The metathalamus is a component of the diencephalon and participates in visual and auditory information processing]. Lesions in this area of the brain can accordingly cause visual and auditory disorders due to, for example, strokes, [[circulatory disorders]], increased intracranial pressure, neurodegenerative diseases, tumors, and traumatic brain injury.
What is the metathalamus?
The metathalamus is an anatomical structure in the brain that is part of the thalamus in the diencephalon (midbrain) and consists of two parts: the corpus geniculatum laterale and the corpus geniculatum mediale. These two structures form projections in the surface of the diencephalon. Because of their shape, neurology therefore also calls them knee bumps. As an anatomical structure of the brain, the metathalamus belongs to the central nervous system. The brain sends signals to the periphery using descending (efferent) pathways. In addition, the brain processes information that originates throughout the body and reaches the central nervous system via ascending (afferent) pathways. Within the brain, these pathways continue to some extent. The visual and auditory pathways are relevant to the function of the metathalamus.
Anatomy and structure
The metathalamus is located in the diencephalon, which is above the brainstem, and includes two popliteal tubercles. These are known as the corpus geniculatum laterale and corpus geniculatum mediale. The corpus geniculatum laterale are located in the lateral region of the metathalamus and are composed of six layers. Because the lowest two layers contain particularly large neurons, they are called magnocellular layers. In contrast, the parvocellular layers have smaller neurons. Within the brain, nerve fibers connect the corpus geniculatum laterale with nuclei of the hypothalamus and with parts of the midbrain and cerebrum. The occipital lobe of the cerebral cortex contains the visual cortex, which is the visual center of the human brain. The corpus geniculatum mediale is part of the auditory pathway and consists of three subunits. The pars ventralis, the pars medialis, and the pars dorsalis each perform different tasks. The corpus geniculatum mediale also has connections to other brain areas, most notably the auditory cortex of the cerebrum and the brainstem and midbrain.
Function and tasks
The corpus geniculatum laterale is predominantly responsible for processing visual information. It works closely with certain areas of the cerebrum in this regard. Information moves between the visual cortex and the corpus geniculatum laterale via the radiatio optica. The lateral popliteal tuberosity plays an important role for the visual pathway, as 90% of its nerve fibers terminate here. The information from the visual pathway, also known as the tractus opticus, comes partly from the aforementioned brain areas and partly directly from the retinal nerve cells. Of the six layers of the corpus geniculatum laterale, three are devoted to processing stimuli originating from the opposite (contralateral) eye, while the other layers are responsible for visual perceptions from the eye on the same (ipsilateral) side. One magnocellular layer and two parvocellular layers each deal with one eye. The corpus geniculatum laterale takes an important role in color vision. Each of the cells has a receptive field: this corresponds to the field of vision covered by the respective cell. The magnocellular layers, with their larger receptive fields, produce a less accurate image, whereas the parvocellular layers, with their smaller receptive fields, produce a sharper image. The corpus geniculatum mediale participates in auditory perception. Cells of the pars ventralis respond to different frequencies. In this process, the synaptic connections between them allow interneuronal exchange and give rise to clusters of neurons. This arrangement enables the division of sounds into octaves and, within a cluster, a finer differentiation of frequencies. The pars ventralis also processes other acoustic information such as modulation. In addition to these processes, the task of the pars medialis is to connect information from the auditory pathway with signals that are not part of acoustic perception. These include emotions, for example.The pars medialis therefore maintains a connection to the limbic system; it is also associated with the inferior colliculi. The pars dorsalis also exerts a combining function by incorporating somatosensory information, among other things.
Diseases
Lesions in the metathalamus manifest in different symptoms; they depend on which part of the brain structure is damaged. The corpus geniculatum laterale and the visual cortex are connected via the radiatio optica. Quadrant anopsia is a visual field defect that may be due to damage to the radiatio optica. The nerve fibers that are still intact continue to transmit information, thereby allowing the affected person partial vision. However, the damaged fibers interrupt the transmission of visual signals and the corresponding visual field fails. However, peculiarities of the metathalamus are not only related to physical symptoms, but also to psychological ones. Based on animal studies of sound discrimination, some researchers hypothesize that the corpus geniculatum mediale is involved in the development of certain language disorders and, in some cases, may play a role in dyslexia. Potential causes of damage include strokes, bleeding and circulatory disorders, increased intracranial pressure, neurodegenerative diseases, tumors, and traumatic brain injury. In addition, other auditory and visual disorders associated with the metathalamus are possible in these diseases. In particular, extensive damage or lesions at critical sites can also lead to complete loss of the respective sensory perception.
