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 abovementioned
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.