Hitherto, the analgesic effect of drugs on animals have been assayed by means of the pain threshold, and reaction to pain. Responses which were employed consisted of: lid reflex, sharp cry, defensive movements, muscle twitch, tail flick, pupillary reflex, movements of the whole body, etc.. Although opiates and other analgesics produce a rise on the threshold of observable reactions to presumably painful stimuli in animals, the validity to employ such reactions for measuring the analgesic effect is open to question, since these drugs, of course, affect, not only the efferent pathways of the nervous system but also the afferent pathways.
In short, the analgesic effect of drugs probably cannot be determined exactly in animal experiments. The decision of sites of action of these drugs in the central nervous system, however, may contribute in part to some interpretation of the mechanism in the action of analgesics.
Electrical responses to afferent stimulation have been obtained after stimulation of the sciatic nerve (1), the splanchnic nerve (2, 3) and the vagus (4), and the details of the afferents of these nerves, or the effect of several drugs on them, have been investigated (5-15), but the physiological meanings of these electrical responses have not yet been clear.
The purpose of the present study is to determine sites of action of several analgesics and related drugs on afferent pathways of various nerves by means of deciding the sites of blockade produced by drugs, and, if possible, through this means to throw light on the nature of the nervous mechanism itself.
View full abstract