Periaqueductal neurons associated to the posterior commissure: a morphological study in the hedgehog, rat and cat.

The existence of a hitherto undescribed PAG neuronal subdivision and its close relationship with the posterior commissure were studied with silver reduced stains and the electron microscope in the hedgehog, rat and cat. The PAG neurons are arranged in discrete groups composed of 2 up to 6 perikaria surrounded by a fiber and glial neuropil. The dendrites of these neurons established synaptic contacts with the ventral commissural fibers and in addition were innervated by PAG intrinsic axons. On the other hand, discrete non-myelinated bundles which apparently arise from the PAG enter through the commissural fibers and makes synaptic contacts with dendritic profiles situated in the intracommissural neuropil zones. These morphological results were discussed in view of a change in the theories that consider the PAG as an non-specific functional group of neurons.