PHGY 210 Lecture Notes - Lecture 44: Myocyte, Resting Potential, Duodenum

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21 Jul 2016
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the%stomach%because%the%SI%is%small.%In%the%distal%stomach%which%is%thicker%walled,%we%find%the%
mechanism%of%motor%activity%is%peristalsis.%We%defined%it%as%a%propagated%wave%of%contraction%that%
moves%over%the%wall%of%the%stomach%and%narrows%the%lumen,%causing%contents%to%move%forward,%
peristalsis%starts%1/3rd%of%the%ay%down%of%the%stomach.%In%the%proximal%stomach,%there%is%no%
peristalsis.%It%passes%over%the%pyloric%sphincter%and%then%stops%there.%The%peristaltic%activity%is%a%
propagated%contraction%which%involves%a%series%of%local%enteric%reflexes%as%a%response%to%local%
distension.%The%amplitude%of%the%contractionN%how%vigorous%the%contraction%isN%is%related%to%the%
magnitude%of%the%stimulus.%This%is%modified%by%neural%and%hormonal%factors.%The%frequencyN%how%
many%peristaltic%waves%there%are%per%minuteN%depends%on%the%electrical%characteristics%of%the%
smooth%muscle%in%the%organ.%The%freq%is%a%factor%of%velocity.%If%oyu%were%to%put%electrodes%in%any%of%
the%muscle%cells%of%the%proximal%stomach,%you%would%find%a%steady%resting%potential,%but%this%isn’t%the%
case%for%the%distal%stomach.%You%would%find%rhythmic%waves%of%partial%depolarization%of%about%10N
15mV%lasting%1N4%seconds%and%recurring%at%regular%intervals%and%they%occur%every%20s,%so%3%a%minute%
!Distal%stomachN%these%waves%are%referred%to%as%the%slow%waves,%and%the%basic%electrical%rhythm%or%
BER%or%ECA.%If%you%know%where%to%put%electrodes%around%the%circumference%of%the%stomach,%you%find%
exactly%the%same%thing%occurring%synchronously.%But%if%you%were%to%put%electrodes%on%the%
longitudinal%axis%of%the%stomach,%each%cell%will%also%show%the%rhythmic%waves%of%depolarization,%but%
they%will%look%as%if%they%are%displaced%in%time.%it%is%the%same%wave,%but%it%occurs%a%fraction%of%the%
second%later.%So%these%waves%clearly%sweep%down%a%long%the%longitudinal%axis%of%the%stomach.%These%
slow%waves%of%electrical%rhythm%are%also%present%in%the%SI%at%all%times%(regardless%of%whether%you’ve%
eaten%or%not),%but%they%are%not%initiative%of%contraction.%However,%on%occasion,%you%see%a%second%
electrical%signal%at%the%peak%of%depolarization%of%the%slow%waves,%which%is%a%series%of%spikes%always%
occurring%at%the%peak%of%depolarization%(not%anywhere%else),%and%when%they%occur,%they%occur%
synchronously%around%the%circumference%and%will%appear%sequentially%along%the%longitudinal%axisN%
second%electrical%signal%or%ERA%or%spikes.%They%are%associated%with%the%contraction.%Whenever%the%
2nd%electrical%signal%occurs,%there%will%be%a%contraction,%but%it%always%occurs%at%the%peak%of%the%ber.%So%
the%BER%sets%up%the%tempospatial%distribution%of%the%contractions.%In%the%stomach,%the%BER%occurs%
1/20s,%so%the%maximum%frequency%of%contraction%for%the%spikes%cannot%exceed%this.%So%the%wave%of%
distribution%occurs%and%then%the%contraction.%The%magnitude%of%distention%determines%how%many%
spikes%there%are%which%in%turn%determines%what%the%amplitude%of%the%contraction%is.%If%we%had%more%
spikes%at%one%spot,%then%the%contraction%would%be%greater.%%
!Where%are%the%pacemaker%cells?%they%are%non%neuronal%and%not%part%of%the%enteric%innervationN%they%
are%specialized%fibroblasts%known%as%the%interstitial%cells%of%cajal%ICC.%StarNshaped%cells%with%long%
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