Unstriped cells, tapered at both ends, each with one central nucleus, about 30 to 200 micrometres long
Smooth muscle tissue — spindle-shaped cells.
Anatomy · Cells & tissues
Smooth muscle tissue
The unstriped, involuntary muscle of hollow organs
Muscle tissue · walls of gut, vessels and other hollow organs
The involuntary muscle tissue found in the walls of hollow organs and tubes: the gut, blood vessels, airways, bladder, uterus and the iris of the eye. Its cells are spindle-shaped, tapered at both ends, each with a single central nucleus, and they show no cross-striations because their filaments are not arranged in sarcomeres. It is controlled by the autonomic nervous system, hormones and local factors, and it contracts slowly and can sustain a contraction for long periods with little energy.
How to spot them
- ▸Cells shaped like a spindle or cigar, pointed at both ends, with no visible stripes
- ▸A single, central, elongated (cigar-shaped) nucleus in each cell
- ▸Cells arranged in sheets, often in two layers, one circular and one longitudinal, as in the gut wall
- ▸In cross-section, round profiles of varying diameter, with a nucleus in only some
- ▸Present in the walls of hollow structures, from arteries to the uterus, not attached to bone
What to know
Wave-like squeezing of the gut wall by circular and longitudinal layers that pushes food along
Smooth muscle tissue — peristalsis.
Thin filaments are tethered to dense bodies instead of Z discs, and are switched on by calmodulin rather than troponin
Smooth muscle tissue — no sarcomeres, hence no striations.
Its tightening in the airways in asthma is relieved by salbutamol, which relaxes it
Smooth muscle tissue — bronchial smooth muscle responds to beta-2 agonists.
Contracts in the walls of arteries to regulate blood pressure, and in the iris to alter the pupil
Smooth muscle tissue — vascular and multi-unit smooth muscle.
It can hold tension for a long time at low energy cost, and can divide to repair damage
Smooth muscle tissue — the latch state, and the ability to proliferate.
Ordered by the autonomic nervous system and hormones, never by conscious thought
Smooth muscle tissue — involuntary control.
Key ideas
- Calcium binds calmodulin, which activates myosin light-chain kinase; the phosphorylated myosin heads then bind actin and pull on filaments anchored to dense bodies, shortening the cell in all directions.
- In single-unit (visceral) smooth muscle, gap junctions link cells so that a wave of contraction passes through the whole sheet; in multi-unit smooth muscle (iris, large arteries) each cell is controlled individually.
Quick-fire facts
- •Cells range from about 30–200 micrometres long, and the pregnant uterus can grow cells up to about 500 micrometres
- •Smooth muscle has no striations because its actin and myosin filaments are not arranged in sarcomeres
- •Contraction begins more slowly than in skeletal muscle and can be sustained for long periods
- •Arrector pili muscles at hair follicles are smooth muscle and produce goose bumps
- •Many drugs act on it: for example salbutamol relaxes the smooth muscle of the airways in asthma
- •It can divide and regenerate after injury, unlike cardiac muscle
- •It forms the tunica media of arteries and the muscularis of the gut, ureters and bladder
- •Unlike skeletal muscle, it can be stimulated by stretch, hormones (such as oxytocin) and local chemicals as well as by nerves
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