ABSTRACT
The suprachiasmatic nuclei (SCN) have been subjected to rigorous morphological analysis in a variety of species. Characterization of the phenotypic diversity of SCN neurons has dramatically advanced our understanding of the functional organization of these nuclei. Certainly, the best characterized projection to the SCN, both from an anatomical and functional standpoint, arises from the retina. Demonstration of the retinohypothalamic projection in 1972 ultimately led to identification of the SCN as the circadian pacemaker, and it is now known that the projection is essential for entrainment of the circadian activity of SCN neurons. Although the aforementioned afferents constitute the best characterized projections to the SCN, a number of other regions of the neuraxis project upon this nucleus. A number of hypothalamic projections have been identified, including the preoptic, arcuate, ventromedial, and dorsomedial nuclei; the bed nucleus of the stria terminalis; lateral hypothalamic area; and caudal hypothalamus.
