Aging and lower motor neuron pathologies can cause what change in the muscle?

• A. Hyperplasia
• B. Atrophy
• C. Hypertrophy
• D. Dystrophy

Answer: (B)

Loss of motor neuron input to muscle causes the fibers to shrink. When lower motor neurons are lost or impaired, the muscles they control become denervated and lose trophic support, leading to decreased protein synthesis and increased protein breakdown in the muscle fibers. Over time this results in smaller fibers and reduced overall muscle mass, i.e., atrophy. Aging contributes to this by gradual motor unit dropout, promoting sarcopenia. Hypertrophy would require increased load to grow fibers, hyperplasia is not how skeletal muscle typically increases, and dystrophy implies degenerative replacement with fibrous or fatty tissue rather than simple fiber shrinking.