Hypothalamus

In this note, we are going to discuss the anatomy and function of the hypothalamus.

It is one of the primary control centers of our body. Anatomically and functionally, it is connected to the pituitary gland, forming what is known as the hypothalamic-pituitary system.

Pituitary gland in the hypothalamic-pituitary system

The hypothalamus carries out its functions through interactions with other endocrine organs, the autonomic nervous system, and the limbic system.

The functioning of the hypothalamus is focused on maintaining homeostasis, including thermoregulation, regulation of blood pressure and circulation, appetite, fluid-electrolyte balance, metabolism, and reproductive functions.

Anatomy of the hypothalamus

The hypothalamus, along with the thalamus and epithalamus, is a part of the diencephalon.

It is situated beneath the hypothalamic sulcus,

Hypothalamic sulcus<br />
(sulcus hypothalamicus) — Hypothalamic sulcus
(sulcus hypothalamicus)

next to the third ventricle from below, forming its floor and the lower portion of the lateral wall.

The optic chiasm is located anteriorly to the organ.

The hypothalamus consists of several parts:

The hypothalamic nuclei

The mammillary bodies

The tuber cinereum, which has a median eminence

The infundibulum, extending from the median eminence,

connects the hypothalamus to the pituitary gland (hypophysis)

Let’s discuss the hypothalamic nuclei in greater detail.

Hypothalamic nuclei

In the frontal section, the hypothalamic nuclei can be divided into three zones or groups:

Periventricular
Intermediate or medial
Lateral

Within the intermediate zone, in the sagittal section, four groups of nuclei can be distinguished:

Preoptic

Anterior (or supraoptic)

Tuberal

Posterior (or mammillary)

Let’s start with the periventricular zone. It contains the periventricular nucleus, which produces some of the hormones that regulate the function of the adenohypophysis.

The preoptic group of nuclei includes the medial preoptic nucleus and the lateral preoptic nucleus. The latter can also be considered as part of the lateral group.

The medial preoptic nucleus regulates fluid-electrolyte balance and the synthesis of the pituitary’s gonadotropin-releasing hormone (GnRH).

The supraoptic group includes the supraoptic nucleus,

the paraventricular nucleus,

the suprachiasmatic and the anterior hypothalamic nucleus.

The supraoptic nucleus produces antidiuretic hormone (ADH or vasopressin), while the paraventricular nucleus synthesizes oxytocin. These hormones are transported to the neurohypophysis via the hypothalamo-hypophyseal tract, and only then are they released into the bloodstream.

The effects of these hormones can be seen in the diagram.

The suprachiasmatic nucleus is involved in the regulation of circadian rhythms because it has connections with the retina and the pineal gland.

The anterior hypothalamic nucleus primarily plays a role in maintaining body temperature, acting as the so-called “cooling center”. By interacting with the parasympathetic system, this nucleus helps to lower body temperature.

The tuberal group contains three nuclei: the ventromedial nucleus,

the dorsomedial nucleus,

and the arcuate nucleus.

The ventromedial nucleus functions as the so-called satiety center.

The dorsomedial nucleus also participates in feeding behavior and plays a role in behavioral patterns related to the expression of aggression.

The arcuate nucleus produces some of the hormones that regulate the function of the adenohypophysis and is involved in the regulation of appetite, reproductive functions, and circulation.

The mammillary group, also known as the mammillary complex, contains the mammillary nuclei (lateral and medial) as well as the posterior hypothalamic nucleus.

The mammillary nuclei are involved in emotions, short-term memory, and states of wakefulness.

They receive impulses from the hippocampus via the fornix and redirect them to the thalamus via the mammillothalamic tract.

The posterior nucleus, in addition to regulating emotions, also controls circulation and body temperature. Stimulation of this nucleus activates the sympathetic nervous system, thereby increasing body temperature.

The lateral zone of the hypothalamus contains the previously mentioned lateral preoptic nucleus and the lateral hypothalamic nucleus.

The former participates in sleep regulation, while the latter is one of the centers for appetite regulation. When stimulated, it induces a feeling of hunger in an individual.

Hypothalamic-pituitary connections

Within the infundibulum of the hypothalamus, there are two systems that provide the functional interaction between the pituitary gland and the hypothalamus.

One of these is the hypophyseal portal system, which consists of blood vessels connected to the adenohypophysis (or anterior pituitary).

Pituitary vessels, including portal system

And the hypothalamo-hypophyseal tract (tractus hypothalamohypophysialis), which consists of nerve fibers connected to the neurohypophysis (or posterior pituitary).

We discussed these fibers when talking about the supraoptic and paraventricular nuclei.

Some hypothalamic nuclei (such as the periventricular, medial preoptic, and arcuate) produce so-called releasing factors, which stimulate the anterior pituitary, as well as statins, which have the opposite effect. These substances are transported to the pituitary gland via the hypophyseal portal system.

The releasing factors include: