I.T. Campbell D. Liu
Central control of muscle tone and movement
Smooth muscle contraction
COMPARISON BETWEEN SKELETAL, CARDIAC AND SMOOTH 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 cells or fibres are quite large, typically about 100 |im 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 fascicles. These fascicles 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 MP. 1).
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 MP.2).
The sarcomere is composed principally of myosin filaments and actin filaments. The myosin filaments occupy the central part of the sarcomere and comprise the A band. The A band is transected by the M line, which keeps the myosin filaments in side by side alignment. The myosin filaments interdigitate with the thin actin filaments whose ends are joined to the Z line or disc which maintains their spatial arrangement. The area of myosin filaments in the middle of the sarcomere not overlapped by actin filaments is known as the H zone. A cross section of the sarcomere in areas where the two types of filaments overlap shows that the two types of filament are arranged in a hexagonal pattern (Figure MP.3). The myoplasm between the filaments contains glycogen, myoglobin, the enzymes involved in glycolysis and mitochondria.
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