The body has three types of muscle tissue, each built for a different job. They differ in how they look under the microscope and in whether you control them voluntarily.
Skeletal muscle
Long, striated fibers with many nuclei attach to bones and move the skeleton. It is under voluntary control and contracts only when a motor nerve signals it.
Cardiac muscle
Found only in the heart, it is striated but made of branched cells joined by intercalated discs. These junctions pass electrical signals from cell to cell, so the heart contracts as one unit without conscious control.
Smooth muscle
Spindle-shaped cells without striations line the walls of blood vessels, the gut, the airways and the bladder. It contracts slowly, can stay contracted for long periods and is controlled by the autonomic nervous system and hormones.
| Type | Control | Where found |
|---|---|---|
| Skeletal | Voluntary | Attached to bones |
| Cardiac | Involuntary, own pacemaker | Heart wall |
| Smooth | Involuntary | Vessels, gut, airways, bladder |
At the bedside
Intramuscular injections go into large skeletal muscles such as the deltoid and vastus lateralis because their rich blood supply absorbs the drug well. Bronchospasm in asthma is smooth muscle tightening in the airways, which is what bronchodilators relax.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).