Avian somatosensory system: II. Ascending projections of the dorsal column and external cuneate nuclei in the pigeon

Abstract

The ascending projections of the dorsal column and external cuneate nuclei (DCN/CuE) in the pigeon were investigated in anterograde tracing experiments by using autoradiography or wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). The results show that the majority of ascending projections decussate via internal arcuate fibers to form a contralateral medial lemniscus which ascends in a ventral position. In the brainstem, terminal fields were observed in the ventral lamella of the inferior olive (OI), the parabrachial nuclei (PB) of the dorsolateral pons, the intercollicular nucleus (ICo) of the midbrain, and the nucleus pretectalis diffusus (PD). In the diencephalon there were terminal fields in the strata cellulare externum and internum (SCE and SCI) of the caudal hypothalamus; in the intercalated (ICT), ventrolateral (VLT), and reticular nuclei of the ventral thalamus; in the nuclei principalis precommissuralis (PPC), spiriform medialis (SpM), and dorsolateralis posterior, pars caudalis (cDLP) of the caudal thalamus; and in the nuclei dorsalis intermedius ventralis anterior (DIVA), dorsolateralis posterior, pars rostralis (rDLP), dorsolateralis anterior (DLA), and dorsolateralis anterior, pars medialis (DLM) of the rostrodorsal thalamus. The origins of these projections within the DCN/CuE complex were verified in retrograde tracing experiments with WGA-HRP and were found to be partly differentiable with respect to their targets. The projections to DIVA, rDLP, DLA, DLM, cDLP, and SpM arise from all rostrocaudal levels of the DCN/CuE complex; those to ICo arise from caudomedial nuclear regions, while those to the hypothalamus and ventral thalamus arise from rostrolateral nuclear regions. Projections to PB arise from lamina I neurons of the dorsal horn of upper cervical spinal cord segments and from CuE. No evidence was found of a projection to the cerebellum. The distribution of the cells of origin of the medial lemniscus (ML) within the DCN/CuE complex was found to be largely coextensive with the areas of termination of primary spinal (Wild: J. Comp. Neurol. 240:377-395, '85) and some trigeminal (Dubbledam and Karten: J. Comp. Neurol. 180:661-678, '78) afferents. Furthermore, the areas of termination of the ML within the rostrodorsal and caudal thalamus are also either coextensive or closely associated with nuclei which provide a somatosensory projection to separate regions of the telencephalon (Wild: Brain Res. 412:205-223, '87). There are thus clear similarities in the overall pattern of somatosensory projections in the pigeon and in many mammalian species.