27 Propulsion and Mixing of Food Flashcards by Nicholas de Guzman

What are the two muscular layers in the for every organ of the GI tract, and what do they do? What is the exception to this?

Inner circular – Contracts to decrease diameter
Outer longitudinal – Contracts to shorten segment

Exception: stomach

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What are the two types of contractions and where might they occur?

phasic and tonic

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what is the phasic contraction and where does it occur?

occur in the esophagus, gastric antrum and small intestine that contract and relax periodically

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what is the tonic contraction and where does it occur?

occur in the lower esophageal sphincter, orad stomach, and ileocecal and internal anal sphincters

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What is the first phase of consumption/digestion and what happens during it? Know that there are two halves, describe the first half of the first phase

First half of first phase is chewing
-Forms bolus
-Mixes food with saliva to lubricate and begin digestive process
▪ Saliva is hypotonic and alkalinic
-Chewing helps to break down large portions of food for swallowing and breaks open indigestible cellulose to obtain nutrients
-After saliva is mixed, it exposes food to the taste buds

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what is the second half of the first phase?

swallowing

Transports food into the esophagus while protecting the airway and so this is Initiated voluntarily, but then relies on involuntary brainstem reflexes

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what are the three phases in the second half of the first phase?

oral (voluntary)
pharyngeal (involuntary)
esophageal (involuntary)

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describe the oral phase? what phase is this in?

Oral phase (voluntary) of the second half of the first phase

Food is propelled to pharynx

Swallowing reflex is initiated by sensors in pharynx

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describe the pharyngeal phase? what phase is this in?

Pharyngeal phase (involuntary) of the second half of the first phase

Soft palate moves upward to protect nasal cavity

Epiglottis closes anterior to posterior to protect airway

Upper esophageal sphincter relaxes

Bolus travels into esophagus

Note: During this phase, respiration is inhibited

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describe the esophageal phase? what phase is this in?

Esophageal phase (involuntary) of the second half of the first phase

Upper esophageal sphincter constricts

Swallowing initiates primary peristaltic wave

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what is unique about the first of consumption/digestion?

This is the only phase in which the parasympathetic and sympathetic system work together, for the rest of digestion, parasympathetic has primary control. If there is sympathetic activity, it’ll mostly try to slow GI down.

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What is the second phase of consumption/digestion and what happens during it?

The second phase involves the movement of food down the esophagus

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T/F, The esophagus has both skeletal/striated muscle (upper 1/3) and smooth muscle (lower 2/3)

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how much of the esophagus is skeletal/striated muscle and smooth muscle?

skeletal/striated muscle (upper 1/3) and smooth muscle (lower 2/3)

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how many sphincters does the esophagus have?

two, upper and lower sphincter

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what happens to the esophagus during swallowing?

Pharynx constricts
UES opens allowing for bolus to pass
LES and proximal stomach relax

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what is the sequence of esophageal motility?

Pharynx constricts
UES opens allowing for bolus to pass
LES and proximal stomach relax
UES constricts
Primary peristalsis: a progressive wave of distension and contraction passes through the esophageal body
LES closes after bolus passes to proximal stomach

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what is the transit time through the second phase of digestion and consumption?

~10 sec

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Describe primary peristalsis? When does secondary peristalsis occur?

triggered when mechanoreceptors sense distension made by the bolus and so:

Signal travels in afferent fibers to the swallowing center (in the medulla)

Proximal to the bolus – Excitatory efferents cause contraction via acetylcholine (ACh)

Distal to bolus – Inhibitory efferents cause relaxation via nitric oxide

Result – Bolus travels forward

*If all food does not clear in primary peristalsis, secondary peristalsis occurs

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T/F, peristalsis occurs the entire length of the esophagus and involves the vagus nerve and enteric nervous system?

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what NT relaxes the LES?

VIP

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What happens in Achalasia?

There is a loss of inhibitory neurons in the LES
-Results in LES being unable to open

Leads to build up of boli in esophagus

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where does the third phase of digestion/consumption occur?

occurs in the stomach

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what does the stomach consist of?

Stomach consists of four sections: fundus (very top), body (majority of the stomach), antrum (lower portion of the stomach right before pyloric valve), and pylorus

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what is the main function of gastric motility?

The main function of gastric motility is to mix bolus with digestive juices, grind to smaller pieces (until chyme is created), and pass the chyme through the pyloric sphincter into the duodenum

how is gastric motility accomplished?

This is accomplished through tonic contraction in the proximal stomach (which is mediated by the vagus nerve/ACh) ▪ Relaxation (through vasovagal reflex) occurs in response to swallowing and distension of the proximal stomach ▪ Vasoactive intestinal peptide (VIP) is mediator for gastric relaxation ▪ Proximal portion of stomach is mostly used for storage

what is the distal stomach responsible for?

propulsion and retropulsion and so Chyme particles must be <2 mm in order to enter duodenum

which parts of the stomach are involved in gastric emptying?

All parts of the stomach are involved in gastric emptying

how does gastric emptying occur?

- Proximal tone increases (ACh), chyme moves distally - Antral contractions become more forceful (ACh, gastrin) and open pylorus - Several mL of chyme are propelled per contraction - Residual food is emptied in the interdigestive period by the migrating motor complex

where does the fourth phase of consumption/digeston occur?

occurs in the small intestine

what are the three parts of the small intestine?

Duodenum, jejunum, and ileum with highest frequency slow waves in the duodenum

what are the three type of motility?

Segmentation contraction Peristaltic contraction Migrating motor complex

what are segmentation contractions?

Mixes chyme in a two-steps forward, one step back pattern

what are Peristaltic contractions?

Propels chyme distally

what is the Migrating motor complex?

Peristaltic movement that occurs every 90 minutes during the interdigestive period from the distal stomach to the terminal ileum

when does segmentation contraction occur? what part of the nervous system controls this?

Contraction occurs at the sight of distention and pushes contents forward and back (mediated by ACh) Controlled by the ENS

how does peristaltic contractions propel chyme?

it propels chyme distally

what is the effect of peristaltic contractions and distension?

causes the release of 5-HT from ECL cells and binds afferent ENS neurons

what NT is linked to the peristaltic contractions that cause reflex contraction proximal to the bolus?

via Ach and Substance P and reflex relaxation distal to bolus via VIP and NO

what is the migrating motor complex?

Peristaltic movement that occurs every 90 minutes during the interdigestive period from the distal stomach to the terminal ileum and functions to clear the stomach and small intestine in preparation for the next meal

what hormone is responsible for initiating the MMC?

The hormone motilin

when is the migrating motor complex inhibited?

during meal

where does the fifth phase occur?

occurs in the large intestine, rectum, and anus

what is motility like in the large intestine?

Motility in the colon is the same in fed vs. fasting states (no migrating motor complex) and is less coordinated

what are the segmentation contractions for mixing?

Inner circular narrow lumen in segments creating haustra Tenia coli shorten segment

what are the mass movements for the Large Intestines?

Moves feces along the colon to rectum Occur 3-10 times per day Do not always result in defecation

what are the key reflexes of the large intestine?

Gastrocolic Reflex | Colonoileal “Ileal break”

what is the Gastrocolic Reflex?

Long reflex involving ANS Afferent limb: distension of stomach Efferent limb: increased mass movements in the colon

what is the Colonoileal “Ileal break”?

Afferent limb: Fat in the proximal large intestine Efferent limb: Slowing of small intestinal motility and closing of ileocecal valve

what is the rectum and anus the reservoir for?

stool

what is the purpose for the valves in the rectum and anus?

Valves slow the movement of contents

what do the rectum and anus join too?

anal canal

what are the two anal spinsters in the rectum and anus?

internal anal sphincter | external anal sphincter

what is the internal anal sphincter?

Circular smooth muscle | Involuntary (autonomic nerves)

what is the external anal sphincter?

Striated muscle | Voluntary (somatic nerves)

what is the final phase of consumption/digestion?

defecation

what is defecation?

coordinated action of smooth and striated muscle in rectum and anus and the pelvic floor

in terms of the final phase of consumption, what is the puborectalis muscle?

relaxes changing the anorectal angle (does not occur when passing flatus)

relaxation of the external anal sphincter is voluntary or involuntary?

Voluntary relaxation

how does rectal contraction work?

relaxation of IAS, followed by mass movement of colon to excrete feces

how often does defecation?

Normal can be every 2-3 days up to 3x/day\

Where does most digestive absorption take place?

Small intestine

this serves as the pacemaker for the GI smooth muscle?

interstitial cells of Cajal

frequency of slow waves is lowest where?

in the stomach with 3 slow waves/min

frequency of slow waves is highest where?

in the duodenum with 12 slow waves/min

where is swallowing coordinated?

in the medulla

where does peristalsis begin?

in the pharynx

what is this medical condition called when the tone of the lower esophageal sphincter is decreased and gastric contents reflux into the esophagus?

GERD (reflux)

this can occur if the lower esophageal sphincter does not relax during swallowing with impaired esophageal peristalsis?

Achalasia

increased segmentation contractions is also known as this?

constipation

decreased segmentation contractions is also known as this?

diarrhea

this is the name of a disease in which there is an absence of the colonic enteric nervous system?

megacolon or Hirschsprung disease

so for swallowing, it is initiated voluntarily but then relies on involuntary brainstem reflex, what is this called?

the swallowing reflex

where is the swallowing center found? what muscles are these?

medulla; skeletal

what are the protective steps in the pharyngeal phase?

the phase is involuntary and the steps are the first two Soft palate moves upward and protect nasal cavity Larynx moved forward and up, epiglottis deflected down to protect airway

what is the sequence of esophageal motility?

``` swallowing UES constricts primary peristalsis LES closes second wave of peristalsis, only if food doesn't clear ```

in the primary peristalsis for mechanism, proximal to the bolus, excitatory efferents cause contraction via _____?

ACh

in the primary peristalsis for mechanism, distal to the bolus, Inhibitory efferents cause relaxation via _____?

nitric oxide

in the primary peristalsis for mechanism, the LES relaxes in response to _____?

VIP

in gastric emptying, what increase LES pressure?

gastrin

in gastric emptying, what increases antral contraction?

Gastrin and ACh