Quantitative relationship between the stimulus intensity and the response magnitude in the tail flick reflex

The purpose of the present study was to investigate the quantitative relation between the stimulus intensity and the response magnitude of the tail flick reflex. The EMG of a tail muscle was recorded from the extensor caudae medialis (ECM) muscle in the side contralateral to heat stimulation, and the area of integrated EMG for 1 sec was measured as the magnitude of EMG activity. The minimum temperature to the onset of the EMG was 42.3 +/- 0.4 degrees C. The relation between the stimulus intensity and the magnitude of an integrated EMG followed a power function with an exponent of about 8.5. The magnitude of an integrated EMG was decreased by about 50% of the control by an intraperitoneal administration of morphine (0.5 mg/kg). These results suggest that tail flick reflex is closely related to painful sensation, and that EMG activity of the ECM muscle is applicable as an electrophysiological indicator to noxious stimulation of the tail and an expressible indicator of the magnitude of pain.