Pacemaker action potential

Pacemaker action potential refers to the unique electrical activity that occurs in certain cells of the heart, which are responsible for initiating and regulating the heart's rhythm. These cells, known as pacemaker cells, are primarily located in the sinoatrial (SA) node, but can also be found in the atrioventricular (AV) node and other parts of the heart's conduction system. The pacemaker action potential is crucial for the heart's automaticity, allowing it to beat rhythmically without external stimuli.

Phases of Pacemaker Action Potential

The pacemaker action potential differs significantly from the action potential in other cardiac cells, such as myocardial cells, in both its shape and the ionic currents involved. It consists of three main phases:

Phase 4 - Spontaneous Depolarization

This phase is also known as the pacemaker potential. Unlike other cardiac cells, pacemaker cells do not have a stable resting membrane potential. Instead, they gradually depolarize due to an influx of sodium ions (Na+) through funny channels (I_f channels) and a decrease in the efflux of potassium ions (K+). This gradual depolarization continues until the threshold potential is reached, triggering the next phase.

Phase 0 - Upstroke

Upon reaching the threshold potential, voltage-gated calcium channels (mainly L-type Ca²⁺ channels) open, leading to a rapid influx of Ca²⁺ ions into the cell. This influx of calcium ions causes the rapid depolarization phase of the pacemaker action potential, which is less steep compared to the upstroke in myocardial action potentials due to the absence of fast Na+ channels.

Phase 3 - Repolarization

Repolarization in pacemaker cells is initiated by the closure of calcium channels and the opening of potassium channels, allowing K+ to exit the cell. This efflux of K+ ions brings the membrane potential back towards its most negative level, setting the stage for another cycle of spontaneous depolarization.

Regulation of Pacemaker Activity

The rate of pacemaker activity and, consequently, the heart rate, is modulated by the autonomic nervous system through the release of neurotransmitters. The parasympathetic nervous system, via the neurotransmitter Acetylcholine, slows down the heart rate by increasing potassium efflux and decreasing calcium influx during spontaneous depolarization. Conversely, the sympathetic nervous system increases heart rate by releasing Norepinephrine, which enhances calcium influx during phase 0 and decreases potassium efflux during phase 3.

Clinical Significance

Abnormalities in pacemaker function can lead to various arrhythmias, such as Bradycardia (abnormally slow heart rate) or Tachycardia (abnormally fast heart rate). In some cases, artificial pacemakers may be required to maintain a normal heart rhythm. These devices mimic the natural pacemaker action potential to regulate the heartbeat.

See Also

• Cardiac electrophysiology
• Electrocardiogram
• Heart block
• Sinoatrial node dysfunction