Ascending connections of a thalamic auditory area in a crocodile, Caiman crocodilus

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Abstract
Nucleus reuniens pars centralis is the major recipient of the ascending auditory lemniscus from the midbrain in the crocodile, Caiman crocodilus. Its ascending connections were studied by means of the Fink‐Heimer procedure with the light microscope after anodal, stereotaxic lesions. Following unilateral destruction of nucleus reuniens pars centralis, degenerating axons assemble on the lateral aspect of the pars centralis, pass through the pars diffusa, and enter the dorsal peduncle of the lateral forebrain bundle. These fascicles travel rostrally in the lateral forebrain bundle where they remain restricted to the medial aspect of this tract. At more rostral levels, these degenerating axons turn dorsad, pass through and perhaps synapse on interposed neurons of the medial portion of the ventrolateral area, and end on cells located in the overlying dorsolateral area. This terminal field is composed of small, round, tightly packed neurons located in a medial caudal part of the dorsolateral area.This experiment together with a previous report demonstrates the existence of a specific and sharply localized auditory pathway leading from the central nucleus of the torus semicircularis by way of nucleus reuniens pars centralis to the telencephalon. The findings of this study are compared with similar research on thalamotelencephalic projections of diencephalic auditory areas in other amniotes.