End-plate spikes in electromyography are fusimotor unit potentials

Spontaneous negative-positive potentials (NPPs) in electromyography were often activated by movement of the needle electrode or by tetanic stimulation. Coactivation of NPPs was sometimes observed in voluntary effort. Circulatory arrest or hyperventilation had no consistent effect. These facts and the differences of firing of NPPs from usual ectopic firing patterns indicate that NPPs are not driven by ectopic activity of motor axons. Instead, the firing of NPPs is similar to activity of muscle spindle afferents during fusimotor drive. We believe that NPPs are action potentials of intrafusal muscle fibers driven by gamma motor neurons.