When the heart is externally stimulated just after the start of the contraction cycle,?
this is b/c once a contraction has started the AP has already fired the muscle fibers. once they are fired in the heart there is : depolarization a plateau a rapid re-polarization then a small refractory period only after the refractory period can the heart fire again, this is b/c the Na+ and K+ balances must return to the state they were just before depolarization.
