Smooth muscle contraction is initiated when cytosolic calcium rises. Which sequence correctly describes the regulatory mechanism unique to smooth muscle?

• A Calcium binds troponin C, exposing myosin binding sites on thin filaments
• B Calcium binds calmodulin, activating myosin light chain kinase which phosphorylates the regulatory light chain of myosin ✓
• C Calcium binds troponin T, triggering tropomyosin displacement via actomyosin ATPase
• D Calcium directly activates myosin light chain phosphatase, causing crossbridge cycling

Explanation

Smooth muscle has no troponin. Calcium entering from caveolae or released from sarcoplasmic reticulum binds calmodulin, and this complex activates myosin light chain kinase, which phosphorylates the 20 kDa regulatory light chain of myosin II, permitting actomyosin ATPase activity and crossbridge cycling. Myosin light chain phosphatase dephosphorylates the light chain to cause relaxation, so option D states the opposite action. Dense bodies containing alpha actinin anchor actin filaments but do not regulate contraction.

Reference: Ganong Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.