Excitatory responses of the dorsal root discharge to stimulation of cutaneous and muscle afferents in the cat

Abstract

Excitatory responses of the dorsal root discharge (DRD) consisting in transient increase in its frequency have been studied in non-anaesthetized low spinal cats. They were evoked by stimulation of cutaneous nerves (superficial peroneal and posterior tibial) and muscle nerves (gastrocnemius-soleus and posterior biceps-semitendinosus), recorded from the central cut ends of single fibres of L7 dorsal roots. The excitatory responses were elicited by single volleys in low threshold cutaneous and group II muscle afferents. Group III afferents of gastrocnemius-soleus nerve were also effective. Increase in strength of stimulation of posterior biceps-semitendinosus nerve from 4T to 40T which activated group III muscle afferents significantly decreased the excitatory effects of the DRD. Incidence of excitatory responses of the DRD to volleys both in low and high threshold cutaneous afferents, was 100%. Frequency of occurrence of excitatory effects to volleys in group II muscle afferents ranged from 23% to 43% (for responses to posterior biceps-semitendinosus and gastrocnemius-soleus volleys, respectively). It was increased to 47% and 72% when excitatory effects were elicited by group III afferent volleys. These findings indicate that only some types of afferent fibres evoke excitatory responses of the DRD. Variable incidence of the excitatory responses to stimulation of cutaneous and muscle afferents suggests important difference in effectiveness of connections between both types of fibres and interneurones generating the DRD.