Brain stem neurons projecting to neocortex: a HRP study in the cat.

The cortical projections of the brain stem were investigated in detail in the cat by means of the horseradish peroxidase (HRP) retrograde axonal transport. Most of the cells providing ascending fibers to the neocortex were located in the pons (locus coerulus and related structures, central gray substance, dorsal tegmental nucleus, raphe nuclei, reticular nuclei); labeled neurons were also identified in the mesencephalon, mainly in the periaqueductal gray and in the nucleus linearis rostralis. These projections, and particularly the pontine fibers, were diffusely distributed throughout the cerebral cortex.