Skeletal, cardiac and smooth muscle: striation and voluntary control
Skeletal, cardiac and smooth muscle are sorted by whether they are striated (striped under the microscope) and whether they are under voluntary control (you can decide to move them).
Remember
Skeletal muscle is striped and voluntary, cardiac muscle is striped but involuntary (it works without conscious control), and smooth muscle is involuntary with no stripes.
In skeletal muscle and cardiac muscle, the contractile proteins actin and myosin are laid out in regular repeating units called sarcomeres. That regular layout makes dark and light bands, the striations (stripes), so these muscles are called striated. Smooth muscle has no organised sarcomeres, so it shows no stripes.
Skeletal muscle is mainly attached to bones by tendons (tough cords of connective tissue), so it pulls on a bone when it contracts, which is how it moves the skeleton. A somatic motor neuron (a nerve cell that carries voluntary movement signals) supplies each fibre (a long muscle cell), so you can decide when to use the muscle: this is voluntary control. Its cells are very long and hold many nuclei.
Cardiac muscle is found only in the heart. It is striated too, but it contracts on its own rhythm, and is not under conscious control, so it is involuntary. Its cells are shorter than skeletal cells and usually have one nucleus. They are joined by intercalated discs, specialised junctions between heart cells that let the electrical signal pass from cell to cell.
Smooth muscle is involuntary and not striated. Its cells are spindle-shaped (tapered at both ends), and each has a single nucleus. It occurs in the walls of blood vessels, the gut and the bladder, where slow, sustained contraction suits it. In the gut it produces peristalsis, the waves of contraction that push food along.
So you can sort the three types by whether the muscle is striped and whether it is under voluntary control.
Key facts
- Skeletal muscle
- striated and voluntary, with very long cells that hold many nuclei; the muscle is mainly attached to bones and moves the skeleton.
- Cardiac muscle
- striated but involuntary, found only in the heart, with cells that are shorter than skeletal cells and usually have one nucleus.
- Smooth muscle
- involuntary and not striated, with spindle-shaped cells that each have a single nucleus.
- Intercalated discs
- junctions that join heart cells and pass the electrical signal along, so they point to cardiac muscle.
- Where smooth muscle lies
- for example the walls of blood vessels, the gut and the bladder.
- Simple way to sort
- ask if the muscle is striped and if you can choose to move it.
Exam traps
- Cardiac muscle is striated too, but its cells are branched, usually have a single nucleus and contract automatically, so it does not fit long voluntary fibres that hold many nuclei.
- Skeletal muscle is still classified as voluntary when it contracts without a conscious decision, as in shivering (a response to cold), because it contracts when somatic motor neurons signal it.
- Cardiac muscle is involuntary and rhythmic, but it is found only in the heart, so the muscle layers of the gut wall are not cardiac.
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