Thalamus

The thalamus (plural: thalami) is the largest of the structures comprising the diencephalon.

Role

The thalamus acts as a relay center, receiving and distributing information between the peripheries and higher centers such as the cerebral cortices. It contributes to functions such as:

• consciousness
• sleep
• memory
• sensory and motor functions

Gross anatomy

Structurally, the thalamus is composed of two symmetrical egg-shaped masses (thalami) which are usually connected at the midline by a band of grey matter, the interthalamic adhesion.

The anterior pole narrows to form the posterior boundary of the interventricular foramen. Posteriorly the thalamus expands to form the pulvinar. On the posteroventral surface of the thalamus lie two rounded protrusions called the medial and lateral geniculate nuclei.

Relations

• superior: the superior surface is covered by the stratum zonale and is separated from the body of the fornix by the tela choroidea. More laterally, the superior surface is separated from the head of the caudate nucleus by the stria terminalis and the thalamostriate vein.
• lateral: the lateral surface contacts the descending posterior limb of the internal capsule.
• medial: the medial walls, as previously mentioned, are joined in most cases at the interthalamic adhesion. This medial surface also forms the lateral wall of the third ventricle.
• inferior: inferomedially, the thalamus joins the hypothalamus. It is also continuous with the lemnisci of the tegmentum.
• anterior: the anterior pole of the thalamus forms the posterior wall of the interventricular foramen.

Nuclei

The thalamus is surrounded by two thin layers of white matter. On its superior surface, it is covered by the stratum zonale and on its lateral surface by the external medullary lamina.

The grey matter of the thalamus is separated by a Y-shaped layer of white matter called the internal medullary lamina. This structure divides the thalamus into anterior, medial and lateral parts. These three parts contains the larger groups of thalamic nuclei.

Other smaller groups are located on the medial and lateral surfaces of the thalamus as well as within the internal medullary lamina.

• lateral
  • ventral
    • ventral anterior
    • ventral lateral
    • ventral posterior
      • ventral posterolateral (VPL)
      • ventral posteromedial (VPM)
    • lateral geniculate nucleus
    • medial geniculate nucleus
  • dorsal
    • lateral posterior
    • lateral dorsal
    • pulvinar
• medial
  • dorsomedial
• anterior
  • anterior
• reticular nucleus
• intralaminar nuclei
• midline nuclei

Blood supply

The thalamus is chiefly supplied by small perforating end-arteries arising from the posterior cerebral artery. The thalamus has four vascular territories ^1-3:

• anterior (or tuberothalamic): supplied by thalamotuberal (or polar) arteries which are branches of the posterior communicating artery
• paramedian: paramedian (or thalamoperforating) arteries which branch from the P1 segment of the posterior cerebral artery with significant variability; they can arise unilaterally or bilaterally from P1 as multiple arterial branches or single arterial trunks (such as the Artery of Percheron) ^{4, 5}
• inferolateral: thalamogeniculate arteries which are branches of the P2 segment of the posterior cerebral artery
• posterior:
  • medial posterior choroidal artery (P1 or P2 segment of the posterior cerebral artery)
  • posterior branch of the lateral posterior choroidal artery (P2 segment of the posterior cerebral artery)

Related pathology

• thalamic syndrome: contralateral hemianaesthesia, often accompanied by mood swings
• thalamic infarct
• thalamic glioma