Pacemaker activity comes from specialized atrial cells that are fundamentally different from ordinary cardiac myocytes.

They fire automatically, without external stimulus, with the following ionic sequence:

• Continuous slow inward leak of Na⁺ until the threshold potential of -40 mV is reached
• Main depolarization brought about by Ca²⁺ (not Na⁺) inflow through L-type channels
• Repolarization from K⁺ outflow

There is no resting phase or resting membrane potential, and phases 1 and 2 of the action potential are absent. The cycle length determines the heart rate.

Question: How is the heart rate changed?

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Pacemaker activity comes from specialized atrial cells that are fundamentally different from ordinary cardiac myocytes.

They fire automatically, without external stimulus, with the following ionic sequence:

• Continuous slow inward leak of Na⁺ until the threshold potential of -40 mV is reached
• Main depolarization brought about by Ca²⁺ (not Na⁺) inflow through L-type channels
• Repolarization from K⁺ outflow

There is no resting phase or resting membrane potential, and phases 1 and 2 of the action potential are absent. The cycle length determines the heart rate.

Question: How is the heart rate changed?

Answer: The slope of phase 4 is altered to make the cycle length shorter or longer. This is brought about by increases in permeability to Na⁺ (tachycardia) or K⁺ (bradycardia).