circumventricular organs Areas of the brain that lack a blood-brain barrier and are found on the midline surrounding the third and fourth ventricles. These regions are essential "windows" through which the brain exerts humoral control of peripheral systems and are also responsive to feedback humoral influences from the systems that they modulate.

fornix A well-defined myelinated fiber bundle that serves as an important histological landmark in the hypothalamus and is also the principle conduit through which neurons in the subiculum project to the mammillary bodies.

infundibular stalk The ventral evagination of the floor of the third ventricle that connects the hypothalamus to the pituitary gland. It contains the long portal vessels that transport release and inhibiting factors from hypothalamus to the anterior lobe of the pituitary gland and is also the conduit for axons of magnocellular neurons that terminate in the posterior (neural) lobe of the pituitary.

median eminence The highly vascularized portion of the tuber cinereum that is essential for hypothalamic regulation of the anterior lobe of the pituitary. The blood vessels in this region lack a blood-brain barrier and are therefore capable of transporting peptides and neurotransmitters from the hypothalamus to the anterior lobe.

stria terminalis A major fiber pathway through which the amygdala communicates with the hypothalamus.

tuber cinereum The protuberance on the ventral surface of the diencephalon that contains the median eminence and gives rise to the infundibular stalk.

The hypothalamus is a remarkable region of the central nervous system. This small subdivision of the ventral diencephalon communicates extensively with other regions of the neuraxis via classical synaptic interactions and also has profound influences on the hormonal regulation of peripheral organ systems. In essence, it is directly responsible for the regulatory control of homeostatic systems essential for survival of the parent organism. The ability of this region to exert such profound influence over behavioral state and physiology is reflected in both the properties of hypothalamic neurons and the mechanisms through which they communicate. Thus, the dynamic regulatory capabilities of the hypothalamus are defined by the unique properties (e.g., timekeeping capabilities) of its constituent neurons, the ability to influence peripheral systems by virtue of hormonal or ''humoral'' communication, and the responsiveness of hypothalamic neurons to feedback control by the peripheral systems that they regulate. This article reviews the basic organizational principles fundamental to hypothala-mic function, focusing on well-studied hypothalamic systems that illustrate the functional parcellation of this small but influential region of the brain.

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