The projection of individual axons from the parabrachial region of the brain stem to the dorsal lateral geniculate nucleus in the cat.

In mammals, the retinogeniculocortical pathway is the primary afferent route to visual cortex. The flow of information along this pathway can be modulated at the thalamic level (i.e., at the lateral geniculate nucleus) as a function of arousal, attention, and phenomena such as eye movements. Physiological studies indicate that an important source of this state-dependent influence on geniculate neuronal responsiveness is the parabrachial region of the brain stem. We used the anterograde tracer Phaseolus vulgaris leucoagglutinin to study the anatomical connections between the parabrachial region and the lateral geniculate nucleus. Labeled parabrachial axons are found throughout the thalamus, including all laminae of the lateral geniculate nucleus, the lateral posterior-pulvinar complex, the ventral lateral geniculate nucleus, the perigeniculate nucleus, and the reticular nucleus of the thalamus. Within these nuclei, the axons exhibit sporadically branched terminal arbors with boutons mostly en passant. Serial reconstructions indicate that individual parabrachial axons that innervate the lateral geniculate nucleus may terminate in other visual thalamic nuclei as well, but not in thalamic nuclei that subserve other modalities. Finally, the labeled parabrachial axons are morphologically heterogeneous; they differ in their innervation targets, terminal arbor shape, and the size spectrum of their boutons. These data suggest that this portion of the ascending parabrachial pathway, which may be functionally diverse, coordinates the responsiveness of the varied thalamic nuclei involved with vision.