The Diencephalon

The diencephalon – in English, the interbrain – is actually located in between, namely between the telencephalon and the midbrain. In an intact brain, hardly anything of it can be seen except for its base; the rest is covered on all sides by the hemispheres of the telencephalon.

Inside the diencephalon is an unpaired, vertically positioned, slit-shaped cavity called the third ventricle. The ancients called it the “thalamus,” which means “room” or “chamber.” Many structures in the diencephalon owe their names to this chamber – they were transferred from the ventricle to its walls. From bottom to top, these walls contain the “floors” of the diencephalon: the hypothalamus – the basement, the subthalamus – the ground floor, the dorsal thalamus – the bel étage, and the epithalamus – the attic.

Functionally, the diencephalon is one of the most heterogeneous sections of our brain, with its different floors having many very different functions. They are so many and so varied that we will discuss them in separate texts and limit ourselves here to keywords.

Let's start in the basement, with the hypothalamus. It is the most important interface between the head and the body, the control center of the autonomic nervous system. Numerous individual nuclei regulate such important processes as hunger and thirst, waking, sleeping, and sexual desire. However, the hypothalamus cannot do all this on its own but needs to work together with the pituitary gland: it produces various hormones, which in turn trigger or inhibit the release of other hormones in the pituitary gland. At the front of the hypothalamus, you can see the optic chiasm.

However, this is only the front part of the hypothalamus, which is low in medulla and appears gray to the naked eye. Its rear part appears whiter because it contains thick fibers rich in medulla. This is particularly true of the mammillary bodies, which are part of the limbic system.

The subthalamus, the ground floor, cannot be seen from the ventricular surface. Its cell masses have migrated sideways “into the depths” and can only be seen if the brain is cut off-center, either transversely or longitudinally. Its most important structure is the subthalamic nucleus, which is connected to the basal ganglia and is involved in controlling motor function.

The “bel étage,” the dorsal thalamus, has inherited the name of the third ventricle and is often simply referred to as the “thalamus.” A shallow groove on the edge of the third ventricle, the sulcus hypothalamicus, separates it from the hypothalamus. The thalami on both sides, which protrude slightly toward the ventricle, are usually connected to each other by a tissue bridge of varying thickness across the ventricle. This bridge, the interthalamic adhesion, is not found in every brain; it may be absent, but this has no functional significance.

The numerous nuclei of the thalamus are reciprocally connected to the cortex. Some of them are also responsible for “switching” sensory information (visual, acoustic, gustatory, and that of the skin and muscle senses) to the cortex. At least, that is the view taken in older textbooks. More recent findings reveal a very interesting systemic interaction between the thalamic nuclei and their cortical counterparts.

At the back and above the thalamus, the pineal gland – also known as the epiphysis or glandula pinealis – catches the eye. Over the course of evolutionary history, it has transformed from a light-sensitive organ – a “third eye,” as it is also referred to in the literature – into an endocrine gland. At night, it produces the hormone melatonin. Together with the tiny habenulae, which presumably have olfactory functions, it forms the epithalamus, the attic.

The fornix (Latin for “arc”) spans the dorsal thalamus and epithalamus. It describes a steep, sloping arc toward the front, thus forming part of the anterior boundary of the diencephalon. Just behind this arch – just before the tip of the dorsal thalamus – there is a hole through which the third ventricle connects to the lateral ventricles. Below, in the hypothalamic basement, just above the optic chiasm, a delicate tissue plate, the lamina terminalis, forms the anterior boundary of the diencephalon. Towards the rear – towards the midbrain – the third ventricle narrows to form the hair-thin aqueduct of the brain, the “water conduit of the brain.” At its beginning lies the boundary line to the brain stem.

First published on August 23, 2011
Updated on November 28, 2025