- The properties studied are extrasystole, compensatory pause, refractory period, all or none law, stair case phenomenon and summation of subminimal stimuli.
Q1. What is extrasystole? Why is it followed by a compensatory pause?
When the ventricle is stimulated during Relative Refractory
Period of relaxation, a contraction occurs earlier than the normally expected
one, hence called extrasystole. The next impulse arrives in the Absolute
Refractory Period of the extrasystole, hence fails to evoke a response
resulting in a pause following an extrasystole which is known as compensatory
pause.
The response following the compensatory pause is greater
than the previous one due to accumulation of calcium ions during the pause.
Q2. Why cannot cardiac muscle be tetanized?
Cardiac muscle has long refractory period. The Absolute Refractory period is about 250 ms. and the Relative Refractory Period is 50 ms. During Absolute Refractory Period, a stimulus cannot re-excite the tissue, no matter how strong the stimulus is. Because of this long refractory period the cardiac muscle cannot be stimulated immediately and hence it cannot be tetanised.