A 42-year-old man from Mumbai presents with progressive weakness of the lower limbs and lower back pain for 6 months. On examination, he has proximal muscle weakness (hip and knee extensors), normal reflexes, and no sensory loss. He reports that his symptoms improve slightly with continued activity (unlike fatigue-related weakness). Serum creatine kinase is elevated at 2800 U/L. Electromyography shows brief, small, polyphasic motor unit action potentials with early recruitment. Muscle biopsy reveals fiber-type grouping and angulated atrophic fibers. Which of the following neuromuscular structures is primarily affected in this patient's condition?