Effect of Succinylcholine on Single Motor End‐plates in Man

Abstract

The effect of small doses of succinylcholine on nerJe muscle impulse transmission in situ was studied with single fibre electromyography. Extracellular muscle fibre action potentials were recorded at voluntary activation or on electrical stimulation. The results showed that the effect of succinylcholine was mainly localized in the the motor end-plate. An initial shortening of the neuromuscular delay, due to a slight depolarization, was followed by an increase in the neuromuscular transmission time of up to 3 ms, probably owing to an increased rise-time of the end-plate potential. I n some experiments a neuromuscular block appeared. After this, the neuromuscular transmission time was normalized within 30 s. The jitter, a measure of the variability in the neuromuscular delay, could usually be accurately measured only during the recovery phase after the maximal effect of succinylcholine. During the recovery phase it was considerably increased for a time period of up to 3 min. This jitter increase was most probably due to a reduced end-plate potential, that is, a desensitization to acetylcholine in the motor end-plate.Motor end-plates, even in the same motor unit, showed a great difference in sensitivity to succinylcholine. Tachyphylaxis also developed after the small doses administered, and lasted for at least 15 min.

Motoneuron firing in amyotrophic lateral sclerosis (ALS)

“…After cutaneous stimulation higher threshold units show lower recruitment thresholds, thus altering recruitment order (Kanda and Desmedt, 1983). Low threshold motor unit activity is never entirely regular (Rosenfalck and Andreassen, 1980), especially when a unit is firing very slowly, a feature attributed by Stålberg et al (1973) to spontaneous fluctuations in the trigger level of the motoneuron membrane, variable presynaptic inflow, and short term irregularity in the neuronal depolarization curve, due to “synaptic noise.” In general, motor units in muscles innervated by lumbar roots show a slower and more regular firing pattern than motor units in upper limb muscles (Stålberg et al, 1973). The variability of low threshold motor unit discharge is higher in old than in young adults, a feature consistent with the greater force fluctuation observed at low muscle forces in older people (Tracy et al, 2005), this probably derives from longer AHP associated with aging (Piotrkiewicz et al, 2007).…”