Q.Draw the diagram of a sarcomere of skeletal muscle showing different regions.

A sarcomere is the functional unit of contraction, taken as the stretch of a myofibril lying between two successive Z-lines. When you draw it, show a horizontal myofibril with these labelled regions:

• Two vertical Z-lines at the ends, marking the boundaries of the sarcomere.
• A central dark A-band (anisotropic band) made of thick myosin filaments.
• Light I-bands (isotropic bands) on either side, made of thin actin filaments, each I-band bisected by a Z-line.
• The thin (actin) filaments anchored to the Z-lines and reaching inward to partly overlap the thick filaments.
• The thick (myosin) filaments held together at the centre by the M-line.
• The H-zone in the middle of the A-band, the part of the thick filaments not overlapped by thin filaments.

The sarcomere is the smallest repeating unit that actually does the work of contraction in a skeletal muscle. Along every myofibril the dark and light bands follow one another in a regular pattern, and the piece of myofibril lying between two neighbouring Z-lines is defined as one sarcomere. Because a diagram cannot be handed in here, the parts to draw and label are described so the picture can be reproduced faithfully.

Draw a long myofibril lying horizontally, and mark it out as follows.

• Z-lines: Place two vertical lines, one at each end of the unit you are drawing. These are elastic reference lines, and the region between them is the sarcomere. Each Z-line sits in the centre of an I-band and bisects it.
• I-band (isotropic, light band): On either side, next to the Z-lines, show a light band. This region contains only the thin filaments, so it appears pale.
• A-band (anisotropic, dark band): In the centre draw a broad dark band. This region contains the thick filaments, and it is where the thick and thin filaments overlap towards its edges.
• Thin (actin) filaments: Draw these as the thinner rods. They are firmly fastened to the Z-lines and stretch inward from both sides, their free ends reaching in to partly overlap the thick filaments.
• Thick (myosin) filaments: Draw these as the stouter rods lying in the middle. They are held together at the centre of the A-band by a fine fibrous line, the M-line, which should be drawn vertically through the middle of the A-band.
• H-zone: In the very centre of the A-band, mark the H-zone — the part of the thick filaments that the thin filaments do not reach and therefore do not overlap.