Contraction of cardiac muscle cells to eject blood is triggered by action potentials sweeping across the muscle cell membranes.
The heart contracts, or beats, rhythmically as a result of action potentials that it generates by itself, a property called autorhythmicity (auto means “self ” ).
There are two specialized types of cardiac muscle cells:
- Contractile cells, which are 99% of the cardiac muscle cells, do the mechanical work of pumping. Th ese working cells normally do not initiate their own action potentials.
- In contrast, the small but extremely important remainder of the cardiac cells, the autorhythmic cells, do not contract but instead are specialized for initiating and conducting the action potentials responsible for contraction of the working cells.
Cardiac autorhythmic cells display pacemaker activity.
In contrast to nerve and skeletal muscle cells, in which the membrane remains at constant resting potential unless the cell is stimulated, the cardiac autorhythmic cells do not have a resting potential. Instead, they display pacemaker activity; that is, their membrane potential slowly depolarizes, or drifts, between action potentials until threshold is reached, at which time the membrane fi res or has an action potential.
An autorhythmic cell membrane’s slow drift to threshold is called the
pacemaker potential.Through repeated cycles of drift and fi re, these autorhythmic cells cyclically initiate action potentials, which then spread throughout the heart to trigger rhythmic beating without any nervous stimulation.