Contraction of smooth muscle requires calcium binding to calmodulin, which then activates myosin light chain kinase. Where are the actin filaments anchored in smooth muscle cells, given the absence of sarcomeres?

• A Z discs composed of alpha-actinin spanning the cell width
• B Intercalated discs containing fascia adherens junctions
• C Dense bodies attached to the sarcolemma and scattered in the cytoplasm ✓
• D Costameres linking the sarcolemma to the extracellular matrix

Explanation

Smooth muscle lacks sarcomeres and Z discs. Actin filaments insert into dense bodies, which contain alpha-actinin and are anchored both to the sarcolemma and dispersed through the cytoplasm, allowing the oblique filament array to shorten the whole cell in multiple directions. Z discs belong to striated muscle, intercalated discs are unique to cardiac muscle, and costameres couple the sarcolemma to myofibrils in skeletal muscle.

Reference: Junqueira's Basic Histology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.