An electrophysiological demonstration of the axonal projections of single spinal interneurones in the cat.

1. Single interneurones excited from group Ia afferents and located in the ventral horn of the spinal cord in the cat were activated antidromically by stimulation of their axons with one micro-electrode while recording extracellularly close to their somas with a second micro-electrode. The interneurones studied were those which, according to previous indirect evidence, should mediate the reciprocal Ia inhibition of motoneurones.2. Two subgroups of these interneurones were studied: those excited from group Ia afferents in the quadriceps (Q) and posterior bicepssemitendinosus (PBSt) nerves. Most of them could be activated from a number of separate loci in the PBSt and Q motor nuclei respectively and from the ventral or lateral funiculi.3. The location of the axonal branches and the extent of their branching in the motor nuclei were reconstructed by comparing the latencies of the responses and the thresholds (0.1-5 muA) for antidromic activation of single interneurones from different electrode positions in a number of tracks, having previously established the relation between the threshold and the distance from the stimulated fibres. For the main branches in the white matter the conduction velocity was found to be about 70 m/sec.4. The axonal projections of the investigated interneurones were found to be fully consistent with the hypothesis that they mediate reciprocal inhibition of motoneurones.