Question

Describe in detail excitation-contraction coupling in myocardial cells. Be sure to include the main types of

membrane proteins involved, and the changes in membrane potential, the movements of ions and other

structures involved. Explain how excitation-contraction coupling in cardiac muscle cells is different from

excitation-contraction coupling in skeletal muscle cells. Explain how cardiac muscle cells relax.

Answer 1

Excitation-contraction coupling in myocardial cells

The action potential activates the myocardial cells in order to estabslish the excitation-contraction coupling.It occurs through several steps which is sequentially as follows-

1.Action potential resides in sarcolemma and down in to transverse tubule system facilitates the cell membrane depolarization.

2.In phase 2 of action potential, L-type of calcium channel -voltage sensitive dihydropyridine (DHP) receptor allows the entry of calcium inside cell.

3.Calcium entry activates the release of stored calcium from the sarcoplasmic reticulum through ryanodine receptor and increases the concentration of calcium inside the cell.

4.Then binding of calcium with troponin C occurs,it induces conformational change in regulatory complexes of troponin-I and reveals the actin site which binds to the myosin ATPase present at myosin head..

5.Following to that actin and myosin slide over each other and shotens the length of sarcomere.This process keeps on repeating until calcium level is elevated.

6. At the end of action potential,calcium influx reduces and calcium is moved by Sarcoplasmic reticulum by ATP dependent calcium pump named as sarco endoplasmic reticulum calcium ATPase.Therefore reduces the cytosolic calcium concentration and release calcium from troponin C.

7.Then some of the cytosolic calcium is transported out of the cell by sodium-calcium pump. The release of calcium from trponin C facilitates the conformational change in troponin complex again to troponin-I inhibition of actin filament binding site.At last, new ATP bounds to myosin and displaces the ADP ,sarcomere length restores.

unlike mycardial cell ,in skeletal muscle,the excitation-contraction coupling is a quick signal transduction process where depolarization of the sarcolemma membrane is coupled to opening of calcium influx channel on sarcoplasmic reticulum.This transduction process occurs with the help of DHP receptor calcium channel and RyR channel .The DHPR and RyR combindly associate with junctional protein and interacts causes RyR opening.Therefore,transient increase calcium activates the contractile protein and muscle contracts.

Cardiac muscle relaxation occurs when calcium released out via sodium -calcium pump and the release of calcium from troponin C facilitates the conformational change causes troponin -I inhibition of actin binding site.New ATP bounds to myosin and activates the motor function causing muscle to relax.