Which ion is primarily responsible for distinguishing fast versus slow myocardial action potentials?

The distinction between fast and slow myocardial action potentials is primarily influenced by the ionic currents that characterize each type of response. In fast action potentials, particularly evident in the atrial and ventricular myocardium, sodium ions (Na+) play a crucial role due to their rapid influx during depolarization. This swift action creates a steep upstroke in the action potential.

In contrast, slow action potentials are typically associated with pacemaker cells in the sinoatrial (SA) node and the atrioventricular (AV) node, where calcium ions (Ca2+) are the primary contributors. The influx of calcium through L-type calcium channels during the plateau phase is responsible for the slower depolarization and prolonged action potential duration characteristic of these cells. This calcium-induced depolarization represents the slower conduction and pacemaker activity that is essential for regulating heart rhythm.

Thus, the presence and dynamics of calcium ions distinguish slow myocardial action potentials from the faster ones, making them integral to the physiological differences observed in cardiac tissue.