Pacemaker potential
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In the heart, the pacemaker potential is the voltage created by impulses from an artificial electronic pacemaker or the SA node which drives the rhythmic firing of the heart.
The pacemaker potential brings the membrane potential to the threshold potential and initiates an action potential.
Distinctions among autonomic foci
In reality, the heart has several pacemakers known as autonomic foci, each which fires at its own intrinsic rate:
- SA node: 80 - 100 bpm
- Atrial foci: 60 - 80 bpm
- Junctional foci: 40 - 60 bpm
- Ventrical foci: 20 - 40 bpm
The potentials will normally travel in order SA node -> atrial foci -> junctional foci -> ventricular foci
Pacemaker potentials are fired not only by SA node, but the other foci. However, the other firing frequencies are slower than the one of the AV node (as seen above). Normally, all the foci will end up firing at the SA node rate, not their intrinsic rate. The other foci attempt to fire at their intrinsic rate, but they are activated by the SA node before they can fire. This rapid firing causes all the foci to fire faster than their intrinsic rates, a phenomenon known as overdrive-suppression. Thus, in the normal, healthy heart, only the SA node intrinsic rate is observable.
Pathology
However, in pathological conditions, the intrinsic rate becomes apparent. Consider a heart attack which damages the region of the heart between the SA node and the atrial foci.
SA node -> |block| atrial foci -> junctional foci -> ventricular foci
The other foci will not see the SA node firing; however, they will see the atrial foci. The heart will now beat at the intrinsic rate of the atrial foci.
Induction
The firing of the pacemaker cells is induced, as the firing of action potentials in any human action-potential rising cell (like nerve cells), electrically, by the reaching of the threshold potential of the cell membrane. It is reached by the constant increase of membrane potential (induced by the special "funny channel", which by its Na+-channel activity giving rise to a current, the "funny current"). Upon the reaching of the threshold potential, the action potential is fired, and then reset.
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