Somatovisceral Projections from Spinal Cord and Dorsal Column Nuclei to the Thalamus in Hedgehog Tenrecs

Abstract

In order first to overcome the difficulties in understanding the increasing amount of information available regarding the mammalian somatosensory thalamus, and then to correlate the findings among different species and integrate them into a general concept of thalamic organization, the present study investigated the spinothalamic and medial lemniscal projections in Madagascan hedgehog tenrecs (Echinops telfairi and Setifer setosus). Tracer substances were injected into the dorsal column nuclei and into spinal segments at various levels; additional injections were made into the inferior colliculus.

The ascending somesthetic projections were to predominantly contralateral posterolateral target areas, and were almost mirror-like on both sides to intralaminar and medial thalamic nuclei. The densest and most extensive projections, originating mainly from the high cervical spinal cord and the dorsal column nuclei, reached the posterolateral thalamus caudal to the lateral geniculate nucleus. This region was difficult to subdivide cytoarchitecturally; nevertheless, on the basis of its labeling pattern, several subdivisions could be described and preliminarly named. Some of them compared tentatively with the internal portion of the medial geniculate nucleus (GM) and the ventral posterior nuclear complex (VPC) in more differentiated mammals. The most prominent subdivision, however, located subjacent to the lateral surface of the brainstem, was shown to receive additional fibers from the inferior colliculus. This region might be considered a further subdivision of GM, VPC, a perigeniculate area, and/or a region of its own not comparable at present, with thalamic regions in other mammals. On the other hand, it may also be a remnant of the hypothetical, diffuse mutlimodal region from which GM and VPC have possibly evolved.

Projections to the rostral intralaminar nuclei were shown to arise from the high cervical spinal cord, whereas dorsal column nuclear projections were traced to the zona incerta, the ventral portion of the lateral geniculate nucleus, and (unusual in comparison to other mammals) to the reuniens complex. There was no evidence for a spinal projection to the parafascicular complex; somesthetic information may, however, reach the lateral and the medial subparafascicular nuclei indirectly via the inferior colliculus.

Key Words:

• spinothalamic projections
• medial lemniscus
• medial geniculate nucleus
• posterior thalamus
• ventral posterior complex
• medial/intralaminar nuclei
• audiosomatic convergence
• insectivore