Where is the thalamus located in the brain?
The thalamus is located deep in the center of the brain, in the so-called diencephalon. It consists of two halves, the left and the right thalamus. One part is therefore located in the left hemisphere, the other in the right hemisphere. The halves of the thalamus are about the size of a walnut and are connected to each other (adhaesio interthalamica).
The third ventricle, a cavity filled with cerebrospinal fluid, runs between the left and right halves. The sides of the thalamus lie on the capsula interna. This structure is a kind of road in the brain that transports signals and information from one place to another. The front surface is fused with the hypothalamus.
Structure of the thalamus
The thalamus consists of gray and white matter. The gray matter is divided into numerous nuclei (collections of nerve cell bodies) – the thalamic nuclei – by thin sheets of white matter.
The thalamus has an anterior pole in which the anterior nuclei of the thalamus (nuclei anteriores thalami) are located. The posterior pole points backwards and downwards and forms the cushion (thalamic pulvinar). To the side of the pulvinar is an elevation, the corpus geniculatum laterale (the lateral popliteal tubercle). Under the front edge of the pulvinar lies the corpus geniculatum mediale (the medial knee tuberosity).
What is the function of the thalamus?
The thalamus is the gateway to consciousness. It acts as a filter and distributor of incoming information. It decides which sensory impressions from the environment and the organism should enter consciousness and which are then passed on to the corresponding processing centers. All sensory impressions of feeling, seeing and hearing – but not of smelling – are transmitted via the thalamus.
The thalamic nuclei
The thalamic nuclei in turn contain smaller nuclei and areas with different functions. All somatosensory and sensory pathways (with the exception of the olfactory pathways) that originate in the periphery and lead to the cerebral cortex are switched in the middle and posterior nuclei of the thalamic nuclei.
All connections are double-connected to the corresponding cortical fields. This makes it possible, through concentrated attention, to perceive different sensory impressions to varying degrees: strongly, slightly or almost not at all.
Visual and auditory impressions are switched in the nuclei of the metathalamus (corpus geniculatum laterale and mediale) on their way to the visual and auditory cortex.
Affective and instinctive excitations, emotional sensations are switched in the thalamic nuclei and passed on to the corresponding cortical areas.
Taste information is brought together via the taste nucleus and passed on to the taste cortex via the thalamus.
What problems can the thalamus cause?
The so-called thalamus syndrome (Déjerine-Roussy syndrome) occurs when a blood clot blocks an important vessel of the thalamus (such as the thalamostriate artery) (thrombosis). The result is a loss of the thalamus with visual and sensory disturbances, hemianopsia (hemi-blindness), a strong excitability of the reflexes as well as a reduced sensitivity of the skin and a disturbance of the depth sensitivity.
In general, sensory disorders with reduced sensitivity, hypersensitivity to all sensory stimuli (albeit with an increased stimulus threshold), sensory disturbances and severe central pain in the side opposite the disorder indicate disorders in this region of the brain.
Motor disorders with rigid facial muscles and hyperkinesia (forced movements of the hands and fingers) and mental disorders with reduced attention, irritability, impatience and nervousness can also indicate damage or disease in the thalamus area.
