The Human Hypothalamus
1C: LEVEL 1 CELLS: At this level, a coronal section through the anteriormost part of the hypothalamus cuts through the anterior commissure and the optic chiasm. The hypothalamus lies between these major fiber bundles. On either side of the hypothalamus, in the basal telencephalon, can be seen the darkly staining cells of the basal nucleus of Meynert, a group of cholinergic cells whose axons distribute widely throughout the cerebral cortex, and whose degeneration is a hallmark of Alzheimer's disease. Within the hypothalamus, the section passes through the anterior edge of the suprachiasmatic nucleus, which contains one or more biological clocks. The clock timing is set by light-induced activity in retino-hypothalamic fibers entering the nucleus from the adjacent optic chiasm. Also visible (barely) are the interstitial nuclei of the anterior hypothalamus, minimally discernible clumps of cells in the anterior hypothalamic area. These cell groups are sexually dimorphic, certain groups being larger and more darkly staining in males than in females. This is a male brain. This hypothalamic region corresponds to the preoptic regions of other mammals, elements of which are known to be active in the regulation of gonadal hormones and reproductive behavior.