Smooth muscle resembles the other muscle types in that contraction is triggered by calcium ions (Ca2+), energized by ATP, and achieved by the sliding of thin filaments over the
Saladin: Anatomy & I 11. Muscular Tissue I Text
Physiology: The Unity of Form and Function, Third Edition thick filaments. The mechanism of excitation-contraction coupling, however, is very different. Little of the Ca2+ comes from the sarcoplasmic reticulum; most comes from the extracellular fluid and enters the cell through calcium channels in the sarcolemma. Some of these channels are voltage-gated and open in response to changes in membrane voltage; some are ligand-gated and open in response to hormones and neurotransmitters; and some are mechanically gated and open in response to stretching of the cell.
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