The tuber cinereum refers to the inferior surface of the hypothalamus, which bulges downward from the floor of the third ventricle and gives rise to the infundibulum. Within the median eminence, projections from the infundibulum are intimately related to vessels of the portal system that bathe the anterior lobe of the pituitary gland. This rich network of capillaries derives from the superior hypophyseal artery, which derives from the internal carotid artery (Fig. 26-1). The releasing hormones of the hypothalamus are thus delivered directly to their target cells in the anterior pituitary. In this way the portal system represents the final output for converging pathways from the brain to the pituitary axis.
The infundibulum also serves another function; it contains fibers that extend into the neurohypophysis and contain vasopressin and oxytocin. The main blood supply to the posterior pituitary is from the inferior hypophyseal artery, which is a branch of the cavernous part of the internal carotid artery.
The abundant blood supply of the hypothalamus (from several feeding arteries) is of importance to neurosurgeons who attempt to treat aneurysms that derive from adjacent vessels. Many small radicles that arise from the carotid arteries, posterior and anterior communicating arteries, and the proximal portions of the anterior and posterior cerebral arteries together form a network of such redundancy that infarction of the hypothalamus is very infrequent. The venous drainage from the portal system is to the petrosal sinuses, where hormone levels can be directly sampled.
Readers requiring a more extensive source of information on anatomic and other aspects of the hypothalamus are directed to the comprehensive material by Swaab in the two volumes of the Handbook of Clinical Neurology devoted to this subject, and to the monograph by Martin and Reichlin.
The Hypothalamic Releasing Hormones
The regulatory system of hypothalamic-releasing hormones is complex. The releasing factors have overlapping functions, and the hypothalamic nuclei act on many parts of the brain in addition to the pituitary. Conversely, many parts of the brain influence the hypothalamic-pituitary axis through the actions of neurotransmitters and modulators (including catecholamines, acetylcholine, serotonin, and dopamine). There is feedback control between every part of the hypothalamus and the endocrine structures on which it acts. The factors that influence hypothalamic neurons have been reviewed in detail by Reichlin. In addition to the sections below, some of these relationships will also emerge in later chapters, particularly as they relate to behavioral and psychiatric disorders.
Of particular significance is the role of the hypothalamus in the integration of the endocrine and autonomic nervous systems at both the peripheral and central levels. The best-known example of this interaction is in the adrenal medulla, as indicated in Chap. 25. Similarly, the juxtaglomerular apparatus of the kidney and the islets of Langerhans of the pancreas function as neuroendocrine transducers, in that they convert a neural stimulus (i.e., adrenergic input) to an endocrine signal (i.e., renin from the kidney and glucagon and insulin from the islet cells).