Origin and terminal distribution of the trigeminal projections to the inferior and superior colliculi in the lesser hedgehog tenrec

Abstract

The trigemino-tectal projections were investigated with anterograde and retrograde tracing techniques in the Madagascan lesser hedgehog tenrec, Echinops telfairi. There were prominent contralateral projections to the inferior colliculus (CoI) and the superior colliculus (CoS), each showing its own characteristic pattern of terminations. While the projections to the CoI were confined consistently to a circumscribed region in its ventrolateral, external portion, the projections from particularly the rostral trigeminal subdivision to the CoS were distributed inhomogenously across almost the entire rostro-caudal and mediolateral extents. Comparing these data with the spino-tectal projections published previously, it demonstrates that the somatotopic organization of ascending tectal afferents is more distinct in the CoI than in the CoS. There were roughly twice as many trigeminal neurones projecting to the CoS than to the CoI. This difference might be due to the fact that the cells projecting to CoS were distributed extensively across the trigeminal nuclear complex (peak densities in the principal and interpolar subdivisions), while the neurones projecting to the CoI were largely confined to the interpolar and caudal trigeminal subdivisions. The latter cells were located adjacent to the spinal trigeminal tract; the neurones projecting to the CoS occupied preferentially the ventral trigeminal regions at rostral levels, while from the interpolar subdivision caudalward the labelled cells shifted dorsolaterally. In comparison to other mammals the trigeminal projection to the tenrec's CoI is unique. There is evidence for such a projection in other species too, but it is poorly documented, presumably due to technical reasons.