Neuronal populations of the human periaqueductal gray, nucleus lateralis

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Abstract
The present communication is part of a study designed to explore the neuronal populations and the organization of the periaqueductal gray matter (PAG). The data presented here deals exclusively with the nucleus lateralis (PAGL) and is based upon Golgi studies of 14 human brainstems. In this investigation 400 cells were measured and classified. Axonal origin, axonal and dendritic ramifications, and spine population were studied. The data demonstrate that morphologically the PAGL is comprised of a heterogeneous population of small and medium sized neurons consisting of three major cell types: vertical cells, stellate cells, and horizontal cells. Observations of axonal pathways and dendritic ramifications describes an anatomical substrate for direct communication between the PAGL and the superior colliculus, which includes a system of axonal projections from the PAGL into the superior colliculus. The cells of origin of this pathway have been identified as being primarily vertical and stellate cells. It is anticipated that these data will provide the groundwork for further detailed study of PAGL neurons and for identifying specific neuronal populations with their functional roles.