Define sliding filament theory of muscle contraction.
The sliding filament theory explains the process of muscle
contraction during which the thin filaments slide over the thick
filaments, which shortens the myofibril.
Each muscle fibre has an alternate light and dark band, which
contains a special contractile protein, called actin and myosin
respectively. Actin is a thin contractile protein present in the
light band and is known as the I-band, whereas myosin is a thick
contractile protein present in the dark band and is known as the
A-band. There is an elastic fibre called z line that bisects each
I-band. The thin filament is firmly anchored to the z line. The
central part of the thick filament that is not overlapped by the
thin filament is known as the H-zone.
During muscle contraction, the myosin heads or cross bridges come
in close contact with the thin filaments. As a result, the thin
filaments are pulled towards the middle of the sarcomere. The Z
line attached to the actin filaments is also pulled leading to
the shortening of the sarcomere. Hence, the length of the band
remains constant as its original length and the I-band shortens
and the H-zone disappears.
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