Smooth muscle lines the walls of hollow organs and blood vessels and works without conscious control. It contracts slowly but can hold tension for a long time with little energy.
Structure
Smooth muscle cells are small, spindle shaped and have no stripes, because their actin and myosin are not arranged in sarcomeres. Many cells are linked by gap junctions and contract together as one unit.
Control
The autonomic nervous system, hormones, stretch and local chemicals all change smooth muscle tone. Calcium triggers contraction, and much of it enters from outside the cell.
Vessels and airways
Smooth muscle in the arterioles sets vessel diameter and so blood pressure and blood flow. In the airways it sets bronchial diameter, and its spasm narrows them in asthma.
Gut and bladder
Waves of contraction called peristalsis move food along the gut. The bladder wall (detrusor) stretches to fill and contracts to empty.
| Site | What it controls | Clinical link |
|---|---|---|
| Arterioles | Blood pressure and flow | Vasodilators, shock |
| Bronchioles | Airway diameter | Bronchospasm in asthma |
| Gut wall | Peristalsis | Ileus, colicky pain |
| Bladder (detrusor) | Emptying urine | Urinary retention |
| Uterus | Labor contractions | Oxytocin |
At the bedside
Many medicines act on smooth muscle: bronchodilators relax the airways, nitroglycerin relaxes vessels and oxytocin contracts the uterus. Wheezing, colicky pain and urinary retention are all signs of smooth muscle at work.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).