The flexor reflex mediated by group II afferent fibers: effects of morphine-HCl and mephenesin.

The effects of morphine-HCl and mephenesin on the flexor reflex mediated by group II afferent fibers were investigated. The flexor reflex was recorded by means of the electromyogram (EMG) evoked in the muscle tibialis anterior by stimulation of the ipsilateral tibial nerve in urethane-alpha-chloralose anesthetized rats. Afferent volleys corresponding to the phasic EMG component of the flexor reflex with 7.6-msec latency (flexor EMG: fEMG) were also recorded using the double volley technique. The threshold of the afferent volleys mediating the fEMG was approximately twice as high as that of the most excitable afferent volleys, which were considered the spikes of group I afferent fibers, and the conduction velocity of the afferent volleys was 39.9 +/- 3.2 m/sec. Morphine-HCl (5 mg/kg, i.v.) did not change the amplitude of the fEMG, but mephenesin (40 and 80 mg/kg, i.v.) depressed it dose-dependently. These results suggest that the fEMG is a flexor reflex mediated by group II afferent fibers, which is not affected by morphine-HCl but depressed by mephenesin.