The Special Conducting System of the Heart. The sinoatrial node acts as the heart's pacemaker because it produces action potentials at a faster rate than anywhere in the heart and thereforeassumes control. The heart compensates automatically for extra filling of the ventricles by pumping more forcefully. This extra contractility occurs because a slight stretch of the muscle on filling reduces the overlap of actin and myosin filaments at the outset of contraction. This permits a greater length for ratcheting up the tension, hence a greater force of contraction.
Extra calcium moving into the muscle can have a similar effect.
G. Regulation of Heart Rate. Both the sympathetic and parasympathetic divisions affect heart rate. The resting heart is said to be regulated by vagal tone . When one division of the autonomic NS is more greatly stimulated by sensory input, the other division is temporarily inhibited. When the walls of the atria are stretched by additional venous return, sympathetic nerves stimulate the sinoatarial node to produce an increase in the rate and force of contraction.