GI Motility (Johnson/Costanza) Flashcards
All contractile muscle in the GI tract is smooth muscle except …
pharynx, upper 1/3 esophagus, and external anal sphincter
**these are striated
The smooth muscle of the gastrointestinal tract is _____ smooth muscle, in which the cells are electrically coupled via _____
unitary smooth muscle coupled via gap junctions
what is the significance of the gap junctions that connect the smooth muscle of GI tract?
allow for low resistance pathways to propagate AP quickly so that there are coordinated contractions
defn tonic vs phasic contractions
phasic: period of contraction then relaxation
Tonic: constant low level confraction
in what part of GI tract are tonic contractions found?
upper stomach and lower esophageal, ileocecal, and internal anal sphincters
in what part of GI tract are phasic contractions found?
esophagus, gastric antrum, and small intestine
***all tissues involved in mixing and propulsion.
are slow waves a type of action potential
NO
slow waves are oscillating depolarization and repolarization of _____ in gastric smooth muscle
membrane potential
what lags behind the electrical activity of a slow wave?
mechanical response (i.e contraction/tension)
what determines the frequency of contraction of GI smooth muscle?
freq of AP which is dept on the freq of slow waves
feq of slow waves in stomach
3/min
feq of slow waves in duodenum
12/min
effect of neuronal input on slow waves
not not affect the slow waves!! (they do effect the production of APs and the strength of the muscle contraction)
the “pacemaker of the GI smooth muscle”
interstitial cells of Cajal
transmits cyclic depolarizations/respolarization of smooth muscle via gap junctions
interstitial cells of Cajal
What is the biochem mechanism behind the sloq waves? (i.e. what ions are involved)
depolarization due to Ca influx
repolarization due to K efflux
cyclic depolarizations that do not reach threshhold cause
weak tonic contractions (i.e. no relaxation)
depolarizations that achieve threshold result in
phasic contraction
what is the result of increasing the number of APs on top of the slow wave depolarization
increase the duration of the PHASIC contraction
drives the frequency of slow waves
the pacemaker cells (interstitial cells of Cajal)
where are the interstitial cells of Cajal found?
myenteric plexus
first steps in the processing of ingested food as it is prepared for digestion and absorption
chewing and swallowing
three functions of cheming
(1) It mixes food with saliva, lubricating it (facilitate swallowing)
(2) it reduces the size of food particles (facilitates swallowing)
(3) it mixes ingested CHO with salivary amylase to begin CHO digestion
Initiated by food in the mouth. Sensory information is relayed from mechanoreceptors in the mouth to the brain stem which orchestrates a reflex oscillatory pattern of activity to the muscles in the mouth
involuntary component of chewing
In involuntary chewing, food in the mouth is detected by… and carried to the …which orchestrates …
mechanoreceptors → the brain stem → reflex oscillatory pattern of activity to the muscles in the mouth
can override involuntary or reflex chewing at any time
voluntary chewing
in the mouth swallowing is (voluntary or involuntary)
voluntary
**from then on down it is involuntary
what controls/coordinates involuntary swallowing
swallowing center in the medulla
brain stem controls…
medulla controls…
brain stem controls chewing (involuntary)
medulla controls swallowing (involuntary)
in involuntary swallowing, food in the mouth is detected by… and carried by … to the …
somatosensory receptors near the pharynx → Vagus and glossopharyngeal (IX and X) → medulla
What are the phases of swallowing? What happens in each?
oral → pharyngeal → esophageal
oral: tongue forces a bolus of food back toward the pharynx (where somatosensory receptors are found that will initiate the involuntary swallowing reflex)
pharyngeal:
(1) The soft palate is pulled upward to prevent reflux into the nasopharynx.
(2) The epiglottis moves to cover the opening to the larynx, (∴ breathing is inhibited) and the larynx moves upward against the epiglottis
(3) The upper esophageal sphincter relaxes, allowing food to into the esophagus.
(4) A peristaltic wave of contraction is initiated in the pharynx to propel food through the open upper esophageal sphincter.
esophageal: swallowing reflex closes the sphincter so food cannot reflux into the pharynx and then a primary peristaltic waves propels food along (secondary wave will be initiated if primary does not move all the food)
controls esophageal phase of swallowing
swallowing reflex and enteric nervous system
primary vs secondary peristaltic wave
Primary is coordinated by the swallowing reflex
Secondary occurs if primary does clear the esophagus of food.
Secondary is mediated by the enteric nervous system and is initiated by the continued distention of food in the esophagus.
initiates the secondary peristaltic wave
continued distention of the esophagus by food that was not cleared
how does the primary peristaltic contraction move food down the esophagus?
As each segment of esophagus contracts, it creates an area of high pressure just behind the bolus, pushing it down the esophagus. Each sequential contraction pushes the bolus further along. If the person is sitting or standing, this action is accelerated by gravity.
What happens as the food bolus approaches the lower esophageal sphinceter
Vagus nerve releases VIP (a vasovagal reflex, i.e. afferent and efferent fibers are in the vagus N) → relaxation of lower esophageal sphincter
the pressure of the intrathoracic esophagus is equal to
the intrathoracic pressure
the abdominal pressure is > or < the intraesophageal pressure. Significance?
adb > esophagus = the gastic contents will want to travel from high to low pressure (stomach to esophagus)
***Conditions in which intra-abdominal pressure is increased (e.g., pregnancy or morbid obesity) may cause gastroesophageal reflux, in which the contents of the stomach reflux into the esophagus.
atmospheric pressure is > or < the intraesophageal pressure. Significance?
atm > esophagus = air will want to travel from high to low (atm to esophagus)
What prevents gastric contents and air from entering into the esophagus (with their pressure gradients)
upper and lower esophageal sphincters
What is receptive relaxation? Why is it important?
At the same time that the lower esophageal sphincter relaxes, the orad region of the stomach also relaxes. This decreases the pressure in the orad stomach and facilitates movement of the bolus into the stomach.
At the lower esophageal sphincter’s resting tone, the pressure at the sphincter is (lower or higher) than the pressure in the esophagus or in the orad stomach.
higher
describe the muscle layers of the stomach
outer longitudinal layer, a middle circular layer, and an inner oblique layer
how does the thickness of the muscle wall change?
it increases from proximal to distal stomach
describe the innervation to the stomach
- extrinsic innervation by the autonomic nervous system
2. intrinsic innervation from the myenteric plexus and submucosal plexuses
What are the anatomic divisions of the stomach
fundus, body, antrum
___ region of the stomach is proximal and contains …
orad region: fundus and proximal portion of the body
___ region of the stomach is distal and contains …
caudad region: distal portion of the body and the antrum
orad or caudad region has thicker wall and stronger contractions. Why?>
caudad to mix and propel food into small intestine
the orad region relaxes when …
the lower esophageal sphincter distends (receptive relaxation, a vasovagal reflex) → mechanoreceptors in stomach sense this and send info to CNS via vagus N → efferent info sent back to stomach smooth muscle via vagus N → post gang release of VIP → smooth muscle of orad relaxes
what breaks down bolus into chyme
contractions of thick muscular wall of caudal portion of stomach + gastric secretions
Gatric contractions … as they approach the pylorus
increase in strength
what is retropulsion?
the wave of contractions in the caudad region also closes the pylorus as they move the bolus down → causes most of the gastric contents to be propelled back into the stomach for further mixing and reduction in size
What increases the frequency of action potentials and the force of gastric contractions?
parasympathetic stimulation, gastrin, motilin
What decreases the frequency of action potentials and the force of gastric contractions?
sympathetic stimulation, secretin, and GIP
What are migrating myoelectric complexes?
during fasting, periodic contractions (every 90 mins) that clear the stomach and small intestines of residual food/chyme products
What mediates migrating myoelectric complexes?
motilin
The rate of gastric emptying must be closely regulated to provide adequate time for …
- neutralization of gastric H+ in the duodenum (maintain pH)
- digestion to appropriate size (< 1mm3)
- absorption of nutrients
isotonic contents empty (more or less) rapidly than either hypotonic or hypertonic contents
more rapidly
major factors slow or inhibit gastric emptying
presence of fat and low pH
if there is high fat content in the stomach, how is gastric emptying time increased?
CCK is secreted
- *the mechanism was not in the book
- *in duodenum
if there is high [H+] in the stomach, how is gastric emptying time increased?
H+ receptors activated and relay this information to gastric smooth muscle via interneurons in the myenteric plexus → increased time for bicarb neutralization (why secretin dec force of contractions)
**in duodenum
frequency of slow waves in the ileum
9 waves/min
Parasympathetic innervation of the SI occurs via
vagus N
Sympathetic innervation of the SI originates in the …
celiac and superior mesenteric ganglia.
Neurocrines released from parasympathetic peptidergic neurons of the small intestine include:
VIP, enkephalins, motilin
what type of contraction in the SI mixes chyme
segmentation contractions
what type of contraction in the SI propels chyme
peristaltic contractions
where do the contractions occur in segmental vs peristaltic contractions in the SI
segmentation: within a bolus or chyme to split it in half (will then relax to allow it to merge back together → repeat)
peristaltic: contractions occur behind chyme and relaxations occur in front → propels it forward
he neurotransmitters and involved in the orad contractions in peristaltic contractions of the SI are?
caudad relaxation?
ordad contraction: ACh and substance P
caudad relaxation: VIP and NO
Where in the brain is the vomiting center?
medulla
Describe the innervation of the vomiting reflex
vestibular system → medulla → throat, GI, chemoreceptor trigger zone in 4th ventricle
Describe the motor events of the vomiting reflex
- reverse peristalsis that begins in the SI
- relaxation of the stomach and pylorus
- inspiration to increase abdominal pressure
- relaxation of the lower esophageal sphincter
- forceful expulsion of gastric, and sometimes duodenal, contents
What is retching
vomiting but the upper esophageal sphincter is closed so the gastric contents return to the stomach
After the contents of the small intestine enter the cecum and proximal colon, _____ to prevent reflux into the ileum
the ileocecal sphincter contracts
___ contractions occur in the cecum and proximal colon are are mediated by ____
segmentation mediated by haustra
occur in the colon and function to move the contents of the large intestine over long distances, such as from the transverse colon to the sigmoid colon
mass movements
how frqeuntly do mass movements occur?
1-3 times per day
makes the fecal contents of the large intestine semisolid and increasingly difficult to move
water absorption (in the distal colon)
What is the rectosphincteric reflex?
As the rectum fills with feces, the smooth muscle wall of the rectum contracts and the internal anal sphincter relaxes
why does defecation not occur with the ectosphincteric reflex?
the external anal sphincter is still contracted – it is under voluntary control (striated muscle)
once the rectum fills to ___% of its capacity, there is an urge to defecate
25
he intra-abdominal pressure created for defecation can be increased by a
valsalva maneuver (expiring against a closed glottis)
What is the gastrocolic reflex?
Distention of the stomach by food increases the motility of the colon and increases the frequency of mass movements in the large intestine
What is the innervation controlling the gastrocolic reflex?
The afferent limb in the stomach is mediated by the parasympathetic nervous system.
The efferent limb of the reflex, which produces increased motility of the colon, is mediated by the hormones CCK and gastrin.
