W13 - GI system I (5.2, 5.3) Flashcards by LV D

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

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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

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Describe the generation of slow waves.

(7 steps)

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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}

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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

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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

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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

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How is swallowing done?

in 3 phases, only first phase is voluntary

oral phase
pharyngeal phase
esophageal phase

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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

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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}

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What happens in the esophageal phase of swallowing?

food moves downward into the esophagus, propelled by peristaltic waves + aided by gravity

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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

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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)

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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)

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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

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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

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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** 1. distension → EC cells to secrete 5-HT 2. _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.

1. **mass movement** forces feces into rectum 2. distension of rectal wall → **rectocolic reflex**: peristaltic wave in distal colon 3. **relaxation** of int. anal sphincter 4. **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**