Aims: 

We intended to label and trace the axons of projection neurons in the lateral part of lamina I and in the lateral spinal nucleus (LSN).

Methods: 

For patch-clamp recordings, we injected intracellular biocytin into lamina I and into LSN neurons of young adult rats. The recently described IR-LED illumination technique applied for the visual identification of neurons allowed us to use intact spinal cord preparations of the lumbar enlargement, preserving complete axonal trees within the specimen. Biocytin labelled neurons were detected with the avidin-biotin horseradish-peroxidase method and reconstructed in 3D by using Neurolucida.

Results: 

Although projection neurons in the lateral part of lamina I and in the LSN show a remarkable morphological variability, on the basis of their axon-trajectories, the recovered neurons could be grouped into four distinct categories. The majority of labeled neurons gave rise to a single projection axon that originated from the cell body or one of the primary dendrites, crossed the midline in the anterior commissure and ascended in the contralateral anterolateral tract (ALT). The main axon of a subset of these neurons gave rise to two long axon collaterals one of which ascended and the other descended in the ipsilateral dorsolateral funiculus. A second set of neurons sent their main axon to the contralateral ALT through the posterior commissure. The third set of neurons provided ipsilateral ascending projection. The main axon ran in the posterior commissure and after bypassing the central canal returned to the ipsilateral ALT through the anterior commissure, and gave short collaterals to the deeper laminae on both sides of the spinal cord.

Conclusion: 

Axon trajectories of projection neurons in the lateral part of lamina I and in the LSN share common morphological features. Axon collaterals of these projection neurons may form propriospinal synaptic contacts contributing to the processing of nociceptive information.