Voluntary muscle tissue

Voluntary muscle tissue is made up of bands of elastic or contractile tissue bound together in bundles and enclosed by a connective tissue sheath which protects the muscle and helps to give it a contoured shape.

Voluntary or skeletal muscle tissue has very little intercellular tissue. It consists almost entirely of muscle fibres held together by fibrous connective tissue and penetrated by numerous tiny blood vessels and nerves. The long slender fibres that make up muscle cells vary in size. Some are around 30 cm in length, whereas others are microscopic.

Each muscle fibre is enclosed in an individual wrapping of fine connective tissue called the endomysium. These are further wrapped together in bundles, known as fasciculi and are covered by the perimysium (fibrous sheath) which is then gathered to the muscle belly (main part of the muscle) with its own sheath - the fascia epimysium.

The relatively inelastic parts of each of the muscles are tendons and these are usually made up of the continuation of the endomysium and perimysium.

Each muscle fibre is made up of even thinner fibres called myofibrils, which are the contractile elements of a skeletal muscle fibre. These consist of long strands of microfilaments, made up of two different types of protein strands called actin and myosin. It is the arrangement of actin and myosin filaments which gives the skeletal muscle its striated or striped appearance when viewed under a microscope. Muscle fibre contraction results from a sliding movement within the myofibrils in which actin and myosin filaments merge.

Each person is born with a set number of muscle fibres which cannot be increased. An increase in the size of a muscle is due to exercise which will cause an increase in the individual fibres. However, with disuse these will shrink again as the muscle atrophies. It is interesting to note that men are more able to enlarge their muscles through exercise than women due to the effects of male hormones.

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Most skeletal muscles are made up of a combination of the following types of fibres.

Fast twitch fibres (white)

These have fast, strong reactions but tire quickly. They are well adapted for rapid movements and short bursts of activity. They have a rich blood supply and mainly use the energy stores of glucose in the muscles which can be transferred into mechanical energy without oxygen.

Slow twitch fibres (red)

These fibres have greater endurance but do not produce as much strength as fast twitch fibres. They are therefore suited to slower and more sustained movements and are relatively resistant to fatigue. Their energy comes from the breaking down of glucose by oxygen and they depend on a continuous supply of glucose for endurance. Slow twitch fibres have a good circulation (the red colour comes from both the circulation and from the presence of a red pigmented protein that stores the oxygen).

During low-intensity work, such as walking, the body is working well below its maximal capacity and only slow twitch fibres are working. As muscle intensity increases and the exercise becomes more anaerobic, fast twitch fibres are activated. Whatever the intensity of movement, only a small number of fibres are used at any one time to prevent damage and injury to the tissues.

The way in which the bundles of fibres lie next to one another in muscle will determine its shape. The contractile force of a muscle is partly attributable to the architecture of its fibres. Common muscle fibre arrangements include the following:

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