Heart rate is one of the determinants of
cardiac output. The heart rate is determined by the
pacemaker cells. Typically, heart beats originate from the
sinoatrial node. However, other sites along the conduction pathway also contain pacemaker cells that, while they tend to have a slower rate of
depolarization, can also cause
contraction. The ability of these cells to generate an
action potential involves the slow depolarization driven by the closure of K+ channels, the opening of non-selective cation channels (funny channels) and the ultimate opening of the Ca2+ channels. The autonomic nervous system is the primary modulator of heart rate. This means that activity of sympathetic and parasympathetic neurons must be able to alter the slope of the slow depolarization of the pacemaker cells. There are sympathetic and parasympathetic neurons that innervate the sinoatrial node, atrioventricular node, and other regions of the conduction pathway that can alter heart rate. Increases in sympathetic activity will increase the slope of the slow depolarization, causing heart rate to increase, whereas parasympathetic activity will decrease the slope of the slow depolarization, causing heart rate to decrease. A number of hormones like
epinephrine,
glucagon,
thyroid hormone, and others act in a manner similar to sympathetic activity. Each of the hormones elevates heart rate by increasing the slope of the slow depolarization. Heart rate independent of the
autonomic nervous system tends to be around 100 bts/min. Since normal heart rate for adults tends to be less than that, the heart is said to be under resting parasympathetic tone. However, this is not to say that the
sympathetic nervous system is not active.
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