Smooth muscle is found in the walls of blood vessels and hollow viscera. Bands of smooth muscle cells can be found in the erector pili muscles of the skin.
Gap junctions electrically couple smooth muscle cells.
Smooth muscles contain actin and myosin filaments, but the filaments are not arranged in orderly arrays as in skeletal muscle.
• Bundles of myofilaments course obliquely in the cell, forming a lattice-like arrangement.
• A sliding filament mechanism of contraction is thought to occur.
• Thin filaments insert into dense bodies located within smooth muscle cytoplasm and attach to their membranes.
Smooth muscle contraction may be triggered by various stimuli such as autonomic nerves or hormones. Depolarization of the cell membrane results in an influx of Ca2+ from outside the cell. Ca2+ is sequestered in either the cell membrane or in the sparse SR.
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