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A morphoquantitative study was carried out to clarify the cytoarchitecture of the human nuclear complex of the posterior commissure (NPC), a collection of cell groups which is thought to be involved in the pupillary light reflex and eye movements. On the basis of the topographical features, 5 different parts (rostral, principal, magnocellular, subcommissural and infracommissural) were identified in NPC. In these 5 parts, the neuronal population consisted of small and medium-sized neurons with a structural pattern similar to that of the central reticular formation. The only exception was the magnocellular part in which the neurons, mainly medium-sized and large, did not show the reticular-like features. In the Golgi material, two types of NPC neurons were identified: multipolar and fusiform cells. In the multipolar cells, 2-3 dendrites emerged from the soma and gave off a wide dendritic arborization. Many of the dendrites and axons of the multipolar cells were seen spreading outside NPC. The fusiform cells had 2 dendrites emerging from the opposite poles of their elongated perikaryon. Dendrites and axons of the fusiform cells always lay inside NPC. These observations suggest that the multipolar cells are mainly projective neurons and that the fusiform cells are neurons which mainly perform a local integrative function. Since the present study concludes our investigation of the human accessory oculomotor nuclei, a comparison of their structure was carried out. All the nuclei showed a common reticular-like structure, although each nucleus had its own structural particularities. These results are in agreement with and give an anatomical support to the functional findings.

We studied the periaqueductal gray matter (PAG) in cresyl-violet-stained serial sections from 5 cats applying quantitative determinations. No significant variations were observed in the cytological aspects in the various sites examined (lateral, ventral and dorsal regions, and external and internal portions). The neuronal density was constant in the different regions, but showed a gradual and significant increase in the most external regions of the PAG. Our findings do not, therefore, confirm the existence in the PAG of subnuclei with a specific cytoarchitecture; this does not, however, rule out the possibility that there are specific regions for connections, histochemical properties or functions.