Smooth muscle !

Tissue

Smooth muscle is the non-striated, involuntary muscle in the walls of vessels, airways, the gut, the bladder and the uterus.

Updated 2026-11-01

See it in 3D: Myometrium · Esophagus · Urinary bladder

What it is

Smooth muscle cells are spindle-shaped with a single central nucleus and no visible striations, because their actin and myosin are not arranged in sarcomeres. They contract slowly and can hold tension for long periods with little energy. They are controlled by autonomic nerves, hormones and stretch, and, unlike the other muscle types, they can divide.

Structure

Longitudinal section: tapered cells packed with their thick middles next to the thin ends of neighbours; one elongated central nucleus that twists like a corkscrew when the cell contracts.
Cross section: round profiles of different sizes, only some showing a nucleus (the ones cut through the middle).
Dense bodies in the cytoplasm and dense plaques on the membrane anchor actin filaments, doing the job of Z lines.
Caveolae, small membrane pits, stand in for T tubules; the sarcoplasmic reticulum is sparse.
Each cell is wrapped in its own basal lamina and reticular fibres.

Step by step

A nerve signal, hormone or stretch opens calcium channels; calcium enters from outside and is released from the sarcoplasmic reticulum.
Calcium binds calmodulin (there is no troponin).
Calcium–calmodulin activates myosin light-chain kinase.
The kinase phosphorylates the myosin light chain, and myosin cycles on actin.
Myosin light-chain phosphatase removes the phosphate and the cell relaxes; slow cycling lets it keep tone cheaply (latch state).

The three muscle types compared

Feature	Skeletal	Cardiac	Smooth
Cell shape	Long unbranched cylinder	Short, branched	Spindle
Nuclei	Many, peripheral	One or two, central	One, central
Striations	Yes	Yes	No
Junctions	None between fibres	Intercalated discs	Gap junctions (single-unit)
Calcium sensor	Troponin	Troponin	Calmodulin
T tubules	Triads at A–I junction	Dyads at Z line	None (caveolae)
Control	Voluntary (somatic nerves)	Involuntary, pacemaker	Involuntary (autonomic, hormones, stretch)
Regeneration	Limited (satellite cells)	Very poor (scar)	Good (cells divide; pericytes)

Clinical notes

Asthma: hypertrophy and hyperplasia of bronchial smooth muscle narrow the airway.
Atherosclerosis: smooth muscle cells migrate into the intima and build the fibrous cap of the plaque.
Pregnancy: myometrial cells undergo both hypertrophy and hyperplasia under hormones.
Leiomyoma (uterine fibroid), the most common benign tumour of the uterus, is made of smooth muscle.

Test yourself

Think of the answer, then tap the question to see it.

What replaces troponin in smooth muscle as the calcium sensor?

Calmodulin; calcium–calmodulin activates myosin light-chain kinase, which phosphorylates myosin so it can bind actin.

Which structures replace Z lines in smooth muscle?

Dense bodies in the cytoplasm and dense plaques on the cell membrane, which anchor actin filaments.

Why does smooth muscle lack visible striations?

Its actin and myosin filaments are not organized into aligned sarcomeres, so there are no repeating bands.

How does the myometrium grow during pregnancy?

By both hypertrophy (bigger cells) and hyperplasia (more cells), because smooth muscle cells can divide.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

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Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: 10.8 Smooth Muscle (OpenStax, 2022); 4.4 Muscle Tissue and Motion (OpenStax, 2022); Muscle Tissue (National Cancer Institute, SEER Training Modules, 2024).

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Representative model, for educational purposes only.