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What is a diencephalon?

Diencephalon is a part of the brain which is lies between the cerebrum & brainstem (midbrain). The cavity present within the diencephalon is called the third ventricle. The diencephalon is communicating on either side with the lateral ventricle of the cerebral hemisphere.

Subdivisions of the diencephalon 

The subdivisions by a shallow groove called the hypothalamic sulcus into Pars dorsalis and Pars ventralis.

  • Pars dorsalis: It is a subdivision of the diencephalon that consists of the thalamus, metathalamus, and epithalamus.
  • Pars ventralis: It is a subdivision of the diencephalon that consists of the subthalamus and hypothalamus.

Thalamus

It is a large ovoid mass of gray matter lying above the midbrain. It situated on each side of the third ventricle. The thalami are connected to each other by interthalamic connection (adhesion).

External features of the thalamus

  • Two ends: anterior (narrow and rounded) and posterior (expanded to form pulvinar)
  • Four surfaces: superior, inferior, medial, and lateral.
  • Superior surface: It is a lateral part that forms the floor of the central part of the lateral ventricle. It is medial part is surrounded by the tela choroidea of the third ventricle.
  • Inferior surface: The anterior part is fused with the subthalamus and the posterior part is free, forming the inferior aspect of the pulvinar.
  • Medial surface: It forms the greater part of the lateral wall of the third ventricle.
  • Lateral surface: It forms the medial boundary of the posterior limb of the internal capsule.

The internal structure of the thalamus

The thalamus consists of mainly gray matter & only a small amount of white matter. The white matter has an external medullary lamina, Strarum zonale, and Y-shaped internal medullary lamina. The gray matter consists of nuclei and divided into three parts such as anterior, medial, and lateral by Y-shaped white matter i.e. internal medullary lamina.

The nuclei of gray matter

  • Nuclei in the anterior part: anterior nuclei
  • Nuclei in the medial part: large dorsomedial nucleus & several smaller nuclei
  • Nuclei in the lateral part: subdivided into a dorsal tier & a ventral tier
    • Dorsal tier of the nuclei:
      • Lateral dorsal nucleus
      • Lateral posterior nucleus
      • Pulvinar
    • Ventral tier of the nuclei
      • Ventral anterior nucleus
      • Ventral lateral nucleus
      • Ventral posterior nucleus
        • Ventral posterolateral nucleus
        • Ventral posteromedial nucleus
  • Other thalamic nuclei are
    • Intralaminar nuclei
    • Midline nuclei
    • Reticular nuclei
    • Medial and lateral geniculate bodies

Applied aspect of thalamus: Thalamic syndrome

Metathalamus

It consists of medial and lateral geniculate bodies. It is a relay station for the auditory and visual pathways respectively.

Epithalamus: Pineal Gland (Epiphysis Cerebri)

A midline cone-shaped reddish-grey structure. It is occupying the vertical groove between the two superior colliculi below the splenium of the corpus callosum. It projects back from the posterior wall of the third ventricle of the brain, below the splenium of the corpus callosum. It divides into two laminae.

 

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