Intracellular staining of Renshaw cells and alpha motoneurons was used to determine the spatial distribution of recurrent inhibitory synapses on spinal motoneurons in the cat. In each experiment, a Renshaw cell and one or more possible target motoneurons were labeled with horseradish peroxidase after physiological identification. 2. Paris of labeled neurons were reconstructed and measured at the light microscopic level. As defined by light microscopy, presumed synaptic contacts between nine Renshaw cells and 10 postsynaptic motoneurons were observed. On average, each Renshaw cell made three synaptic contacts (range 1-9) on each motoneuron. 3. Electron microscopic confirmation of several presumed contacts provided evidence that the appositions identified by light microscopic criteria are genuine contacts between Renshaw cell boutons and the labeled motoneuron. 4. All of the identified synapses observed in these experiments were located on motoneuron dendrites, between 65 and 706 microns from the soma. Use of a simplified cable model indicated that the synapses are electrotonically close to the soma, the average location being approximately 0.25 length constants from the soma (range 0.04-0.82 lambda). 5. These observations provide direct evidence to support the hypothesis that Renshaw cell synapses on motoneurons are located on the dendrites and not on the cell body (whereas reciprocal inhibitory synapses, from Ia inhibitory interneurons, are predominantly located on the soma). The functional significance of the observed distribution of Renshaw inhibitory synapses is discussed. One possibility is that the recurrent inhibitory pathway selectively inhibits particular dendritic inputs.
