GI Motility
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- What is the function of he Interstitial Cell of Cajal and where are they located?
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1) pacemaker cells of the gut - oscillate at a higher frequency than the intrinsic rate of SM cells
2) located at myenteric and submucosal juctions - What cells act as an intermediate between muscle cells and neurons in the gut via their processes?
- Interstitial Cells of Cajal
- What determines the amplitude and duration of a plateau phase in slow waves?
- the magnitude of Ca2+ influx
- What can modify the plateau of a slow wave?
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Hormones and NTs
(excitatory increase amplitude and duration - ACh, Sub P, CCK)
(inhibitory decrease amplitude and duration - VIP) - When does contraction occur?
- Only when plateau potential exceeds threshold for voltage-activated Ca2+ channel
- When are "spikes" usually seen?
- above threshold in intestinal cells
- What reverses membrane potential in a "spike"?
- inactivation of voltage-sensitive Ca2+ channels, and opening of K+ channels
- Are spike potentials always accompanied by contraction?
- Yes
- Are spike potentials necessary for contraction?
- No -- I do not understand why??? pg. 579
- What is the amplitude of contraction determined by?
- the # of spikes
- What sets the maximal rate of contraction in the gut?
- slow waves
- What characterized Unitary (gut) muscle?
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1) less tightly controlled by release of NT from nerve varicosities of passing neurons
2) more gap junctions
3) spontaneously active
4) no true neuromuscular junctions - Describe the nerves in longitudinal muscle (thinner layer) .
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1) very few nerve fibers
2) contracted by ACh from myenteric plexus
3) relaxation mainly due to inhibition of ACh release (very few inhibitory nerve fibers) - Describe the nerves in circular muscle (thicker layer).
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1) many neurons of varied type
2) release NT from varicosities
3) spread of excitation or inhibition thru gap junctions
4) relaxation predominates (VIP) - Describe reciprocal innervation.
- Circular muscle relaxes when longitudinal muscle contracts to move food down the GI tract. Comprises the Law of the Intestine and is an example of a Short Arc reflex - Peristaltic Reflex.
- What are the three patterns of motility in the gut?
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1) tonic or sustained contraction (sphincters) - neural innervation usually inhibitory
2) segmental contraction (mixing in small intestine and colon)
3) peristaltic contractions (propulsive, propogated contractions; mvmt toward anus; found throughout gut except proximal stomach) - Is swallowing voluntary or involuntary?
- both -- starts voluntary then becomes reflexive
- Three phases of the swallow:
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1) Oral/voluntary
2) Pharyngeal
3) Esophageal (includes primary and secondary peristalsis) - What is the difference between primary and secondary peristalsis?
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primary - follows pharyngeal and oral phase and is initiated by distension from bolus
secondary - via chemical stimulation of the mucosa - What nerves are responsible for mediating the neural reflex of peristalsis in the esophagus?
- the vagus (both efferent and afferent)
- What kind of muscle makes up the upper 1/3 of esophagus and what innervates it?
- Skeletal - innervated directly by vagus (ACh)
- What kind of muscle makes up the lower 1/3 of esophagus and what innervates it?
- Smooth muscle - innervated indirectly by vagus through enteric NS (ACh - excitatory, or VIP/NO - inhibitory)
- What fibers make up the middle 1/3 of esophagus?
- mixed skeletal and smooth muscle fibers
- What mediates the relaxation of the LES?
- VIP/NO from the myenteric plexus
- What are the major functions of the stomach?
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1) storage!! (reservoir)
2) grind and mix food
3) regulate delivery of nutrients to small intestine - What is unique about the fundus and orad corpus regions of the stomach?
- they are tonically contracted at rest due to low resting membrane potential (-48 mv) -- allow Ca2+ channels to remain open -- NO SLOW WAVES OR MYOELECTRICAL RHYTHM
- Description and function of Proximal stomach:
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1) muscle is thin
2) reservoir function
3) acid secretion - Description and function of Corpus (mid and caudad) stomach:
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1) muscle layer thicker
2) storage and mixing functions - Description and function of Antrum stomach:
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1) very thick muscle layer
2) pump/grinding
3) little secretion - Description and function of Pyloric Sphincter:
- 1) controls particle size and regulates emptying
- What happens to the duration and amplitude of contractions as you proceed down the stomach distally?
- the contractions increase in amplitude and duration
- How frequent are slow waves in the stomach?
- 3/min
- What part of the stomach has the strongest and longest contractions?
- Terminal (caudad) antrum
- Explain receptive relaxation.
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1) peristaltic wave in stomach relaxes the LES, fundus, and orad corpus
2) involves vagal preganglionic nerves and myenteric relaxant nerves w/ VIP/NOS - Where are pacemaker cells located?
- in the corpus of the stomach
- How much material (approx) is pushed thru the pyloric sphincter each time it opens?
- 10-15 ml
- Describe Gastric Emptying:
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1) liquids quicker than solids
2) water is fastest liquid
3) distension of stomach increases the rate of emptying (mechanoreceptors) - short arc and long arc (vago-vagal) reflexes - Do chemoreceptors in intestine speed or slow down gastric emptying?
- Slow down
- Components of Chyme which slow gastric emptying:
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1) Acid
2) Lipid digestion products (with CCK, GIP) ** most effective!!
3) Protein digestion products (with CCK, gastrin)
4) Hypo or Hyper-osmotic solutions - Novel site of action for CCK during gastric emptying:
- on sensory mechanoreceptors in stomach - activate vasovagal reflex - VIP - relaxation of fundus - reduce emptying