Abstract

1. Two of the commonly used indices of pain perception in animals, the "tail flick" of the rat and the "back skin twitch" of the dog, were found to be merely spinal reflexes.

2. These reflexes are directly depressed by morphine, methadone, and meperidine.

3. Depression of the tail reflex by potent analgetics is manifested by both a reduction in the amplitude of the response and an elevation of the response-time threshold as measured by the D'Amour and Smith technique.

4. Quantitative differences were noted in the drug effects in intact and spinal rats which suggest that in addition to their direct effects on the reflex are, morphine and methadone also enhance supraspinal inhibitory mechanisms concerned with the tail reflex.

5. Small, localized tail movements were observed in dead, pithed and deeply etherized rats on radiant heat stimulation. Since these responses cannot be distinguished from reflex responses markedly depressed by analgetics, a method was suggested whereby predetermination of this physical limitation of the D'Amour-Smith technique may be utilized to define a uniform "maximal measurable effect" ceiling for any given intensity of stimulus used.

6. Some similarities between the tail reflex and cutaneous maximus reflex on the one hand, and multineuron reflexes on the other, were discussed with regard to the role that analogous neural mechanisms may play in mediation of pain.