GI Motility Flashcards
which GI contractile tissues are not smooth muscle with gap junctions?
pharynx, upper 1/3 of esophagus, external anal sphincter
what tissue type and distinguishing feature does GI contractile tissue have?
smooth muscle with gap junctions to move as a syncytium
what does longitudinal muscle do?
shorten GI segment for propulsion
what does circular muscle do?
narrow diameter of segment for segmentation
what segments of GI undergo phasic contraction?
- esophagus 2. gastric antrum (posterior stomach) 3. small intestine for mixing and propulsion
what segments of GI undergo tonic contraction?
- upper stomach 2. lower esophagus 3. ileocecal junction (SI and LI) 4. internal anal sphincters
what are phasic contractions
brief periods of both relaxation and contractions
what muscle layer carries out phasic contractions in the posterior stomach and SI?
muscularis externa
what are tonic contractions
contractions that are maintained for several minutes or several hours at a time
what kind of contraction does the posterior stomach do?
phasic
what kind of contraction does the anterior stomach do?
tonic
what type of waves are unique to GI smooth muscle?
slow waves that have oscillating membrane depolarization and repolarization
how are action potentials formed in GI smooth muscle?
AP formed as long as the membrane potential remains above threshold
what are the interstitial cells of cajal?
myenteric interstitial cells of Cajal are the pacemaker which creates the bioelectrical slow wave potential leading to contraction of the smooth muscle
what is the average slow wave rate?
between 3-12 waves/min
what is the slow wave rate in the stomach?
3waves/min
what is the slow wave rate in the duodenum?
12 waves/min
where do slow waves originate?
intersitial cells of cajal (ICC)
how do ICC communicate with smooth muscle cells?
gap junctions – action must go through ICC before acting on smooth muscle
how do NT and hormones work on smooth muscle in GI?
is indirect – must go through ICC
what ion causes depolarization in slow waves
Ca influx
what ion causes membrane repolarization?
opening of K efflux channels
what occurs when membrane depolarizes to threshold?
AP generates short duration phasic contraction and the summation of the phasic contractions produce long continuous tonic contractions
how are tonic contractions produced?
summation of phasic contractions
what unique characteristic does smooth muscle demonstrate?
stretch induced contraction
what do pacemakers present in smooth muscle cells control?
rhythmic contraction
- peristalsis
- segmentation
purpose of migrating motility complex
purging between meals to increase food movement
via motilin via interdigestive myoelectric complexes
what does peristalsis and segmentation allow?
food progress along the digestive tract while ensuring absorption of nutrients, and mixing and grinding
mechanical function of the mouth
teeth for chewing
what are incisors for?
tearing leaves (vegetarian)
what are molars for?
grinding, carnivore
chemical function of the mouth
salivary amyase for sugars and lubrication, while the tongue mixes it together.
when is the end of voluntary action when eating?
swallowing
what does the presence of food in the mouth do?
stimulates mouth mechanoreceptors which signal the brain to stimulate chewing muscles (this is involuntary) but voluntary override of chewing is possible
what stimulates swallowing
mechanoreceptors are activated as food bolus approaches the pharynx, and stimulates the medulla oblongata swallowing center
innvervation of chewing and swallowing
receptors signal to vagus and glossopharyngeal nerve AFFERENTS to the medulla oblongata, which outputs to striated muscle EFFERENTS of pharynx and upper esophagus
3 swallowing phases
- oral phase
- pharyngeal phase
- esophageal phase
what occurs during the oral phase of swallowing?
is initiated by the tongue pushing food bolus toward the pharynx.
activated pharyngeal receptors signal medulla oblongata swallowing center to initiate involuntary swallowing reflex.
what nerves are responsible for swallowing
glossopharyngeal and vagus sense posterior pharynx, then vagus is motor
what occurs during the pharyngeal phase of swallowing
food moves from front of mouth to pharynx to esophagus.
the soft palate lifts
epiglottis covers larynx
upper esophageal sphincter relaxes
how does food not move into the nasopharynx?
soft palate lifts, uvula is a valve to shut off nasopharynx
how does food not move into the larynx?
epiglottis covers larynx
how does food actually move from pharynx to esophagus?
the upper esophageal sphincter relaxes so the food moves from pharynx to esophagus
what is the normal condition of the upper esophageal sphincter?
it is usually contracted/closed.
what occurs during the esophageal phase of swallowing?
food moves from esophagus to stomach
upper esophageal sphincter closes
primary peristaltic wave moves food into stomach
aka swallowing reflex–>UES opens–>peristalsis
what occurs if there is residual food in the esophagus?
residual food continues to stretch the esophagus, initiating secondary peristaltic wave (stretch initiated contraction)
how is food moved into the stomach from the esophagus?
UES closes and primary peristaltic wave moves food into stomach
when does UES close?
once food bolus enters esophagus to prevent reflux
what does the primary peristaltic wave do?
moves food down esophagus regardless of body position (you can stand on head! or in space!)
what opens the Lower esophageal sphincter
vagus nerve releases vasoactive intestinal peptide (VIP) to initiate smooth muscle relaxation to lower LES
how is reflux normally prevented?
upper esophageal sphincter closes to make a one way pathway once food bolus enters esophagus
how does the orad region of the stomach allow movement into the stomach?
undergoes receptive relaxation to allow the food. normal would contract bc stretch activated contraction
what occurs once food enters the stomach?
lower esophageal sphincter closes
what makes up the upper esophageal sphincter
striated muscle of inferior pharyngeal constrictor but is not under conscious control
what triggers opening of the UES?
the swallow reflex
what is the lower esophageal sphincter also called?
cardiac sphincter
gastroesophageal sphincter
esophageal sphincter
describe the area of the lower esophageal sphincter
cardia of the stomach overlaps but does not contain the lower esophageal sphincter.
describe the appearance of the lower esophageal sphincter area (Z line)
the squamous epithelium of esophagus gives way to columnar epithelium of the GI tract
what regions does the esophagus consist of?
- proximal striated muscle portion above the sternal notch (cervical esophagus)
- distal smooth muscle portion (thoracic and abdominal esophagus)
what do most esophageal disorders involve?
hypo or hypermotility
how are esophageal disorders classified?
disorders of ihibitory (nitrergic) or excitatory (cholinergic and noncholinergic) innervation
what are inhibitory innervation disorders also called
nitrergic
what are excitatory innervation disorders also called
cholinergic and noncholinergic
what do disorders of decreased INHIBITORY nerve function involve?
- diffuse esophageal spasm
- achalasia
what does achalasia involve
lower esophageal sphincter AND esophageal body are both involved
what does diffuse esophageal spasm
esophageal body is primarily involved
how is esophageal spasm further subdivided?
- diffuse esophageal spasm (DES)
- nutcracker esophagus
what occurs in diffuse esophageal spasm
contractions are uncoordinated
what occurs in nutcracker esophagus
contractions proceed in coordinated manner, but the amplitude is excessive
what occurs in achalasia
there is a failure of normal relaxation of the lower esophageal sphincter associated with uncoordinated contractions of the thoracic esophagus that results in functional obstruction and difficulty swallowing
symptoms of achalasia?
- dysphagia
- regurgitation of indigested food (may involve liquids as worsen)
- coughing when laying down
- chest pain perceived as heartburn or heart attack
- aspiration
how to treat achalasia?
calcium channel blockers block contraction
nitrates
myotomy
botox
\what is a myotomy
cut the nerves to LES??? prevents strict contraction,
cut muscle?
how does botox help achalasia?
relax muscles on lower part of esophagus (for non-surgical candidates)
\what is the proximal esophagus?
striated muscle (non voluntary)
what is the distal esophagus?
smooth muscle in abdominal and thoracic esophagus
parasympathetic innervation of the esophagus
vagus nerve for peristalsis
what is the sympathetic innervation of the proximal esophagus?
cervical and upper thoracic paravertebral chain ganglia
what is the sympathetic innervation of the lower esophageal sphincter and proximal stomach?
celiac ganglion
how is the innate stretch induced contraction of smooth muscle overridden?
hormonally via vasoactive intestinal peptide
opens LES and relaxes orad
what is the orad region of the stomach
fundus and 1/2 of the body
what is gastric motility
- relaxation of orad region to receive esophageal food bolus
- stomach contractions to mix gastric secretions for form chyme
- stomach chyme empties into SI duodenum via pyloric valve
where does stomach chyme empty?
into SI duodenum via pyloric valve
what does the body of the stomach secrete
mucus, pepsinogen, HCl
what does the antrum of the stomach secrete
mucus, pepsinogen, gastrin
describe how the muscle layers of the stomach change
trilayered muscular stomach increases in thickness from proximal to distal
(body is thinner than antrum)
what innervates the stomach
autonomic system consisting of the extrinsic innervation (SNS, PSNS) and intrinsic/enteric innervation of the myenteric and submucosal plexuses
what is unique about gastric muscles
has 3 layers – outer longitudinal, middle circular, inner oblique layers
what is the general organization of the GI tract?
lumen
mucosa (epithelium, lamina propria, muscularis mucosa)
submucosa containing meissner’s plexus
muscularis propria (circular layer, myenteric plexus, longitudinal)
serosa/adventitia
what mediates the parasympathetic innervation of the stomach
vagus nerve
what mediates the sympathetic innervation of the stomach
receives nerve fibers originating in the celiac ganglion
what does food in the esophagus result in?
relaxation of the LES and orad stomach via vasoactive intestinal peptide
what occurs once food enters the orad stomach?
mechanoreceptors carry sensory input to CNS which sends efferent nerve fibers signal to release VIP
what is the relaxation of the orad stomach called?
receptive relaxation – vagovagal reflex involving both afferent and efferent fibers traveling along vagus nerve
what is a vagovagal reflex?
both afferent and efferent components carried by vagus nerve
what effect would a vagotomy have on receptive relaxation?
would inhibit this receptive relaxation override. would be able to take in a little food, but stomach would not be able to relax to take in more food. aka helps limit ingestion
what follows the orad stomach
thin walled proximal orad stomach is followed by the thick walled caudad stomach
what does the caudad stomach do?
produces segmentation that mixes chyme with gastric secretions and physically breaks chyme into smaller pieces
what increases both frequency of AP firing and force of contraction in the stomach?
parasympathetic stimulation and gastrin and motilin
what decreases frequency of AP firing and force of contraction?
GIP (gastric inhibitory peptide) (K cells)
what hormone is secreted during fasting
motilin in 90 min intervals, leads to gastric clearing
describe the contractility of the stomach
contractions increase in intensity along the length of the caudad stomach, maximal at pyloric junction of the duodenum
where is the max contraction intensity of the stomach?
pyloric junction of the duodenum
what occurs if the pyloric valve is closed?
means that particles aren’t small enough or the small intestine isn’t ready. chyme is propelled backward (retropulsion) for further digestion
how long does gastric retropulsion occur?
contintues until food particles are smaller than 1 cubic mm diameter
how often are gastric contents emptied?
every 3 hours
what regulates rate of release into SI duodenum?
neutralization of stomach acid by SI bicarbonate (secretin, CCK)
what inhibits gastric emptying
presence of duodenal fat and acidity
what does fatty acids in the duodenum stimulate?
CCK release by I cells which slows gastric emptying and provides sufficient time for SI digestion of fat by lipases
what leads to inhibition of gastric smooth muscle motility?
duodenal H+ receptors mediated by myenteric plexus to provide time for HCO3 neutralization (secretin, CCK)
what occurs in the SI?
both digestion and absorption, mixes chyme with digestive enzymes and pancreatic secretions
what determines SI motility?
slow wave frequency! is 12 waves/min in duodenum and 9 waves/min in ileum
slow wave frequency in duodenum
12 waves/min
slow wave frequency in the ileum?
9 waves/min
what happens during fasting in the SI?
SI is cleared every 90 min by migrating myoelectric muscle complexes (MMC)
what detects food bolus in SI?
enterochromaffin like cells (ECL) in the mucosa detect food, 5-HT is released to initiate peristalsis
what initiates SI peristalsis?
5-HT
what excites the circular muscle?
ACh, substance P, neuropeptide Y
what occurs behind the bolus of food in the intestine?
circular muscle is excited and longitudinal muscle is inhibited to narrow the intestine
what occurs in front of the bolus of food in the intestine
circular muscle is inhibited and longitudinal muscle is excited to cause segmental widening and shortening
what inhibits circular muscle
VIP and NO
what does the medulla oblongata contain
cardiac, respiratory, vomiting, swallowing, and vasomotor centers
cone shaped neuronal mass, controls things from vomiting to sneezing
what area of the medulla controls vomiting
medullary area postrema
how does vomiting happen
reverse peristaltic contractions begin in the SI
the stomach and pylorus relax.
respiratory inspiration increases abdominal pressure.
LES is relaxed
glottis is closed
forceful contraction of the stomach and duodenum
UES open
=vomit
what happens during retching?
LES is open but UES is closed and the stomach contents percolate up and down
where does most abosorption occur
in the SI
is 6m=20ft long
where does food go after SI?
SI ileum–>ileocecal valve–>LI cecum –>ascending–>transverse–>descending–>sigmoid colon–>rectum–>anus
what does the LI contain/
after water content adjustment, it is fecal waste
where does segmentation occur in the LI?
cecum and proximal colon
what are LI contractions associated with
haustra
what are haustra
small pouches of the LI caused by sacculation
what are teniae coli?
3 narrow longitudinal bands of smooth muscle which run along entire length of cecum and colon to converge at base of the vermiform appendix on the cecum
what do teniae coli do?
contract lengthwise to produce haustra
what is mass movement
LI motility that moves the contents over long distances from transverse to sigmoid colon
it happens 1-3 times a day
what makes fecal contents difficult to move
reabsorption of H2O
what does final mass movement do?
delivers feces to the rectum for defecation
how often does mass movement occur?
1-3 times a day
what does defecation require?
both the voluntary relaxation of the external sphincter and the contraction of the rectum
what occurs once rectum fills with feces?
rectum’s smooth muscle wall is distended, stimulating the stretched induced contraction of the rectum.
what does the rectosphincteric reflex cause?
relaxation of the internal anal sphincter (smooth muscle)
THE VOLUNTARY EXTERNAL ANAL SPHINCTER STAYS CLOSED (skeletal muscle)
when does the urge to defecate occur?
once the rectum is 25% full. but can override the urge because the external anal sphincter is closed. causes retrograde movement of the fecal matter back into the rectum.
