Friday, 11 October 2013

Q&A : Absolute vs. Refractory period of cardiac muscle cells .

I have been asked by many students about absolute vs. relative refractory period of the cardiac muscle cells . I always notice some confusion in their eyes during the lecture , so i decided to explain that in this essay .
To understand that better we have firstly to understand what does refractory period in excitable cells mean ? then to compare between refractory period of skeletal and cardiac muscle , then to link that to excitatory contraction coupling in both muscle types to extract the benefit of the refractory period.
Refractory period in excitable cells mean: Inability of the cell to respond to any stimulus ( abolishing of excitability ) during a period of time.
In skeletal muscle cells , refractory period is quite short (1-2 ms ) , it usually coincides with the depolarization phase of the action potential . Comparing to the contraction phase duration ( about 50 ms ) and the relaxation phase duration (about 50 ms too) the refractory period of the skeletal muscle is quite short. This mean that the muscle could be re-excited while it contracts , and to start a new contraction before the previous one proceed , and this event is the reason for the physiological tetanus of the skeletal muscle ( repetitive contraction ) .

In cardiac muscle cells , There are absolute and relative refractory periods . Absolute refractory period : is a period during which the cardiac excitability is totally cancelled ( cardiac muscle will not respond to any stimulus) . It prologs from the beginning of the depolarization phase till the end of plateau phase ( about 250 -300 ms ) about hundred times longer that of the skeletal muscle`s refractory period. By the way : This period coincides with the systole (contraction) of the cardiac muscle, so: thankful to this absolute period an other contraction would never occur except after completion of the systole . and thus tetanisation of cardiac muscle is impossible .
Relative refractory period : is a period that follows the absolute period , and the excitability begins to recover gradually until it reaches its normal value. Relative refractory period coincides with the period of rapid polarization following the plateau.

Thankful to short refractory period in skeletal muscle we can lift objects for example and adequately use our muscle , because if skeletal muscles contract and relax for 100 ms each time we would not be able to make any quick action.
On the other hand: Thankful to the long refractory period of the cardiac muscle , it would be impossible to excite the heart before the proceeding of the contraction , which enables the heart to pump blood and then to relax to be filled with blood in what is known as (cardiac cycle ) .

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