Fig. 1.
Example of jaw-opening reflex EMG tracing before and after administration of morphine. The top tracedemonstrates two components, A and B, which show EMG amplitudes of 1.0 and 2.9 mV, respectively. Based on the short latency, component A results from activation of fast-conducting (presumably non-nociceptive) afferents; the latency of component B is compatible with the conduction velocity of Aδ-nociceptors (calculated conduction velocity of ∼16–20 m/sec). The bottom trace, recorded from the same animal 30 min after systemic administration of morphine (10 mg/kg, i.p.), shows attenuation of component B but no change in component A, compatible with the findings of others that opioids do not attenuate non-nociceptive responses. The data in this study were always taken from that part of the EMG signal exhibiting a latency similar to that of component B.