Q.Describe the arrangement of the A band, I band, H zone and Z line within a single sarcomere, and state which of these shorten and which stays constant in length during contraction.

[!TLDR]

The I band and H zone shorten during contraction; the A band stays constant in length.

[!ANSWER]

I band and H zone shorten; A band length stays constant.

Within a sarcomere, going outward from the centre: the M line lies at the exact middle, within the H zone, which is the central part of the A band containing only thick (myosin) filaments; the A band as a whole is the full dark zone spanning the length of the thick filaments; the I band lies on either side of each Z line and contains only thin (actin) filaments; and the two Z lines mark the two ends of the sarcomere. During contraction, the thin filaments slide further in over the thick filaments (the sliding filament mechanism), which pulls the Z lines closer together. Because the thick and thin filaments themselves do not change length, but only their region of overlap increases, the A band (defined by the length of the thick filaments) stays constant, while the I band (where actin does not overlap myosin) shortens, and the H zone (where myosin does not overlap actin) also shortens or can disappear entirely at maximal contraction.

[!ANSWER]

I band and H zone shorten; A band length stays constant.

Recall that only the degree of filament overlap changes during contraction, not filament length itself -- so any band defined purely by the thick filament's length (the A band) cannot change, while bands defined by the absence of overlap (I band, H zone) shrink as overlap increases.

Assuming the A band shortens along with the I band and H zone -- this is exactly what the sliding filament theory rules out, since it is a defining prediction that filament lengths, and so the A band, remain unchanged.