Final answer:
Sympathetic stimulation of the heart through the cardioaccelerator nerves signals the heart to beat faster during exercise.
Step-by-step explanation:
The mechanism that signals the heart to beat faster during exercise is sympathetic stimulation of the heart through the cardioaccelerator nerves. The sympathetic nerves release the neurotransmitter norepinephrine, which speeds up the rate of depolarization and contraction, leading to an increase in heart rate. This sympathetic stimulation is part of the autonomic nervous system's response to increased physical activity.