Thalamus
Thalamus is a large ovoid mass of grey matter, situated bilaterally in the diencephalon. It is a paired symmetrical structure in the brain, perched on top of the brainstem. It is the largest component of the diencephalon (both parts of the thalamus form 80% of it). Both parts of this structure are each about the size of a walnut.
Location
- It is located near the center of the brain.
- Obliquely (about 30°) and symmetrically on each side of the 3rd ventricle.
Thalamic Nuclei
The thalamus can be divided into nuclei that project diffusely to wide regions of the neocortex and nuclei that project to specific discrete portions of the neocortex and limbic system. The thalamic nuclei are classified by 2 methods;
- Anatomical classification
- Physiological classification
Anatomical Classification of Thalamic Nuclei
The thalamus on each side is divided into 5 main nuclear groups by ‘Y’ shaped internal medullary lamina;
- Midline nuclei
- Intralaminar nuclei
- Medial mass of nuclei
- Lateral mass of nuclei
- Posterior group of nuclei
Midline Nuclei
A group of small nuclei, situated on the medial surface of the thalamus near the midline.
Intralaminar Nuclei
They are smaller nuclei present in the medullary septum of the thalamus.
Medial Mass of Nuclei
They are situated medial to the septum and it comprises of 2 nuclei;
- Anterior nucleus
- Dorsomedial nucleus
Lateral Mass of Nuclei
These group of nuclei are situated lateral to the septum and are divided into 2 subgroups;
- Dorsal group of lateral mass with 2 nuclei
- Dorsolateral nucleus
- Posterolateral nucleus
- Ventral group of lateral mass with 3 nuclei
- Ventral anterior nucleus
- Ventral lateral nucleus
- Ventral posterior nucleus – ventral posterolateral nucleus and ventral posteromedial nucleus
Posterior Group of Nuclei
They are the continuation of the lateral mass of nuclei. It has 2 subgroups;
- Pulvinar
- Metathalamus which consists of 2 structures;
- Medial geniculate body; and
- Lateral geniculate body
Thalamic Reticular Nucleus
The thalamus also includes the thalamic reticular nucleus.
- It is a thin layer of neurons covering the lateral aspect of the thalamus.
- It is separated from the thalamus by external medullary lamina.
- This nucleus contains inhibitory neurons which release GABA.
- Its axons do not project to the cortex, rather, it serves as thalamic interneurons that modulate the responses of other thalamic neurons to input coming from the cortex.
- It receives information from the reticular formation, cerebral cortex and other thalamic structures and sends inhibitory signals to the other thalamic nuclei.
Physiological Classification Of Thalamic Nuclei
On the basis of functions and their projections, thalamic nuclei are classified into 5 groups. This type of classification is called Bondok classification. The 5 groups are;
- Specific sensory relay nuclei
- Specific motor nuclei
- Association or less specific nuclei
- Non-specific nuclei
- Limbic system nuclei
Specific Sensory Relay Nuclei
Relay to the auditory, visual and somatosensory cortices.
- Nuclei:
- Ventral posterior nucleus
- Medial geniculate body
- Lateral geniculate body
- Roles:
- Relay specific information from a particular tract or modality. This is not just sensory information, relay nuclei are part of several important modulatory loops in the CNS.
- This is not simple "passing on" of the signal, relay nuclei engage in some complex condensing and processing of the incoming raw information.
Specific Motor Nuclei
Relay from basal ganglia and cerebellum to the motor cortex.
- Nuclei:
- Ventral anterior nucleus
- Ventral lateral nucleus
- Roles:
- Receive signals controlling motor activities from the cerebellum and corpus striatum, and
- Send these signals to motor areas in the cerebral cortex to complete the feedback system of motor control mechanism.
Association or Less Specific Nuclei
Project to association areas.
- Nuclei:
- Dorsolateral nucleus
- Posterolateral nucleus
- Pulvinar
- Roles:
- Send information to association areas of the cerebral cortex (prefrontal and parietal-occipital-temporal cortices).
- The association cortex is involved in higher cognitive function.
Non-Specific Nuclei
Relay to diffused regions of the neocortex.
- Nuclei:
- Midline nuclei
- Intralaminar nuclei
- Reticular nucleus
- Roles:
- Send signals to diffuse areas of the cerebral cortex.
- Produce general changes in cortical function.
Limbic System Nuclei
- Nuclei:
- Anterior nucleus
- Dorsolateral nucleus
- Roles:
- Project to the limbic cortex.
Function Roles
The thalamus has 4 basic functional roles;
- Sensory: All sensory information (except olfaction) is relayed to the cortex via the thalamus.
- Motor: Motor system outputs from the basal ganglia and cerebellum are relayed by the thalamus.
- Emotion/memory: The thalamus is part of the Papez circuit and helps control some emotional and memory information going to the limbic cortex (cingulate gyrus).
- Vegetative: The thalamus has some intrinsic nuclei associated with alertness and arousal. Can be associated with disorders of consciousness.
Cortical Feedback
The thalamus doesn’t just send information to the cortex, it also receives feedback from the cortex.
- This signal helps to regulate what is coming to the cortex.
- Cortical input is a feedback inhibition loop, letting the thalamus know that information has been received and inhibiting further relaying of the information.
Thalamic Radiation/Thalamocortical Pathways
These are a collection of reciprocal myelinated (thalamocortical and corticothalamic) fibers that connect the thalamus and cerebral cortex.
- Fibers of thalamic radiation are divided into four groups, which are called thalamic peduncles/thalamic stalks
- Anterior (frontal) thalamic peduncle or radiation
- Superior (centroparietal) thalamic peduncle or radiation
- Posterior (occipital) thalamic peduncle or radiation
- Inferior (Temporal) thalamic peduncle or radiation
Anterior (Frontal) Thalamic Peduncle or Radiation
Connects the frontal lobe of the cerebral cortex with medial and lateral thalamic nuclei. It contains mostly motor nerve fibers.
Superior (Centroparietal) Thalamic Peduncle or Radiation
Connects the postcentral gyrus (somesthetic area) of the parietal lobe and adjacent area in the frontal cortex with the lateral mass of thalamic nuclei. It contains mainly the sensory fibers.
Posterior (Occipital) Thalamic Peduncle or Radiation
Connects the occipital lobe of the cerebral cortex with the pulvinar and lateral geniculate body. It contains the nerve fibers concerned with vision.
Inferior (Temporal) Thalamic Peduncle or Radiation
Connect the temporal lobe and insula with the pulvinar and medial geniculate body. It contains the nerve fibers concerned with hearing.
Functions of Thalamus
The thalamus is concerned with somatic functions. It plays little role in visceral functions.
- The thalamus serves as a relay center for sensations (except smell).
- Impulses of almost all the sensations reach the thalamic nuclei, particularly in the ventral posterolateral nucleus.
- They reach the cerebral cortex via the thalamocortical fibers.
- The thalamus forms the major center for processing the sensory information.
- All the peripheral sensory impulses reaching the thalamus are integrated and modified before being sent to specific areas of the cerebral cortex.
- The thalamus also assists to determine the quality of sensations, i.e. the affective nature of sensations.
- It serves as the perception center for pain and sexual sensations.
- The thalamus integrates motor signals from the cerebellum and basal ganglia for onward transmission to the motor cortex.
- Because of its connections with nuclei of reticular formation, the thalamus plays an important role in arousal and alertness reactions.
- The thalamus forms the center for many reflex activities, since the sensory fibers relay here.
- The thalamus involves in memory formation.
Clinical Correlates
- Thalamic stroke/syndrome – lesion:
- Loss of consciousness/coma
- Attention/arousal problems
- Widespread disruption of cortical function
- Severe cognitive deficits
- Hypersensitive to pain
- Loss of Sensations (anesthesia)
- Asterognosis: Inability to recognize a known object by touch with closed eyes.
- Anosognosia: Lack of awareness or denial of the existence of a neurological defect or general illness or any disability.
- Ataxia: Incoordination of voluntary movement due to loss of kinaesthetic sensation.
- Spontaneous Pain and Thalamic Over-reaction: Hyperalgesia occurs and sometimes, the patient feels pain even in the absence of a pain stimulus. It becomes worst in conditions such as emotional disturbance and exposure to cold or heat. Pain is due to overactivity of the medial mass of nuclei of the thalamus, which escapes the lesion.
- Thalamic Phantom Limb: Unable to locate the position of a limb with closed eyes.
- Involuntary Movements (athetosis, chorea, intention tremor) occurs.
- Thalamic Hand or Athetoid Hand: Abnormal attitude of hand in thalamic lesion. It is characterized by moderate flexion at the wrist and hyperextension of all fingers.
