Structure of the Hypothalamus

The hypothalamus in the mammalian brain encompasses the most ventral part of the diencephalon, where it forms the floor and, in parts, the walls of the third ventricle. Its upper boundary is marked by a sulcus in the ventricular wall, the ventral diencephalic or hypothalamic sulcus, which separates the hypotha lamus from the dorsally located thalamus (Fig. 2). Caudally, the hypothalamus merges without any clear limits with the periventricular gray and the tegmentum of the mesencephalon. However, it is customary to define the caudal boundary of the hypothalamus as represented by a plane extending from the caudal limit of the mammilary nuclei ventrally and from the posterior commissure dorsally. Rostrally, the hypothalamus is continuous with the preoptic area, which lies partly forward to and above the optic chiasm.

By means of the previously mentioned external landmarks at the ventral surface of the brain, the hypothalamus can be subdivided in the anterior-posterior direction into an anterior part that includes the preoptic area, a middle part, and a posterior part. Another subdivision in the lateral-medial direction consists of three longitudinal zones recognized as the periventricular, the medial, and the lateral zones. The

Figure 2 The location of the main hypothalamic nuclei shown in a medial view. The hypothalamus contains a large number of neuronal circuits that regulate vital functions, such as body temperature, heart rate, blood pressure, blood osmolarity, water and food intake, emotional behavior, and reproduction.

Figure 2 The location of the main hypothalamic nuclei shown in a medial view. The hypothalamus contains a large number of neuronal circuits that regulate vital functions, such as body temperature, heart rate, blood pressure, blood osmolarity, water and food intake, emotional behavior, and reproduction.

Figure 3 The posterior lobe of the pituitary gland. In the posterior lobe, axons from hypothalamic cell groups called supraoptic and paraventricular nucleus release vasopressin and oxytocin into the systemic circulation (inferior hypophyseal artery). OC, optic chiasm.

periventricular zone consists mostly of small cells that, in general, are oriented along fibers parallel with the wall of the third ventricle. The medial zone is cell rich, containing most of the well-delineated nuclei of the hypothalamus that include the preoptic and supra-chiasmatic nuclei in the anterior region; the dorsome-dial, ventromedial, and paraventricular nuclei in the middle region; and the posterior nucleus and mam-millary bodies in the posterior region (Fig. 2). The lateral zone contains only a small number of cells interposed between the longitudinal fiber system of the medial forebrain bundle. This region possesses long fibers that project to the spinal cord and cortex as well as extensive short-fiber, multisynaptic ascending and descending pathways. The basal portion of the medial region and the periventricular region contain many of the small hypothalamic neurons that secrete the substances that control the release of anterior pituitary hormones. Most fiber systems of the hypothalamus are bidirectional. Projections to and from areas caudal to the hypothalamus are carried in the medial forebrain bundle, the mammillo-tegmental tract, and the dorsal longitudinal fasciculus. Rostral structures are interconnected with the hypothalamus by means of the mammillo-thalamic tracts, fornix, and stria terminalis. However, there are two important exceptions to the rule that fibers are bidirectional in the hypothalamus. First, the hypothalamo-hypophyseal tract contains only descending axons of paraventricular and su-praoptic neurons, which terminate primarily in the posterior pituitary (Fig. 3). Second, the hypothalamus receives one-way afferent connections directly from the retina. These fibers terminate in the suprachias-matic nucleus, which is involved in generating light-dark cycles. The role of these rhythms in the control of motivated behaviors is discussed later.

The following section describes the interrelated functions of the hypothalamus and the pituitary gland as well as some of the major functions of the limbic system.

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