The thalamus is located in the nucleus of the diencephalon, which is a part of the forebrain that also contains the hypothalamus, epithalamus and subthalamus.
The thalamus is often described as the relay station of the brain, since much of the information that reaches the cerebral cortex stops first in the thalamus before being sent to its destination.
All of our senses, with the exception of smell, pass through the thalamus before being directed to other areas of the brain for processing.
There are two thalami, one in each hemisphere of the brain. They are located above the brainstem and midbrain, allowing nerve fiber connections to reach the cerebral cortex in all directions.
This brain structure is capable of relaying and integrating a variety of motor and sensory signals between the higher brain centers and the peripheries.
The thalamus consists mainly of gray matter, but is also surrounded by two layers of white matter. Its appearance is oval, almost egg-shaped, with two protrusions on the surface.
One of these is known as the medial geniculate bodies, which are important for processing auditory information. The other is the lateral geniculate bodies, which are responsible for processing visual sensory inputs.
The thalamus is made up of different types of nuclei, each of which has a unique function, from transmitting sensory and motor signals to regulating consciousness and alertness.
What functions does the thalamus perform?
Because the thalamus is heavily involved in the transmission of information between the cortex and brainstem, as well as within different cortical structures, it contributes to many brain processes.
Although historically believed to be related only to sensory transmission in the visual, auditory, somatosensory and gustatory systems, it has been found to be involved in many other functions.
Some of the associated functions are listed below:
- Contribution to perception
- Transmission of motor information
- Transmission of sensory information
- Role in memory
- Alertness and attention
- Consciousness and awareness
- Role in cognition
- Connections with structures such as the hippocampus and other parts of the limbic system suggest that the thalamus plays a role in memory, especially episodic memory, as well as learning and emotion.
The thalamus is also thought to be involved in the regulation of sleep, wakefulness, and arousal. The thalamus filters information between the brain and the body.
Each sensory function, except olfactory (sense of smell), has a thalamic nucleus that receives, processes and transmits information to associated areas within the cerebral cortex.
In general, connections between the sensory organs of the body and the thalamus are contralateral, i.e., they communicate with the opposite side of the body.
Whereas connections between the thalamus and the cerebral cortex are ipsilateral, meaning that they communicate on the same side of the brain.
Thalamic nuclei
The thalamus is made up of a series of nuclei, all of which are responsible for the transmission of different sensory signals.
The nuclei are both excitatory and inhibitory in nature and receive sensory or motor information from the body, presenting the selected information via nerve fibers to the cerebral cortex.
Some of the main groups of nuclei of the thalamus and their functions are described below:
Posterior lateral nucleus
The posterior lateral nucleus is thought to be involved in the integration of sensory information and its association with cognitive functions. Its other functions include being able to determine which visual stimuli are most salient and visually guided behaviors.
Pulvinar nucleus
The pulvinar nucleus is thought to be involved in the processing of visual stimuli and to have strong connectivity with the visual cortex.
The pulvinar nucleus projects to the amygdala and striatum (an area involved in decision making, reinforcement and motivation).
It helps transmit visual information to guide precise movements, as well as transmitting visual information to the amygdala.
Reticular nucleus
The reticular nucleus forms a sheet that forms the outer covering of the thalamus and can influence the activity of other nuclei within the thalamus.
It receives information from the cerebral cortex and the dorsal thalamic nuclei.
It is the only nucleus of the thalamus that does not project to the cerebral cortex, but modulates information from other nuclei of the thalamus.
Anterior nucleus
Researchers believe that the anterior nucleus is involved in memory because of its extensive connectivity with the hippocampus.
It is also connected to the mammillothalamic tract (from the mammillary nucleus of the mammillary bodies to the hypothalamus) and the cingulate gyrus (involved in emotion processing and behavioral regulation).
As these areas are linked to the limbic system, they are involved in the organization of memory and emotions. The anterior nucleus essentially receives information from the limbic system and projects to the cingulate gyrus.
Dorsomedial nucleus
The dorsomedial nucleus is involved in emotional behavior and memory.
This nucleus transmits information from the amygdala and olfactory cortex, which then projects to the prefrontal cortex and limbic system, and in turn transmits them to the prefrontal association cortex.
Thus, the dorsomedial nucleus plays an important role in attention, organization, planning and higher cognitive thinking.
Ventral posteromedial and posterolateral nucleus
Both act as relay nuclei that send somatosensory information to the somatosensory cortex, a region that receives and processes sensory information about the body.
In addition, the ventral posteromedial nucleus receives sensory information from the trigeminal nerve about the face.
Lateral and medial 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 is projected to the visual cortex of the occipital lobe.
The medial geniculate nucleus receives auditory information 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.
Ventral anterior and ventrolateral nucleus.
These two nuclei are the relay motor nuclei, receiving inputs 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.
Which arteries supply the thalamus?
The main blood supply to the thalamus comes from the posterior cerebral artery. Contributing branches of the posterior communicating artery also supply the thalamus after passing through the posterior perforated substance.
These arteries arise from the vertebrobasilar arterial system, which anastomoses indirectly with the carotid artery through the circle of Willis.
Consequences of thalamic damage and injury
Because of the location of the thalamus, any injury or insult to the organ will impact adjacent structures.
For example, a neoplasm in the anterior part of the thalamus may obstruct the interventricular foramen of Monro. A similar neoplasm in the posteromedial thalamus may obstruct the third ventricle and, more importantly, the cerebral aqueduct of Sylvius.
In both cases, not only would the respective functions of the thalamus be compromised, but the patient may develop noncommunicating hydrocephalus.
However, most lesions of the thalamus are ischemic in nature. The cause of ischemia may be iatrogenic (caused during a therapeutic procedure) or due to vascular compromise (thrombotic or hemorrhagic). If this damage affects the VPM or VPL nuclei, all contralateral sensory inputs would be lost.
Vascular accidents of that thalamus can also produce ataxic choreoathetosis (uncoordinated and involuntary movements). In addition, the thalamus is intrinsically involved in the transmission of pain to the cerebral cortex. This is important, as surgical cauterization of these fibers can be used to alleviate the immense pain of terminal cancer patients.
A phenomenon known as thalamic pain-in which the thalamus overreacts to pain impulses from the contralateral side-has been observed following thalamic infarction.
Since the thalamus acts as a relay station from which it sends inputs to and receives outputs from many brain structures, damage to this area can affect many brain functions.
The following is a list of symptoms that may be associated with damage to the thalamus:
- Amnesia
- Aphasia
- Attention difficulties
- Movement difficulties
- Postural impairment
- Chronic pain
- Drowsiness
- Loss of alertness and activation
- Impaired sensory information processing
- Apathy
Because the thalamus plays a key role in the regulation of sleep and wakefulness, damage to this area has been implicated in disorders related to consciousness, as well as in the comatose state of individuals.
Because the thalamus is important in generating the normal thalamocortical rhythms of sleep, sleep disorders can result from damage such as insomnia.
Language deficits due to thalamic damage, known as thalamic aphasia, can result in difficulties with lexical semantics. It can also lead to verbal paraphasia, which is a speech disturbance, presenting as jumbled words or nonsensical speech.
Disorders of the thalamus can also manifest as sensory loss, movement disorders, pain syndromes and visual disturbances. Stroke is a common cause of many disorders of the thalamus.
Thalamic pain syndrome can occur when there are disturbances in one of the pathways of the thalamus affecting temperature sensation after a stroke. This can result in tingling or burning pain, as well as discomfort with temperature changes.
Months after suffering a thalamic stroke, this can lead to severe chronic pain. Strokes of the thalamus have also been found to produce symptoms of uncoordinated involuntary body movements because they affect the pathways associated with motor movements.
It has been found that patients with schizophrenia had significantly smaller thalamic volume compared to those without schizophrenia.
Reduced thalamic size was suggested to correlate with worse neuropsychological functioning and specific deficits in language, motor, and executive skills.
This implies that differences in thalamic structure are significantly related to some of the symptoms of schizophrenia. Another study was conducted to investigate thalamic differences in people with autism.
Autistic males were found to have lower and weaker increases in functional connectivity with their thalamus.
