Prolonged response of skeletal muscle in the absence of penetrating anions

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Abstract
Replacement of Ringer's chloride by a variety of nonpenetrating anions results in prolonged electrical and mechanical responses of muscle to stimulation. The ‘negative after-potential’ is characterized by a slowly increasing secondary depolarization which reaches a stable plateau lasting as long as 2 minutes. After-discharge frequently occurs during early depolarization. In most fibers repolarization is relatively abrupt, but in some, slow oscillations resembling relaxation oscillations arise following the plateau, grow gradually in amplitude and, only when they are of sufficient amplitude, does the membrane repolarize. Prolonged depolarization can still be produced when the spike has failed. At times, fibers may respond with short-duration action potentials, but may be primed to give prolonged responses by previous stimuli or by increase of external potassium. Addition of chloride has no effect below a critical concentration. Reduction of sodium to 25% of normal does not change plateau level or duration. Duration of the plateau phase is decreased by potassium.