Diencephalon

Structure of the thalamus

The thalamus (thalamus) is a large mass of gray matter with an irregular ovoid shape. Inside it, layers of white matter run through, dividing the thalamus into numerous individual nuclei. Anatomically, the thalamus is conventionally divided into two parts: the ventral thalamus (thalamus ventralis) and the thicker dorsal thalamus (thalamus dorsalis).

Topographically, the thalamus has several important surfaces and landmarks:

• Medial surface. It faces the cavity of the third ventricle. Inferiorly, this surface is bounded by the hypothalamic sulcus (sulcus hypothalamicus). The right and left medial surfaces of the thalami are often connected to each other by the interthalamic adhesion (adhesio interthalamica).
• Superior surface. It is free. It is separated from the medial surface by the medullary stria of the thalamus (stria medullaris thalami). Posteriorly, the superior surface broadens and forms the habenular trigone (trigonum habenulae).

In the anterior part of the superior surface, the anterior tubercle of the thalamus (tuberculum anterius thalami) stands out, while the posterior part forms a massive thickening designated as the pulvinar (pulvinar). The choroid plexus of the third ventricle attaches to the thalamus; if removed, the taenia thalami (tenia thalami) remains on the surface.

Lateral to the corpus striatum, the thalamus is separated by the internal capsule. Within the thalamus itself run white matter medullary laminae: the internal medullary lamina (lamina medullaris interna) and the external medullary lamina (lamina medullaris externa).

Thalamic nuclei and subthalamic region

The medullary laminae of the thalamus divide its gray matter into specific groups of nuclei (nuclei thalami) that have various topographic and functional connections. Emerging from the nuclei is the thalamic fasciculus (fasciculus thalamicus), which provides connection between the thalamus and the cerebral cortex.

Main groups of thalamic nuclei:

1. Anterior nuclei. Located in the anterior tubercle of the thalamus (include nucleus anterodorsalis, anteroventralis, anteromedialis).
2. Medial nuclei. Located near the medial surface (include the dorsal medial nucleus — nucleus medialis dorsalis).
3. Ventrolateral group. This is the largest group, lying lateral to the anterior and medial nuclei. It includes nucleus lateralis posterior, lateralis dorsalis, the anterior, lateral, and medial ventral nuclei, as well as the posterior ventral nuclei (nucleus ventralis posterolateralis and posteromedialis).
4. Intralaminar nuclei. Small clusters of gray matter within the substance of the medullary laminae (e.g., nucleus centromedianus, paracentralis).
5. Posterior nuclei. Located in the region of the pulvinar (nuclei posteriores).
6. Reticular nuclei. Located in the ventral thalamus (nuclei reticulares).

Between the thalamic fasciculi lies the zona incerta (zona incerta), a thin layer of gray matter. Along the course of the fasciculi, the nuclei of field H (nucleus areae H1 and H2) are also described here.

In the inferior part of the ventral thalamus lies the subthalamic nucleus (nucleus subthalamicus). This is a cluster of gray matter interspersed with fibers, which functionally belongs to the extrapyramidal system. It connects to the globus pallidus via fibers of the subthalamic fasciculus (fasciculus subthalamicus).

Metathalamus and Epithalamus

The metathalamus (metathalamus) is formed by the geniculate bodies, which lie as elongated elevations posterior and inferior to the pulvinar of the thalamus. They are closely connected with the auditory and visual analyzers. Two geniculate bodies are distinguished:

• Medial geniculate body (corpus geniculatum mediale).
• Lateral geniculate body (corpus geniculatum laterale).

Each of these bodies contains corresponding nuclei (nucleus corporis geniculati medialis and lateralis). The geniculate bodies have posterior parts (partes dorsales) located within the metathalamus proper, and anterior parts (partes ventrales) that extend into the ventral thalamus.

The epithalamus (epithalamus) includes structures that form the roof of the third ventricle and are functionally connected to the limbic system of the brain. It comprises:

• Pineal body, or epiphysis (corpus pineale / glandula pinealis). This is an endocrine gland that develops from the posterior region of the roof of the diencephalon.
• Habenulae (habenulae). They are the direct continuation of the habenular trigone.
• Habenular commissure (commissura habenularum).

The bulk of nerve fibers originating from the habenular nuclei forms the habenulo-interpeduncular tract (tractus habenulo-interpeduncularis), which heads toward the interpeduncular nucleus (nucleus interpeduncularis).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Anatomy and connections of the intralaminar thalamic nuclei.
• Topography of the zona incerta and the nuclei of field H.
• Structural features of the anterior and posterior parts of the geniculate bodies.
• Functional projections of the ventrolateral group of thalamic nuclei.

Other topics in the "CNS" section

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