General Topography of the Hypothalamus

In general, the hypothalamus can be divided into three tiers of nuclei. Most medially, along the wall of the third ventricle, is the periventricular nucleus, shown here in green. Along the base of the periventricular nucleus is an expansion laterally along the edge of the median eminence, known as the arcuate or infundibular nucleus. The periventricular stratum contains many neurons that make releasing or release-inhibiting hormones (see Plate 5-6) and whose axons end on the capillary loops of the hypophysial portal vessels in the median eminence. Many axons from the brainstem run through the periventricular gray matter, in the posterior longitudinal fasciculus, and into the periventricular region of the hypothalamus.

The next tier of nuclei is sometimes called the medial tier. These nuclei are generally involved in intrinsic connections within the hypothalamus that allow integration of various functions. The most rostral of the medial nuclei is the medial preoptic region (orange), which sits along the wall of the third ventricle as it opens. Along the anterior wall of the third ventricle is the median preoptic nucleus (not shown here). These two cell groups are involved in integrating control of body temperature with fluid and electrolyte balance, wake-sleep cycles, and reproductive function.

The next most caudal region is called the anterior hypothalamic area (purple). At the base of the anterior hypothalamic area, just above the optic chiasm, is the suprachiasmatic nucleus (see Plate 5-5). These structures are involved in regulating circadian rhythms. The suprachiasmatic nucleus is the body’s main biologic clock, and it sets the timing of rhythms of sleep, feeding, body temperature, and reproduction. These functions are controlled by means of outputs to the portion of the anterior hypothalamic area between the suprachiasmatic nucleus and the paraventricular nucleus (blue), called the subparaventricular zone.

The supraoptic and paraventricular nuclei are also at this anterior level in the medial tier. Both nuclei contain large numbers of oxytocin and vasopressin neurons, whose axons travel through the pituitary stalk in the tuberohypophysial tract, to the posterior pituitary gland, where they release their hormones into the circulation. The paraventricular nucleus also contains neurons that make releasing hormones (especially corticotrophic-releasing hormone) and project to the median eminence. A third population of neurons in the paraventricular nucleus sends axons through the medial forebrain bundle in the lateral hypothalamus to the brainstem and spinal cord, to control both the sympathetic and parasympathetic nervous systems. Many of these neurons use either oxytocin or vasopressin as a central neurotransmitter in this autonomic pathway, but they are an entirely separate set of neurons from those that send axons to the posterior pituitary gland.

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