Smooth muscle tissue
Textus muscularis levis
Also known as: smooth muscle, visceral muscle, involuntary muscle, non-striated muscle
Non-striated, involuntary muscle of spindle-shaped cells with a single central nucleus. It forms the walls of hollow organs and blood vessels.
Overview
Smooth muscle cells are fusiform (≈3–10 µm wide and ≈30–200 µm long, reaching ≈500 µm in the pregnant uterus), with one central, often cigar-shaped nucleus. Their actin and myosin are not arranged in sarcomeres, so they show no striations. Thin filaments, far more numerous than thick ones, anchor to cytoplasmic dense bodies (which contain α-actinin and correspond to Z discs) and to membrane plaques, and intermediate filaments (desmin, vimentin) link them. Contraction therefore pulls the cell into a shorter, twisted shape. In place of T-tubules, the membrane has flask-shaped caveolae, and the SR is modest. Much of the activating Ca²⁺ enters from the extracellular fluid.
Contraction is regulated by Ca²⁺–calmodulin → myosin light-chain kinase (MLCK), not by troponin. Control is involuntary: autonomic nerves release transmitter from varicosities onto a diffuse junction, and hormones, local metabolites, stretch and intrinsic pacemaker cells (interstitial cells of Cajal in the gut) also act on the cells. In single-unit (visceral) smooth muscle, found in the gut, uterus and small vessels, gap junctions couple the cells and spontaneous waves spread between them. Multi-unit smooth muscle (iris, ciliary muscle, large arteries, arrector pili, vas deferens) is controlled more finely by nerves. Smooth muscle contracts slowly and economically. Through the latch state it can hold tone for long periods, and it adapts to stretch (stress–relaxation). Unlike skeletal and cardiac muscle, smooth muscle cells can divide (hyperplasia).
- Location
Walls of the gastrointestinal, respiratory, urinary and reproductive tracts; blood and lymphatic vessels; iris and ciliary body; arrector pili muscles; capsules of the spleen and some glands.
Parts & subdivisions
- Dense bodiesCytoplasmic and membrane-associated anchoring points for thin filaments, rich in α-actinin. They are the functional equivalent of Z discs.
- CaveolaeMembrane invaginations that concentrate Ca²⁺ channels and signalling molecules, taking the place of T-tubules.
- Gap junctionsCouple the cells of single-unit smooth muscle electrically.
Anatomy
- Histology
Sheets or bundles of overlapping spindle cells with central nuclei. In longitudinal section, the nuclei of contracted cells look corkscrew-shaped. In transverse section, profiles vary in size, and only the widest profiles contain a nucleus. In the gut, the muscularis externa is arranged as an inner circular and an outer longitudinal layer.
- Development
Most smooth muscle comes from splanchnic mesoderm and local mesenchyme. The iris sphincter and dilator pupillae and the myoepithelial cells derive from ectoderm, and vascular smooth muscle of the head comes from neural crest.
Function
- Peristalsis and mixing in the gut; propulsion of urine, gametes and bile
- Regulation of vessel diameter, and so of blood pressure and flow distribution
- Regulation of airway calibre
- Expulsion during micturition and parturition
- Pupil size and lens accommodation
- Physiology
Ca²⁺ enters through voltage-gated L-type channels, receptor-operated channels, and IP₃-triggered SR release. MLCK phosphorylates the regulatory light chain, and myosin light-chain phosphatase reverses this. Rho-kinase inhibits the phosphatase, so contraction strengthens without any extra Ca²⁺ (Ca²⁺ sensitisation). NO → cGMP and β₂ → cAMP cause relaxation.
Clinical significance
Asthma and hypertension
Bronchial smooth muscle contraction and hypertrophy narrow the airways in asthma, which is treated with β₂-agonists and antimuscarinics. Increased arteriolar smooth muscle tone raises peripheral resistance in hypertension, which is treated with Ca²⁺-channel blockers among other drugs.
Leiomyoma (fibroids)
Benign smooth muscle tumours of the myometrium. They are the most common tumour in women and cause menorrhagia, pain and subfertility.
Test yourself
In smooth muscle, which enzyme is activated by Ca²⁺–calmodulin to start contraction?
- Troponin C
- Myosin light-chain kinase
- Myosin light-chain phosphatase
- Creatine kinase
Answer: Myosin light-chain kinase
Smooth muscle has no troponin. Ca²⁺–calmodulin activates MLCK, which phosphorylates the regulatory light chain of myosin so cross-bridges can cycle. MLC phosphatase causes relaxation.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.