title: [Neurophysiologic assessment of disorders affecting the neuromuscular junction].
abstract: The neuromuscular junction (NMJ) is a specialized synapse with a complex structural organization. Muscle contraction involves several steps: (1) nerve conduction to depolarize the motor nerve terminals, (2) opening of voltage-gated calcium channels (VGCCs) in the presynaptic membrane, (3) generation of endplate potential in the postsynaptic membrane via acetylcholine receptors, (4) depolarization of muscle sodium channels, and (5) excitation-contraction (E-C) coupling. Each step can be affected by various diseases. Guillain-Barre syndrome involves distal axons and possibly the presynaptic NMJ. The abnormalities can be detected by nerve conduction studies and single-fiber electromyography (SFEMG). Myasthenia gravis (MG) with anti-acetylcholine receptor (AChR) antibodies, is the most common NMJ disorder, and ~5% of myasthenia patients are positive for anti-muscle specific kinase (MuSK) antibodies. Patterns and severity of neuromuscular transmission failure detected by repetitive nerve stimulation test and SFEMG are somewhat different in AChR-MG and MuSK-MG. Excitation-contraction (E-C) coupling can be affected by MG, possibly via antibodies against ryanodine receptors. The E-C coupling time can be assessed with an accelerometer. Lambert-Eaton myasthenic syndrome is caused by antibodies against presynaptic VGCCs. This review will focus on neurophysiological testing, including SFEMG, and measurements of E-C coupling time with an accelerometer. In addition to confirming or excluding the diagnosis, these techniques can provide new insights into the pathophysiology of a variety of neuromuscular disorders.
authors: Department of Neurology, Graduate School of Medicine, Chiba University, Chiba, Japan.
authors: Satoshi
authors: S
authors: Kuwabara
mesh_terms: D004576:Electromyography; D006801:Humans; D009157:Myasthenia Gravis / Q000503:physiopathology; D020511:Neuromuscular Junction Diseases / Q000175:diagnosis / Q000503:physiopathology*