Q.Reading a myofibril from one Z line to the next, which region lies exactly in the middle of the A band and, in a resting sarcomere, contains only the rod-like portions of the myosin filaments with no overlapping actin?

[!TLDR]

The H zone is the central part of the A band where only thick (myosin) filaments are present, with no actin overlap.

[!ANSWER]

(b) H zone

Within a sarcomere, the A band is the whole dark zone spanning the length of the thick filaments. Its very centre, the H zone, is paler than the rest of the A band because only the thick filaments are present there, with no overlapping thin (actin) filaments reaching that far in. A fine M line runs through the exact middle of the H zone, anchoring the thick filaments in place, but the M line is a thin line, not a band. The I band, by contrast, lies on either side of each Z line and contains only thin filaments, with no thick filaments present at all.

[!ANSWER]

(b) H zone

Recall the sarcomere map: Z line -- I band (actin only) -- A band (myosin, with overlap at its edges) -- H zone (myosin only, at the centre of the A band) -- M line (a line, not a band, at the centre of the H zone) -- and symmetrically back out to the next Z line.

Mixing up the I band (actin only, at the sarcomere's outer edges) with the H zone (myosin only, at the sarcomere's very centre) -- they are opposite ends of the sarcomere and contain opposite filament types.