Plateau phase in Cardiac Action Potential

[WolfmanHarris]

So there's no answer key in my A&P workbook and I have a question which I answered, but can't seem to verify against my text.

The question is:

"State the significance of the prolonged depolarization of cardiac muscle compared to the brief depolarization of skeletal muscle."

My answer in that the plateau phase allows for greater inflow of Ca+ as the channels are kept open against the balance of K+ outflow from the contractile fibre. Since Ca+ binds with troponin to allow the contraction, the greater amount of Ca+ moving into the muscle fibre over the extended depolarization results in a stronger force of contraction than skeletal muscle.

Am I correct on this or have I missed something entirely. I've already reread the relevant section of the text a few times and haven't found a problem here, but this text doesn't spoon feed questions that match the text exactly. Makes for better questions but harder to verify answers.

Thanks again for the study help.

- Matt

[Mateo_1387]

Great question Doc !

I may have the answer, but if not, sorry for wasting your time.

Here goes:

While more calcium influx into the myocardial cells causes a stronger contraction, it does not truly explain the length of the plateau phase of cardiac conduction. It may be that the plateau phase is longer so that the action potential can be spread to all ventricular tissue. Skeletal muscle does not all contract at once. Hence we are able to control the amount of skeletal muscle movement. For the heart though, contraction of the cardiac muscle needs to be synchronous. My guess is that the prolonged plateau phase allows time for all ventricular tissue to contract synchronously. Also, since ventricular tissue relies on gap junctions to transmit electric signals, it will need 'extra time' since it lacks specialized conduction tissue.

I am not too sure though. Great question to help guide a review of important A&P concepts.