Smooth Muscle: Histology, Physiology, and Anatomical Distribution

Smooth muscle (non-striated, involuntary) forms the contractile layer of hollow viscera and vascular walls.

Histology

• Spindle-shaped cells (20–500 μm), single central nucleus.
• No sarcomeres → no striations.
• Dense bodies (α-actinin) anchor actin filaments; intermediate filaments (desmin, vimentin) transmit force.
• Low myosin:actin ratio (~1:15) → slower, sustained contractions.
• Gap junctions allow electrical coupling → syncytial behavior (single-unit) or multi-unit innervation.

Physiology

• Excitation-contraction – Ca²⁺ enters via voltage-gated or ligand-gated channels; binds calmodulin → activates MLCK → phosphorylates myosin light chain → contraction.
• Latch state – Dephosphorylated myosin remains attached → force maintenance with low energy (tonic contraction).
• Plasticity – Adapts to length changes; stress-relaxation response.

Anatomical Locations & Functions

Organ System	Location	Function
GI	Esophagus (distal) → anus	Peristalsis, segmentation, sphincter tone
Cardiovascular	Arteries, arterioles, veins	Vasoconstriction/dilation → BP, flow distribution
Respiratory	Bronchi, bronchioles	Bronchoconstriction/dilation
Urogenital	Ureters, bladder, uterus, vas deferens	Peristalsis, voiding, parturition, ejaculation
Integumentary	Arrector pili (hair erection)	Piloerection (goosebumps)
Ocular	Iris (sphincter/dilator), ciliary body	Pupil size, accommodation

Control

Autonomic (sympathetic, parasympathetic), hormonal, local factors (stretch, pH, O₂). No voluntary control.