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The action of volleys in contralateral primary afferents on transmission in the Ia inhibitory pathways to motoneurones was investigated with intracellular recording from motoneurones. Ia IPSPs in flexor as well as most extensor motoneurones were regularly facilitated by volleys in contralateral high threshold muscle, cutaneous and joint afferents in spinal cats under chloralose anaesthesia. In decerebrate cats with a low pontine lesion transmission in Ia inhibitory pathways was not facilitated but rather depressed by volleys in these afferents. The recurrent effects from motor axon collaterals were investigated on inhibitory transmission from different contralateral afferents to motoneurones. Previous investigations have shown that the interneurones mediating the reciprocal Ia inhibition receive recurrent inhibition via motor axon collaterals and Renshaw cells. Now a strong positive correlation was revealed between recurrent depression of IPSPs evoked from different contralateral afferents and facilitation of Ia IPSPs by the same afferent volleys. These results suggest that the recurrent depression of IPSPs from different contralateral primary afferents depends on their excitatory convergence onto the Ia inhibitory interneurones, which then partly mediate the IPSP evoked in the motoneurone from these afferents.

A method for the injection of horseradish peroxidase (HRP) into the spinal cord (SC) is described, based on the principle of fluid expansion upon application of heat. HRP solution in the tip of a glass micropipette (tip diameter 10-15 microm) was extruded when the sealed stem of the pipette, containing distilled water, was warmed. The pipette was calibrated (temperature/volume relation) before it was inserted into the SC and the injection of HRP was made according to the calibration curve. Using the pipette as an electrode it was possible to locate its tip with reference to recorded field potentials. After the injection, the volume of HRP solution actually injected was determined.