Hans J . ten Donkelaar and Akira Hori The Hypothalamus and Hypothalamohypophysial Systems

The rather small hypothalamus contains a large number of more or less well-defined cell groups that are of utmost importance for preserving the individual and the species. The hypothalamus is involved in a wide variety of functions in the brain and is characterized by numerous connections with practically every major part of the central nervous system (CNS), including the cerebral cortex, the hippocampus, the amygdala, the thalamus, the cerebellum, the brain stem and the spinal cord. Alterations in hypothalamic nuclei are found in various endocrine diseases such as diabetes insipidus (DI), Wolfram and Prader–Willi syndromes, and in various neurodegenerative diseases such as Alzheimer, Parkinson and Huntington diseases. In two volumes of the Handbook of Clinical Neurology, Dick Swaab described almost everything so far known about the hypothalamus and its role in health and disease (Swaab 2003, 2004). This chapter merely represents a brief summary of these volumes. Through its intimate neuronal and vascular relationships with the pituitary gland, the hypothalamus controls the release of the pituitary hormones, thereby bringing the entire endocrine system under the control of the CNS. In 1940, Ernst and Berta Scharrer presented their findings on neurosecretory neurons in the hypothalamus that secrete hormones directly into the blood stream (Scharrer and Scharrer 1940). In 1949, Wolfgang Bargmann presented the first evidence for a magnocellular secretory system, composed of supraoptic and paraventricular neurons, giving rise to axons that innervate the posterior lobe of the pituitary via the tuberohypophysial tract (Bargmann 1949). All other hypothalamic control of pituitary function is achieved through neurohumoral mechanisms via the portal plexus in the external zone of the median eminence. Neurosecretory neurons throughout the hypothalamus, more in particular the arcuate nucleus, project to the median eminence. This parvocellular secretory system controls the anterior pituitary. Hans J. ten Donkelaar and Akira Hori The Hypothalamus and Hypothalamohypophysial Systems 13