Key Points

• The isoforms of actin and myosin in smooth muscle differ from those in striated muscle.

• Filaments are not arranged in transverse registry and the proportion of thin filaments is higher than that found in striated muscle.

• Contraction is initiated by the phosphorylation of the 20,000-Da light chains of myosin by the enzyme myosin light-chain kinase, which is activated by increases in intracellular-free calcium. Contraction is due to crossbridge cycling and requires ATP.

• Relaxation occurs when kinase activity decreases and the light chains are dephosphorylated by phosphatase; for relaxation to occur, calcium is actively transported back into the sarcoplas-mic reticulum and/or out of the cell.

• In many smooth muscles, excitation is due to increases in calcium permeability in response to the opening of voltage-gated channels and/ or ligand-gated channels.

• In unitary smooth muscles, individual muscle cells are electrically coupled to one another and respond as a unit; in multiunit smooth muscles, cells are not coupled.

• Smooth muscles exhibit length-force and force-velocity relationships qualitatively similar to those seen for striated muscle.

• Maximal force per unit of smooth muscle is as great as, if not greater than, that of striated muscle; Vmax of smooth muscle is much lower.

• Many smooth muscles are supplied by both excitatory and inhibitory nerves.

• Consumption of ATP is low compared with striated muscle because of the relatively lower myosin ATPase activity in smooth muscle; many smooth muscles can sustain contraction, once developed, with little crossbridge cycling or use of ATP.

Essential Medical Physiology, Third Edition

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