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).
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