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MGU - PHGY 210 - Class Notes - Week 8

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MGU - PHGY 210 - Class Notes - Week 8

School: McGill University
Department: OTHER
Course: MAMMALIAN PHYSIOLOGY 2
Professor: John White
Term: Spring 2018
Tags:
Name: phgy 210
Description: phgy 210 notes
Uploaded: 03/20/2018
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background image Esophageal forces 1. Gravity: minor importance – small effect for liquids
2. Peristalsis: main force – esophageal contents carried toward stomach
-
Peristalsis: wave of contraction (circular muscles) moving over the wall of the 
organ, narrowing the lumen and setting up a gradient of pressure favoring aboral
movement 
o Each time we swallow, a single primary peristaltic wave is generated (rate of peristalsis = rate of swallowing) o It takes 8-10 seconds to propagate the length of the esophagus
o Primary peristalsis is part of the deglutition reflexes
- Primary peristalsis: although the first 1/3 of the esophagus is striated muscles 
and the rest is smooth muscle, the peristaltic wave moves as a smoothly 
propagated contraction
 over the entire esophagus
o Wave of activity in striated muscle: vagus somatic fibres fire sequentially  activation from proximal to distal regions o Wave of activity in smooth muscle: vagus autonomic fibres are synchronous activation (at the same time), but there is
an increased latency in distal esophagus (short delay) 
Once some enteric neurons are activated, they are
capable of relaying and activating other enteric
neurons (in an orderly fashion)  evokes contraction in
aboral direction, independent of extrinsic nerves
Integrity of the enteric innervation is critical to the
propagation of the peristaltic wave in the distal
esophagus 
- If we cut the vagus n. high in the neck (lose
somatic and autonomic fibres)  no primary
peristaltic wave
- If we cut the vagus n. transthoracically (only
cuts a section of the autonomic fibres)  primary
peristalsis continues, ENS continues the
propagation of the contraction
- Vagus n. is essential for initiating peristalsis in proximal esophagus - Intact ENS is essential for continuing and propagating peristalsis in
distal esophagus
- Frequency of peristalsis is dependent on the frequency of swallowing  
background image - What happens if the bolus gets stuck in the esophagus? (NOT choking) o The bolus creates a local distention  initiates secondary peristalsis 
o Secondary peristalsis is mediated by:
Enteric reflexes (short, local reflexes) Long reflexes (vagal sensory afferents to CNS
which reinforces efferent vagal input  vagal-
vagal reflexes
)
o Secondary peristalsis waves may be generated until bolus has been displaced  o i.e. “Steakhouse syndrome” Lower esophageal sphincter (LES) - Found in the terminal 4 cm of the esophagus – half above and half below the 
diaphragm 
- No visible enlargement of circular muscle - Stomach pH = 2 (vs. esophagus pH = 7) - Though anatomically insignificant, the LES is functionally important  o Ability to act as a sphincter is a property of the musculature, residual resistance in absence of NES of
autonomic innervation
o Closure is myogenic – muscle is under constant tone (always contracted), but it is under neural and
hormonal influences
o Relaxation is neurogenic – local ENS release of NANC (-) or vagal stimulation activate inhibitory ENS
neurons 
LES relaxation is a part of deglutition reflexes  - LES is under high pressure (thoracic cavity = -5mm Hg, abdominal cavity = 
+5mm Hg)
o LES has an intrathoracic segment (-) and intraabdominal segment (+)
o The presence of an intraabdominal segment of LES
assists the sphincter in maintaining an effective
barrier between the stomach and esophagus
- Increasing the intraabdominal pressure increases pressure
equally on the stomach and LES  no effective change in
gradient of pressure between stomach and esophagus (no
reflux)
o Ex: leaning forward increases the abdominal pressure 
background image o If the LES is entirely displaced into thorax (hiatus hernia),  intraabdominal pressure increases do not increase LES pressure  - LES characteristics: 1. Intrinsic physiologic sphincter – tonically contracted din the absence of  swallowing 2. Anti-reflux mechanism assisted by presence of an intraabdominal  segment - Incompetent LES: sphincter fails to close o LES is a target for pharmaceutical companies  - Heartburn (pyrosis): burning sensation radiating upwards in chest towards 
neck, due to acid refluc into esophagus
- Hormonal modulation of LES o Originally, they thought that gastrin would tighten LES (false, no effect at  physiological doses) o Progesterone lowers the resistance of LES (reduces the tone)  High progesterone in the 3 rd  trimester of pregnancy  pregnant  women have issues with reflux  Motor functions of the stomach - 3 motor functions of the stomach: 1. Temporary storage (1-2L)
2. Physical disruption and mixing of contents (semi-liquid consistency = 
chyme, no longer called the bolus)  3. Propulsion into duodenum: slow, regulated propulsion so we do not fill the  intestine too quickly - Structure of the stomach: o Upper portion (proximal): thin-walled, storage Composed of fundus, ½ body o Lower portion (distal): thick-walled, mixing and propulsion Composed of ½ body, antrum, pylorus - Intragastric pressure = +5mm Hg - How does the stomach accommodate the meal? o The empty stomach has a small volume ~50mL  o It can expand to 1.5-3L
Receptive relaxation (in the proximal stomach): increase in volume 
without significant increase in intraluminal pressure  4. Gastric phase
-
Receptive relaxation is a deglutition reflex, restricted to the proximal stomach o It is the ability of the stomach to accommodate a large meal without  significant increase in intragastric pressure If the vagus n. to the proximal stomach are cut, receptive relaxation
is limited (no initial response and no feedback), resulting in great 
increase in intragastric pressure

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School: McGill University
Department: OTHER
Course: MAMMALIAN PHYSIOLOGY 2
Professor: John White
Term: Spring 2018
Tags:
Name: phgy 210
Description: phgy 210 notes
Uploaded: 03/20/2018
9 Pages 35 Views 28 Unlocks
  • Better Grades Guarantee
  • 24/7 Homework help
  • Notes, Study Guides, Flashcards + More!
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