pacemaker

One who sets the pace in a race.

A leader in a field.

A part of the body, such as the mass of muscle fibers of the sinoatrial node, that sets the pace or rhythm of physiological activity.

Any of several usually miniaturized and surgically implanted electronic devices used to stimulate or regulate contractions of the heart muscle.

A substance that regulates a series of related reactions.

One who sets the pace for others, as in racing.

One who or that which makes a record for speed.

a specialized bit of heart tissue that controls the heartbeat.