Is the beating of the heart initiated by the heart itself or by the central nervous system? What role does the autonomic nervous system play in affecting the heart? Explain how "nodal tissue" initiates a heart beat.
The beating of the heart is due to a system of stimulation inherent to the heart itself and does not require stimulation from the central nervous system. Two sets of nerves from the autonomic system do modulate the heartbeat, though, one set tending to accelerate the heart rate and one set tending to slow the heart rate. The left and right sides of the heart beat in unison each half being stimulated by a single set of heartbeat initiating mechanisms. The initiating mechanisms are composed of nodal tissue, a specialized type of cardiac muscle that can generate ionic action potentials. The sino-atrial node or S-A node is located near the site where the anterior vena cava empties into the right atrium. The S-A node initiates the heartbeat creating an impulse that causes the walls of both atria of the heart to contract. The second node, the atrio-ventricular node or A-V node, is located in the partition between the two atria. When the wave of contraction initiated by the S-A node reaches the A-V node, it sends an impulse through a branching bundle of nodal tissue fibers, called the bundle of His, to all parts of both ventricles causing the ventricles to contract.
