Organization of brain stem afferents to the ventral lateral geniculate nucleus of rats.

We have examined the patterns of projections from different nuclei of the brain stem to the ventral lateral geniculate nucleus (vLGN) of the thalamus. Injections of biotinylated dextran were made into different nuclei of the brainstem (i.e., midbrain reticular nucleus, pontine reticular nucleus, deep layers of superior colliculus, periaqueductal grey matter [ventrolateral, dorsolateral, and lateral columns], pedunculopontine tegmental nucleus, parabrachial nucleus, lateral dorsal tegmental nucleus, substantia nigra [pars reticulata], locus coeruleus, and dorsal raphe) of Sprague-Dawley rats using stereotaxic coordinates. Our results show that all of the above mentioned brain-stem nuclei have overlapping projections to the medial regions of the vLGN, within the parvocellular lamina of the nucleus. This if the first instance of the parvocellular lamina being shown to receive a major set of projections. Very few labelled terminals from the brain stem were ever seen within the larger more lateral magnocellular lamina, which has been shown by previous studies to receive heavy inputs from visually associated structures, such as the retina and occipital cortex. Since many of the brain-stem nuclei injected in this study have little to do with visual processing, our results suggest that one can perhaps package the vLGN into distinct visual (magnocellular) and nonvisual (parvocellular) components.