Diencephalon - structure, subdivision and functional organization

The diencephalon is a part of the brain located between the brainstem and the cerebrum. It is one of the three major parts of the forebrain, the other two being the telencephalon and the hypothalamus.

It is made up of five functionally and morphologically distinct parts. Dorsoventrally they are: epithalamus, thalamus, metathalamus, subthalamus and hypothalamus.

Anatomy and function

It is the caudal part of the forebrain (prosencephalon) that occupies the central region of the brain.

The roof: a thin tela choroidea, which is a combination of two membranes, the ependyma and mater. Within the tela choroidea are two plexuses of blood vessels that bulge downwards into the cavity of the third ventricle. These are the choroid plexuses of the third ventricle which functions as a point of production of the cerebrospinal fluid.

The floor: the optic chiasm, the tuber cinereum and infundibulum, the mammillary bodies, the posterior perforated substance and the mesencephalic tegmentum.

The anterior wall: the lamina terminalis, the anterior commissure and anterior column of the fornix..

The posterior wall: the stalk of the pineal gland, posterior commissure and the Habenular commissures.

The lateral walls: are of the cavity are formed by the medial walls of each thalami.

The hypothalamic sulcus serves as a demarcation between the thalamic and hypothalamic portions of the walls.

Epithalamus is located in the posterior part of the roof of the third ventricle, playing a crucial role in various neural and endocrine functions. It consists of several parts:

stria medullaris: A bundle of nerve fibers that connects the thalamus to other brain areas, including the habenular nuclei. These fibers help link different parts of the brain and allow communication between the two sides of the brain.

posterior commissure: A group of fibers at the bottom of the pineal stalk, helping to connect parts of the brain related to eye movement and balance.

habenular nuclei (medial and lateral): Clusters of nerve cells that are involved in regulating mood, emotions, and pain.

pineal body: A small gland that produces melatonin, a hormone that controls sleep-wake cycles. It also affects other hormones in the body. Over time, the pineal gland may become calcified, forming "brain sand."

paraventricular nuclei (anterior and posterior): Small clusters of nerve cells near the third ventricle, involved in regulating hormone release and bodily functions.

Thalamus is located near the third ventricle in the brain. It sits between the cerebral cortex and the midbrain. It has several surfaces, including an anterior, posterior, superior, inferior, medial, and lateral surface.

The thalamus contains several important nuclei (clusters of nerve cells):

Anterior nuclei: Involved in memory and emotion.

Medial nuclei: Help with memory, mood, and emotional responses.

Lateral nuclei: Deal with sensory and motor information.

Pulvinar: Important for visual processing.

Intralaminar nuclei: Play a role in alertness and consciousness.

Reticular nucleus: Regulates the activity of other thalamic nuclei.

Main functions: Sensory and motor signals: The thalamus acts as a relay station for signals between the brain and body. Consciousness, sleep, and alertness: It helps regulate sleep and wakefulness.

Subthalamus is a region of the brain located beneath the posterior part of the thalamus, behind and to the side of the hypothalamus. It contains several key structures, including:

Zona incerta: A region that sends GABA (a neurotransmitter) to the thalamus, helping to regulate its activity.

Reticular nucleus: A thin layer of neurons that regulates the thalamocortical pathway with GABA, affecting how the thalamus communicates with the cortex.

Perigeniculate nucleus: Involved in visual processing, particularly related to the lateral geniculate nucleus.

Connections:

The pre-geniculate nucleus has connections to the lateral geniculate nucleus, retina, superior colliculus, and other visual regions, influencing vision and eye movements.

The zona incerta connects with the reticular nucleus and is thought to help control visceral functions like hunger, thirst, sexual activity, and cardiovascular regulation.

Reticular nucleus fibers connect with the thalamus and midbrain regions, playing a role in processing sensory and motor signals.

Metathalamus is a part of the diencephalon and consists of two structures called the geniculate bodies. They are located bellow the thalamus.

Medial geniculate body: involved in auditory processing. It receives sound information from the inferior colliculus (a part of the midbrain) and sends this information to the primary auditory cortex in the brain, where it is processed and interpreted.

Lateral geniculate body: this structure is involved in visual processing. It receives visual information from the retina through the optic tract. This information is organized according to the visual field and relayed to the primary visual cortex via the optic radiation.

Hypothalamus is a small but crucial part of the brain located below the thalamus. It is involved in regulating many of the body's essential functions and is part of the diencephalon. It is divided into three main zones (periventricular, medial, and lateral) and three regions (anterior, middle, and posterior).It forms the wall of the third ventricle and extends to the floor of the third ventricle, where it connects to several important structures:

Tuber cinereum

Infundibulum: connects to the pituitary gland

Mammillary bodies: important for memory and limbic system functions

The hypothalamus contains various nuclei (clusters of neurons), which are grouped by location and function:

Preoptic Nuclei (anterior region): Involved in reproductive behavior, body temperature regulation, and sleep.

Supraoptic (Chiasmatic) Nuclei: Regulate hormones, water balance, and are involved in circadian rhythms.

Infundibular (Tuberal) Nuclei (middle region): Control hunger, satiety, and metabolism.

Mammillary Nuclei (posterior region): Involved in memory processing and autonomic functions.

Used literature[edit | edit source]

ČIHÁK, Radomír – GRIM, Miloš. Anatomie 3. 2., upr. a dopl edition. Grada, 2004. 673 pp. pp. 560. ISBN 80-247-1132-X.