Ace the CVS Pharmacy Exam 2025 - Your Prescription for Success!

The correct choice identifies excitatory and conductive muscle fibers as those that depolarize spontaneously. These specialized muscle fibers, primarily found in the sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje fibers of the heart, play a crucial role in the heart's electrical conduction system. Unlike other cardiac muscle fibers, which are primarily responsible for contraction, these fibers are designed to initiate and propagate electrical impulses, allowing for the automatic rhythm of the heartbeat.

Spontaneous depolarization in these fibers occurs due to unique ion channel activity that results in a gradual change in membrane potential until a threshold is reached, leading to an action potential. This ability to self-generate impulses is essential for coordinating the heart's rhythm and ensuring effective blood flow.

In contrast, atrial and ventricular muscle fibers do not possess this intrinsic ability to depolarize on their own. They rely on signals from the excitatory and conductive fibers for stimulation, which trigger their own contraction. Therefore, while all cardiac muscle fibers play vital roles in heart function, only excitatory and conductive muscle fibers have the capacity for spontaneous depolarization.