Synaptic effects from chemically activated fine muscle afferents upon alpha-motoneurones in decerebrate and spinal cats.

In spinal and decerebrate cats fine muscle afferents (group III and IV) were selectively activated by intra-arterial injection of bradykinin and KCl into the gastrocnemius-soleus muscle. By this method the synaptic responses induced in lumbar alpha-motoneurones by fine muscle afferents could be examined without interference of effects from large afferents. alpha-Motoneurones receiving EPSPs evoked by electrical stimulation of cutaneous and high threshold muscle afferents (mainly flexor motoneurones) responded to the activation of fine muscle afferents with a depolarization of their membrane and an increase in synaptic noise, while motoneurones in which IPSPs were evoked by electrical stimulation of cutaneous and high threshold muscle afferents (mainly extensor motoneurones), responded with hyperpolarization of their membrane. Cells with mixed excitatory-inhibitory electrically induced response characteristic responded with an increase in synaptic noise without substantial change in the level of their membrane potential to chemical stimulation of fine muscle afferents. The results indicate that one function of group III and IV muscle afferents is to participate in the complex reflex control performed by the flexor reflex system.