The thalamus: its characteristics and functions

The thalamus is an oval-shaped structure that constitutes the dorsal portion of the diencephalon.. It transmits sensory input to the primary sensory areas of the cortex, but it is more than a simple relay station.

The main function of the thalamus is the integration of sensory information directed to the cortex. Transmits most of the information that reaches the cerebral cortex and, furthermore, it integrates various sensory modalities and facilitates or inhibits projections towards one lobe or another.

It is essential for the maintenance of cortical activity. It is also responsible for transmitting information from the cerebellum and striatum. to the cerebral cortex. The cerebellum and striatum are centers that modulate the descending motor pathways of the cerebral cortex. Almost all sensorimotor information passes through the thalamus before reaching its destination (cortex).

The thalamus is organized into different nuclei with different functions. The only sensory information that does not relay to the thalamus is olfactory information, which passes directly from the olfactory bulb to the cerebral cortex. The rest of the senses: taste, vision, touch and hearing are relayed in the nuclei of the thalamus and then projected to their corresponding area in the cortex.

But in addition to ascending information, it also deals with descending information, the information that the cortex sends to other areas. Although its main function is to regulate sensorimotor projections, It is also involved in the functioning of some higher functions, such as memory or language.

Organization of the thalamus

The thalamus is part of the diencephalon located at the base of the brain, between the telencephalon and the brainstem. In addition to the thalamus, three other subdivisions are part of the diencephalon: the hypothalamus, the subthalamus and the epithalamus. It is the largest of the four subdivisions and consists of groups of neurons that project to all areas of the cerebral cortex.

Let’s say It is in a strategic location that allows it to be “the functional gate of the cerebral cortex. Virtually all information that arrives or descends from the cortex first passes through the thalamus. The nuclei of the thalamus are organized around the internal medullary lamina, which is “Y” shaped, dividing the thalamus into three parts: anterior, medial and lateral.

Regarding the function played by each of the nuclei, we can classify them into:

Thalamic relay nucleiwhich are intermediate processing stations in the sensory transit to the cerebral cortex, sending their projections to specific areas of said cortexDiffuse projection, which are nonspecific: they do not send projections to specific areas.Association nuclei. These are part of a circuit that connects the cerebral cortex with subcortical structures.

Main nuclei of the thalamus

Former: The anterior nucleus is thought to be related to memory due to its extensive connectivity with the hippocampus.Dorsomedial: It is involved in emotional behavior and memory. This nucleus transmits information from the amygdala and olfactory cortex that is then projected to the prefrontal cortex and limbic system.Ventral posterolateral and ventral posteromedial: both act as relay nuclei that send somatosensory information to the sensory cortex.Ventral anterior and ventrolateral: They are motor relay nuclei that receive information from the cerebellum and basal ganglia. They are thought to be involved in motor functions and both have pathways leading to the substantia nigra, premotor cortex, reticular formation, and striatum.Lateral back: It is involved in the integration of sensory information and its association with cognitive functions. Among its other functions are being able to determine the visual stimuli that stand out the most and visually guided behaviors.Pulvinar: The pulvinar nucleus is thought to be involved in the processing of visual stimuli and has strong connectivity with the visual cortex.Medial geniculate and lateral geniculate: These nuclei are important for transmitting auditory and visual information, respectively. The lateral geniculate nucleus receives visual information from the retinas of the eyes, which projects to the visual cortex of the occipital lobe. The medial geniculate nucleus receives auditory input from the inferior colliculus (a part of the midbrain that is the main auditory center) and projects it to the primary auditory cortex within the temporal lobe.Reticular: It receives information from the cerebral cortex as well as the dorsal thalamic nuclei.

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Thalamic-cortical connections

Communication between the cortex and the thalamus is reciprocal and bidirectional. The thalamus consists of projection neurons that aim to communicate with other areas, such as the cortex, cerebellum or basal ganglia. In addition, it also has interneurons that send information between the nuclei of the thalamus. Thus, the relay nuclei send projections to specific areas of the cortex.

Each sensory modality has its relay nucleus, except for smell. The sensory relay nuclei transmit their signals to specific areas of the cortex located in the parietal, occipital, temporal and insula lobes. In addition, each thalamic sensory relay nucleus receives projections from the brain area to which it projects.

Those with diffuse projection include the intralaminar group, the midline group, and the reticular group. The first two are characterized because Information from different areas of the brain and spinal cord converges on them.and, in turn, they distribute it to large areas of the cerebral cortex, without respecting the limits of the cortical areas.

The thalamus not only functions as a station that processes and transmits the signals it receives to the cerebral cortex, but also acts as a center that controls the signals that reach it. The reticular nucleus carries out this function of controlling cortical activity. The function of the thalamus is crucial for the functioning of our CNS, its injury would lead to various deficits.

Groups of relay and diffuse projection nuclei

Up to 50 nuclei have been identified. The relay nuclei of the thalamus are usually classified into four groups with respect to the internal medullary lamina (fiber bundle):

Former: participates in memory and emotions connected to regions of the cingulate and frontal cortex. medial: consists of three subdivisions, each connected to a specific region of the frontal cortex. It receives afferents from the basal ganglia, the amygdaloid nucleus and the midbrain. He has been involved with memory. Ventral: important for motor control, transmitting information from the cerebellum and basal ganglia to the motor cortex. Later: consists of the medial geniculate, lateral geniculate and pulvinar. The medial geniculate is a component of the auditory system, it transmits tonetopically organized auditory information to the temporal lobe. The lateral geniculate receives information from the retina and sends visual information to the primary visual cortex of the occipital lobe. And the pulvinar projects to parieto-occipito-temporal association areas, more developed in humans.

As for the diffuse projection nuclei, they are located in the midline of the thalamus or in the internal medullary lamina. The intralaminar nuclei project to limbic structures., which govern the activation of the cortex, and perhaps participate in the integration of sensory submodalities. The reticular nucleus regulates the activity of other thalamic nuclei based on their control.

It is not only a relay station for simple transfer to the neocortex. Arguably, this is a complex brain region where a substantial degree of information processing is possible. Acts as a gatekeeper of information to the cerebral cortexpreventing or facilitating the passage of specific information depending on the person’s state of consciousness.

The multiple functions of the thalamus

It is a complex structure that It has implications for various functions of the central nervous system.these are some of them:

Sensorimotor: is responsible for the analysis and integration of sensitive functions. These can be mechanical, thermal and painful stimuli and collaborate in their perception. Its injury can cause sensory loss contralateral to the injury, paresthesia or thalamic pain. At the motor level, it is involved in voluntary and involuntary movements and walking.Attention: Evidence has been found about its involvement in the sleep-wake cycle and the degree of consciousness.Emotions: is responsible for the integration of visceral information with affect, emotions and thought.Language: Mainly, it influences the motor aspects of language, although damage to the thalamus can also cause naming or syntactic problems.Memory: There is a clear involvement of the thalamus in anterograde memory, that is, the formation of new memories. In addition, it collaborates in the temporal organization of recent and old memories. His injury is also related to diseases characterized by amnesia such as Korsakoff syndrome.executive function: in its projections to the prefrontal cortex it also modulates executive functions, attention, initiative, inhibition and the temporal organization of behavior.

As can be seen, its mission cannot be limited to a single function.: modulates multiple sensory and motor modalities and even higher functions.

It can be stated, therefore, that The thalamus plays an important role in different brain functions including memory, emotions, sleep-wake cycle, executive functions. Mediates general cortical responses of alertness, sensorimotor control, and sensory information processing.

Thalamus injury

The thalamus is a gray nucleus located at the base of our brain that serves as an intersection point for multiple neuronal pathways. The clinical manifestations produced by thalamic lesions are very diverse.little systematizable and relatively little known.

At the same time, These injuries respond to multiple causes, within which the vascular predominates, although those of metabolic, neoplastic, inflammatory and infectious origin also stand out.

It integrates somatic, visceral and visual information related to emotional sensations and subjective states, influences the activity of the motor cortex and serves as a relay towards consciousness. Hence Levels of consciousness and alertness may be alteredemotional tone or recent memory, in addition to problems with sensitivity and movements.

We can talk about specific pathologies directly related to damage to the thalamus.. Two of these injuries would be Korsakoff syndrome or…