An 8-year-old boy is evaluated in the office for walking difficulties. Running, jumping, and walking up stairs are all difficult for the patient. There is no family history of neurological disorders. A physical examination reveals bilateral weakness of lower extremity muscles. The image below shows electron microscopy of a calf muscle biopsy sample. 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 for walking difficulties. Running, jumping, and walking up stairs are all difficult for the patient. There is no family history of neurological disorders. A physical examination reveals bilateral weakness of lower extremity muscles. The image below shows electron microscopy of a calf muscle biopsy sample.  Which of the following labeled regions contains only thick filaments and no thin filaments?

Quizplus · Accepted Answer

Eductional Objective: The H band is the sarcomere region that only has thick (myosin) filaments. The H band is a portion of the A band that spans the M line in electron microscopy. The A band corresponds to the sarcomere's thick filaments and includes portions overlapped by thin (actin) filaments. The thin filaments slide over the thick filaments toward the M line during muscle contraction, shortening the H band. The A band is always the same length.

An 8-Year-Old Boy Is Evaluated in the Office for Walking