Salient structural features of smooth muscle: Lack T-tubules, poorly developed SR is associated with membrane invaginations- caveolae Voltage-gated L-type Ca++ channel and 3Na+-1Ca++ antipo are associated with caveolae. Proteins caveolin and cholesterol are both critical for the formation of caveolae Instead of Z lines, there are dense bodies in the cytoplasm and are bound by a-actinin to actin filaments.They are held in place by desmin or vimentin. Actin is more plentiful in smooth muscle than in striated muscle, actin:myosin =10-15 :1, compared with 2-4 :1 in striated muscle. Calmodulin is present in smooth muscle in place troponin. Tropomyosin is also present.