Read the following description carefully: "In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments. This central part is called 'X', and it is bisected by a thin fibrous membrane called 'Y'." Identify 'X' and 'Y'.

Step-by-step solution

In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments. The central part of the thick filament, not overlapped by thin filaments, is called the H-zone ('X'). The thick filaments in the A-band are held together in the middle of this band by a thin fibrous membrane called the M-line ('Y'). Therefore, X is the H-zone and Y is the M-line. Answer: X - H-zone, Y - M-line