Skeletal muscle

Skeletal muscle fibres are attached to bone by tendons of strong connective tissue. The proximal attachment is known as the ‘origin’ and the distal attachment is the ‘insertion’, the two being connected by the muscle ‘belly’. The precise shape and distribution of a muscle, or muscle group, about a joint depends on its particular function. In summary, the functions of skeletal muscle are to provide:

• A mechanical response to environmental stimuli

• A short-term store of glycogen and glucose

• A long-term metabolic reserve of protein for gluconeogenesis

Microscopic structure

Muscle cell

Muscle cells or fibres are quite large: they are typically about 100 μm in diameter, and may run the full length of the muscle. They are multinucleated, are surrounded by a membrane, the endomysium, and are bound into fasciculi. These fasciculi are surrounded by the perimysium, and combine to make up the whole muscle. This, in turn, is covered by connective tissue sheet, the epimysium (Figure MP1).

Myofibril

The muscle cells or fibres are made up of myofibrils enclosed by the cell membrane (sarcolemma). The myofibrils consist of two types of myofilaments. There are thick filaments composed of myosin and thin filaments that are made up of actin.

Sarcomere

The myofibril is made up of basic contractile units called sarcomeres. The microscopic appearance of the sarcomere identifies various regions, which are known by letters (Figure MP2).

The sarcomere is composed principally of myosin filaments and actin filaments.