6 / 56
14
1
The Basics
If we record one electrical cycle of depolarization and repolarization
from a single cell, we obtain an electrical tracing called an
action
potential
. With each spontaneous depolarization, a new action
potential is generated, which in turn stimulates neighboring cells to
depolarize and generate their own action potential, and so on and on,
until the entire heart has been depolarized.
The action potential of a cardiac pacemaker cell looks a little
different than the generic action potential shown here. A pacemaker
cell does
not
have a true resting potential. Its electrical charge
drops to a minimal negative potential of approximately −60 mV,
which it maintains for just a moment (it does not rest there), and
then gradually rises until it reaches the threshold for the sudden
depolarization that is an action potential. These events are illustrated
on the following tracing.
A pacemaker cell depolarizing spontaneously.
Maximum voltage at
peak depolarization
Resting potential
A typical action potential.