Smooth Muscle: Structure, Locations & Involuntary Functions

Smooth muscle is non-striated, involuntary muscle found in hollow viscera walls. Single nucleus, spindle-shaped, slow sustained contractions.

Key Locations & Functions

Control Mechanisms

• Autonomic: Sympathetic (α/β adrenergic), parasympathetic (muscarinic), enteric (ENS).
• Hormonal: Oxytocin, ADH, angiotensin II, histamine, serotonin.
• Local: Stretch, pH, O₂, CO₂, nitric oxide, endothelin.

Pathology Examples

• Hypertension (vascular remodeling)
• Asthma (bronchial hyperreactivity)
• IBS (visceral hypersensitivity, dysmotility)
• Overactive bladder (detrusor overactivity)
• Uterine fibroids (leiomyomas — benign smooth muscle tumors)
• Achalasia (loss of LES relaxation)

Smooth muscle is the engine of involuntary physiology — moving, mixing, regulating, and protecting without conscious effort.

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• Muscle Types: Why No Muscle Is Both Involuntary and Striated
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Organ System	Location	Function
GI	Esophagus (distal), stomach, intestines, gallbladder	Peristalsis, segmentation, mixing, propulsion
Cardiovascular	Arteries, arterioles, veins (tunica media)	Vasoconstriction/dilation → BP, flow distribution
Respiratory	Trachea, bronchi, bronchioles	Bronchoconstriction/dilation → airway resistance
Urinary	Ureters, bladder (detrusor), urethra	Peristalsis (ureters), voiding (bladder)
Reproductive	Uterus (myometrium), vas deferens	Labor contractions, sperm transport
Integumentary	Arrector pili (hair follicles)	Piloerection (goosebumps)
Ocular	Iris (sphincter/dilator pupillae), ciliary body	Pupil size, lens accommodation