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Location and external features of Thalamus.

Thalamus ( also known as ‘Gateway to Cerebral Cortex’) is part of diencephalon which is part of brain between the cerebrum above and midbrain below.

Each cerebral hemisphere contains a thalamus (dorsal thalamus), a large egg-shaped mass of grey matter that lies immediately lateral to the third ventricle. The paired thalami (right and left) are connected by the interthalamic adhesion. It has

  • Two ends/poles: anterior and posterior. anterior end lies just behind the interventricular foramen and posterior end called pulvinar lies just above and lateral to superior colliculus.
  • Four surfaces: superior, inferior, medial and lateral.
    • Superior surface: lateral part of superior surface forms the floor of central part of lateral ventricle and the medial part is separated from the lateral ventricle by fornix and tela choroidea.
    • Inferior surface is related to hypothalamus.
    • Medial surface: Medial surfaces of the two thalami are connected by a mass of grey matter called interthamamic adhesion. It forms the lateral wall of third ventricle and is lined by ependyma.
    • Lateral surface is related to internal capsule.

Functions of Thalamus

Thalamus is regarded as integrating centre, where information from all sources is brought together. Thalamus has four basic functional roles:

  • Sensory: It is a centre for relay of sensory pathways except olfaction. All sensory information (except olfaction) is relayed to the cortex via the thalamus.
  • Motor: Motor system outputs from the basal ganglia and cerebellum are relayed to cerebrum via the thalamus.
  • Emotion &memory: Thalamus is part of the Papez circuit and helps control some emotional and memory information going to limbic cortex (cingulate gyrus).
  • Consciousness: Thalamic nuclei are connected to reticular activating system , therefore associated with alertness and consciousness.

Structure of Thalamus

Thalamus is mainly composed of gray matter (nuclei) and a small amount of white matter.

  • White matter:
    • Lateral surface of thalamus is covered by a thin layer of myelinated fibers called External medullay lamina.
    • Internally a ‘Y’ shaped  white matter sheet called Internal medullary lamina divides thalamus into 3 parts: Anterior, Medial and Lateral.
  • Grey matter/ Thalamic nuclei: there are five groups of thalamic nuclei, each with specific connections.

* The medial and lateral geniculate bodies, traditionally described under Metathalamus are now considered part of thalamus.

Functional classification of nuclei of Thalamus

Connections of Thalamus

Afferents:  Thalamus receives

  • General and proprioceptive sensory information via ascending tracts spinothalamic tracts, medial lemniscus and trigeminothalmic tract.
  • Taste sensation reaches via solitariothalamic fibers.
  • Visual and auditory impulses reach via  optic radiation and from inferior colliculus respectively.
  • Fibers from hypothalamus, hippocampus and reticular formation also reach thalamus.
  • It also receives profuse connections from cerebral cortex, corpus striatum and cerebellum.

Efferents: The information received by thalamus is projected to cerebral cortex via thalamocortical fibers which form thalamic radiations. These are :

  • Anterior to frontal lobe (cingulate gyrus and prefrontal cortex)
  • Superior to parietal lobe (motor and premotor cortex, sensory cortex)
  • Posterior to occipital lobe ( primary visual area)
  • Inferior to temporal lobe ( primary auditory area)

Projections from thalamus also reach corpus striatum, hypothalmus and reticular formation.

Applied

Thalamic syndrome/ Dejerine Roussy syndrome

This occurs due to occlusion of the branch of posterior cerebral artery that supplies the posterolateral part of thalamus. It is characterized by:

  • Pansensory loss on contralateral side of lesion ( due to involvement of ventral posterior nucleus).
  • Sudden severe pain ( involvement of non-specific nuclei).
  • Transient haemiparesis ( involvement of internal capsule).
  •  Hameataxia and choreiform movements ( involvements of fibers from cerebellum and globus pallidus).
  • Homonymous hemianopia ( involvement of lateral geniculate body).

2 thoughts on “Thalamus

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