Hearts can be classified into two groups based on how the heart contraction is initiated: myogenic and neurogenic. Neurogenic hearts depend on an extrinsic neural innervation to initiate the contraction. Without the innervation the heart doesn't beat. Neurogenic hearts are found in crustaceans, spiders, and some insects (Kay 1998).
Myogenic hearts contract spontaneously. Myogenic hearts are found in vertebrates and molluscs. In vertebrates a specialized region of cells called the sinoatrial (SA) node, acts as the pacemaker for the heart. Cells throughout vertebrate cardiac muscle are self-excitable, meaning they contract without any signal from the nervous system. The SA node manages to coordinate the contraction of individual cardiac muscle cells through electrical innervation, impulses. Interestingly, if the SA node (the primary pacemaker region) fails, another region of the heart can assume pacemaker activity because other cells in the heart can spontaneously and regularly contract (Schmidt-Nielson 1997).
The cell membrane of the SA node has an unstable resting membrane potential. The instability allows the membrane potential to drift towards the threshold. Each time the threshold is reached, an action potential is generated and a heartbeat is initiated. This depolarization spreads through the rest of the cardiac muscle and causes a contraction.