A possible interpretation for difference in neostigmine-induced changes of spontaneous activities and evoked muscle potentials between rat medial gastrocnemius and soleus muscles.

1. Under urethane-anaesthesia, changes of spontaneous activities and evoked muscle potentials in rat medial gastrocnemius (MG) and soleus (SOL) muscles induced by the injection of neostigmine were investigated. 2. Before the administration of neostigmine, spontaneous activities were rarely observed and muscle potentials evoked by single shocks were simply biphasic in both MG and SOL muscles. When the anti-cholinesterase drug was applied, spontaneous discharges of motor units were often observed in bursts in the MG muscles, in contrast to single spikes in the SOL muscles. The biphasic wave of evoked potentials in the MG muscles was followed by one or more oscillations, which were scarcely observed in the SOL muscles. These burst discharges and oscillations in the MG muscles [corrected] also occurred even after transection of the sciatic nerve proximal to the stimulating site. 3. A late component with a small amplitude could be observed in the evoked potentials of MG and SOL muscles. It was potentiated in amplitude after the administration of the drug. 4. Prolonged action of acetylcholine (ACh) has been known to occur in the presence of anticholinesterase drugs, and this was taken into account for the mechanism of the phenomena proposed in the present study. Contrasting changes between MG and SOL muscles were observed, which may be explained by different modes of ACh release in the MG and SOL muscles.