Inside a skeletal muscle

A single skeletal muscle fibre can be up to 30 cm long, and it is still one cell.

An organ made of bundles

A skeletal muscle is an organ rather than a lump of a single tissue. Inside it are muscle fibres, blood vessels, nerve fibres and connective tissue, all working together. The connective tissue comes in three layers, one inside the other. If you have ever pulled a piece of slow-cooked meat apart along its grain, you have felt them.

The outermost layer, the epimysium, wraps the whole muscle. It holds the muscle together while it pulls hard, and it separates it from neighbouring muscles and organs so each one can move on its own. Inside, the fibres are grouped into bundles called fascicles, each wrapped in a middle layer, the perimysium. Bundling lets the nervous system switch on part of a muscle rather than all of it at once. Inside each fascicle, every fibre has its own thin sleeve, the endomysium, which holds the fluid and nutrients that support it.

Where the pull goes

At the ends of most skeletal muscles, the collagen of all three layers runs together into a tendon, and at its far end the tendon fuses with the periosteum, the covering of the bone. So when the fibres contract, the tension passes through the wrappings to the tendon and on to the bone. Some muscles end in a broad, flat sheet called an aponeurosis instead; the latissimus dorsi of the back is one. Every muscle also has a rich blood supply for oxygen, nutrients and waste removal, and every single fibre gets its own branch of a motor neuron.

One cell, many nuclei

A skeletal muscle fibre is a single cell, but an unusual one. Fibres can be up to 100 micrometres across and, in the sartorius of the thigh, up to 30 cm long. Each one forms before birth when many embryonic cells called myoblasts fuse, so a mature fibre has many nuclei. More nuclei mean more copies of the genes needed to make the large amounts of protein a fibre uses. Because the Greek word sarco means flesh, the parts of a muscle cell have their own names. The cell membrane is the sarcolemma, the cytoplasm is the sarcoplasm, and the store of calcium ions inside is the sarcoplasmic reticulum.

Myofibrils and sarcomeres

Packed inside the fibre are myofibrils, rods only about 1.2 micrometres across. One fibre holds hundreds to thousands of them, and each runs the whole length of the cell. Along every myofibril the same short unit repeats thousands of times. This is the sarcomere, about 2 micrometres long from one Z disc to the next, and it is where contraction actually happens. When all the sarcomeres in a myofibril shorten, the myofibril shortens, and when the myofibrils shorten, the whole fibre does.

How the bundles are laid out

Fascicles are not arranged the same way in every muscle, and the layout affects how hard and how far a muscle can pull. Most muscles are parallel: their fascicles run along the length of the muscle. The biceps is a parallel muscle with a thick middle, or belly, that tapers at both ends, a shape called fusiform. Circular muscles, or sphincters, ring an opening and close it when they contract; the orbicularis oris around the mouth is one. In a convergent muscle such as the pectoralis major of the chest, a wide fan of fibres comes together at one tendon. In pennate muscles the fibres join a tendon at an angle, like the barbs of a feather. They cannot move the tendon very far, but they fit in more fibres and so produce more tension for their size. The deltoid of the shoulder is a multipennate muscle.

In the model, choose each level from the Part list to see its wrapping and what it holds.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.