Skip to main content

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 ) .

Comments

  1. plzzz tell me the exact defination of refrectory period

    ReplyDelete
  2. Useful.... I would like to have the graph of skeletal muscle tension like that of the cardiac muscle..

    ReplyDelete
  3. 😄😄 .....very good ....thank you very much

    ReplyDelete
  4. amazing explanation thanks a lot

    ReplyDelete
  5. I hv undrstood wel...but tel what are the effect if woukd no r

    ReplyDelete
  6. Authentic submit today! I genuinely like this excellent resource with a pile up details regarding the subject. Thumbs up for you! residency personal statement

    ReplyDelete
  7. Very helpful!! Life saver :)

    ReplyDelete
  8. i didn't understand the Relative Refractory period
    could you explain it more, please?

    ReplyDelete
  9. Very nice Article good information Great post.Do you want to ask any queries related to your Yahoo from our experts will answer you in no time. For Ishq Pakeezah Zee TV Ishq Pakeezah Serial Zee TV 2018, Star Cast & Crew Promo, Start Date, Timings Ishq Pakeezah Serial on Zee TV Wiki 2018,Ishq Pakeezah Full Episode If you really facing an issues in AOL emails service, for more information visit AOL customer support.

    ReplyDelete

Post a Comment

Popular posts from this blog

Q&A : Consequences of chordae tendineae rupture & papillary muscles paralysis

Q : Hello doctor,What would happen if the corda tendinea becames ruptured or if papillary muscle becames paralyzed?Sabreen Yaqoub
A: Rupturing of chordae tendineae , usually results from infective endocarditis  as a primary cause and from other inflammatory diseases as a secondary cause. Healed infective endocarditis may be a significant cause too. Rupturing will lead to a valvular insufficiency , that could be severe , leading to cardiac decompensation and death.  Rupturing of chordae tendineae of the mitral valve is more common than the tricuspid valve. The severity of complications depends on the larger involvement of the chordae . ( More ruptured chordae causes more severe complications ) To explain the pathophysiological mechanism , lets consider that the ruptured chordae are those of the mitral valve : Insufficiency of the valve will lead to regurgitation ( back flow ) of blood from the left ventricle to the left atrium . This will lead to increased  intraatrial pressure , which will …

Neurotransmitters

Neurotransmitters are biologically active , endogenous , chemicals that transmit signals from a neuron to an other neuron or other target cell across a synapse .
They are synthesized in the presynaptic neuron from precursors and then packaged in synaptic vesicles , and released after reaching of an action potential to the presynaptic neuron , usually by Exocytosis.
After releasing of a neurotransmitter into the synaptic cleft , it binds to a receptor on the post-synaptic membrane and then causes a response in the postsynaptic target cell.
Electrical signal ( action potential) is necessary for releasing of a neurotransmitter.

* criteria used to determine a neurotransmitter :
Classical criteria were used to determine a given chemical as neurotransmitter , some of them are:
1- To exist in the presynaptic neuron in sufficient quantity
2- To be produced in the presynaptic neuron ( to have precursors and enzyme system that
3- to be present within a neuron
3- to have specific postsynaptic r…