The refractory period in cardiac muscle is the period after the ventricular contraction during which both the atria and the ventricles are at rest. It is the time needed for repolarization of the muscle cell membranes before they can be stimulated again by an impulse from either the sinoatrial node or the atrioventricular node. This period lasts for 0.4 seconds (Applegate, 2000, 251). The repolarization time in cardiac muscle is roughly the same length of time (250 msec) as the time for a contraction and so contractions cannot overlap (Kimball, 2004). The refractory period in skeletal muscle is only 1 - 2 msec, which is much shorter than the contraction/relaxation cycle of skeletal muscle (50 msec for contraction, 60-100 msec for relaxation). This means the skeletal muscle cells can be stimulated by a series of stimuli in rapid succession. At about five to 10 stimuli per second, the responses to stimuli begin to fuse together in a process known as clonus, and when the rate reaches 50 stimuli per second, a smooth sustained contraction known as tetanus occurs. In this state, the muscle cannot respond to further stimuli.
The refractory period of cardiac muscle has two stages: the Absolute refractory period and the Relative refractory period (Nurse, 2004). In the Absolute refractory period, the muscle cannot respond to an impulse. In the Relative refractory period, the cells are not fully repolarized, but will try to contract if they receive an impulse. However, the response will not be as strong as a normal response to such an impulse. Premature contractions due to responses of cardiac muscle during the Relative refractory period result in poorly filled ventricles (since 70 percent of the filling of the ventricles occurs during the refractory period) and a poor ejection of blood from the heart (Applegate, 2000, 251; Nurse, 2004). It can also result in a life-threatening arrhythmia which compromises the heart's abi...