Action of physostigmine (eserine) sulfate on peripheral nerve.

Abstract

The effect of eserine (physostigmine) sulfate on frog and rabbit nerve has been studied. Concentrations ranging from 0.005–0.08 m caused: a) a reversible block of conduction in from 50–120 minutes in frog nerve, 20–40 minutes in rabbit nerve; b) the threshold (rheobase) to rise to three times normal value in frog nerve and up to five times normal value in rabbit nerve; c) No change in the value of the excitability constant k; d) The refractory periods to be greatly lengthened. Absolute periods of 10 msec., relative periods of 50 msec were obtained; e) a 60% reduction in the conduction velocity; f) a slight depolarization of less than 2 mv. A constant anodal voltage applied along 3 cm length of nerve partially relieves the eserine blocking action, cathodal voltage enhances the drug action. Nerve conduction is blocked much quicker with continuous stimulation, due to the lengthening of the refractory period. Following a brief interruption of continuous stimulation, a second or less, an action potential can be elicited again. This spike is almost identical with the action potential of a nonstimulated nerve subjected to eserine for the same period of time. The blocking action cannot be due to acetylcholine accumulation by the anticholinesterase activity of eserine, but rather is due to the inactivation of the sodium mechanism by the drug. Eserinized nerve loses some sodium quickly, large amounts of potassium more slowly.