<P> The ratio of actin to myosin is between 2: 1 and 10: 1 in smooth muscle, compared to ~ 6: 1 in skeletal muscle and 4: 1 in cardiac muscle . </P> <P> Smooth muscle does not contain the protein troponin; instead calmodulin (which takes on the regulatory role in smooth muscle), caldesmon and calponin are significant proteins expressed within smooth muscle . </P> <Ul> <Li> Tropomyosin is present in smooth muscle, spanning seven actin monomers and is laid out end to end over the entire length of the thin filaments . In striated muscle, tropomyosin serves to block actin--myosin interactions until calcium is present, but in smooth muscle, its function is unknown . </Li> <Li> Calponin molecules may exist in equal number as actin, and has been proposed to be a load - bearing protein . </Li> <Li> Caldesmon has been suggested to be involved in tethering actin, myosin and tropomyosin, and thereby enhance the ability of smooth muscle to maintain tension . </Li> </Ul> <Li> Tropomyosin is present in smooth muscle, spanning seven actin monomers and is laid out end to end over the entire length of the thin filaments . In striated muscle, tropomyosin serves to block actin--myosin interactions until calcium is present, but in smooth muscle, its function is unknown . </Li>

Contraction in smooth muscle is directly activated by