MEDizzy - A patient with muscle weakness is evaluated, and his arm

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Musculoskeletal and Integumentary Systems

A patient with muscle weakness is evaluated, and his arm muscles become progressively weaker during repeated lifting of a weight. Clinical nerve conduction tests indicate that repetitive stimulation of the motor nerve leads to normal action potentials recorded from the nerve, but impaired compound action potentials recorded from the innervated muscle fibers. Direct stimulation of the muscle with needle electrodes produces normal muscle action potentials and contractions. Direct application of acetylcholine to the motor end plate also leads to normal muscle responses. Based on these test results, which of the following is most likely leading to the muscle weakness in this patient?

ExplanationBecause the nerve action potential generation and conduction appear to be normal, and direct stimulation of the muscle fiber with either electrodes or acetylcholine (ACh) results in a normal muscle contraction, the problem must be at the synapse between the nerve and the muscle, which is the neuromuscular junction. (A) The contractile mechanism of the muscle is not defective, because direct stimulation of the muscle fiber elicits a normal muscle contraction. (C) The ACh receptors are functional, because application of ACh at the motor end plate leads to a normal muscle response. (D) Because direct stimulation of the motor nerve generates normal nerve action potential and conduction, the nerves are not likely demyelinated.