Rhythmical beating of the heart at a rate of approximately 100 beats/min will occur in the complete absence of any nervous or hormonal influences on the SA node. This is the inherent autonomous discharge rate of the SA node. The heart rate may be much lower or higher than this, however, since the SA node is normally under the constant influence of nerves and hormones. A large number of parasympathetic and sympathetic postganglionic fibers end on the SA node. Activity in the parasympathetic (vagus) nerves causes the heart rate to decrease, whereas activity in the sympathetic nerves increases the heart rate. In the resting state, there is considerably more parasympathetic activity to the heart than sympathetic, and so the normal resting heart rate of about 70 beats/min is well below the inherent rate of 100 beats/min.
Sympathetic stimulation increases the slope of the pacemaker potential by increasing the If (sodium) and T-type calcium currents. This causes the SA node cells to reach threshold more rapidly and the heart rate to increase. Stimulation of the parasympathetics has the opposite effect—the slope of the pacemaker potential decreases due to a reduction in the inward currents. Threshold is thus reached more slowly, and heart rate decreases. Parasympathetic stimulation also hyperpolarizes the plasma membrane of SA node cells by increasing the permeability to potassium. The pacemaker potential thus starts from a more negative value (closer to the potassium equilibrium potential). Factors other than the cardiac nerves can also alter heart rate. Epinephrine, the main hormone liberated from the adrenal medulla, speeds the heart by acting on the same beta-adrenergic receptors in the SA node as norepinephrine released from neurons.
The heart rate is also sensitive to changes in body temperature, plasma electrolyte concentrations, hormones other than epinephrine, and a metabolite—adenosine— produced by myocardial cells. These factors are normally of lesser importance, however, than the cardiac nerves. sympathetic and parasympathetic neurons innervate not only the SA node but other parts of the conducting system as well. Sympathetic stimulation also increases conduction velocity through the AV node and other cells of the conducting system, whereas parasympathetic stimulation decreases the rate of spread of excitation through the AV node.