The Hypothalamus

The lowest level, the basement of the diencephalon, is called the hypothalamus – literally “subspace,” i.e., the basement. As we all know, a lot of things accumulate in a basement over time. Similarly, the hypothalamus contains a whole smorgasbord of structures and functions. Put simply, the anterior and middle hypothalamus control vegetative functions and serve to preserve the species and the individual. The posterior hypothalamus is part of the limbic system.

Towards the top, at the vertex, a shallow groove in the wall of the third ventricle (sulcus hypothalamicus) marks the boundary between the hypothalamus and the dorsal thalamus. Below this groove, in the walls of the diencephalon, lie the hypothalamic core areas. The hypothalamus is divided into anterior, middle, and posterior parts: The anterior part lies between the optic chiasm (chiasma opticum) and the anterior commissure (commissura anterior). In the middle part, a funnel, the infundibulum, descends into the floor of the diencephalon. At the end of its “spout” lies the eminentia mediana, to which the pituitary gland is attached. In the posterior part, at the transition to the midbrain, the mammillary bodies protrude downward from the floor of the hypothalamus on both sides of the midline. They are part of the limbic system.

The posterior hypothalamus consists essentially only of the mammillary bodies. It is also known as the white hypothalamus because it is traversed by thick, medullary nerve fibers. These are the axons of the fornix, which extend to the mammillary bodies, and the axons of the nerve cells there, which ascend to the anterior dorsal thalamus. All these structures belong to the limbic system and are involved in memory formation (Papez circuit).

The anterior and middle medullary sections of the hypothalamus, which are traversed by thinner nerve fibers, are much more heterogeneous. Here there are several dozen distinct nuclear areas, which are referred to as magnocellular or parvocellular nuclei according to the size of their perikarya, i.e., their cell bodies. They serve vegetative, unconscious bodily functions and control the body's “internal environment.” In addition, the white matter hypothalamus is an endocrine gland that produces many different hormones. And that's not all. It is also packed with receptors to which hormones from other peripheral endocrine glands (thyroid, adrenal cortex) bind. In short, we find ourselves here at one of the interfaces between nervous and hormonal regulation of bodily functions.

Let's take a closer look at some of the “famous” core areas of the hypothalamus. There is, for example, the bilaterally arranged suprachiasmatic nucleus. It is located directly above the optic chiasm and is the seat of the so-called internal clock. The biochemical processes inside the interconnected nerve cells in this core area – and thus also their electrical activities – exhibit spontaneous, endogenous oscillations with a phase length of about 24 hours, which they maintain even in the absence of external stimuli. Special ganglion cells in the retina of the eye, known as intrinsically photoreceptive ganglion cells, send their axons to the suprachiasmatic nucleus, synchronizing this “roughly day-long” (circadian) rhythm with the actual daily cycle of light and dark. The respective phase position of the internal clock determines the metabolic processes and activities of the entire organism. It communicates with the rest of the body via neural and humoral pathways that originate in the suprachiasmatic nucleus.

An important target organ of the neural pathways originating in the suprachiasmatic nucleus is the epiphysis cerebri (pineal gland). This is where melatonin is produced night after night and released into the bloodstream. Postganglionic nerve fibers from the superior cervical ganglion of the sympathetic nervous system control the biosynthesis of melatonin. They form the last link in a polysynaptic pathway that originates in neurons of the suprachiasmatic nucleus. Melatonin, in turn, influences the activity of the suprachiasmatic nucleus in the sense of a feedback loop. It also acts on the pars tuberalis of the pituitary gland and other organs in the body's periphery.

The anterior hypothalamus, which is low in myelin, also contains two magnocellular nuclei: the paraventricular nucleus and the supraoptic nucleus. These two are located where their names suggest – in the optic tract and right next to the ventricle, very close to the optic nerve. They contain glandular nerve cells, i.e. endocrine neurons that produce hormones. However, these hormones do not enter the circulating blood at the site of their production, i.e., in the nuclei themselves. Instead, they are transported in the axons of the glandular nerve cells via the pituitary stalk to the neurohypophysis, where they are released into the circulation.

The hormones in question are oxytocin and antidiuretic hormone (ADH). ADH reduces the amount of urine excreted. If it is lacking due to damage to the magnocellular nuclear areas, diabetes insipidus occurs, a rare hormone deficiency disease with extremely high urine excretion (5 to 25 liters per day) and a corresponding feeling of thirst. 

Oxytocin literally means “rapid birth.” Among other things, the hormone triggers contractions at the end of pregnancy. But it is not only related to birth, but also to the reproductive activities that take place before it. It is therefore also known as the “cuddle hormone,” released during orgasm, among other things and promotes physical closeness and trust. This also applies to men. The release of oxytocin during breastfeeding, on the other hand, is reserved for women and controls milk ejection – not milk production. This, in turn, is stimulated by prolactin.

Numerous other parvocellular endocrine nuclei are located in the middle section of the hypothalamus, near the opening of the funnel, the infundibulum mentioned above, from which the pituitary gland hangs downwards. Since these nuclear areas form a small bump behind the funnel opening (and in front of the mammillary bodies), this area is called the tuber cinereum (gray bump) and the nuclei are accordingly called the tuber nuclei. They are also endocrine glands. Their nerve cells produce hormones – releasing hormones, i.e., triggering hormones, and inhibiting hormones – which in turn cause the adenohypophysis (anterior and middle lobes and pars tuberalis) to produce hormones – or to refrain from doing so.

For example, the infundibular nucleus (= arcuate nucleus) produces dopamine, which, as an inhibiting hormone, inhibits the release of prolactin in the adenohypophysis. Prolactin, in turn, acts on the mammary gland, where it causes milk production. Other cells in the arcuate nucleus produce a releasing hormone called “GnRH,” which stands for “gonadotropin-releasing hormone.” In response to this hormone, the anterior lobe of the pituitary gland produces gonadotropins, which in turn stimulate the ovaries and testicles to produce the “actual” sex hormones, namely estrogen and testosterone. The infundibular nucleus, together with the Eminetia mediana and the Nucleus dorsomedialis, plays an important role in controlling food intake (“feeding and satiety center”) and metabolism. 

The low-marrow hypothalamus does not only influence bodily functions via hormonal pathways. Its various core areas are locally and reciprocally interconnected with each other and with the nucleus lateralis hypothalami (= lateral hypothalamic area). The lateral hypothalamic nucleus is an extensive core area: it borders the paraventricular nucleus and the tuber nuclei on the side, as well as the mammillary bodies deep in the wall of the hypothalamus. And further to the side, it borders the internal capsule. The nucleus is the starting point for extensive axonal projections to many other distant brain regions, from the cortex to the spinal cord, and thus serves, so to speak, as a kind of neural “distributor” for the hypothalamic influences on the rest of the nervous system's functions – including cognitive ones.

The lateral hypothalamic nucleus is involved in a variety of functions: eating behavior, i.e., the conscious experience of hunger and thirst, modulation of the pain and reward systems, regulation of attention, the perception of stress and exhaustion, but also purely vegetative functions such as thermoregulation, blood pressure regulation, and control of the motor activity of the stomach, intestines, and bladder. The neurotransmitters involved in these processes are diverse. They are predominantly neuropeptides such as orexin and dynorphin, but many other transmitters such as GABA, glutamate, and galanin also play a role.

First published on August 28, 2011
Last updated on July 1, 2025