Boundaries

The boundaries of the hypothalamus are as follows (see Figures 26.1 and 26.2):

Figure 26.1 Hypothalamic nuclei and hypophysis, viewed from the right side. DMN, dorsomedial nucleus; DN, dorsal nucleus; MB, mammillary body; PN, posterior nucleus; PVN, paraventricular nucleus; TN, tuberomammillary nucleus; VMN, ventromedial nucleus. The lateral hypothalamic nucleus is shown in pink.

Figure 26.2 Hypothalamic nuclei, and related neural pathways, in a coronal section. ARC, arcuate nucleus; DMN, dorsomedial nucleus; LN, lateral nucleus; MFB, medial forebrain bundle; PAR, paraventricular nucleus; PER, periventricular nucleus; VMN, ventromedial nucleus; ZI, zona incerta.

Subdivisions and nuclei

In the sagittal plane, it is customary to divide the hypothalamus into three regions: anterior (supraoptic), middle (tuberal), and posterior (mammillary). The descriptive use of ‘regions’ has been convenient for animal experiments involving placement of lesions. Named nuclei in the three regions are listed in Table 26.1.

Table 26.1 Hypothalamic nuclei

In the coronal plane, the hypothalamus can be divided into lateral, medial, and periventricular regions. The full length of the lateral region is occupied by the lateral hypothalamic nucleus. Merging with the lateral nucleus is the medial forebrain bundle, carrying aminergic fibers to the hypothalamus and to the cerebral cortex.

Hypothalamic control of the pituitary gland

The arterial supply of the pituitary gland comes from hypophyseal branches of the internal carotid artery (Figure 26.3). One set of branches supplies a capillary bed in the wall of the infundibulum. These capillaries drain into portal vessels which pass into the adenohypophysis (anterior lobe). There they break up to form a second capillary bed which bathes the endocrine cells and drains into the cavernous sinus.

Figure 26.3 Hypothalamic neuroendocrine cells. The blood supply to the hypophysis, including the endocrine cells of the adenohypophysis, is also shown (arrow indicates direction of blood flow in the portal system).

The neurohypophysis receives a direct supply from another set of hypophyseal arteries. The capillaries drain into the cavernous sinus, which delivers the secretions of the anterior and posterior lobes into the general circulation.

Secretions of the pituitary gland are controlled by two sets of neuroendocrine cells. Neuroendocrine cells are true neurons in having dendrites and axons and in conducting nerve impulses. They are also true endocrine cells because they liberate their secretions into capillary beds (Figure 26.4). With one exception (mentioned below), the secretions are peptides, synthesized in clumps of granular endoplasmic reticulum and packaged in Golgi complexes. The peptides are attached to long-chain polypeptides called neurophysins. The capillaries concerned are outside the blood–brain barrier, and are fenestrated.

Figure 26.4 Morphology of a peptide-secreting neuroendocrine cell.

The somas of the neuroendocrine cells occupy the hypophysiotropic area in the lower half of the preoptic and tuberal regions. Contributory nuclei are the preoptic, supraoptic, paraventricular, ventromedial, and arcuate (infundibular). Two classes of neurons can be identified: parvocellular (small) neurons reaching the median eminence, and magnocellular (large) neurons reaching the posterior lobe of the pituitary gland.