Which of the following is NOT true for a smooth muscle?: (A) no Z-discs present, (B) Actin : myosin ratio = 2 : 1, (C) The contraction is "thick filament-driven", (D) "Latch bridge" observed.
Answer is B. There are no Z-lines in smooth muscle. Z-lines are replaced with dense bodies. Thin : thick filament ratio is 15 : 1. When Ca2+ combines with calmodulin, there is activation of the enzyme myosin light chain kinase (MLCK), and the actin-myosin interaction can begin. Myosin light chains in the thick filament thus regulates the contraction. "Latch-bridge" phenomenon causes smooth muscle to develop sustained tension for a lesser energy consumption.