The heart is made up of a specialized tissue - the cardiac muscle, giving rise to an intrinsic, regular heartbeat. Although the heart beats spontaneously (requires no external stimulation), it receives continuous input from the sympathetic and parasympathetic nervous systems. Sympathetic nerves increase both rate and force of contraction. Parasympathetic effects are essentially opposite. 

A measured heart rate - derived from ECG, Blood Pressure, or another pulsatile measurement indicating a heart contraction - can be used as a simple indicator of the acute or chronic condition of a subject, or in-vitro isolated heart. Because of its simple derivation and overall utility, it is one of the most common signals recorded in physiological research.