The heart initiates its activity through a property known as auto-rhythmicity. This is the ability of the autorhythmic cells, to spontaneously generate and conduct electrical potentials at a fixed rate. Unlike skeletal or smooth muscle cells, these cardiac cells have the unique capability to spontaneously generate action potentials, a feature essential to the heart’s function as a self-activating pump.
Action Potential Generation in Autorhythmic Cells: Autorhythmic cells, unlike most other cells, do not have a stable resting potential. These cells gradually depolarize, a phase known as the pacemaker potential, which leads to the generation of an action potential. This gradual depolarization is due to a slow influx of sodium ions (Na⁺) and a decrease in the efflux of potassium ions (K⁺). When the pacemaker potential reaches a certain threshold, voltage-gated calcium channels open, allowing calcium ions (Ca²⁺) to enter the cell. This influx of calcium ions leads to the rapid depolarization phase of the action potential. The unique role of calcium ions in the depolarization phase is a key difference between autorhythmic cells and other excitable cells, like neurons, where the rapid depolarization is mainly due to sodium ions.
