Physiology Ch 63 p763-772
Propulsion and Mixing of Food in the Alimentary Tract
-Amount of food a person ingests is due to intrinsic desire for food called hunger. Type of food a person
seeks is called appetite
Mastication – teeth are designed for chewing
-incisors for cutting and molars for grinding
-can close teeth with a force as great as 55 pounds on incisors and 200 pounds on molars
-most muscles innervated by motor branch of trigeminal nerve, and controlled by brainstem for
rhythmic chewing movements
-Much of the process is caused by a chewing reflex – presence of bolus in mouth initiates reflex
inhibition of muscles of mastication which allows jaw to drop. Jaw drop initiates a stretch reflex which
leads to rebound contraction
-compresses bolus of food in linings of mouth which inhibits jaw muscles again, repeating
process
-Chewing is important for all foods, but especially for fruits and veggies with indigestible cellulose
membranes that must be broken before digested
-digestive enzymes also only act on surface of foot particles
Swallowing (deglutition) – complicated process because pharynx subserves respiration and swallowing
-pharynx converts for only a few seconds into a tract for food propulsion. Usually for respiration
-swallowing can be divided into voluntary stage, a pharyngeal stage, which is involuntary and constitutes
passage of food through pharynx into esophagus, and an esophageal stage that is involuntary and
transports food from esophagus into stomach
1. Voluntary stage of swallowing – when food is ready to be swallowed, it is voluntarily pushed into the
pharynx by the tongue upward and backward against the palate, becoming automatic afterwards
2. Pharyngeal Stage of swallowing – bolus entering the pharynx stimulates epithelial swallowing
receptor areas around the pharynx (tonsillar pillars), and impulses from these pass to brainstem to
initiate a series of contractions as follows
A. soft palate is pulled up to close the posterior nares to prevent reflux into nose
B. palatopharyngeal folds pulled medially to form a sagittal slit through which food passes
C. Vocal cords are approximated and larynx is pulled upward and anteriorly, causes epiglottis to
swing backward over opening of larynx, preventing food from entering trachea
D. Movement of larynx also pulls up the opening to esophagus, upper esophageal sphincter
relaxes
-between swallows, sphincter is contracted to prevent air in esophagus
E. entire muscular wall of pharynx contracts which propels food by peristalsis
Nervous Initiation of Pharyngeal Stage of Swallowing – impulses travel to the sensitive areas of
posterior mouth and tonsillar pillars through the trigeminal and glossopharyngeal nerves into medulla
and into the tractus solitarius which receives all senses from mouth
-successive stages controlled by automatic innervation by medulla and pons, areas of
swallowing are called deglutition or swallowing center
-motor impulses from the swallowing center to the pharynx and upper esophagus that cause swallowing
travel through 5th, 9th, 10th, and 12th cranial nerves
-pharyngeal swallowing is a reflex act initiated by voluntary movement of food into back of mouth which
excites involuntary pharyngeal sensory receptors to elicit swallowing reflex
Effect of Pharyngeal Stage of Swallowing on Respiration – Entire stage occurs in less than 6 seconds,
interrupting respiration for only a fraction of its cycle
-swallowing center inhibits respiratory center of medulla during this time
Esophageal Stage of Swallowing – esophagus conducts food rapidly from pharynx into the stomach,
exhibiting both primary and secondary peristalsis
1. Primary Peristalsis – continuation of peristaltic wave that begins in the pharynx and spreads
into the esophagus during pharyngeal stage of swallowing in about 8-10 seconds
2. Secondary Peristalsis – retained food after primary peristalsis causes distention and initiates a
secondary peristaltic waves through intrinsic neural circuits in myenteric nervous system
a. also partly through pharyngeal reflexes traveling through vagal afferents to medulla and
back through glossopharyngeal and vagal efferent fibers
-upper third of esophagus is striated muscle controlled by CN IX and X. Lower 2/3rds of esophagus are
smooth muscle controlled by CN X through connections with esophageal myenteric nervous system
-if vagus nerves to esophagus are cut, myenteric plexus becomes excitable enough after several
days to cause strong secondary peristaltic waves even without vagal support
Receptive Relaxation of Stomach – myenteric inhibitory neurons cause a wave of relaxation to precede
the peristalsis. Entire stomach and duodenum become relaxed
Lower Esophageal Sphincter – esophageal circular muscle 3cm above junction with stomach functions
as a sphincter, which is normally constricted
-when receptive relaxation occurs ahead of the peristaltic wave, food is propulsed easily into the
stomach.
Achalasia- sphincter does not relax properly
-stomach secretions are acidic and can damage the esophagus, which is resolved by tonic constriction of
the sphincter
-there exists a valvelike mechanism of a short part of esophagus that extends into the stomach to
prevent esophageal reflux
-increased abdominal pressure caves esophagus inward which causes valvelike closure of lower
esophagus to prevent stomach contents moving backward into esophagus
Motor Functions of the Stomach – three functions aer storage of large quantities of food, mixing of food
with gastric secretions until it forms chime, and slow emptying of chyme from stomach into small
intestine at a rate suitable for digestion and absorption
1. Storage Function of Stomach – food entering stomach forms concentric circles of food in orad
portion of stomach, newest food closest to esophageal opening
a. Vagovagal reflex from stomach to brain stem and back reduces tone in muscular wall of
stomach so that the wall bulges outward, accommodating greater and greater
quantities of food up to a limit
2. Mixing and Propulsion of Food in Stomach – gastric glands secrete digestive juices along entire
wall of body of stomach
-weak peristaltic constrictor waves called mixing waves begin in mid to uppor portions
of stomach and move to antrum.
-initiated by basic electrical rhythm from the gut wall, (slow waves occurring
spontaneously in stomach
-constrictor waves of stomach become more intense as food reaches antrum, providing
powerful peristaltic action potential-driven constrictor rings, forcing antral contents
toward pylorus
-constrictor rings also mix stomach contents by digging deeply into foot in antrum, since
pyloric opening is still small, it churns the food and only a few particles get through
-pyloric muscle contracts too, further impeding flow into duodenum
(retropulsion)
Chyme – after food has been mixed in stomach, the resulting mixture passes into duodenum as chyme.
Hunger Contractions – besides peristalsis, contractions known as hunger contractions occur when
stomach has been empty for hours
-Contractions of the body itself, can cause continuous ‘tetanic contraction’ lasting 2-3 minutes
-Most intense in young, healthy people with high GI tonicity and people with high blood sugar
-mild pain in pit of stomach, called Hunger Pangs – which do not begin until 12-24 hours
of last ingestion of food
Stomach Emptying – promoted by intense peristaltic contractions in the stomach antrum
-opposed by varying degrees of resistance to passage of chyme at pylorus
Pyloric Pump – stomach contractions usually weak and act just to mix food together with gastric
secretions
-20% of the time the contractions become strong, beginning in midstomach and spreading to lower part
-ring-like and tight, can cause stomach emptying
-as stomach becomes more empty, these contractions move higher and higher up, pushing food
to the chyme in the antrum
-when pyloric tone is normal, each strong peristaltic force moves several mL of chyme into duodenum,
called the pyloric pump
Pylorus – distal opening of stomach, where circular wall muscle is thick and tonically contracted, called
the pyloric sphincter
-usually allows water and other fluid to empty with ease, but prevents food particles until mixed with
chyme to almost fluid consistency.
Regulation of Stomach Emptying – duodenum regulates stomach emptying more than the stomach
does, because you can’t pass food through faster than it can be digested and absorbed by the small
intestine
-increased food volume promotes emptying from stomach by eliciting myenteric reflexes that
accentuate activity of the pyloric pump
-digestive products of meat secrete gastrin from antral mucosa, which secretes acidic gastric juice, also
has stimulatory effects on motor functions in the body of the stomach
-gastrin enhances activity of the pyloric pump
Inhibition of Stomach Emptying by Duodenum – food entering duodenum causes nervous reflexes to
be initiated from duodenal wall. Pass up to stomach to slow or stop stomach emptying if volume of
chyme is too much
-reflexes are mediated by three routes:
1. Directly from duodenum to stomach through enteric nervous system
2. Through extrinsic nerves that go to prevertebral sympathetic ganglia and back through inhibitory
nerves
3. Slight extent through vagus nerves all the way to brain stem
-all of these strongly inhibit the pyloric pump propulsive contractions, and increase tone of pyloric
sphincter, can be influenced by:
-degree of distention in duodenum
-presence of any degree of irritation of the duodenal mucosa
-degree of acidity of duodenal chyme
-degree of osmolality of chyme
-presence of certain breakdown products in chyme
-inhibitory reflexes extra sensitive to irritants and acids in chyme, and strongly activated
Hormonal Feedback from Duodenum Inhibits Gastric Emptying – stimulus for secretion of these
hormones is mainly fats entering duodenum
-fats entering duodenum extract several hormones by binding to receptors on epithelial cells, such as
cholecystokinin (CCK), released from mucosa of jejunum in response to fatty chyme, acts to block
stomach motility caused by gastrin
-secretin is released from duodenal mucosa in response to gastric acid passed from stomach through
pylorus
-gastric inhibitory peptide (GIP) is an inhibitor of stomach emptying (also called glucose-dependent
insulinotropic peptide), released from upper small intestine in response to fat in chyme and carbs.
-can inhibit gastric motility, but main effect is to stimulate insulin from pancreas
-hormones, especially CCK can inhibit gastric emptying when excess chyme with acid or fat enters
duodenum
Movements of Small Intestine – can be divided into mixing and propulsive contractions
1. Mixing Contractions – caused by distention of small intestine by chyme, results in concentric
contractions spaced at intervals along intestine and lasting less than a minute
a. Cause segmentation of intestine into spaces that appear like chain sausages. As one set
of contractions relaxes, a new one begins at new points between previous contractions
to chop up the chyme
b. Frequency is determined by electrical slow waves in intestinal wall
c. Maximum frequency is 12 per minute
2. Propulsive movements – chyme is propelled through peristaltic waves in any part of small
intestine faster in proximal and slower in distal regions, normally weak and die after 2-5cm,
meaning it takes 3-5 hours for food to travel from pylorus to ileocecal valve
a. Controlled by Nerve and Homorones – intestinal peristalsis increased after meal, first
by the gastroenteric reflex initiated by distention of stomach and conducted through
myenteric plexus from stomach to small intestine
b. Gastrin, CCK, insulin, motilin, and serotonin – enhance intestinal motility
c. Secretin/glucagon – decreased intestinal motility
-function of peristaltic movements not only for movement of chyme to ileocecal valve but
also to spread it along intestinal mucosa
-on reaching ileocecal valve, chyme is blocked for several hours until person eats another
meal
-until then, gastroileal reflex intensifies peristalsis in ileum and forces remaining chyme
to ileocecal valve
Propulsive effect of Segmentation – the segmentation movements also travel 1cm in anal direction and
help propel food down the intestine
Peristaltic Rush – intense irritation of intestinal mucosa, such as in infectious diarrhea, can cause
powerful and rapid peristalsis called peristaltic rush. Initiated by nervous reflexes that involve
autonomic nervous system and brainstem and partly by intrinsic enhancement of myenteric plexus. The
powerful contractions sweep chyme through intestine in minutes, and into colon thereby relieving
irritating substance from small intestine.
-muscularis mucosa can cause short folds to appear in the intestinal mucosa, and also fibers from this
muscle extend into villi to cause them to contract intermittently
-Mucosal folds increase surface area exposed to chyme
-muscle fibers in villi cause shortening and lengthening them for lymph to flow freely from
lacteals of villi into the lymphatic system
Function of the Ileocecal Valve – prevent backflow of fecal contents from colon into the small intestine
-protrudes into lumen of the cecum and is forcefully closed when excess pressure builds up in cecum
and tries to push backward
-upstream from the ileocecal valve is thickened circular muscle tissue called ileocecal sphincter
-sphincter is normally constricted and slows emptying of ileal contents into cecum
-gastroileal reflex intensifies peristalsis in the ileum and empties contents into cecum
-contraction of the ileocecal sphincter and intensity of peristalsis are significantly controlled by reflexes
from cecum
-when cecum is distended, contraction of the ileocecal sphincter intensifies and ileal peristalsis
decreases
-any irritant in the cecum delays emptying (inflamed appendix causes spasm of sphincter)
-reflexes mediated by myenteric plexus in gut wall and extrinsic autonomic nerves (sympathetics)
Movements of the Colon – function of colon is absorption of water from chyme to form solid feces,
storage of fecal matter until it can be expelled
-proximal colon deals with absorption of water, distal half deals with storage, movements normally
sluggish
Mixing Movements “Haustrations” of Colon – constrictions also occur in the large intestine, large
circular constrictions that constrict almost to the point of occlusion
-at the same time, longitudinal muscle called teniae coli contracts, causing unstimulated portion of large
intestine to bulge outward to form sacs called haustrations
-provide minor forward propulsion toward anus in ascending colon
-fecal matter in large intestine is dug into and rolled over, exposing all of it to surface of colon
-80-200mL of feces expelled each day
Propulsive Movements “Mass Movements” – most of propulsion occurs from slow haustral
contractions, requiring 8-15 hours to move chyme from ileocecal valve through colon
-between cecum and sigmoid, mass movements can occur to propel fecal matter
-A mass movement occurs in this series of events
1. Constrictive ring appears in response to distended/irritated point in colon (usually in transverse)
2. 20cm of colon distal to constrictive ring lose their haustrations and contract as a unit, propelling
fecal matter in mass down the colon.
3. A series of mass movements persists for 10-30 minutes, can reappear to push mass into rectum,
when desire for defecation is felt.
Mass Movement Initiation by Gastrocolic and Duodenocolic Reflexes – these reflexes result from
distention of the stomach and duodenum transmitted by autonomic nervous system
-irritation of colon can also influence mass movements
-ulcerative colitis has mass movements that persist all the time
Defecation – rectum is empty most of the time because a weak sphincter exists 20cm from the anus at
the junction between sigmoid and rectum
-when a mass movement forces feces into rectum, desire for defecation occurs immediately, including
reflex contraction of rectum and relaxation of anal sphincters
-Dribble of fecal matter through anus is prevented by tonic constriction of
Internal Anal Sphincter – thickening of circular smooth muscle lies immediately inside anus
External Anal Sphincter – striated voluntary muscle surrounding internal sphincter and
extending distally
-controlled by pudendal nerve
-almost always constricted unless conscious signals inhibit constriction
Defecation Reflexes – one of these is an intrinsic reflex mediated by enteric nervous system in rectal
wall
-when feces enter rectum, distention of rectal wall initiates afferent signals spreading through myenteric
plexus to initiate peristaltic waves in descending and sigmoid colon with rectum, forcing feces toward
anus
-internal anal sphincter is relaxed by inhibitory signals from myenteric plexus. If external sphincter is
voluntarily relaxed at the same time, defecation occurs
-intrinsic myenteric defecation reflex is weak; to be effective, must be fortified with a parasympathetic
defecation reflex involving sacral cord
-parasympathetic pelvic nerves intensify peristaltic waves to relax internal anal sphincter
-defecation signals entering the spinal cord initiate effects such as taking a deep breath, closure of
glottis, contraction of abdominal wall muscles to force fecal contents down colon and cause pelvic floor
muscles to relax downward and pull outward on anal ring to evaginate feces
Other reflexes affecting bowel 1. Peritoneointestinal Reflex results from irritation of peritoneum, inhibits excitatory enteric
nerves and causes intestinal paralysis
2. Renointestinal inhibit intestinal activity as a result of kidney irritation
3. Vesicointestinal inhibit intestinal activity as a result of bladder irritation