An 8-year-old boy is evaluated in the office due to difficulty walking. The patient has problems running, jumping, and walking up steps. There is no history of neurological disorders in the family. Physical examination reveals weakness of lower extremity muscles bilaterally. Electron microscopy of a biopsy sample taken from calf muscle is shown below. Which of the following labeled regions contains only thick filaments and no thin filaments?

Question

An 8-year-old boy is evaluated in the office due to difficulty walking.  The patient has problems running, jumping, and walking up steps.  There is no history of neurological disorders in the family.  Physical examination reveals weakness of lower extremity muscles bilaterally.  Electron microscopy of a biopsy sample taken from calf muscle is shown below.  Which of the following labeled regions contains only thick filaments and no thin filaments?

Quizplus · Accepted Answer

11ec160b_3d02_39f0_9249_cbf624eb593d_MD0001_00 The H band is the region of the sarcomere containing only thick (myosin) filaments. On electron microscopy, the H band is a portion of the A band that straddles the M line. The A band corresponds to the thick filaments in the sarcomere and includes portions overlapped by thin (actin) filaments (Choice B). During muscle contraction, the thin filaments slide over the thick filaments toward the M line, reducing the length of the H band. The A band always remains the same length. (Choice A) The Z line is typically distinctly darker than the remainder of the sarcomere. Thin filaments, which are composed of actin, tropomyosin, and troponin, anchor at the Z line. (Choice C) The segment of the sarcomere between the H and I bands is where thick and thin filaments overlap. (Choice D) The I band contains the Z line and only those sections of the thin filaments that do not overlap with thick filaments. During muscle contraction, both the I band and H band decrease in length. Educational objective: The H band is the region of the sarcomere that contains only thick (myosin) filaments. The H band is the part of the A band (which is on either side of the M line) where thick filaments have no overlapping thin (actin) filaments. __________ References: The muscle ultrastructure: a structural perspective of the sarcomere. (http://www.ncbi.nlm.nih.gov/pubmed/15583864) The sarcomeric cytoskeleton: from molecules to motion. (http://www.ncbi.nlm.nih.gov/pubmed/26792323)

An 8-Year-Old Boy Is Evaluated in the Office Due to Difficulty