Studies on the repetitive discharges evoked in motor nerve and skeletal muscle after injection of anticholinesterase drugs

Abstract

Repetitive discharges recorded from the ventral root and from the gastrocnemius muscle in response to single motor nerve shocks applied close to the muscle after injection of edrophonium, neostigmine or ambenonium were studied in cats anaesthetized with chloralose. Two closely spaced volleys with an interval of 1 to 5 msec between them produced more repetitive firing than did a single shock. With longer intervals, the repetitive firing was not potentiated by the second volley. All frequencies of tetanic stimulation depressed the repetitive firing and, for successive stimuli to produce a degree of repetitive firing equivalent to the first, it was necessary to stimulate at frequencies below 2 shocks/sec. With stimulation frequencies higher than 100 shocks/sec, repetitive firing did not occur unless the duration of the tetanus was shorter than about 30 msec when slight repetition followed the last stimulus of the train. With stimulation frequencies of 100 down to 20 shocks/sec, repetitive firing was produced only by the first volley of the tetanus. Subsequent nerve action potentials of the tetanus occurring during the repetitive firing in the nerve following the first volley were partially extinguished by collision with the back discharge. This effect contributed to the waning tetanus, which is characteristic of treatment with an anticholinesterase, but the main depression of tetanic contractions appeared to be a consequence of depolarization block through accumulating acetylcholine. Tubocurarine and benzoquinonium reversed the initial "extinction" phase of the depressed tetani by abolishing the repetitive discharge in the nerve and in larger doses reversed the secondary depressant phase presumably by reducing the excessive end-plate depolarization. The results are discussed in relation to the hypothesis that anticholinesterases may effect transmission by acting at three sites at the neuromuscular junction-on acetylcholinesterase, at the motor nerve ending and at the motor end-plate-and that reaction at any one site may be augmented by the production of reverberating activity across the junction.