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Structure

Thalamus

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The thalamus is the paired diencephalic relay and integration centre through which almost all sensory pathways, except olfaction, reach the cerebral cortex. It also relays motor loops and participates in arousal and attention.

Terminologia Anatomica (Latin): Thalamus · Open in the 3D atlas

Location

On either side of the third ventricle, bounded laterally by the posterior limb of the internal capsule and inferiorly by the hypothalamic sulcus; the two thalami are often joined by the interthalamic adhesion.

Structure

An ovoid grey-matter mass divided by the internal medullary lamina into anterior, medial and lateral nuclear groups. Key relay nuclei: VPL (body somatosensation), VPM (face, taste), LGN (vision), MGN (hearing), VA and VL (motor loops), anterior nucleus (limbic).

Blood supply

Mostly from perforating arteries of the posterior circulation: paramedian (thalamoperforating) arteries from the P1 segment of the posterior cerebral artery, tuberothalamic perforators from the posterior communicating artery, thalamogeniculate branches of the posterior cerebral artery (lateral thalamus) and the posterior choroidal arteries. The artery of Percheron is a variant in which a single trunk from one P1 segment supplies both paramedian thalami and the rostral midbrain.

Innervation

Connections: reciprocal thalamocortical and corticothalamic fibres through the internal capsule, afferents from the spinothalamic tract, medial lemniscus, optic tract, brachium of the inferior colliculus, cerebellum and globus pallidus, and the reticular nucleus as a modulating shell.

Function

It relays and filters sensory input to the primary cortices, relays cerebellar and pallidal output to motor cortex, and supports consciousness through intralaminar and reticular connections. The thalamus acts as a gate rather than a passive relay.

Clinical correlations

Thalamic infarction or haemorrhage may cause contralateral hemisensory loss; the thalamic pain syndrome (Dejerine-Roussy) can follow, with persistent contralateral pain. Bilateral paramedian infarction (artery of Percheron) may impair consciousness and vertical gaze.

Common exam points

  • Almost all sensory pathways relay in the thalamus before the cortex; olfaction is the exception.
  • VPL carries body sensation, VPM face and taste, LGN vision, MGN hearing.
  • VA and VL relay basal ganglia and cerebellar output to the motor cortex.
  • Supplied mainly by posterior circulation perforators (paramedian from P1, tuberothalamic from the posterior communicating artery, thalamogeniculate, posterior choroidal).
  • Thalamic stroke: contralateral sensory loss, possible central pain syndrome.

References

  1. Kandel ER, Koester JD, Mack SH, Siegelbaum SA. Principles of Neural Science. McGraw-Hill, 6th edition (2021) — Tier 3 (textbook)
  2. Haines DE, Mihailoff GA. Fundamental Neuroscience for Basic and Clinical Applications. Elsevier, 5th edition (2018) — Tier 3 (textbook)
  3. Drake RL, Vogl AW, Mitchell AWM. Gray's Anatomy for Students. Elsevier — Tier 3 (textbook)

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