Thalamus: One of the Essential Parts of the Human Brain

The diencephalon’s thalamus, which is primarily composed of gray matter, plays a number of vital roles in human physiology. The thalamus is made up of several nuclei, each of which has a distinct function. These functions include the regulation of consciousness and alertness as well as the transmission of sensory and motor impulses. There are very few clinical diseases that are associated with dysfunction and damage to the thalamus. While the majority of these illnesses are uncommon, MRI can reveal significant structural alterations in some of the more prevalent ones. Though there have been limits with thalamic surgical procedures in the past, this discipline is rapidly evolving due to enhanced neuroimaging and increased accessibility through the development of microsurgical techniques.

Structure and Functions

Location:

Situated close to the brain’s center, the thalamus is a paired gray matter component of the diencephalon. Each thalamus is connected to the other by the interthalamic adhesion, which enables nerve fiber connections to the cerebral cortex in all directions. It is located above the midbrain or mesencephalon. The dorsal surface is a portion of the floor of the lateral ventricle’s body, whereas the thalamus constitutes the third ventricle’s upper and lateral walls. The thalamus lateralizes with the internal capsule’s posterior arm. It borders the head of the caudate and the ventral nucleus, which houses the hypothalamus and subthalamus, anterolaterally.

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Anatomy:

The diencephalon gives rise to two symmetrical components that make up the thalamus. The thalamus appears ovoid because of its elongation on the anteroposterior axis in both halves. The anterior end of it is the narrowest, and the posterior end is the widest. The internal medullary lamina, a “Y” shaped white matter structure, divides the grey matter that makes up the thalami.

Each thalamus is essentially separated into the anterior, medial, and lateral thalamus due to the placement of the internal medullary lamina. While the medial and lateral parts are located on each side of the main stem of the “Y,” the anterior component is situated between the short limbs of the internal medullary lamina. The interthalamic adhesion facilitates communication between the left and right thalamus.

Apart from two protuberances on the posteroventral surface, the thalamus is somewhat ordinary overall. These are the lateral and medial geniculate bodies, which interpret sensory information from the visual and auditory modalities, respectively. Moreover, two layers of white matter encircle each thalami. The external medullary lamina, which divides the thalamic reticular nucleus and the subthalamus from the lateral and ventral thalamus, covers it laterally. Dorsally, it is covered by a layer called the stratum zonale.

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The many nuclei that make up the thalamus are in charge of relaying the various sensory information. Excitatory and inhibitory neurons make up the majority of these nuclei. The cerebral cortex receives specific information from the thalamocortical neurons through nerve fibers known as thalamocortical radiations, which carry sensory or motor information from the rest of the body. The mammillothalamic tract connects the thalamus to the hippocampal, mammillary, and fornix regions. Learning and episodic memory are processes that the thalamus is involved in because of the connections between its anterior nuclei and limbic system elements. The control of alertness and sleep is another function of the thalamus.

Functions:

• Bringing information into your brain by nerve signals from all of your senses—taste, touch, hearing, and sight—aside from the smell. A thalamic nucleus is responsible for receiving, processing, and transmitting information linked to each sensory function to the corresponding area of your cerebral cortex.
• Transferring motor (movement) data: Similar to sensory data, the thalamus is where all motor routes travel.
• Making attention a priority: The thalamus assists in selecting from among the copious amounts of information it receives.
• The function of awareness: Your ability to stay awake and attentive is influenced by the thalamus.
• Function in memory and thinking (cognition). The limbic system, which is involved in memory creation and storage, processing, and regulation of emotions, sexual arousal, and learning, is linked to the thalamus.

Disorders and Implications

Given the thalamus’s pivotal role in the information transfer to and from the cerebral cortex, particular nuclei are important in a range of illness conditions. The thalamus is crucial for controlling arousal and sleep-wake cycles. Thus, deep brain stimulation has been tried as a therapeutic approach in cases of coma and related diseases of consciousness, such as vegetative and minimally conscious states, where thalamic pathology has been linked to the condition.

1. Strokes:

Strokes, also known as cerebrovascular accidents, are a prevalent neurological disease state that can occur in different thalamic areas. They may result in hemorrhagic cerebral blood vessel disruption, ischemic cell death from the insufficient blood supply, or hemorrhagic secondary hemorrhagic bleeding from tissue ischemia. The thalamus is a common site of ischemic (lacunar) and hemorrhagic infarcts, particularly with disease in small blood vessels (microvascular disease) that arises from several chronic conditions (such as diabetes and hypertension).

Dejerine-Roussy syndrome, also known as thalamic pain syndrome, is an uncommon disorder that can develop following a thalamic stroke. It is characterized by tingling and loss of feeling in the body’s contralateral side.

2. Epilepsy:

Abnormal and repetitive discharges in the cortex are the hallmark of Epilepsy. There are numerous varieties of epilepsy, and research has indicated that certain thalamic nuclei may be involved in certain of them. Given its close ties to the limbic and medial temporal regions, deep brain stimulation of the anterior nucleus has been demonstrated in both animal and human experiments to reduce the frequency of seizures in patients with refractory partial epilepsies. It has also been demonstrated that deep brain stimulation of intralaminar nuclei, such as the centromedian nucleus, reduces the frequency of seizures in individuals with medically refractory epilepsies.

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3. Essential Tremors:

A frequent degenerative neurological condition called essential tremor often involves a low-frequency action-induced tremor, usually in the hands and neck. It has been demonstrated that deep brain stimulation of the ventral intermediate thalamic nucleus (VL area) is an efficient surgical treatment for crippling essential tremor.

Damage to the thalamus, mammillary body, or mammillothalamic fasciculus is the cause of alcoholic Korsakoff syndrome. The thalamus degenerates in fatal familial insomnia, a hereditary prion disease that eventually results in total insomnia and the patient’s death. The disease causes progressive loss of sleep over time. On the other hand, thalamic injury may cause coma.