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The frequency of discharge of pacemaker cells is primarily determined by the rate of phase 4 depolarization. Pacemaker cells, such as those found in the sinoatrial (SA) node, exhibit a unique action potential profile characterized by spontaneous depolarization during phase 4. This gradual depolarization leads to the eventual threshold being reached, which triggers an action potential.

The rate at which phase 4 depolarization occurs directly influences how quickly these cells reach the threshold for firing an action potential, hence determining their discharge frequency. An increased rate of phase 4 depolarization results in a higher frequency of action potentials, while a slower rate leads to a lower frequency.

While threshold potential and resting membrane potential are important in setting the overall excitability of the cells, they do not alone dictate the frequency of discharge. Rather, they serve to establish the baseline conditions under which phase 4 depolarization occurs. Similarly, the rate of phase 2 depolarization does not play a significant role in pacemaker activity as it pertains to the fast responses of myocardial cells rather than the gradual automaticity of pacemaker cells.

Thus, the essence of pacemaker activity is encapsulated in the characteristics of phase 4 depolarization,