Emerging Principles of Organization of the Midbrain Periaqueductal Gray Matter

Abstract

Throughout this book the term periaqueductal gray (PAG) has been used in preference to the term central gray, since PAG refers specifically to the portion of the ventricular gray matter which surrounds the midbrain aqueduct. Rostrally, the PAG is continuous with the periventricular gray matter surrounding the third ventricle in the hypothalamus and thalamus. Caudally, it is continuous with the periventricular gray matter which in the dorsal pons forms the ventral and ventrolateral border of the fourth ventricle. Although the oculomotor related group of nuclei (i.e., oculomotor and trochlear nuclei, the Edinger-Westphal nucleus, the nucleus of Darkschewitsch and the interstitial nucleus of Cajal) and the dorsal raphe nucleus, constitute the major part of the gray matter ventral to the midbrain aqueduct, they are usually considered, on functional and anatomical grounds, separable from the PAG. The midbrain tegmentum laterally adjacent to the PAG has also usually been considered to be a separate entity. However, as discussed by Holstege (this volume), the PAG and the laterally adjacent tegmentum together, likely form a common neuronal pool which is divided by the fiber stream formed by the tectobulbospinal fibers and the fibers of the mesencephalic trigeminal tract.