Final answer:
The cells responsible for the autonomous rhythmic contractions of the heart are the pacemaker cells. They are self-excitable and located in the sinoatrial node, initiating the sinus rhythm and coordinating the heart's beating through the action potentials they generate.
Step-by-step explanation:
The cells that rhythmically and spontaneously generate action potentials in the heart are called pacemaker cells. These cells are unique because they possess autorhythmicity, which means they can depolarize to threshold and fire action potentials without any external stimuli. Located primarily in the sinoatrial (SA) node, pacemaker cells establish the heart's rhythm through a regular sequence of electrical impulses. This rhythm is known as the sinus rhythm, which is the normal, rhythmic beating of the heart. The action potentials generated travel through the heart's conduction system via gap junctions and ensure a coordinated contraction of the heart muscle, effectively pumping blood throughout the body.
Pacemaker cells also respond to signals from the autonomic nervous system (ANS) and hormones, adjusting the heart rate according to the body's needs. Their self-excitability and ability to influence the cardiac muscle contraction make them vital for the autonomous regulation of the heart.