Question

Describe in detail the action potential of myocardial contractile cells. Be sure to names the phases, include the main types of membrane proteins involved, how they are opened, the movements of ions, and the changes in membrane potential. Explain how this type of action potential is different from that seen in a skeletal muscle cell and why the difference(s) are significant.

Answer 1

Action potential of contractile cells of heart

Duration of action potential of cardiac muscle is .25 to .35 seconds

Phases of action potential

1. Initial depolarisation
2. Initial repolarization
3. Plateau or final step of depolarisation
4. Final repolarization

Explainarion of different phases

1 ) initial phase- it is rapid and last for about 2 milliseconds. In this rapid opening of fast sodium channels and rapid influx of sodium occur

2) initial repolarization

It is the immediatebphase after depolarisation which is a rapid repolarization for about 2milliseconds . This is due to opening of pottasium channel and efflux of small amount of pottasium ions . As well as closure of fast sodium channel and opening of slow sodium channel occur

3) plateau or final depolarisation

It last for about 200milliseconds in atrial muscle fibers and 300milliseconds in ventricular muscle fibers. In this stage the muscle fibers initially stay in depolarisation state for some time and then quick repolarization take place resulting in formation of a plateau. It is due to slow opening calcium channels . They are open for long time resulting in influex of large number of calciuyioms . Calcium and sodium enter into cells and pottasium is moved out result indepolarization

4) final repolarization

It last for 50 to 80milliseconds . In this stage the number of pottasium ions moving out exceeds the number of calcium ions moving in that resukte in negativity inside resulting in final repolarization.

Difference of caridiac action potential from skeletal muscle

The skeltal muscle action potential time is two to five milliseconds where as in cardiac muscle it is about .25- .35 milliseconds

This is because cardiac muscle are those muscles that work in our body without rest and for efficiency of the muscles this change occur