Lecture #12: Anatomy and Physiology of the Thalamus

What is the thalamus?

The thalamus is the largest component of the diencephalon and functions as a major relay and integration center for sensory, motor, limbic, and associative information traveling to the cerebral cortex.

Where is the thalamus located?

The thalamus lies between the brainstem and the cerebral hemispheres and forms the lateral walls of the third ventricle.

What is the primary function of the thalamus?

The thalamus relays specific information to defined cortical areas and modulates cortical excitability, attention, arousal, and consciousness.

How is the thalamus structurally organized?

It is divided into anterior, medial, and lateral nuclear groups by the internal medullary lamina, which also contains intralaminar nuclei.

What is the reticular nucleus of the thalamus?

The reticular nucleus is a thin lateral shell of inhibitory GABAergic neurons that regulate thalamic output to the cerebral cortex.

What is the difference between relay nuclei and association nuclei?

Relay nuclei transmit specific sensory or motor information to discrete cortical regions, whereas association nuclei connect association cortex with other association areas.

What information does the VPL nucleus relay?

The VPL relays pain, temperature, touch, vibration, and proprioception from the body to the primary somatosensory cortex.

What information does the VPM nucleus relay?

The VPM relays facial sensation and taste information to the somatosensory cortex and insular/frontal opercular regions.

What is the function of the VA nucleus?

The VA nucleus relays motor information from the basal ganglia to premotor and supplementary motor cortex.

What is the function of the VL nucleus?

The VL nucleus relays motor information from the cerebellum to the primary motor cortex.

What is the function of the lateral geniculate nucleus?

The lateral geniculate nucleus relays visual information from the optic tract to the primary visual cortex.

What is the function of the medial geniculate nucleus?

The medial geniculate nucleus relays auditory information from the inferior colliculus to the auditory cortex.

What is the role of the pulvinar?

The pulvinar is an association nucleus involved in visual attention, sensory integration, and coordination between parietal, occipital, and temporal association cortices.

What is the function of the dorsomedial nucleus?

The dorsomedial nucleus connects with the prefrontal cortex and is involved in emotion, behavior, motivation, and executive function.

What are intralaminar nuclei responsible for?

They regulate arousal, alertness, and cortico-thalamo-cortical communication involved in consciousness.

What is the role of corticothalamic projections?

Corticothalamic fibers provide feedback from the cortex to modulate thalamic relay activity.

Which pathways do not synapse in the thalamus?

Major descending motor pathways such as corticospinal, corticobulbar, and corticopontine tracts bypass the thalamus.

What are the two firing modes of thalamic neurons?

Thalamic neurons operate in tonic mode for accurate sensory transmission and burst mode during sleep or altered consciousness.

What characterizes tonic firing mode?

Tonic firing occurs during wakefulness and allows precise relay of sensory information to the cortex.

What characterizes burst firing mode?

Burst firing occurs during sleep and involves rhythmic bursts that reduce accurate sensory transmission.

How does the thalamus contribute to sleep–wake regulation?

It interacts with the ascending reticular activating system to regulate cortical arousal and transitions between sleep states.

What arteries supply the thalamus?

The thalamus is primarily supplied by penetrating branches of the posterior cerebral artery, with contributions from the posterior communicating and anterior choroidal arteries.

What is Dejerine–Roussy syndrome?

A central post-stroke pain syndrome caused by ventral/posterior thalamic lesions resulting in delayed severe neuropathic pain.

What symptoms characterize Dejerine–Roussy syndrome?

Initial numbness followed by allodynia and burning, aching, or temperature-related pain weeks to months after stroke.

What is the artery of Percheron?

A rare vascular variant where a single artery supplies both paramedian thalami, causing bilateral thalamic infarction when occluded.

What is the internal capsule?

The internal capsule is a compact white matter structure containing ascending sensory and descending motor fibers connecting cortex with subcortical structures.

Where is the internal capsule located?

It lies between the caudate nucleus and lentiform nucleus anteriorly and between the thalamus and lentiform nucleus posteriorly.

What are the five parts of the internal capsule?

Anterior limb, genu, posterior limb, sublenticular part, and retrolenticular part.

What fibers pass through the anterior limb of the internal capsule?

Frontopontine fibers and thalamocortical fibers connecting the thalamus with frontal cortex.

What fibers pass through the genu of the internal capsule?

Corticobulbar fibers controlling motor function of the face and head.

What fibers pass through the posterior limb of the internal capsule?

Corticospinal motor fibers and ascending sensory fibers from the thalamus.

What fibers pass through the sublenticular part?

Auditory radiations connecting the medial geniculate nucleus to auditory cortex.

What fibers pass through the retrolenticular part?

Optic radiations connecting the lateral geniculate nucleus to visual cortex.

Why are internal capsule lesions clinically devastating?

Dense packing of fibers means small lesions can cause widespread motor and sensory deficits.

How do thalamic lesions affect consciousness and cognition?

Depending on nuclei involved, lesions can impair arousal, attention, sensory perception, emotion, and memory.