The Hypothalamus Can Be Subdivided in Both Longitudinal and Medial-Lateral Directions

Parts of the hypothalamus are exposed at the base of the brain, surrounded by the circle of Willis. The mammillary bodies form the most posterior part of the hypothalamus and lie adjacent to the cerebral peduncles. Between the mammillary bodies and the optic chiasm and tract is a small swelling called the tuber cinereum. The median eminence arises from the tuber cinereum and narrows into the infundibulum, to which the pituitary gland is attached. These landmarks on the base of the brain are used to divide the hypothalamus longitudinally (Fig. 23-2) into an anterior region (above the optic chiasm, extending anteriorly to the lamina terminalis), a tuberal region (above and including the tuber cinereum), and a posterior region (above and including the mammillary bodies).

Figure 23-2 Longitudinal subdivisions of the hypothalamus, and the two lobes of the pituitary gland. III, Oculomotor nerve; LT, lamina terminalis (the membrane at the anterior end of the third ventricle); OC, optic chiasm.

The hypothalamus also gets divided up in a medial to lateral direction. The periaqueductal gray of the midbrain continues into the thin periventricular zone in the wall of the third ventricle. The fornix runs right through the longitudinal zones on its way to the mammillary body and is used as a landmark to divide the rest of the hypothalamus on each side into medial and lateral zones.

Hypothalamic Inputs Arise in Widespread Neural Sites

The hypothalamus receives lots of inputs (Fig. 23-3), but most of them are from two general categories: those from nuclei in the brainstem and spinal cord conveying information about the state of your body, and those from limbic structures like the amygdala, hippocampus, and septal nuclei. Inputs about the state of the body (“It’s getting warm in here,” or “Blood glucose is getting low”) arrive from places like the nucleus of the solitary tract by way of the dorsal longitudinal fasciculus, which travels through the periaqueductal gray into the periventricular zone; through the medial forebrain bundle, which travels through the reticular formation into the lateral hypothalamus; and as branches from tracts like the spinothalamic tract. Limbic inputs arrive from the amygdala, from the hippocampus (through the fornix), and from the septal nuclei and other sites (through the medial forebrain bundle); collectively they keep the hypothalamus updated on other aspects of the environment (“Not a good place to take off my shirt”).

Figure 23-3 Inputs to the hypothalamus.

Inputs also reach the hypothalamus from the retina and in the form of direct physical stimuli. Axons of some retinal ganglion cells terminate in the small suprachiasmatic nucleus on each side of the anterior hypothalamus. The suprachiasmatic nucleus is the “master clock” for most circadian rhythms, and information from the retina helps get these rhythms synchronized with the 24-hour day (THB6 Figure 23-5, p. 585). Finally, some hypothalamic neurons are sensory receptors themselves, directly responsive to temperature, blood osmolality, or the concentration of some chemicals in blood passing through the hypothalamus.

Hypothalamic Outputs Largely Reciprocate Inputs

Hypothalamic connections with visceral nuclei and limbic structures are largely reciprocal (Fig. 23-4). Projections through the dorsal longitudinal fasciculus and the medial forebrain bundle reach sites like the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, and the intermediolateral cell column