Which line bisects the H zone vertically?

• A. Z disc
• B. M line
• C. A band
• D. I band

Answer: (B)

The central line that runs through the middle of the sarcomere, called the M line, bisects the H zone vertically. The H zone is the central part of the A band where only thick (myosin) filaments are present, with no actin overlap, and it sits around the M line. The M line anchors thick filaments and marks the exact center of the sarcomere, so it divides the H zone into two equal halves. By contrast, the Z discs mark the boundaries of the sarcomere and define the start of the I band (thin filaments only), while the A band includes the full length of the thick filaments and areas of overlap with actin.