Course 16.1Lesson 2 of 3

The kinds of muscle and the structure of a muscle

Muscle is a tissue that can contract. The body has three kinds: skeletal muscle with stripes under voluntary control, smooth muscle in the walls of hollow organs, and cardiac muscle in the heart. A skeletal muscle is built from bundles of fibres, and each fibre is a single cell with many nuclei that holds thread-like myofibrils.

about 25 min1 simulation1 practice question

Lesson 2 of 3Show all lessons

Try the code and the simulations beside the lesson.

What happens when you bend your arm

Put your hand on your upper arm and bend your elbow. A hard bulge appears under the skin. That bulge is a muscle that has become shorter and thicker.

Muscle is a specialised tissue of mesodermal origin. It makes up about 40 to 50 per cent of the weight of an adult. It has four special properties.

Excitability: it responds to a stimulus, such as a nerve signal.
Contractility: it can shorten.
Extensibility: it can be stretched.
Elasticity: it goes back to its length after it is stretched.

The three kinds of muscle

Muscles are grouped by where they lie. There are three kinds.

Three kinds of muscle tissue drawn side by side. Skeletal muscle: long cylindrical fibres with fine cross stripes and several nuclei at the edge of each fibre, labelled striated and voluntary. Visceral muscle: short spindle shaped cells with pointed ends, one nucleus in the middle of each cell and no stripes, labelled smooth and involuntary. Cardiac muscle: branching fibres with cross stripes and one nucleus in the middle of each cell, joined end to end by thick dark lines called intercalated discs, labelled striated and involuntary.

Skeletal: next to the bones. Striated (striped). Voluntary.
Visceral: in the walls of hollow organs. Smooth, with no stripes. Involuntary.
Cardiac: in the heart. Striated and branched. Involuntary.

Skeletal muscle takes part in locomotion and in changes of the posture of the body. The nervous system controls it by our will, so it is a voluntary muscle.

Visceral muscle lies in the inner walls of hollow organs of the body, such as the alimentary canal and the reproductive tract. It has no stripes, so it is called smooth muscle. It is involuntary. It helps to move food along the digestive tract and gametes along the genital tract.

Cardiac muscle is the muscle of the heart. Many cells join in a branching pattern. The cells are striated, but the muscle is involuntary, because the nervous system does not control it directly. The cells stick to each other at junctions called intercalated discs.

A muscle is joined to a bone by a tendon. A tendon is a strong cord of dense connective tissue.

Inside a skeletal muscle

A whole skeletal muscle is made of smaller and smaller parts.

The muscle is made of a number of muscle bundles, or fascicles. A layer of collagenous connective tissue called the fascia holds the bundles together.
Each bundle holds a number of muscle fibres.
Each fibre is lined by the plasma membrane, which is called the sarcolemma. It encloses the cytoplasm, which is called the sarcoplasm.
The sarcoplasm has many nuclei, so a muscle fibre is a syncytium. It is one cell with many nuclei.
The endoplasmic reticulum of the fibre is called the sarcoplasmic reticulum. It is the store of calcium ions.
A large number of parallel threads lie in the sarcoplasm. They are the myofilaments, or myofibrils. Each myofibril shows alternate dark and light bands.

A skeletal muscle taken apart in four steps from left to right. First, the whole muscle, wrapped in a layer of connective tissue called the fascia, with bundles of fibres inside it. Second, one bundle, or fascicle, holding many muscle fibres side by side. Third, one muscle fibre, a long cell lined by the sarcolemma, enclosing the sarcoplasm, with many nuclei at its edge and the sarcoplasmic reticulum around the myofibrils. Fourth, a myofibril, a thread inside the fibre with alternate dark and light bands, which is made of thin and thick filaments.

You should find that a muscle with many bundles holds a very large number of fibres, and a still larger number of nuclei.

Try it in Python

The program counts the fibres and the nuclei of a model muscle. Then it lists the three kinds of muscle.

Practise with new numbers

Common mistakes

Saying that cardiac muscle is voluntary because it is striped. It is striated, but involuntary.
Saying that smooth muscle is striated. Visceral muscle has no stripes.
Mixing the sarcolemma and the sarcoplasm. The sarcolemma is the membrane. The sarcoplasm is the cytoplasm inside it.
Saying that a muscle fibre has one nucleus. A skeletal muscle fibre is a syncytium with many nuclei.
Saying that a fibre and a myofibril are the same. A fibre is a whole cell. A myofibril is a thread inside the cell.

Remember these

Skeletal muscle: striated, voluntary, next to the bones. Visceral muscle: smooth, involuntary, in hollow organs. Cardiac muscle: striated, branched, involuntary, in the heart.
Order of size: muscle, bundle (fascicle), fibre, myofibril, filament.
Fascia holds the bundles. The sarcolemma lines the fibre. The sarcoplasm holds many nuclei. The sarcoplasmic reticulum stores calcium ions.