Skeletal Muscle Structure

Although skeletal muscles come in different shapes and sizes, the main structure of a skeletal muscle remains the same. If you were to take one whole muscle and cut through it, you would find that the muscle is covered in a layer of connective tissue known as the epimysium. The epimysium protects the muscle from friction against other muscles and bones.

It also continues at the end of the muscle to form (along with other connective tissues) the muscles tendon. Looking at the cross section of the muscle you can see bundles of fibres, known as fascicule, which are surrounded by another connective tissue, called the perimysium. Each fascicule contains anywhere between 10 and 100 muscle fibres, depending on the muscle in question.

A large strong muscle, such as those forming quadriceps would have a large number of fibres within each bundle. A smaller muscle used for precision movement, such as those in the hand would contain far fewer fibres per fascicule.

Looking at each muscle fibre in detail (Figure 2), we can see they are covered with a fibrous connective tissue, known as endomysium which insulates each muscle fibre. Muscle fibres can range from 10 to 80 micrometers in diameter and may be up to 35 cm long.

Beneath the endomysium and surrounding the muscle fibre is the sarcolemma, which is the fibre's cell membrane and beneath this is the sarcoplasm, which is the cells cytoplasm, a gelatinous fluid which fills most cells. This contains glycogen and fats for energy and also mitochondria which are the cells powerhouses, inside which the cells energy is produced.

Figure 2. Muscle fibre

Each muscle fibre itself contains cylindrical organelles known as myofibrils. Each muscle fibre contains hundreds to thousands of myofibrils. These are bundles of actin and myosin proteins which run the length of the muscle fibre and are important in muscle contraction.

Surrounding the myofibril there is a network of tubules and channels called the sarcoplasmic reticule in which calcium is stored which is important in muscle contraction. Transverse tubules pass inwards from the sacrolemma throughout the Myofibril, through which nerve impulses travel.