Smooth muscle

Unlike skeletal and cardiac muscle it lacks visible cross-striations.

Contains actin and myosin-II that produce contraction.

Dense bodies are present in in the cytoplasm and are attached to the cell membrane and by alpha-actinin to actin filaments.

Contains tropomyosin but not troponin.

The sarcoplasmic reticulum is poorly developed and there are few mitochondria with a dependence on glycolysis for metabolic needs.

Divided into visceral, or unitary smooth muscle and multiunit smooth muscle.

Visceral smooth muscle occurs in large sheets, associated with many low resistance gap junction bridges between individual muscle cells.

Visceral smooth muscle functions in a syncitial manner.

Visceral smooth muscle found mainly in the walls of hollow viscera and include the intestine, uterus and ureters.

Multiunit smooth muscle has individual units without interconnecting bridges and is found in the iris in which fine graded contractions occur.

Multiunit smooth muscle are not under voluntary control.

Visceral smooth muscle has instability in it membrane potential and has continuous, irregular contractions independent of its nerve supply.

Visceral smooth muscle continuous state of partial contractions that exist is ref2242ed to as tonus.

In periods of restfulness visceral smooth muscle membrane potential averages about -50 mV.

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