Abstract

1. Anatomic and physiologic studies indicate that the skin-twitch response to nociceptive stimulation in the dog is integrated over a spinal reflex arc whose afferent limb arises from the cutaneous innervation of portions of about T-2 to L-7 dermatomes (corresponding roughly to the receptive zone of the scratch reflex), and whose efferent limb traverses mainly the C-8 ventral root, innervating the cutaneous maximus muscle through the external thoracic nerve.

2. The amplitude of the skin-twitch response is enhanced by section of the spinal cord at appropriate levels. Morphine, in doses of 5 and 10 mgm./kgm., depresses the skin-twitch response in both intact and spinal dogs, but to a greater degree in the former. The depressant effects of morphine on the skin-twitch in intact dogs is due partly to a direct spinal cord depressant action, and partly to augmentation of supraspinal inhibitory mechanisms.

3. The effects of morphine on the skin-twitch in dogs are non-specific since this response is also depressed by adequate doses of thiopental or mephenesin.

4. The effects of depressant drugs on the amplitude of reflex responses to supramaximal stimuli are more reliable indices of their effectiveness than are prolongations of time required for a constant stimulus, or elevations of intensities of stimuli of fixed duration, to elicit "threshold" responses. The connotations of the term "threshold" when used in connection with such measurements, are different from those which are implied in electrophysiology.

5. The significance of these findings is discussed briefly with reference to the general validity of current analgesic testing methods in animals.