The autonomic nervous system (ANS) regulates internal organ activities, usually involuntarily and automatically.

This system regulates sweating, alters the size of the pupils, and directs many other body adjustments. The ANS influences the heart rate and myocardial contractility (the ability of cardiac muscle cells or tissues to shorten when stimulated) by sympathetic and/or parasympathetic stimulation.

Sympathetic impulses increase heart activity; parasympathetic impulses decrease heart activity. Both of these systems must be in balance for the heart to function properly. If one or the other system is stimulated abnormally or blocked, the result will be heart arrhythmias. See figure 1-3 for autonomic nervous system control of the heart.