The thalamic nuclei are the clusters of densely packed neuronal cell bodies that comprise the thalamus. The thalamus is an ovoid, paired gray matter structure, found in the center of the brain, just superior to the brainstem.
Each side of the thalamus contains six groups of nuclei;
- Anterior nuclei of thalamus
- Lateral nuclei of thalamus
- Medial nuclei of thalamus
- Intralaminar nuclei of thalamus
- Paraventricular (midline) nuclei of thalamus
- Reticular nucleus of thalamus
The thalamic nuclei relay and modulate information incoming from the periphery to the cerebral cortex. Basically, almost all ascending neural pathways first synapse within a thalamic nucleus, where the information is sorted, integrated, and analysed by the thalami before they are sent further to the cerebral cortex. This fact makes the thalamus a so-called “gateway” to the cerebral cortex for limbic, motor, and all sensory modalities besides olfaction, including vision, hearing, taste, and somatic sensation.
|Functions||Relaying limbic, sensory and motor signals to the cerebral cortex, Regulating consciousness, sleep, alertness|
|Anterior nuclei of thalamus||Anteroventral, anterodorsal, anteromedial nuclei|
|Lateral nuclei of thalamus||Medial and lateral geniculate nuclei, dorsal group (lateral dorsal, lateral posterior, pulvinar), ventral group (ventral anterior, ventral lateral, ventral posterior)|
|Medial nuclei of thalamus||Dorsomedial nucleus (parvocellular, magnocellular parts)|
|Intralaminar nuclei of thalamus||
Anterior group: Central medial, paracentral, central lateral nuclei
Posterior group: Centromedian, parafascicular nuclei
|Periventricular nuclei of thalamus||Rhomboid, reuniens, parataenial nuclei|
- Gross anatomy
- Anterior nuclei of thalamus
- Lateral thalamic nuclei
- Medial thalamic nuclei
- Paraventricular nuclei
- Reticular nucleus
- Connections and functions
- Clinical aspects
The thalamus is an egg shaped, bilateral, symmetrical mass of grey matter which forms a major part of the diencephalon, along with the hypothalamus and epithalamus. There are two thalami in total, one situated on each side of the third ventricle. They are connected by a band of grey matter called the interthalamic adhesion.
Each side of the thalamus is divided into three main areas that each contain a collection of nuclei which are explained in the sections to follow;
- Anterior part, that contains the anterior nuclei of thalamus.
- Lateral part, that contains the lateral nuclei of thalamus.
- Medial part, that contains the medial nuclei of thalamus.
These groups of nuclei are separated by a Y-shaped vertical sheet of white matter called the internal medullary lamina. The internal medullary lamina itself contains a group of nuclei called the intralaminar nuclei. In addition, the thalamus contains an area of thin, paraventricular nuclei of thalamus, scattered in the periventricular gray matter that separates the medial part of the thalamus from the ependyma of the third ventricle. Lastly, the thalamus contains another nucleus called the thalamic reticular nucleus that envelopes each lateral aspect of the thalamus.
In addition to the just mentioned grouping by their anatomic location, the thalamic nuclei can also be classified according to their function into the:
- Relay nuclei, which receive information (e.g. visual, acoustic, primary somatosensory, motor input) that is very specific and well-defined, and project this information to very specific, functionally discrete cortical areas. These nuclei include the ventral anterior (VA), ventral lateral (VL), ventral posterior (VP), medial geniculate and lateral geniculate nuclei.
- Association nuclei, which receive most of their information from the cerebral cortex itself, and then project back to the “association areas” in the cortex to regulate the integration and interpretation of the sensory information. These nuclei include the anterior nucleus, pulvinar, and dorsomedial nucleus.
- Nonspecific nuclei, which show broad and diffuse projections through the cerebral cortex, and thus may be involved in general functions such as consciousness and attention. These nuclei include the reticular nucleus, and intralaminar and midline nuclei.
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Anterior nuclei of thalamus
Anterior thalamic nuclei, also called the anterior nuclear group, is a collection of neuronal cell bodies on the anterior part of the thalamus, situated between the limbs of the Y-shaped internal medullary lamina. There are three subdivisions of the anterior part of the thalamus: anteroventral, anteromedial and anterodorsal nuclei.
These nuclei receive information from the limbic system, thus having important functions and influence upon emotional states, such as attention and alertness and memory acquisition. Specifically, the anterior thalamic nuclei receive afferent fibers input from the mammillary bodies via the mammillothalamic tract.
- The medial mammillary nucleus, that projects onto the ipsilateral anteroventral and anteromedial thalamic nuclei.
- The lateral mammillary nucleus, that projects bilaterally onto the anterodorsal nucleus.
After acquiring the information from the mammillary bodies, the anterior thalamic nuclei in turn project efferent fibers to the cerebral cortex. These pathways are reciprocal (bidirectional) and are projected to the following cortical structures:
- Cingulate gyrus
- Anterior limbic area
- Parahippocampal gyrus
- Dorsolateral prefrontal and posterior areas of neocortex
Lateral thalamic nuclei
Lateral thalamic nuclei, also referred to as lateral nuclear group, are a collection of neuronal cell bodies found laterally to the internal medullary lamina. These are the largest division of the thalamic nuclei, divided into dorsal and ventral tiers of nuclei.
- The ventral tier nuclei are the ventral anterior (VA), ventral lateral (VL) and ventral posterior (VP) nuclei.
- The dorsal tier nuclei are the medial geniculate nucleus (MGN), lateral geniculate nucleus (LGN), lateral dorsal (LD) nucleus, lateral posterior (LP) nucleus and pulvinar.
Ventral anterior nucleus
The ventral anterior (VA) nucleus lies on the anterior pole of the ventral group of nuclei. It is bordered by the reticular nucleus anteriorly, the ventral lateral nucleus posteriorly, and the internal and external medullary laminae bilaterally. It consists of two parts; a principal part and a magnocellular part.
The ventral anterior nucleus is located on the path between the basal ganglia and the motor areas of the premotor cortex, relaying information from both. Specifically, the afferent fibers to the ventral anterior nucleus stem from the following:
- Internal segment of the globus pallidus, that projects onto the principal part of the ventral anterior nucleus.
- Substantia nigra, that projects onto the magnocellular part of the ventral anterior nucleus.
In turn, the ventral anterior nucleus sends efferent fibers to Brodmann area 6 and area 8 of the premotor cortex. These fibers are reciprocal. Additional projections from the ventral anterior nucleus are directed to the intralaminar thalamic nuclei, the frontal lobe and anterior parietal complex.
Through these connections, the ventral anterior nucleus modulates and relays the signals from the basal ganglia to the premotor cortex, thus playing a role in planning and initiating movements. It is also involved in transmitting the cortical “recruiting response”.
Ventral lateral nucleus
This subgroup has two major divisions; pars oralis, located anteriorly, and pars caudalis, located posteriorly.
The afferent fibers to the ventral lateral nucleus stem from the following:
- Ipsilateral internal globus pallidus projects onto the pars oralis via the thalamic fasciculus.
- Contralateral deep cerebellar nuclei project onto the pars caudalis.
- Additional projections to both pars oralis and pars caudalis stem from the spinothalamic tract, vestibular nuclei, and precentral motor cortex.
The ventral lateral nucleus projects efferent fibers to the following areas:
- Pars oralis: Supplementary motor cortex, lateral premotor cortex
- Pars caudalis: Area 4 of primary motor cortex
The ventral lateral nucleus is active during both passive and active movements of the contralateral part of the body. It also has a role in relaying motor feedback from the cerebellum to the cerebral cortex.
Ventral posterior nucleus
The ventral posterior nucleus is the main relay nucleus for the somatosensory pathways. It is subdivided into two parts: ventral posteromedial (or VPM) and ventral posterolateral (or VPL). This nucleus establishes several neuronal pathways with different regions within the brain.
The afferent fibers to the ventral posterior nucleus stem from the following:
- Medial lemniscal and spinothalamic pathways project their fibers onto the ventral posterolateral (VPL) nucleus
- Trigeminothalamic pathway projects onto the ventral posteromedial (VPM) nucleus
The trigeminothalamic pathway (ventral trigeminal tract) carries temperature, conscious proprioception, crude touch and pain sensations from the face, head and neck. In turn, the medial lemniscal and spinothalamic pathways carry sensory information from the skin and joints. Neurons receiving the inputs are organised into curved lamellae, with one lamella usually representing one region of the body. Within one lamella, different regions receive inputs distinguished by the level of sensorial “depth”. For instance, when referring to the hand, the anterodorsal part receives information from deep stimuli such as joint, tendon and muscle movement. The ventral part receives more superficial stimuli from tapping, while the neurons located between these two parts receive the most superficial inputs, such as cutaneous touch.
The ventral posterior nucleus projects efferent fibers to the following areas:
- Brodmann areas 1 and 3b of the primary somatosensory cortex
- Secondary somatosensory cortex
The efferent fibers running to the somatosensory cortex pass through the posterior limb of the internal capsule and corona radiata. This illustrates their involvement in relaying sensory sensations for conscious identification.
Medial geniculate nucleus
The medial geniculate nucleus is found within the medial geniculate body, an ovoid projection of the ventrolateral surface of the thalamus, right underneath the pulvinar. It should be noted that some authors differ on opinion whether the medial and lateral geniculate bodies are part of the thalamus, or constitute a part of the dorsal diencephalon.
The medial geniculate nucleus represents the thalamic relay station of the auditory pathway, between the inferior colliculus and the auditory cortex, which receives various inputs from other auditory nuclei and, in turn, projects its fibers via the acoustic radiation to auditory centers of the temporal cortex.
The afferent fibers to the medial geniculate nucleus stem from the following:
- Central nucleus of the ipsilateral and contralateral inferior colliculi
- Pericentral nucleus of the inferior colliculus
- Brainstem nuclei of auditory pathways
- Reticular nucleus of the thalamus
- Inferior and superior colliculi
The medial geniculate nucleus projects efferent fibers to the following areas:
- Primary auditory cortex
- Auditory areas around the primary auditory cortex
- Insular and opercular cortical fields
Medial geniculate nucleus seems to be responsible for tonotopy (organization of sound frequencies along an axis or structure) and detection of sound duration and intensity. Sound tones trigger specific cortical patterns which in turn project back to the medial geniculate body. This reciprocal relationship provides a feedback control system of ascending auditory input.
Lateral geniculate nucleus
The lateral geniculate nucleus is located within the lateral geniculate body, an ovoid projection of the posterior aspect of the thalamus. The lateral geniculate nucleus represents the thalamic relay station of the visual pathway.
The nucleus consists of six to eight laminae, which are separated by interlaminar zones. The laminae receive the fibers from the optic tract that carries visual information from the ganglionic cell layer of the retina of both eyes. In turn, the lateral geniculate nucleus projects to the visual cortex around the calcarine fissure via the geniculocalcarine tract.
The afferent fibers to the lateral geniculate nucleus stem from the following:
- Contralateral nasal hemiretina project via the optic tract to laminae 1, 4 and 6
- Ipsilateral temporal hemiretina project via the optic tract to laminae 2, 3 and 5
The lateral geniculate nucleus projects efferent fibers to the following areas:
- Area 17 of primary visual cortex
- Extrastriate visual areas in the occipital lobe
Lateral dorsal nucleus
The lateral dorsal nucleus is the most anterior of the dorsal group of nuclei, situated in the splitting of the internal medullary lamina.
This nucleus receives afferent fibers from the pretectum and the superior colliculus.
Additional reciprocal connections of the lateral dorsal nucleus are with the cingulate gyrus, parahippocampal gyrus, parietal cortex and presubiculum of the hippocampal formation.
Lateral posterior nucleus
The lateral posterior nucleus lies dorsally to the ventral posterior nucleus.
It receives afferent fibers from the superior colliculus and has reciprocal connections to the superior parietal lobe.
Additional connections of the lateral posterior nucleus are the inferior parietal lobe, cingulate gyrus, and medial parahippocampal cortex.
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The pulvinar is an expansion found on the posterior aspect of the thalamus. The pulvinar is subdivided into the medial, lateral and inferior pulvinar nuclei.
The pulvinar receives inputs from the nuclei of the visual pathway and projects efferent fibers to the visual cortex, as well as to association areas of the parietal and temporal cortices. The exact function of the pulvinar is unknown, but due to its connections, it is suggested that it takes part in modulating higher functions (perception, cognition, memory) in regards to vision.
Specifically, the afferent fibers to the pulvinar stem from the following:
- Superior colliculus
The pulvinar nuclei each project their own efferent fibers to the following areas:
- The medial pulvinar nucleus projects onto the parietotemporal complex, inferior parietal cortex, posterior cingulate gyrus and the temporal lobe.
- The lateral pulvinar nucleus projects onto the visual areas in the occipital and posterior temporal lobes, occipital cortex, temporal association cortex, parietal cortex and rostromedial prefrontal cortex.
- The inferior pulvinar nucleus projects onto the visual areas in posterior temporal lobes and the striate and extrastriate cortex in occipital lobe.
Medial thalamic nuclei
The medial thalamic nuclei are clusters of neuronal cell bodies located medial to the internal medullary lamina and lateral to the midline nuclei of the thalamus. This group contains one component called the dorsomedial nucleus.
The dorsomedial nucleus, also called the medial dorsal (MD) nucleus, is subdivided into two parts: anteromedial magnocellular and posterolateral parvocellular part. The anteromedial magnocellular part communicates with several regions of the brain and projects its fibers mainly to the frontal cortex. Some fibers are purely a one-way communication (afferent/efferent), while others are two way (reciprocal).
The anteromedial magnocellular division establishes the following connections:
- It receives olfactory input (afferent fibers) from the piriform cortex and the adjacent olfactory areas, the ventral pallidum and the amygdala.
- It establishes reciprocal connections with the anterior and medial prefrontal cortex, mainly to the olfactory areas on the orbital surface of the frontal lobe; the ventromedial cingulate cortex and the inferior parietal cortex and anterior insula.
The posterolateral parvocellular division establishes the following connections:
- It establishes reciprocal connections with the dorsolateral and dorsomedial prefrontal cortex, the anterior cingulate gyrus and the supplementary motor area.
- It projects efferent fibers to the posterior parietal cortex.
Through all these connections, the dorsomedial nucleus is responsible for integrating sensory, motor, visceral and olfactory information and subsequently relating it to the individual’s emotional state. Its overall functions are similar to those of the prefrontal cortex.
These are collections of nerve cells within the internal medullary lamina, the “Y” shaped white matter sheet discussed before. There are two groups of nuclei involved: the anterior (rostral) and posterior (caudal).
The intralaminar nuclei play a role in the activation of the cortex from the brainstem reticular formation. It is also involved in sensory-motor integration. The posterior intralaminar nuclei also seem to be important in speaking and motivation.
Anterior (rostral) group
The anterior group consists of subdivisions, namely the central medial, paracentral and central lateral nuclei.
These nuclei have widespread reciprocal connections with the following cortical areas of the brain:
- The central lateral nucleus connects mainly to parietal and temporal association areas.
- The paracentral nucleus connects to the occipitotemporal and prefrontal cortex.
- The central medial nucleus connects to the orbitofrontal and prefrontal cortex and to the cortex on the medial surface.
These nuclei also establish purely one-way connections with the following:
- All anterior group nuclei receive afferent fibers from the brainstem reticular formation, superior colliculus, and pretectal nuclei
- Central lateral nucleus receives afferent fibers from the spinothalamic tract.
- All anterior group nuclei project efferent fibers to the striatum.
- Central medial nucleus projects efferent fibers to the orbitofrontal and prefrontal cortex and to the cortex on the medial surface.
Posterior (caudal) group
The posterior group is subdivided into centromedian and parafascicular nuclei. The centromedian nucleus is related to the globus pallidus, deep cerebellar nuclei and motor cortex. The smaller parafascicular nucleus lies more medially within the lamina.
The posterior group of the intralaminar nuclei receives afferent fibers from the following:
- Internal segment of globus pallidus
- Pars reticulata of substantia nigra
- Deep cerebellar nuclei
- Pedunculopontine nucleus of midbrain
- Spinothalamic tract
The posterior group projects efferent fibers to the striatum, and establishes reciprocal connections with the motor, premotor and supplementary motor areas.
The paraventricular nuclei, also commonly referred to as the midline nuclei, are clusters of neuronal cell bodies that are found in the periventricular gray matter underneath the ependyma of the third ventricle and within the interthalamic adhesion. The entire group is located ventrally to the central medial nucleus. The midline nuclei can be subdivided into three nuclei; rhomboid, reuniens and parataenial nuclei.
The midline nuclei receive afferent fibres from the following:
- Periaqueductal grey matter of the midbrain
- Spinothalamic tract
- Medullary reticular formation
- Pontine reticular formation
Efferent fibers from the midline nuclei pass to the following areas:
- Hippocampal formation
- Nucleus accumbens
The midline nuclei also establish reciprocal connections with the cingulate gyrus and possibly the orbitofrontal cortex.
As illustrated by their connections, the midline nuclei are important in the limbic system. In addition, they may also play a role in memory and arousal.
The reticular nucleus is a narrow, shell-shaped nucleus that wraps around the lateral margin of the thalamus and extends from the rostral pole to the pulvinar and the lateral geniculate nucleus. It is found adjacent to the lateral thalamic nuclei, separated from them by the external medullary lamina.
The reticular nucleus is the only nucleus that does not project to the cortex. This nucleus receives inputs from other thalamic nuclei and the cerebral cortex and sends out projections that circle back to the thalamus.
Thus, it is believed that this nucleus is involved in gating and modulating information that is relayed through the thalamus, thereby regulating the activity of the thalamus itself. In addition, it is believed that the reticular nucleus receives input from the reticular formation and in turn projects to other thalamic nuclei, regulating the flow of information through these to the cortex. This would imply that the reticular nucleus plays a role in regulating alertness and attention.
The reticular nucleus receives afferent fibers from the following:
- Nucleus cuneiformis of the midbrain
- Frontal cortex
- Temporal cortex
- Occipital cortex
The efferent fibers that pass from the reticular nucleus are directed to the following:
- Body of the thalamus
- Contralateral dorsal thalamus
Connections and functions
The thalamus has multiple functions. Generally, it acts as a relay station, exchanging and modulating information between the body and the brain. In addition, the thalamus plays a role in regulating consciousness and alertness.
As we’ve seen throughout this article, the thalamic nuclei have many connections. Each nucleus receives unique afferent pathways as inputs that mainly stem from subcortical structures including the diencephalon, limbic structures and the basal ganglia. These signals are first analyzed, sorted and integrated by the thalamic nuclei, and only then projected further as efferent pathways to the cerebral cortex. Thereby, the thalamic nuclei act as a hub that filter and modulate all motor, limbic and most sensory information incoming from the subcortical structures to the cerebral cortex. This fact makes the thalamus the defining factor of how the subcortical structures communicate with and influence the cerebral cortex.
This is particularly the case with ascending sensory pathways that pass from the periphery to the cerebral cortex. Namely, every sensory modality besides olfaction has a thalamic nucleus that receives, analyses, and selects the information to be sent to an associated cortical area. To recap the previous sections, here is a list of the main thalamic nuclei that process the incoming sensory information:
- The lateral geniculate nucleus receives visual sensory information from the retina and relays it to the visual cortex of the occipital lobe.
- The medial geniculate nucleus receives auditory sensory information from the inferior colliculus and relays it to the primary auditory cortex of the temporal lobe.
- The ventral posterolateral nucleus receives information for pain, temperature and crude touch via the spinothalamic tract
- The ventral posteromedial nucleus receives sensory information from the face via the trigeminal nerve.
Some thalamic nuclei are also involved in processing information necessary for motor control, by relaying inputs from the cerebellum and basal ganglia to the cerebral cortex.
- The ventral lateral nucleus is involved in relaying cerebellar signals to the primary motor area and plays a role in fine motor control.
- The ventral anterior nucleus is involved in relaying the basal ganglia signals to the premotor cortex and plays a role in planning and initiating movements.
Finally, the thalamus plays a role in regulating consciousness, sleep and alertness. This is due to the fact that the thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are thought to be involved with consciousness. It is also because certain thalamic nuclei have connections with the brainstem and basal forebrain such as the reticular nucleus and intralaminar nuclei.
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Thalamic lesions can paradoxically present with different non-sensory clinical symptoms despite being a central sensory hub, thus complicating diagnostic problems.
Dejerine-Roussy syndrome (thalamic pain syndrome) is caused by a stroke (ischemic or hemorrhagic). This condition is most commonly preceded by numbness in the affected side. Subsequently, numbness is replaced by burning, tingling sensations, or even allodynia (pain from a stimulus that normally would not cause pain), widely varying in degree of severity across patients.
The reticular nucleus of the thalamus plays an important role in the rhythmic activation of the cerebral cortex, which some studies consider it being the cause of idiopathic generalized epilepsy.
Korsakoff syndrome is a syndrome seen in alcoholic patients due to the damage (by thiamine B1 deficiency) of their mamillary bodies which can extend into the thalamus through the mammillothalamic fasciculus.
Fatal familial insomnia is a hereditary condition where patients gradually lose their ability to sleep which invariably leads to death. Patients can have associated psychiatric symptoms such as panic attacks, paranoia, phobias and even hallucinations. Rapid weight loss follows with dementia and mutism until death occurs. It is caused by a mutation in the PRNP gene located on the 20th chromosome.
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