EXCITATION CONTRACTION COUPLING IN SMOOTH MUSCLE
The Excitation-Contraction coupling process in smooth muscle differs from that in striated muscles. In smooth muscle thin (actin) filaments are arranged in bundles attached to dense plaques (α-actinin) either on the sarcolemma (unipolar) or bound to the desmin cytoskeleton (bipolar). Thick (myosin) filaments (bipolar) interdigitate among the thin filaments, but there is no regular alignment, hence the absence of striations.
Tropomyosin is present on thr actin filaments, but troponin is absent. Depolarisation, or the opening of chemically (hormonally) gated Ca2+ channels in the membrane causes Ca2+ entry. The Ca2+ combines with a protein calmodulin which is very similar to the calcium binding subunit of troponin (TnC). The (Ca2+)4-calmodulin complex activates myosin light chain kinase which phosphorylates a small protein myosin light chain which is bound to the myosin head near the neck, and which prevents it from swiveling normally, thus hindering combination with the actin. The phosphorylation activates the myosin head, initiating contraction.
At very low Ca2+ levels, a phosphodiesterase (always present) dephosphorylates the Myosin Light Chain (dephosphorylation outweighs phosphorylation), inactivates the myosin, and so stops contraction. At intermediate Ca2+ levels (orange portion of the cross-bridge cycle), dephosphorylation of some of the myosin heads while they are still attached to actin, may prolong attachment time, slow down cycling, and allow prolonged maintenance of tone at a given length, without the use of large amounts of ATP (latch mechanism).
In skeletal, cardiac and smooth muscle therefore the trigger for contraction is Ca2+ ion. In skeletal and cardiac muscles it operates by combination with troponin bound to the actin filament. In smooth muscle it operates by combination with calmodulin in the cytosol, and thus activating the light chain attached to the myosin head.