The ability of cardiac cells to contract or depolarize spontaneously without external influence is called

• A. Automaticity
• B. Excitability
• C. Conductivity
• D. Refractoriness

Answer: (A)

Automaticity is the heart’s ability to generate spontaneous depolarizations and contractions without external stimulation. This property mainly resides in pacemaker cells, especially in the sinoatrial node, where the cell membrane slowly depolarizes during phase 4 (diastolic depolarization) until it reaches threshold and fires an action potential. This intrinsic rhythmic activity sets the heart’s pace and can be modulated by autonomic input, without requiring a nervous impulse from outside.

This concept is distinct from excitability, which is the ability to respond to a stimulus; conductivity, which is the propagation of the impulse through cardiac tissue; and refractoriness, the period after an action potential during which cells are temporarily unresponsive to new stimulation.