H-zone: a narrow zone in the centre of each A-band contains thick but NOT thin filaments. Supporting proteins that hold the thick filaments together at the centre of the H-zone, form the M-line.
M-line: the middle of the sarcomere
Muscle Fibre Proteins
Myofibrils are built from 3 kinds of proteins: Contractile, Regulatory, Structural.
1. Contractile Proteins
Generate force during contraction. There are 2 principle types which are the main components of thick and thin filaments: Actin : Individual actin molecules join to form an actin filament that is twisted into a helix. This is a thin filament. On each actin molecule is a myosin-binding site where a myosin head can attach. Myosin : functions as a motor protein in all 3 types of muscle tissue. Motor proteins push or pull various cellular structures to achieve movement by converting chemical energy in ATP to mechanical energy of motion or production of force. As stated earlier, about 300 molecules of myosin form a single thick filament. Each myosin molecule is shaped like 2 golf clubs twisted together with the tail pointing toward the M-line in the centre of the sarcomere. Tails of neighboring myosin molecules lie parallel to one another forming the shaft of the thick filament.
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