All of the following are true statements about electromechanical coupling, EXCEPT:
The three types of muscle cells (smooth muscle, cardiac muscle, skeletal muscle) utilize different mechanisms by which membrane depolarization results in muscle contraction, i.e., electromechanical coupling. In smooth muscle, Ca2+ ions entering through opening of voltage-gated Ca2+ channels or following release from the SR, bind to calmodulin. The Ca2+-calmodulin complex activates myosin light chain kinase, and phosphorylation of myosin light chain initiates smooth muscle contraction. This is in contrast to cardiac and skeletal muscle, where contraction is triggered by a conformational change induced by Ca2+ handling. In cardiac muscle, opening of voltage-gated Ca2+ channels results in Ca2+. In skeletal muscle, depolarization of the T tubule triggers release of Ca2+ from the SR. In contrast to cardiac muscle, skeletal muscle contractions can summate during repeated stimulation, resulting in an increase in force with incomplete relaxation between the stimuli (tetanic contraction). This difference is due to the much shoer action potential duration in skeletal muscle compared to cardiac muscle.
In smooth muscle, Ca2+ ions bind to calmodulin
Smooth muscle contraction is initiated by myosin light chain phosphorylation
In cardiac muscle, Ca2+ ions bind to troponin
The major source of Ca2+ ions for contraction of skeletal muscle is influx of Ca2+ ions from the extracellular space following voltage-dependent opening of L-type Ca2+ channels.