Contraction

Smooth muscles contain the major contractile proteins actin and myosin, together with tropomyosin. The relative proportion of myosin is much less than in skeletal muscles. The activation mechanism is calcium-dependent, but does not act via troponin, which is absent.

The actin occurs in thin filaments which are readily seen by electron microscopy. Many of them are attached to dense bodies in the cytoplasm, others are attached to dense patches next to the cell membrane. The structure of the thick filaments is different from that in skeletal muscle. The myosin molecules are probably oriented in opposite directions on the two faces of a filament. This arrangement allows a thin filament to be pulled over the whole of its length by a thick filament, so that the muscle can operate at near maximum tension over a wide range of lengths (Fig. 11.9).

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