Question

I understand that depolarization is driven by sodium in most cases. However, in smooth muscle, is the ion that is driving depolarization calcium? Since calcium is the ion that increases in concentration for smooth muscle action potential propagation, would that make it the ion that depolarizes?

Answer 1

1) In smooth muscle, depolarizaton occurs by Same ions as on case of skeletal and cardiac i.e with the help of sodium ion.

Only difference in smooth muscle is that their is direct phosphorylation of myosin by Ca++ which result in myosin ATPase activity. But in case of skeletal muscle, Ca++ causes activation of troponin tropomyosin complex which then causes activation of actin myosin binding.

So sequence of events in smooth muscle contraction and relaxation are:

1) Binding of Acetylcholine to Muscaraniv receptors

2) Increased influx of Ca++ into the cell

3) Activation of Calmodulin dependent myosin light chain kinase

4) Phosphorylation of myosin

5) Increased myosin ATPase activity and binding of myosin to actin

6) contraction

7) dephosphorylation of myosin by myosin light chain phosphatase

8) Relaxation or sustained contraction due to the latch bridge and other mechanism