Final answer:
The SA node's firing and the cardiac cycle are controlled by the heart's pacemaker function, with modulation by the ANS and hormonal signals.
Step-by-step explanation:
The firing of the SA node, which triggers the start of the cardiac cycle, is controlled by the autonomous pacemaker function of the heart. This pacemaker generates electrical signals that begin at the SA node located in the right atrium and spread through the heart to coordinate the contraction of the atria and ventricles.
The autonomic nervous system (ANS) and various hormones can fine-tune the heart rate by signaling the pacemaker cells, either to increase or decrease the heart's pulsing rate as needed. The cardiac cycle is a carefully timed series of electrical and mechanical events that ensure efficient pumping of blood throughout the body.
During the cardiac cycle, the electrical impulse begins at the SA node and then travels to the atrioventricular (AV) node where it is briefly delayed, allowing the atria to complete their contraction and the ventricles to fill with blood.
Afterwards, the electrical impulse continues through the bundle of His, down the left and right bundle branches, and into the Purkinje fibers. This network ensures that the ventricles contract from the bottom up, effectively pushing blood out of the heart. Influences such as ANS inputs, hormones, and intrinsic rhythmicity regulate this sequence to maintain proper heart function.