Sympathetic reflex responses presumably specific for nociceptive stimulation

1. In superficially anaesthetized and in unanaesthetized decerebrate cats, tetanic stimulation of A or A + C afferent fibres of the tibial nerve evoked in renal and inferior cardiac nerves a three-phasic change of activity, which consisted of a short powerful burst of discharges, followed by an inhibitory pause and then, by a moderate excitation. When such a command appeared in sympathetic nerves, reflex rising of systemic arterial pressure was extremely fast. 2. The latencies of the three-phasic command and of the so-called very late A response are almost the same. The very late A response is evident if the central nervous system is in the same state as when repetitive stimulation of A afferents evokes the three phasic command in sympathetic nerves and a steep rise of systemic arterial pressure. This permits the assumption that the very late A response reflects the excitation of just those central structures that on repetitive stimulation organize the three-phasic command. 3. A command of this kind is shown to ensure the optimal-time-control of the heart and blood vessels. Such a control (three-phasic command) is supposed to be evoked by nociceptive stimulation, i.e., to be specific for the defence reaction.