Regulation of Smooth Muscle Contraction
Several mechanisms regulate the contraction and relaxation of smooth muscle cells. Smooth muscle contraction is under the control of regulatory proteins, the nervous system and a mechanism called the stress-relaxation response . A regulatory protein used is Calmodulin which binds to Ca++ in the cytosol (similar to troponin’s role in striated muscle fibres). After Calmodulin binds to Ca++, it activates an enzyme called Myosin Light Chain Kinase. This enzyme uses ATP to add a phosphate group to a portion of the myosin head. Once the phosphate group is bound, the myosin head can bind to actin, and contraction can occur. Not only do Ca++ ions enter the smooth muscle fibre slowly, but they also move slowly out of the muscle fibre, which delays relaxation. This prolonged presence of Ca++ in the cytosol provides for Smooth Muscle Tone – a state of continued partial contraction. Smooth muscle fibres contract and relax in response to action potentials from the nervous system (Autonomic Nervous System), stretching, hormones, changes in pH, O2 and CO2 levels, temperature, and ion concentrations. Stress-Relaxation Response: When smooth muscle fibres are stretched, they initially contract developing increased tension. Within a minute or so, the tension decreases. This allows smooth muscle to undergo great changes in length while retaining the ability to contract effectively (ex: stomach, intestines, urinary bladder).
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