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Anti-arrhythmicsPhysiology
Cardiac action potential
The heart is composed of pacemaker, conducting and contractile tissue. Each has a
different action potential morphology allowing the heart to function as a coordinated
unit.
The SA node is in the right atrium, and of all cardiac tissue it has the fastest rate
of spontaneous depolarization so that it sets the heart rate. The slow spontaneous
depolarization (pre-potential or pacemaker potential) of the membrane potential
is due to increased Ca^2 +conductance (directed inward). At−40 mV, slow voltage-
gated Ca^2 +channels (L channels) open resulting in membrane depolarization. Na+
conductance changes very little. Repolarization is due to increased K+conductance
while Ca^2 +channels close (Figure14.1a).
Contractile cardiac tissue has a more stable resting potential at−80 mV. Its action
potential has been divided into five phases (Figure14.1b):
Phase 0 – describes the rapid depolarization (duration <1 ms) of the membrane,
resulting from increased Na+(and possibly some Ca^2 +) conductance through
voltage-gated Na+channels.
Phase1–represents closure of the Na+channels while Cl−is expelled.
Plateau phase2–dueto Ca 2 +influx via voltage-sensitive type-L Ca 2 +channels
and lasts up to 150 ms. This period is also known as the absolute refractory period
in which the myocyte cannot be further depolarized. This prevents myocardial
tetany.
Phase 3 – commences when the Ca 2 +channels are inactivated and there is an
increase in K+conductance that returns the membrane potential to its rest-
ing value. This period is also known as the relative refractory period in which
the myocyte requires a greater than normal stimulus to provoke a contrac-
tion.
Phase 4 – during this the Na+/K+ATPase maintains the ionic concentration gra-
dient at about−80 mV, although there will be variable spontaneous ‘diastolic’
depolarization.