Thalamus: Anatomy, Function and Medical Significance

The thalamus, also known as thalami, is one of two massive ovoid organs that make up the majority of the third ventricle of the brain's lateral walls. The thalamus is responsible for translating neural impulses from various receptors to the cerebral cortex. 

• While the thalamus is best known as a sensory relay in the visual, auditory, somatosensory, and gustatory systems.
• It also plays important roles in motor activity, emotion, memory, arousal, and other sensorimotor association functions. 
• The thalamus is an egg-shaped structure in the middle of the brain. 
• It acts as a relay station for all sensory data flowing from the body to the brain about hearing, taste, sight, and touch. 

Read More: Thermoregulation

Key Terms: Brain, Pituitary gland, Thalamus, Hormone, Forebrain, Nuclei, Cell.

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Where is Thalamus Located?

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The human brain is made up of ventricles, or fluid-filled spaces, and the thalamus surrounds the third ventricle. 

• It is a division of the human brain known as the diencephalon.
• It is also the largest structure of diencephalon division during embryonic development.

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Development of Thalamus

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The thalamic complex is made up of three parts: the perithalamus (or prethalamus, previously known as ventral thalamus), the mid-diencephalic organiser (which later forms the zona limitans intrathalamica (ZLI), and the thalamus (dorsal thalamus).

1. Early Development of Brain

The anlage of the prethalamus and thalamus is induced within the neural tube following neurulation. 

• Data from various vertebrate model organisms support a model in which the interaction of two transcription factors, Fez and Otx, is crucial. 
• Otx1 and Otx2, which border the Fez expression domain posteriorly, are necessary for healthy thalamic development.

2. Progenitor domain formation

Early in thalamic development, two progenitor domains form, one caudal and one rostral. The caudal domain generates all glutamatergic neurons in the adult thalamus, whereas the rostral domain generates all GABAergic neurons in the adult thalamus.

3. The development of the mid-diencephalic organizer

The mid-diencephalic organizer (MDO, also known as the ZLI organiser) is induced within the thalamic anlage at the interface between the expression domains of Fez and Otx. 

• The MDO is the thalamic's primary signaling organizer. 
• The absence of the organizer results in the absence of the thalamus.

4. Thalamus maturation and parcellation

Following its activation, the MDO begins to orchestrate the development of the thalamic anlage through the release of signaling molecules such as SHH.

• SHH exposure causes thalamic neurons to differentiate. 
• SHH signaling from the MDO causes a posterior-to-anterior wave of expression of the proneural gene Neurogenin1 in the major (caudal) part of the thalamus and Ascl1 (formerly Mash1) in the remaining narrow stripe of rostral thalamic cells immediately adjacent to the MDO, as well as in the prethalamus.

Read More: Conduction of Nerve Impulse

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Anatomy of Thalamus

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The thalamus is a paired structure of grey matter located in the forebrain, superior to the midbrain, near the brain's centre, with nerve fibres projecting out in all directions to the cerebral cortex.

• The thalamic medial surface forms the upper part of the third ventricle's lateral wall and is connected to the corresponding surface of the opposite thalamus by a flattened grey band known as the interthalamic adhesion.
• The lateral thalamus, or "neothalamus," is the thalamus' most recent evolutionary division and is made up of the lateral nuclei, the pulvinar, and the medial and lateral geniculate nuclei.
• The thalamus contains white matter areas such as the stratum zonale, which covers the dorsal surface, and the external and internal medullary laminae.
• The lateral surface is covered by the external lamina, and the nuclei are divided into anterior, medial, and lateral groups by the internal lamina.

Read More: Pineal Gland

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Functions of Thalamus

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A few mentionable functions of the thalamus are–

• Sensory information is relayed: Taking in information in the form of nerve signals from all of the senses (except smell) and storing it in the brain. Each sensory function has its own thalamic nucleus, which receives, processes, and transmits information to the appropriate area of cerebral cortex.
• Motor (movement) information is relayed: Motor pathways, like sensory information, all pass through thalamus.
• Setting priorities: Thalamus helps to choose what to focus on from the vast amount of information one receive.
• Function in consciousness: Thalamus is responsible for keeping one awake and alert.
• Thinking (cognition) and memory: Thalamus is linked to structures in limbic system, which is involved in emotion processing and regulation, memory formation and storage, sexual arousal, and learning.
• Thalamus is also involved in perception and sleep and wakefulness.

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Working of Thalamus

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Sensory impulses go from body by nerve fibers to thalamus through brain structures. Each nucleus of thalamus is responsible for processing different sensory or motor impulses received from body and then sending the selected information to the related area of cerebral cortex for interpretation.

This chart lists some of the most well-known nuclei and their functions.

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Medical Significance of Thalamus

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Given the thalamus's central role in the flow of information to and from the cerebral cortex, specific nuclei are important in a variety of disease states. The thalamus is critical for sleep-wake regulation and arousal.

• As a result, thalamic pathology has been linked to coma and other consciousness disorders, such as vegetative and minimally conscious states, where deep brain stimulation has been tested therapeutically.
• The thalamus is involved in some sleep disorders, such as fatal familial insomnia, and plays an important role in generating normal sleep thalamocortical rhythms. Certain general anesthetic agents' mechanisms of action may include thalamic function modulation.

Read More: Central Nervous System

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Conditions that Affect Thalamus

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Certain conditions that affect the Thalamus are listed below:

• Family insomnia: Fatal familial insomnia is a hereditary prion (protein) disease that targets a specific chromosome. People who are affected experience worsening insomnia, panic attacks, paranoia, phobias, hallucinations, and an inability to sleep. This is followed by rapid weight loss, dementia, and a loss of ability to communicate until death.
• Fabry disease and Creutzfeldt-Jakob disease: The pulvinar sign is a feature that helps diagnose these diseases. On an MRI scan, a change in density at the back (posterior) of thalamus appears in the shape of hockey sticks.
• The Korsakoff syndrome: This syndrome, which is caused by alcohol, can harm a structure in brain called the mammillothalamic fasiculus, which extends into thalamus.

Read More: Peripheral Neural System

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Symptoms and Causes of Damaged Thalamus

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The thalamus can be damaged due to several reasons. The common ones are:

• Stroke in Thalamus
• Tumour in Thalamus

The symptoms to detect the damaged Thalamus include:

• Memory lapses (amnesia).
• Lack of interest or enthusiasm (apathy).
• Loss of ability to understand language or speak (aphasia).
• Attention problems, loss of alertness
• Processing sensory information is difficult.
• Movement is hampered.
• Sleepiness.
• Chronic discomfort.

Read More: Neuroscience

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Effects of a Damaged Thalamus

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Thalamus serves as a central relay station for sensory and motor information. This information is then sent to other parts of brain by thalamus. As a result, thalamic damage can have a wide range of consequences.

• Unconsciousness, and possibly coma
• Insomnia and fatal familial insomnia are two examples of sleep disorders (inability to sleep, leading to death).
• Aphasia of the thymus (jumbled words, meaningless speech).
• Tremors and other movement disorders
• Syndromes of pain
• Vision loss or light sensitivity.
• The syndrome of thalamic pain (tingling or burning pain).

Read More: Difference between Nervous and Endocrine System

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Connections of the Thalamus

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The thalamus is located at the centre of the diencephalon. As a result, it is surrounded by a number of significant structures.

• Anterior

Its anterior pole forms the posterior wall of the Monro foramen, which allows communication between the lateral and third ventricles.

• Medial

The lateral walls of the aforementioned third ventricle are formed by the medial walls of each thalamus.

• Dorsal

The stria terminalis, the choroid plexus of the third ventricle, and the body of the fornix are all close to the dorsal surface.

• Posterior

The pulvinar is the posterior most aspect of the thalamus. Each pulvinar is superior to the medial and lateral geniculate bodies and lateral to the pineal gland, the Habenular and posterior commissures, and the corpora quadrigemini.

• Inferior

There are two major structures beneath the thalamus. The hypothalamus is located anteroinferiorly, caudal to the hypothalamic sulcus. The cerebral peduncle and the Sylvius cerebral aqueduct are located directly beneath the thalamus.

Read More: Animal Nervous System

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Things to Remember

• The thalamus is made up of various nuclei, each of which serves a specific function, such as relaying sensory and motor signals or regulating consciousness and alertness.
• The thalamus is about 4 cm long and is located deep within the brain, accounting for 80% of the diencephalon. 
• The thalamus is also a component of the lateral ventricle's floor.
• Although it appears to be a single structure, but actually there are two, one in each hemisphere (side) of the brain.
• The volume of the thalamus decreases with age in a linear fashion, which is associated with slower cognitive speed.
• The thalamus is a diencephalon structure that is mostly grey in colour and plays many important roles in human physiology.
• Relaying movement and sensory information to the cerebral cortex is its main duty.
• It is also responsible for controlling sleep, attentiveness, emotion, memory, and wakefulness. Neural impulses from diverse receptors are translated by the thalamus and sent to the cerebral cortex.

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Previous Year Questions

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