As smooth muscle is not as organized as skeletal muscle, its contraction takes place in all directions.
A simplified version of steps involved in smooth muscle contraction is -
Entry of Calcium ions into cytoplasm of smooth muscle due to nerve stimulation or stretching of the muscle fibers.
Binding of Calcium to Calmodulin, a regulatory protein forming a complex.
Activation of the enzyme, myosin kinase, The enzyme complex breaks up ATP into ADP and transfers the inorganic Phosphate directly to myosin.
Activated myosin forms crossbridges with actin.
If Calcium is pumped out of the cell, the Phosphate gets removed from myosin by another enzyme, called myosin phosphatase. Myosin then becomes inactive, and the muscle relaxes.
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