What characteristic of the heart allows cells to respond to external stimuli?

The characteristic of the heart that allows cells to respond to external stimuli is excitability. This property refers to the ability of cardiac cells to respond to electrical impulses, which can initiate an action potential. When these cells are stimulated by changes in the electrical environment, they can create and transmit electrical signals that lead to heart contractions. This responsiveness is crucial for the heart's function, as it allows it to react to various stimuli, such as changes in heart rate due to exercise or stress, thus maintaining proper blood circulation.

Automaticity, while related to the generation of impulses without external stimuli, focuses more on the heart's inherent ability to initiate its rhythm. Contractility refers to the strength of the heart's contraction, and conductivity relates to how quickly electrical signals pass through the heart tissue. These characteristics are essential for overall heart function but do not specifically address the ability of cardiac cells to react to external stimuli.