Muscular system: how the muscles in your body work

Three tissues, three different jobs

Like every other tissue in the body, muscle tissue is made of specialised cells called myocytes. The difference between the three types of muscle comes down mostly to the shape of these cells and how the proteins that generate movement, actin and myosin, are arranged inside them.

Skeletal muscle moves the bones and is under voluntary control, even though in some cases it works semi-automatically (think of the diaphragm, which breathes for you even when you’re not thinking about it). Cardiac muscle forms the heart and is involuntary: its cells are short, branched, and linked by junctions called intercalated discs, which spread the electrical signal and make the heart contract in a coordinated way. Smooth muscle, finally, lines the walls of hollow organs such as the stomach, intestines and blood vessels: it is involuntary, and its spindle-shaped cells lack the orderly arrangement of contractile proteins seen in the other two types.

Feature	Skeletal	Cardiac	Smooth
Cell shape	Long, cylindrical, unbranched	Short, branched	Spindle-shaped
Nuclei	Multiple, at the periphery	One, central	One, central
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones via tendons	Only in the heart	Walls of hollow organs and blood vessels

Skeletal muscle: a structure like nested boxes

A skeletal muscle is not a single block: it is a set of layers nested one inside the other. On the outside, a layer of connective tissue called the epimysium wraps the entire muscle. Inside, the muscle is divided into bundles, each wrapped by the perimysium; each bundle contains many muscle fibres (the individual cells), each in turn wrapped by the endomysium.

The muscle fibre is an unusual cell: it is multinucleate, meaning it contains several copies of DNA inside a single cell that can run several centimetres long. Inside the fibre sit the myofibrils, extremely thin filaments (about a thousandth of a millimetre across) made of sarcomeres lined up one after another.

The sarcomere, where the muscle actually shortens

The sarcomere is the basic contractile unit of skeletal muscle: the stretch of myofibril between two Z-discs. Under a microscope you can see alternating light and dark bands, which is where the name “striated muscle” comes from: the A band, dark, contains the thick myosin filaments, partly overlapped by the thin actin filaments; the I bands, light, sit on either side of the A band and contain only actin; the M line runs through the centre, and near it sits the H zone, where only myosin is present. An elastic filament made of the protein titin holds everything under tension.

Two “regulatory” proteins, troponin and tropomyosin, sit on the actin filaments and control when actin and myosin are allowed to link up.

How a contraction switches on

The signal starts in the nervous system. At the neuromuscular junction, the nerve releases the neurotransmitter acetylcholine, which opens channels on the muscle fibre’s membrane and depolarises it. This electrical signal tells the sarcoplasmic reticulum — a network of internal membranes inside the fibre that stores calcium — to release large amounts of calcium ions.

Calcium binds to troponin, which in turn shifts tropomyosin and exposes the spots on actin where myosin can attach. From here starts what is known as the cross-bridge cycle: the myosin head attaches to actin, consumes ATP to detach and reset, attaches again a little further along, and pulls the actin filament toward the centre of the sarcomere. Repeated rapidly across thousands of sarcomeres in a row, this sliding shortens the whole fibre: this is the sliding filament theory.

Practical example: when you pick up a bag of groceries, the signal starts in the brain, travels down a motor neuron, and releases acetylcholine onto the biceps: within a fraction of a second calcium is released, actin and myosin begin sliding past each other, and the arm bends. Stop thinking about it and the nerve signal stops: calcium goes back into the sarcoplasmic reticulum and the muscle relaxes.

Voluntary or involuntary: who decides

Only skeletal muscle responds to a conscious command: you can choose to bend an elbow or squeeze an eye shut. Cardiac and smooth muscle work without any need for thought: the heart beats on its own, the intestines push food along, blood vessels narrow or widen to regulate blood pressure, even while you sleep.

Muscle cells, like all others, still arise from the division of existing cells — the same principle explained in detail for mitosis. In adult skeletal muscle, though, growth happens mostly by enlarging fibres that already exist: that is the mechanism behind muscle hypertrophy, the muscle’s response to training and rest.

The main muscle groups

Skeletal muscles are grouped by body region. In the limbs they move the arms and legs — biceps, quadriceps, hamstrings. In the trunk they support posture and protect the organs (abdominals, back muscles); in the head and neck they govern expressions and chewing instead. A special case is the diaphragm, the skeletal muscle that contracts and relaxes automatically to move air in and out of the lungs, while still staying under voluntary control when needed (for example, when holding your breath).

Among the body’s records: the gluteus maximus is the largest muscle, the masseter (the jaw muscle) is the strongest by absolute force exerted, and the stapedius, in the middle ear, is the smallest of all, measuring less than a millimetre in length.