Smooth Muscle Characteristics
Smooth muscle cells are NOT striated and are NOT multinucleated—they are mononucleate cells that lack the striations characteristic of cardiac and skeletal muscle. 1, 2
Structural Features of Smooth Muscle
Nuclear Characteristics
- Smooth muscle cells contain a single nucleus (mononucleate), distinguishing them from skeletal muscle fibers which are multinucleate 1
- These cells exist either isolated or grouped in layers within various tissues 1
Absence of Striations
- Smooth muscle lacks the striations that are characteristic of both cardiac and skeletal muscle 2
- The absence of aligned filament arrays in registers explains why smooth muscle appears "smooth" rather than striated 3
- Unlike striated muscle with its permanent filament lattice, smooth muscle has structural malleability that precludes formation of regularly aligned sarcomeric structures 3
Comparison with Other Muscle Types
Skeletal Muscle (Striated and Multinucleated)
- Skeletal muscle fibers are multinucleate cells with visible striations 1
- These fibers are responsible for voluntary movements 1
Cardiac Muscle (Striated but Mononucleate)
- Cardiac muscle cells are mononucleate striated cells 1
- They exhibit fast, rhythmic, spontaneous contractions 1
Smooth Muscle (Non-striated and Mononucleate)
- Smooth muscle contraction is slow and depends on the autonomic nervous system, not under voluntary control 1, 2
- Contractile activity is initiated by Ca²⁺-calmodulin interaction rather than the troponin system used in striated muscle 2
Clinical Relevance
When examining tissue histologically, the presence of spindle-shaped cells without striations and with single nuclei is characteristic of smooth muscle 4. This is particularly important when differentiating smooth muscle tumors (leiomyomas) from other pathologies, as these tumors stain positively for smooth muscle actin and are composed of well-differentiated smooth muscle cells 4.