Sudden Destruction of Motor End Plates by Lactic Acid ∗

Summary

Lactic acid in concentrations of 0.05 to 0.3% injected into the exposed zone of innervation of the sternomastoid muscle in the white rat produced sudden destruction of most of the motor end plates within 30 seconds to 25 minutes dependent upon the concentration of the acid. This liquefaction of the hypolemmal axons of the end plates was preceded by a first phase of retraction and a second phase of expansion and fragmentation. Lactic acid caused a progressive depletion, in both the hypolemmal and the epilemma1 axons, of auriphilous substances. The blood vessels were enlarged, hyperemic, and contained clumps of agglutinated red blood cells in the rigorous muscle paralyzed by lactic acid. I t is suggested that the acid metabolites which accumulate during rigor mortis of muscle are responsible for the destruction of the end plates and that during life certain processes that result in the intramuscular accumulation of acid, such as vascular congestion and anoxia, likewise may result in the destruction of the motor end plates.