The response of some sympathetic neurones to volleys in various afferent nerves.

1. Responses of sympathetic neurones to various afferent inputs have been analysed both in anaesthetized cats with intact neuraxis and in spinal cats.2. In anaesthetized cats electrical stimulation of low threshold group III afferent fibres in skin and muscle nerves inhibited sympathetic neurones and gave depressor responses. The silent period following a sympathetic reflex discharge is most likely due to inhibition brought on by a particular subgroup within this afferent group.3. High threshold group III and group IV afferent fibres excited sympathetic neurones and elicited pressor responses.4. Sympathetic reflex arcs could be temporally facilitated during the somatic afferent induced inhibition, by group IV, and to a lesser extent by group III volleys.5. Section of the spinal cord shortened the time course and lessened the degree of group III inhibition suggesting that suprasegmental pathways are involved in the long-lasting depression following a reflex.6. Baroreceptor afferent stimulation inhibited group III- and group IV- evoked reflexes even with some temporal facilitation in the pathway.7. The special properties of the sympathetic reflex arcs and the relation of the results to other work on B.P. reflexes are discussed. It is suggested that the group III and IV muscle afferents have a chemoreceptor function and are responsible for mediation of the pressor reflex during muscle exercise.