List the mechanical functions of motility.
- grinding = increasing surface area
- storage = slows down propulsion for optimized digestion/absorption
- mixing = renewal of surface
- propulsion
- reflux inhibition
What are slow waves?
Function?
continuous fluctuation of membrane potential generated by interstitial cells of Cajal to propagate electric activity via gap junctions to nearby long./circ. muscle layer
⇒ constitute tonic/basal contractivity of GI tract
Describe the generation of slow waves.
(7 steps)
How are APs generated in the GI tract?
Function?
if temporal summation of slow waves occurs and threshold is reached → depolarization due to opening of Na+ and Ca2+ channels
⇒ constitues phasic contraction of GI tract for propulsion
<u>NOTE:</u> force of contraction incr. with incr. AP frequency
What are the main functions of the oral cavity?
- mastication
- swallowing
- lubrication of food
- sampling of food for pathogen’s (Waldeyer’s lymphatic ring)
- vocalization
- taste
What are the 2 types of teeth which mainly contribute to mastication?
Function.
- incisors → cutting
- molars → grinding
⇒ increase surface area of ingested food, facilitated swallowing
Explain the reflex mechanism of mastication.
BUT… ?
cycles of reflex inhibition of muscles → jaw drops
and
stretch reflex → contraction
BUT: can also be initiated voluntarily
How is swallowing done?
in 3 phases, only first phase is voluntary
- oral phase
- pharyngeal phase
- esophageal phase
What happens in the oral phase of swallowing?
tongue pushes bolus back against soft palate/back of mouth containing somatosensory receptors
⇒ trigger swallowing reflex
NOTE: only voluntary phase of swallowing
What happens in the pharyngeal phase of swallowing?
- upper esophageal sphincter relaxes while airways are closed to keep swallowed material out
- stimulation of epithelial swallowing receptor area cause autonomic contractions of pharyngeal muscle
⇒ preciously timed
<u>also:</u> vocal cords approximate, palatopharyngeal folds pulled med. so only small food particles can pass
What happens in the esophageal phase of swallowing?
food moves downward into the esophagus, propelled by peristaltic waves + aided by gravity
Differentiate btw primary and secondary peristalsis.
Where can it be seen?
during esophageal phase of swallowing
- primary peristalsis: continuation of peristaltic wave originating in pharynx
- secondary peristalsis: results from distension of esophagus
How is swallowing terminated?
receptive relaxation of lower esophageal sphincter + proximal stomach
= wave of relaxation transmitted by myenteric inh. neurons (VIP) + vagovagal reflex
NOTE: precedes peristalsis (obv)
How does the pressure during swallowing change in the upper GI tract?
stimulation of the pharynx in the presence of a bolus initiates
- ↓P (= opening) of the upper esophageal sphincter
- ↓P of the lower esophageal sphincter to prepare for entry of food
peristaltic wave of contraction along the esophagus
(progressively incr. P)
Describe the symptoms of achalasia.
How is it treated?
= lower esophageal sphincter fails to relax
consequences:
- enlargement of esophagus → infections
- reflux = regurgitation → esophagitis
can be resolved surgically
What are the 3 main functions of gastric motility?
- storage of ingested food - 1.5l
- grinding and mixing of the food w/ gastric secretion
- slow/controlled emptying of chyme into duodenum
What are the functions of the fundus and proximal stomach?
reservoir function
stable membrane potential → no phasic contractions
- receptive relaxation before filling
- adapative relaxation during filling
⇒ tonic activity reestabilished after meal
What are the functions of the distal stomach?
grinding, mixing, fractional propulsion
slow waves begin spontaneously in the middle of the stomach → constrictive rings pass down to antrum
Which substances incr., resp. decrease the contractive force of gastric propulsions?
- incr.: gastrin, vagal activity (parasymp.)
- decr.: CCK, NE (symp.)
Describe the function of the pyloric pump.
- propulsion: progressively stronger propulsive contractions push bolus towards pylorus
- grinding: antrum grinds trapped material
- retropulsion: pyloric sphincter allows passage of fluids/particles < 2mm, bigger particles pushed back into proximal stomach
Which factors can slow the rate of gastric emptying?
Mechanism?
- acidity
- high fat content → CKK, GIP
- hypertonicity
⇒ cause entero-gastric reflex via chemoreceptors: directly ↓ gastric emptying or via ↑ symp. output
Which factors increase the rate of gastric emptying?
- incr. food volume → distension of stomach wall
- release of gastrin from antral mucosa
Explain the physiology of vomiting.
-
retching
- antiperistalsis: contraction of stomach/duodenum
- contraction of diaphragm/abs → ↑↑ intragrastric P, “inverted” swallowing, LES relaxes, UES closed
- expulsion: after several retching cycles UES relaxes → expulsion
What are the 2 types of movement of the GI tract?
- segmental contraction
- peristaltic contraction
Describe the mechanism of segmental contraction.
Function?
distension of wall → stretching + contractions of circular muscle layer spaced at intervals
⇒ mixing of chyme
Describe the mechanism of propulsive contraction.
Function?
contraction right behind bolus while muscle in front of bolus relaxes ⇒ peristaltic waves (1cm/sec)
→ propels chyme
What does the law of the intestine state?
Mechanism.
= peristaltic reflex
⇒ contraction above and relaxation below a stimulated point in the intestine
- distension → EC cells to secrete 5-HT
-
behind bolus: exc. transmitters released → contraction of circ. muscle, rel. of long. muscle
in front of bolus: inh. transmitters released → contraction of long. muscle., rel. of circ. muscle
describes the mechanism of propulsive contraction
How is propulsive movement in the small intestine regulated?
-
enhanced by:
- gastroenteric reflex (myenteric plexus)
- gastrin, CCK, insulin
- serotonin
- motilin
-
decreased by:
- secretin, glucagon
What is important to mention when we talk about the 2 muscle layers of the GI tract?
reciprocally innervated
→ contract independently
As a summary..
Which transmitters act excitatory, resp. inhibitory on GI muscle layers?
- excitatory:
- *ACh, substance P**
- inhibitory:
- *NO, ATP, VIP, PACAP**
What are MMCs?
When and where do they occur?
migrating myoelectric complexes
motor activity of stomach/small intestine, faciliated by motilin ⇒ transportation of indigestible substances into the colon
every 90-120 min. during the interdigestive phase → responsible for the rumbling when hungry
How is the ileocecal valve regulated?
Function?
-
closed in resp. to:
- distension/irritation of cecum
- reflexes (myenteric plexus, prevert. sympathetic ganglia)
-
opened in resp. to:
- gastroileal reflex, gastrin
- fluidity of contents (diarrhea)
⇒ prevention of reflux + colonisation of small int. by bacteria
What are the 2 main functions of the colon?
- absorption of water/electrolytes
- storage of fecal matter
Which movements can be observed in the colon?
Where?
- haustration: ascending - descending colon
- antiperistalsis: ascending colon
- mass peristalsis: transverse colon - sigmoid
Define haustration.
Where does it occur?
Function?
in ascending - descending colon
- slow segmenting, uncoordinated movements that occur ∽ every 25 minutes
- done by circular muscle + taeniae coli
⇒ contributes to peristalsis (from haustrum to haustrum)
What is antiperistalsis?
Where does it occur?
in ascending colon
peristalsis in upward direction
and during vomiting
Where can mass peristalsis be seen?
How often does it happen daily?
in transverse colon - sigmoid
- 1 - 3/day
- tonic contraction of long (20cm) segments followed by relaxation for 2-3 min.
Which reflexes can be seen in the colon?
Function?
-
gastrocolic/duodenocolic reflex:
distension of stomach/duodenum → facilitate mass movements -
colonocolic reflex:
propells stool caudally by proximal muscle contraction and distal dilatation
both transmitted by ANS
Describe the mechanism of the defecation.
- mass movement forces feces into rectum
- distension of rectal wall → rectocolic reflex: peristaltic wave in distal colon
- relaxation of int. anal sphincter
- vol. relaxation of ext. anal sphincter
⇒ rectal smooth muscle wall contracts, intra-abdominal pressure can be created with valsalva maneuver
As a summary..
In which segments of the GI tract can tonic and propulsive contractions be observed?
-
tonic:
in all sphincters, upper third of stomach -
propulsive:
esophagus, lower 2 thirds of stomach, small int., rectum
What is the peristaltic rush?
Mechanism.
very powerful peristaltic movement which occurs when the intestinal mucosa is irritated intensely (e.g. by antigens, stress)
⇒ diarrhea
What is a megacolon?
Causes?
severe constipation → enlargement of colon
e.g. due to
- Hirschsprung’s disease (lack of myenteric plexus in a segment of sigmoid)
- Chagas disease