27 Propulsion and Mixing of Food Flashcards
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
What are the two types of contractions and where might they occur?
phasic and tonic
what is the phasic contraction and where does it occur?
occur in the esophagus, gastric antrum and small intestine that contract and relax periodically
what is the tonic contraction and where does it occur?
occur in the lower esophageal sphincter, orad stomach, and ileocecal and internal anal sphincters
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
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
what are the three phases in the second half of the first phase?
oral (voluntary)
pharyngeal (involuntary)
esophageal (involuntary)
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
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
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
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.
What is the second phase of consumption/digestion and what happens during it?
The second phase involves the movement of food down the esophagus
T/F, The esophagus has both skeletal/striated muscle (upper 1/3) and smooth muscle (lower 2/3)
T
how much of the esophagus is skeletal/striated muscle and smooth muscle?
skeletal/striated muscle (upper 1/3) and smooth muscle (lower 2/3)
how many sphincters does the esophagus have?
two, upper and lower sphincter
what happens to the esophagus during swallowing?
Pharynx constricts
UES opens allowing for bolus to pass
LES and proximal stomach relax
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
what is the transit time through the second phase of digestion and consumption?
~10 sec
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
T/F, peristalsis occurs the entire length of the esophagus and involves the vagus nerve and enteric nervous system?
T
what NT relaxes the LES?
VIP
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
where does the third phase of digestion/consumption occur?
occurs in the stomach
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
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
