Projections of the nuclei of the periaqueductal gray matter in the cat

Abstract To obtain more evidence in support of the theory of tri‐nuclear division of the periaqueductal gray matter (PAG) in the cat, the projection systems of each of the nuclei were investigated. Direct current lesions were stereotaxically placed in the three nuclei of the PAG and the resulting fibers of degeneration were stained by reduced silver impregnation techniques. Degenerating axons from the lesion in the nucleus medialis were traced ventrally to the tegmentum in a radiating pattern and rostrally to the fields of Forel and the ventral tegmental area. Descending fibers were traced to their termination in the dorsal tegemental nucleus. Lesions in the nucleus dorsalis resulted in no radiating or descending fibers of degeneration, but a bundle of degenerating fibers running within the PAG could be followed to the level of the mesencephalic‐diencephalic junction. Here the axons terminated in the pretectal area and the lateral habenular nucleus. The pattern of degeneration was completely ipsilateral for the two nuclear lesions described above and predominately ipsilateral for the third nucleus, the lateralis. From it, all the ascending fibers were ipsilateral, but the heavy radiating fibers to the tectum and tegmentum did extend somewhat to the contralateral side. The large bundle of axons of degeneration ran rostrally within the PAG to diencephalic levels and divided into numerous fasciculi and terminated in the periventricular gray, the posterior hypothalamus and numerous thalamic nuclei. The evidence that each nucleus projects differently supports the theory of tri‐nuclear division of the PAG.