General description and structure of skeletal muscle

Skeletal or striated muscle is also known as voluntary muscle since we have conscious control over its use. Other muscles, such as those lining the wall of the intestine, are involuntary smooth muscle, which lacks striations.

The cells of skeletal muscle are also known as muscle fibres since they are long and fibril-like - they may be a few cm long. Individual cells are grouped together into bundles known as fasciculi (each one is a fasciculus), each surrounded by a sheath of connective tissue. A number of fasciculi are then grouped together and surrounded by a further covering of connecting tissue, the epimysium or deep fascia; this whole structure is what we call a muscle (Fig. 4.9).

Muscle Fibre Structure

Muscle Muscle fibre bundle Single muscle fibre

Fig. 4.9 Structural organisation of skeletal muscle. One cell is a muscle fibre. The whole muscle is made up of bundles of fibres, each filled with myofibrils. Reproduced from Jones, D.A. & Round, J.M. (1990) Skeletal Muscle in Health and Disease. A Textbook of Muscle Physiology, with permission of Manchester University Press.

Muscle Muscle fibre bundle Single muscle fibre

Fig. 4.9 Structural organisation of skeletal muscle. One cell is a muscle fibre. The whole muscle is made up of bundles of fibres, each filled with myofibrils. Reproduced from Jones, D.A. & Round, J.M. (1990) Skeletal Muscle in Health and Disease. A Textbook of Muscle Physiology, with permission of Manchester University Press.

Within each muscle fibre are many myofibrils, themselves highly organised bundles of long polymers of the proteins myosin and actin. These form, respectively, 'thick' and 'thin' filaments, that overlap in a characteristic pattern to form the striations that give skeletal muscle its alternative name. Muscle contraction is brought about through head-groups, that protrude from the myosin thick filaments, binding to the actin filaments. The head-groups can 'rock' to move the myosin relative to the actin, detach, and rebind further along the actin. This process requires energy in the form of ATP, which is hydrolysed to release ADP + P., and it is regulated by Ca2+ binding to a protein known as troponin-C that is associated with the thin (actin) filaments.

This is necessarily a brief description of the molecular basis of muscle contraction, and the reader is referred to Further Reading for more details of this process. The important point is that a supply of ATP is required at the point of action at the appropriate time. There is a 'buffer store' in the form of phospho-creatine, which is in equilibrium with ATP through the action of the enzyme creatine kinase (Fig. 4.10). Since this is an equilibrium reaction, any fall in the ATP concentration will lead to the formation of further ATP from ADP, using the energy of phosphocreatine. This section covers the major fuels used by skeletal muscle to form the ATP required for contraction, and for the many other functions involved in cellular metabolism.

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