 Moving peristaltic constrictive ring plus upstream squeezing action called retropulsion
Hunger contractions – often occurs when stomach has been empty for several hours or more; rhythmical
peristaltic contractions in body of stomach; when successive contractions become extremely strong, they often
fuse to cause continuing tetanic contraction that sometimes lasts 2-3 minutes
o Most intense in young, healthy people who have high degrees of GI tonus; greatly increased by persons
with lower than normal levels of blood sugar
o Not usually felt until 12-24 hours after last ingestion of food
o In starvation, they reach their greatest intensity in 3-4 days and gradually weaken in succeeding days
Stomach emptying promoted by intense peristaltic contractions in stomach antrum; at same time, emptying
opposed by varying degrees of resistance to passage of chyme at pylorus
o Most of time, rhythmical stomach contractions weak and function mainly to cause mixing of food and
gastric secretions
o About 20% of time while food in stomach, contractions become intense, beginning in mid-stomach and
spreading through caudad stomach; contractions strong peristaltic, very tight, ring-like constrictions that
can cause stomach emptying
o As stomach becomes progressively more and more empty, constrictions begin farther and farther up
body of stomach, gradually pinching off food in body of stomach and adding this food to chyme in
antrum
o When pyloric tone normal, each strong peristaltic wave forces up to several mL of chyme into
duodenum, so these peristaltic waves cause mixing in stomach as well as pyloric pump action
o Pylorus circular wall muscle thickness is greater than that of antrum and remains slightly contracted at
all times (pyloric sphincter)
o Pyloric sphincter usually open enough for water and other fluids to empty form stomach into duodenum
with ease; constriction usually prevents food from passing until it has become mixed in chyme to almost
fluid consistency
o Degree of constriction of pylorus increases or decreases under influence of nervous and humoral reflex
signals from both stomach and duodenum
Duodenum provides by far the more potent signals, controlling emptying of chyme into duodenum at rate no
greater than rate at which chyme can be digested and absorbed in small intestine
Increased food volume in stomach promotes increased emptying from stomach; stretching of stomach wall
elicits local myenteric reflexes in wall that greatly accentuate activity of pyloric pump and inhibit pylorus
o Presence of certain types of foods in stomach (particularly presence of certain types of foods like meat)
elicit release of hormone gastrin from antral mucosa; gastrin has potent effects to cause secretion of
highly acidic gastric juice by stomach glands and has mild to moderate stimulatory effects on motor
functions in body of stomach; gastrin enhances activity of pyloric pump
When food enters duodenum, multiple nervous reflexes initiated from duodenal wall, pass back to stomach to
slow or stop stomach emptying if volume of chyme in duodenum becomes too much
o Reflexes mediated
 Directly from duodenum to stomach through enteric nervous system in gut wall
 Through extrinsic nerves that go to prevertebral sympathetic ganglia and then back through
inhibitory SNS nerve fibers to stomach

Through vagus nerves all the way to brain stem, where they inhibit normal excitatory signals
transmitted to stomach through vagi
o All parallel reflexes strongly inhibit pyloric pump propulsive contractions and increase tone of pyloric
sphincter
o Types of factors that are continually monitored in duodenum and can initiate enterogastric inhibitory
reflexes include
 Degree of distention of duodenum
 Presence of any degree of irritation of duodenal mucosa
 Degree of acidity of duodenal chyme
 Degree of osmolality of chyme
 Presence of certain breakdown products in chyme, especially breakdown products of proteins
and to a lesser extent fats
o Enterogastric inhibitory reflexes especially sensitive to presence of irritants and acids in duodenal chyme
 Whenever pH of chyme in duodenum falls below 3.5-4, reflexes block further release of acidic
stomach contents into duodenum until duodenal chyme can be neutralized by secretions
o Breakdown products of protein digestion elicit inhibitory enterogastric reflexes so sufficient time
ensured for adequate protein digestion in duodenum and small intestine
o Either hypotonic or hypertonic fluids elicit inhibitory reflexes, so too rapid flow of nonisotonic fluids into
small intestine prevented, thereby preventing rapid changes in electrolyte concentrations in whole-body
extracellular fluid during absorption of intestinal contents
 Stimulus for releasing inhibitory hormones from small intestine is mainly fats entering duodenum
o On entering duodenum, fats extract several different hormones from duodenal and jejunal epithelium,
either by binding with receptors on epithelial cells or in some other way
o Hormones carried by way of blood to stomach, where they inhibit pyloric pump and at same time
increase strength of contraction of pyloric sphincter
o Most potent hormone is CCK (released from mucosa of jejunum in response to fatty substances in
chyme; hormone acts as inhibitor to block increased stomach motility caused by gastrin
o Gastric inhibitory peptide (GIP) – has general but weak effect of decreasing GI motility; released from
upper small intestine in response mainly to fat in chyme, but to a lesser extent carbs as well; main effect
at physiologic concentrations is to stimulate secretion of insulin by pancreas
o Secretin – released mainly from duodenal mucosa in response to gastric acid passed from stomach
through pylorus
Movements of Small Intestine
 Essentially all movements of small intestine cause at least some degree of both mixing and propulsion
 When portion of small intestine becomes distended with chyme, stretching of intestinal wall elicits localized
concentric contractions spaced at intervals along intestine and lasting fraction of a minute
o Contractions cause segmentation of small intestine
o As one set of segmentation contractions relaxes, a new set often begins, but contractions occur mainly
at new points between previous contractions, chopping the chyme 2-3x per minute, promoting
progressive mixing of food with secretions of small intestine
o Max frequency of segmentation contractions determined by frequency of electrical slow waves in
intestinal wall
o Segmentation contractions become exceedingly weak when excitatory activity of enteric nervous system
blocked by atropine
o Even though slow waves in smooth muscle cause segmentation contractions, they are not effective
without background excitation mainly from myenteric nerve plexus
o Move toward anus (about 1 cm)
 Chyme propelled through small intestine by peristaltic waves; can occur in any part of small intestine, and move
toward anus faster in proximal intestine and slower in terminal intestine
o Normally weak and die out after traveling a short distance
o Activity greatly increased after meal, partly by beginning entry of chyme into duodenum causing stretch
of duodenal wall
o
Activity increased by gastroenteric reflex initiated by distention of stomach and conducted principally
through myenteric plexus from stomach down along wall of small intestine
o Gastrin, CCK, insulin, motilin, and serotonin all enhance intestinal motility and are secreted during
various phases of food processing
o Secretin and glucagon inhibit small intestine motility
o Peristaltic waves function to spread out chyme along intestinal mucosa
o On reaching ileocecal valve, chyme sometimes blocked for several hours until person eats another meal;
at that time gastroileal reflex intensifies peristalsis in ileum and forces remaining chyme through
ileocecal valve into cecum of large intestine
 Intense irritation of intestinal mucosa (as occurs in some severe cases of infectious diarrhea) can cause both
powerful and rapid peristalsis (peristaltic rush); initiated partly by nervous reflexes that involve ANS and brain
stem and partly by intrinsic enhancement of myenteric plexus reflexes within gut wall itself
o Powerful peristaltic contractions travel long distances in small intestine within minutes, sweeping
contents of intestine into colon and thereby relieving small intestine of irritating chyme and excessive
distention
 Muscularis mucosae – cause short folds to appear in intestinal mucosa; individual fibers from muscularis extend
into intestinal villi and cause them to contract intermittently
o Mucosal folds increase surface area exposed to chyme, thereby increasing absorption
o Contractions of villi (shortening, elongating, and shortening again), milk the villi so lymph flows freely
from central lacteals of villi into lymphatic system
o Mucosal and villous contractions initiated mainly by local nervous reflexes in submucosal nerve plexus
that occur in response to chyme in small intestine
 Principal function of ileocecal valve is to prevent backflow of fecal contents from colon into small intestine
o Ileocecal valve protrudes into lumen of cecum and therefore is forcefully closed when excess pressure
builds up in cecum and tries to push cecal contents backward against valve lips
o Wall of ileum for several cm immediately upstream from ileocecal valve has thickened circular muscle
(ileocecal sphincter) that normally remains mildly constricted and slows emptying of ileal contents into
cecum; immediately after meal, gastroileal reflex intensifies peristalsis in ileum, and emptying of ileal
contents into cecum proceeds
 Degree of contraction of ileocecal sphincter and intensity of peristalsis in terminal ileum controlled significantly
by reflexes from cecum
o When cecum distended, contraction of ileocecal sphincter becomes intensified and ileal peristalsis
inhibited
o Any irritant in cecum delays emptying (i.e., inflamed appendix)
o Reflexes from cecum to ileocecal sphincter and ileum mediated by way of myenteric plexus in gut wall
itself and extrinsic autonomic nerves, especially by way of prevertebral sympathetic ganglia
Movements of Colon
 Proximal half of colon concerned principally with absorption of water and electrolytes, and distal half with
storage of fecal matter until it can be expelled
 Movements of colon normally sluggish
 Haustrations – mixing movements; large circular constrictions in large intestine; large circular constrictions
occur, and longitudinal muscle of colon (aggregated into 3 longitudinal strips called teniae coli) contracts
o Combined contractions of circular and longitudinal strips of muscle cause unstimulated portion of large
intestine to bulge outward into baglike sacs (haustrations)
o Can at times move slowly toward anus, especially in cecum and ascending colon
o Fecal material in large intestine slowly dug into and rolled over in much same manner as one spades
earth, so all fecal material gradually exposed to mucosal surface of large intestine, and fluid and
dissolved substances progressively absorbed
o Provides much of propulsion in cecum and ascending colon
 Mass movements – propulsive movements; mainly from cecum to sigmoid colon; occur usually only 1-3x per
day, usually during first hour right after eating breakfast
o Constrictive ring occurs in response to distended or irritated point in colon, usually in transverse colon
o 20 or more cm of colon distal to constrictive ring lose haustrations and instead contract as a unit
o
Contraction develops progressively more force for about 30 seconds, and relaxation occurs during next
2-3 minutes; then another mass movement occurs, this time farther along the colon
o Series of mass movements usually persists for 10-30 minutes, then they cease but return half day later
o When they have forced mass of feces into rectum, desire for defecation felt
 Appearance of mass movements after meals facilitated by gastrocolic and duodenocolic reflexes, resulting from
distention of stomach and duodenum
o Need extrinsic autonomic nerves to work
o Irritation of colon can initiate intense mass movements (ulcerative colitis) frequently have mass
movements that persist almost all the time
 Most of the time, rectum is empty of feces, partly because weak functional sphincter exists about 20 cm from
anus at juncture between sigmoid colon and rectum
o Sharp angulation at sphincter that contributes additional resistance to filling of rectum
o When mass movement forces feces into rectum, desire for defecation occurs immediately, including
reflex contraction of rectum and relaxation of anal sphincters
o Continual dribble of fecal matter through anus prevented by tonic constriction of internal anal sphincter
(circular smooth muscle that lies immediately inside anus) and external anal sphincter (composed of
striated voluntary muscle that surrounds internal sphincter and extends distal to it)
o External sphincter controlled by nerve fibers in pudendal nerve
o Subconsciously, external sphincter usually kept continuously constricted unless conscious signals inhibit
constriction
 Defecation initiated by defecation reflexes
o Intrinsic reflex – mediated by local enteric nervous system in rectal wall
 When feces enter rectum, distention of rectal wall initiates afferent signals that spread through
myenteric plexus to initiate peristaltic waves in descending colon, sigmoid, and rectum, forcing
feces toward anus
 As peristaltic wave approaches anus, internal anal sphincter relaxed by inhibitory signals from
myenteric plexus
 If external anal sphincter consciously, voluntarily relaxed at same time, defecation occurs
 Relatively weak reflex
o Parasympathetic defecation reflex – involves sacral segments of spinal cord
 When nerve endgins in rectum stimulated, signals transmitted into spinal cord and then reflexly
back to descending colon, sigmoid, rectum, and anus by way of PNS fibers in pelvic nerves
 PNS signals greatly intensify peristaltic waves and relax internal anal sphincter, converting
intrinsic myenteric defecation reflex from weak effort into powerful process of defecation
sometimes effective in emptying large bowel all the way from splenic flexure to anus
o Defecation signals entering spinal cord initiate other effects, such as taking deep breath, closure of
glottis, and contraction of abdominal wall muscles to force fecal contents of colon downward and at
same time, cause pelvic floor to relax downward and pull outward on anal ring to evaginate feces
 When it becomes convenient for person to defecate, defecation reflexes can purposely be activated by taking a
deep breath to move diaphragm downward and then contracting abdominal muscles to increase pressure in
abdomen, forcing fecal contents into rectum to cause new reflexes
o Reflexes initiated in this way almost never as effective as those that arise naturally, which is why people
who too often inhibit their natural reflexes likely to become severely constipated
Other Autonomic Reflexes that Affect Bowel Activity
 Peritoneointestinal reflex – results from irritation of peritoneum; strongly inhibits excitatory enteric nerves and
can cause intestinal paralysis, especially in patients with peritonitis
 Renointestinal and vesicointestinal reflexes inhibit intestinal activity as result of kidney or bladder irritation