SA node

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• SA node
• ?????
• AV node
• Bundle of His
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• ????? ?- AV node
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• ????? 0.5 m/sec
• AV node 0.05 m/sec
• Bundle of His 2 m/sec
• ??? ???????? 4 m/sec
• ????? 0.5 m/sec

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When these cells are rapidly depolarized to a
threshold voltage of about -70 mV (e.g., by
another conducted action potential), there is a
rapid depolarization (phase 0) that is caused by
a transient increase in fast Na-channel
conductance. At the same time, gK falls. These
two conductance changes move the membrane
potential away from EK and closer toward ENa. 
Phase 1 represents an initial repolarization that
is caused by the opening of a special type of K
channel (not shown in the figure).  However,
because of the large increase in slow inward
gCa, the repolarization is delayed and there is
a plateau phase in the action potential (phase
2).  This inward calcium movement is through
long-lasting (L-type) calcium channels that open
up when the membrane potential depolarizes to
about -40 mV.  Repolarization (phase 3) occurs
when gK increases and gCa decreases.
Therefore, the action potential in non-pacemaker
cells is primarily determined by changes in fast
Na, slow Ca and K conductances as summarized
below and in the figure to the right.
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• ?? QRS ?????????? ??????
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• ?? T ?????????? ??????

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• ????? PR (PQ) ??? ????? ??? ????? ??????
• ????? ????? ????? ?????? ?- AV node
• ????? QT ??? ???????? ????? ????
• ????? PP ??? ??????
• ????? QQ ??? ??????

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EKG ????? ?????
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EKG ?? ????
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SA node ????? ? -