MOLECULAR STRUCTURE OF THE SKELETAL MUSCLE::
The characterisitic feature of skeletal muscle is the alternate light and dark shades electron micrography reveals that it is due to presence of myofibrils of different densities on the molecular basis the functional unit of contractiles system in muscle is sarcomere each sarcomere consists of two types of myofibrils thick and thin thick fibres are 150 a in diameter and consists of protein myosin they form the dark band thin fibres are 50 a in diameter they form the light band and consists of the protein actin on an average a muscle fibre consists of 16 billion thick and 64 billion thin fibres
part of thin fibres overlays thick fibres forming different bands as follows :
H= Lighter band formed only by myosin (thick filament )
A= dark band formed by myosin and actin (thin fibre ) overlapping it
I= Light band formed only by actin .it is the region between A bands of the adjoining sarcomeres
Z= Consists of short elements which interconnects thin filaments from adjoining sarcomeres
It the muscle is observed in transverse section it will reveal that each thick fibre is surrounded by six thin fibres actin and myosin are the structural proteins in the muscle a part from these other proteins such as troponin and tropomycin are also present these are regulatory proteins which do allow actin and myosin to interact for the contraction of muscle troponin and tropomysin are inhibited ca++ and stimulated by k+
calcium is another important constituent of the muscle required for the contraction the source of ca++ is sarcoplasmic reticulum the membrane the membrane of sarcoplasmic reticulum has ability to concentrate calcium with the help of ATP a part from sarcoplasmic reticulum there is a network of sarcoplasmic tubules where calcium is concentrated sarcoplasmic reticulum and tubules surround myofibrils of the muscle
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