Organization of projection-specific interneurons in the spinal cord of the red-eared turtle

Using differential retrograde axonal tracing, we identified motoneurons (MNs) and projection-specific interneuron (IN) classes in lumbar segment D9 of the adult red-eared turtle spinal cord. We characterized the distribution of these neurons in the transverse plane, and estimated their numbers and proportions. Different labeling paradigms allowed us to distinguish ipsilateral INs (IINs) from commissural INs (CINs), and to identify IINs and CINs with either ascending (a) axons, descending (d) axons, or axons that bifurcate to both ascend and descend (ad). Local interneurons with axons shorter than 1 segment in length were not studied. We show that most retrogradely labeled INs are located dorsal to the MNs, in the ventral horn, the intermediate zone and the dorsal horn. IINs predominate in the dorsal horn. CINs are located on average more medially than the IINs in the ventral horn and intermediate zone. Within the IIN and CIN populations, aINs and dINs overlap extensively. The adIINs and adCINs make up only a small fraction of the total number of INs and are scattered throughout much of the respective IIN and CIN domains. The proportions of IINs and CINs are about equal, as are the proportions of aIINs versus dIINs, of aCINs versus dCINs, and of adIINs versus adCINs. The findings are compared to the organization of lumbar spinal INs in other vertebrate species.