thalamus

Thalamus

-Dr. Sachin A. Adukia

Headings

IntroductionAnatomy: Gross and Location (with normal

MRI sections)PhysiologyFunctional AnatomyNeurobiochemistryThalamus infarction and SyndromesTheraputic importance

Introduction

Thalamus (Greek) means “inner chamber” or “meeting place”

Historical interest Earlier – “Optic Thalamus” and chamber of vision- 2nd

century AD; Galen.

Later - Prefix “optic” was dropped - when discovered that sensory modalities other than vision are also processed in the thalamus

Primary Divisions of the Brain

The Diencephalon

Relay between the brainstem & cerebral cortex

Dorsal-posterior structures Epithalamus Thalamus Metathalamus

Medial geniculate body – auditory relay Lateral geniculate body – visual relay

Ventral-anterior structure Hypothalamus

Thalamus Largest component of the diencephalon. Paired symmetrical structure in the brain, perched on top of the brainst

em Near the center of the brain, located obliquely (about 30°) and symmetrically

on each side of the third ventricle Both parts of this structure are each about the size and shape of a waln

ut.

Dimensions: Rostrocaudal (AP) - 30 mm, height - 20 mm, width - 20 mm, estimated 10 million neurons in each half

Location and Relation: Thalamus lies medially in the cerebrum. bounded medially by III venticle laterally by internal capsule and basal ganglia ventrally continuous with subthalamus.

Thalamus- Axial View

Thalamus- Sagittal View

Thalamus- Coronal View


3rd ventricle


Internal capsule


Mediodorsal nucleus Internal capsule

Thalamus- Blood SupplyPosterior communicating artery

paramedian thalamic- subthalamic arteries

inferolateral (thalamogeniculate) arteries

posterior (medial and lateral) choroidal arteries.

All are branches of PCA.

ADDITIONALLY…

The ICA, via its anterior choroidal branch, supplies the lateral thalamic territory.

Thalamus Physiology (Overview)

Primarily - a relay station that modulates and coordinates the function of various systems

Locus for integration, modulation, and intercommunication between various systems

Has important motor, sensory, arousal, memory, behavioral, limbic, and cognitive functions

The largest source of afferent fibers to thalamus is cerebral cortex and cortex is the primary destination for projection fibres from the Thalamus

Characteristically, thalamic connections are reciprocal, that is, the target of the axonal projection of any given thalamic nucleus sends back fibers to that nucleus. Nevertheless, thalamocortical projections are often larger than their corticothalamic counterparts

Functional Anatomy of the Thalamus

It is subdivided into the following major nuclear groups on the basis of their rostrocaudal and mediolateral location within the thalamus:

AnteriorMedialLateralIntralaminar and reticularMidlinePosterior

Thalamic Nuclei

Internal medullary lamina divides the thalamus into medial and lateral groups of nuclei

It houses the intralaminar nuclei, which include the centromedian(CM) and parafascicular (PF) nuclei, among others

lamina splits into two leaves anteriorly and encloses the anterior nucleus

The medial group has only one nucleus: the dorsomedial nucleus (DM)

lateral group - several nuclei divided into small superior or dorsal tier much larger inferior or ventral tier of

nuclei.

Thalamus is traversed by a band of myelinated fibers, the internal medullary lamina, which runs along its rostrocaudal extent

The internal medullary lamina contains intrathalamic fibers connecting the different nuclei of the thalamus with each other

External medullary lamina, forms the lateral boundary of the thalamus medial to the internal capsule. It contains nerve fibers leaving or entering the thalamus on their way to or from the adjacent capsule

Between the external medullary lamina and the internal capsule is the Thalamic reticular nucleus.

Anterior Nuclear Group

consists of two nuclei: principal anterior and anterodorsal.

The anterior group of thalamic nuclei has reciprocal connections with the hypothalamus (mamillary bodies) and the cerebral cortex (cingulate gyrus).

The anterior group also receives significant input from the hippocampal formation of the cerebral cortex

Schematic diagram showing the reciprocal connections among the anterior nucleus of the thalamus, mamillary body, and cingulate gyrus.

MEDIAL NUCLEAR GROUP

The dorsomedial nucleus is reciprocally connected with the prefrontal cortex (areas 9, 10, 11, and 12), via the anterior thalamic peduncle, and with the frontal eye fields (area 8)

Receives inputs from the temporal neocortex (via the inferior thalamic peduncle) amygdaloid nucleus and substantia nigra pars reticulata adjacent thalamic nuclei (lateral and intralaminar groups)

Concerned with affective behavior, decision making and judgment, memory, and the integration of somatic and visceral activity.

Schematic diagram showing the major afferent and efferent connections of the Dorsomedial Nucleus of the thalamus

Clinical Importance of DM nucleus

Bilateral lesions of the dorsomedial nucleus result in a syndrome of lost physical self-activation, manifested by apathy, indifference, and poor motivation.

The reciprocal connections between the prefrontal cortex and the dorsomedial nucleus can be interrupted surgically to relieve severe anxiety states and other psychiatric disorders.

This operation, known as prefrontal lobotomy is rarely practiced nowadays, having been replaced largely by medical treatment

Lateral group of Nuclei (Overview)

Lateral nuclear group is subdivided into two groups: dorsal and

ventral

Dorsal Subgroup from rostral to caudal: lateral dorsal - LD, lateral posterior,

and pulvinar nuclei

Similar to the anterior group nuclei, the LD nucleus receives inputs from the hippocampus (via the fornix) and mamillary bodies and projects to the cingulate gyrus

The Pulvinar - Lateral posterior complex has reciprocal connections

caudally with the lateral geniculate body rostrally with association areas of parietal, temporal and occipital

cortices It also receives inputs from the pretectal area and superior colliculus

The pulvinar is a relay station between subcortical visual centers and their respective association cortices in the temporal, parietal, and occipital lobes.

Has a role in selective visual attention.

Plays a role in speech mechanisms.

Stimulation of pulvinar of dominant hemisphere has produced nominal aphasia

Has role in pain mechanisms.Lesions in the pulvinar : effective in the treatment of intractable pain

Ventral Subgroup(ventral anterior, ventral lateral, and ventral posterior nuclei)

Share the following characteristics:They receive a direct input from the long ascending tracts.They have reciprocal relationships with specific cortical areas.They degenerate on ablation of the specific cortical area to which they project

Connections of Ventral Anterior Nuclei

INPUTSGABAergic inhibitory input

Globus pallidus:- Pallidal fibers terminate in the lateral portion of the ventral anterior nucleus

Substantia nigra pars reticulata:- Nigral afferents terminate in the medial portion

Excitatory Premotor and prefrontal cortices (areas 6 and 8)

RECIPROCAL CONNECTIONS Intralaminar thalamic nuclei

OUTPUTS Major output to:- premotor cortices and to wide areas of the prefrontal

cortex, including the frontal eye fields. A major relay station in the motor pathways from the basal ganglia to the

cerebral cortex (is involved in regulation of movement)

Ventral lateral nucleus located caudal to the Ventral Anterior nucleus, plays a role in motor

integration

ventral anterior and ventral lateral nuclei together comprise MOTOR THALAMUS.

AFFERENT FIBERS TO THE VENTRAL LATERAL NUCLEUS: Deep cerebellar nuclei Globus pallidus (internal segment) Primary motor cortex (area 4)

EFFERENT FIBERS mainly go to primary motor cortex nonprimary somatosensory areas in the parietal cortex (areas

5 and 7) premotor and supplementary motor cortices

Ventral posterior nucleus located in the caudal part of the thalamus

Receives the long ascending tracts conveying sensory modalities (including taste) from the contralateral half of the body and face.

The ventral posterior nucleus is made up of two parts: ventral posterior medial (VPM) nucleus- receives the trigeminal

lemniscus and taste fibers ventral posterior lateral (VPL) nucleus- receives the medial

lemniscus and spinothalamic tracts. Both nuclei also receive input from the primary somatosensory

cortex

The output from both is to primary somatosensory cortex (area 1, 2, and 3).

The VPL and VPM nuclei collectively comprise - ventrobasal complex

Intralaminar Nuclei

The Intralaminar nuclei- divided into caudal and rostral groups. The Caudal group includes the centromedian and parafascicular nuclei The Rostral group includes the paracentral, centrolateral, and centromedial

nuclei

Afferent connections Reticular formation of the brain stem (major input) Cerebellum : The dentatorubrothalamic system Spinothalamic and trigeminal lemniscus Globus pallidus Cerebral cortex

Efferent Connections Other thalamic nuclei (influences cortical activity via other thalamic nuclei) The striatum (caudate and putamen)

Reticular nucleus

Is a continuation of the reticular formation of the brainstem into the diencephalon.

Afferents from the cerebral cortex and other thalamic nuclei. The former are collaterals of corticothalamic projections, and

the latter are collaterals of thalamocortical projections.

Efferent projections to other thalamic nuclei. The inhibitory neurotransmitter in this projection is GABA.

It plays a role in integrating and gating activities of thalamic nuclei

METATHALAMUS (refers to two thalamic nuclei, the Medial Geniculate and Lateral

Geniculate)

Lateral Geniculate Nucleus This is a relay thalamic nucleus in the visual system. It receives fibers from the optic tract conveying impulses from

both retinae The efferent outflow from the lateral geniculate nucleus forms the

optic radiation of the internal capsule (retrolenticular part) to the primary visual cortex in the occipital lobe

Medial Geniculate NucleusThis is a relay nucleus in the auditory system. It receives fibers from the lateral lemniscus directly or, more

frequently, after a synapse in the inferior colliculus. efferent outflow from the MG nucleus forms the auditory radiation of

the internal capsule (sublenticular part) to the primary auditory cortex in temporal lobe (areas 41 and 42)

Other nomenclature system

This system groups thalamic nuclei into the following categories: (1) motor (2) sensory (3) limbic (4) associative (5) nonspecific and reticular.

The motor group receives motor inputs from the basal ganglia (ventral anterior, ventral lateral) or the cerebellum (ventral lateral) and projects to the premotor and primary motor cortices.

The sensory group receives inputs from ascending somatosensory

(ventral posterior lateral and medial), auditory (medial geniculate), and visual (lateral geniculate) systems.

The limbic group is related to limbic structures (mamillary bodies,

hippocampus, cingulate gyrus).

Neurobiochemistry of Thalamus

Neurotransmitters identified in the thalamus: GABA is the inhibitory neurotransmitter in

Afferent terminals from the GP and in local circuit neurons, and in efferent projections of the reticular nucleus and lateral geniculate nucleus; and

Glutamate and aspartate are the excitatory neurotransmitters in

corticothalamic and cerebellar terminals and in thalamocortical projection neurons.

Others: substance P, somatostatin, neuropeptide Y, enkephalin, and

cholecystokinin

Thalamic Infarcts and Syndromes

The conglomerate of signs and symptoms associated with thalamic lesions includes the following: sensory disturbances, thalamic pain, hemiparesis, dyskinesias, disturbances of consciousness, memory disturbances, affective disturbances, and disorders of language

Most thalamic infarcts are reported in the posterolateral and the medial thalamic territories supplied by the geniculothalamic and paramedian arteries, respectively

Posterolateral Thalamic Territory (geniculothalamic artery, a branch of the posterior cerebral artery)

signs and symptoms associated with posterolateral thalamic territory infarcts comprise the thalamic syndrome of Dejerine and Roussy.

In this syndrome, severe, persistent, paroxysmal, and often intolerable pain

(thalamic pain) resistant to analgesic medications occurs at the time of injury or following a period of transient hemiparesis, hemiataxia, choreiform movements, and hemisensory loss

Cutaneous stimuli trigger paroxysmal exacerbations of the pain that outlast the stimulus. Because the perception of “epicritic” pain (from a pinprick) is reduced on the painful areas, this symptom is known as anesthesia dolorosa, or painful anesthesia

An athetoid posture of the contralateral hand (thalamic hand) may appear 2 or more weeks following lesions in this territory.

The hand is flexed and pronated at the wrist and MCP joints and extended at the interphalangeal joints.

The fingers may be abducted. The thumb is either abducted or pushed against the palm.

Thalamic hand with the corresponding MRI

T2-weighted axial magnetic resonance image (MRI) showing an infarct (arrow) in the posterolateral thalamic

territory

Medial territory of the thalamus

Infarcts in the medial territory of the thalamus are associated with occlusion of the paramedian branches of the basilar root of the posterior cerebral artery.

hallmark - drowsiness.

In addition, there are abnormalities in recent memory, attention, intellect, vertical gaze, and occasionally, mild hemiparesis or hemiataxia.

No sensory deficits as a rule.

Utilization behavior, although characteristic of frontal lobe damage, has been reported

Two syndromes asso. with medial thalamus territory :

Akinetic mutism and Kleine-Levin syndrome

In akinetic mutism (persistent vegetative state), patients

appear awake and maintain a sleep-wake cycle but are unable to communicate in any way.

The Kleine-Levin syndrome (hypersomnia-bulimia syndrome) is characterized by recurrent periods (lasting 1 to 2 weeks every 3 to 6 months) in adolescent males of excessive somnolence, hyperphagia (compulsive eating), hypersexual behavior (sexual disinhibition), and impaired recent memory, and eventually ending with recovery.

A confusional state, hallucinosis, irritability, or a schizophreniform state may occur around the time of the attacks

T2-weighted axial MRI showing an infarct (arrow) in the medial thalamic territory

Thalamic pain syndromesThe four types are differentiated from each other on the basis of

the presence or absence in each of central (thalamic) pain proprioceptive sensations (vibration, touch, joint) exteroceptive sensations (pain and temperature) and abnormalities in somatosensory evoked potentials

Memory deficits (may be transient or permanent)

Discrete lesions of the thalamus can cause severe and lasting memory deficits

three distinct behavioral and anatomic types of memory impairment Severe encoding defects A milder form characterized by severe distractibility Disturbances in verbal memory (retrieval, registration, and

retention)

Memory disturbances are most common with bilateral thalamic lesions but do occur with unilateral lesions of either side

Alien hand syndrome

Defined as unwilled, uncontrollable movements of an upper limb together with failure to recognize ownership of a limb in the absence of visual cues

Most cases are associated with lesions in the corpus callosum and mesial frontal area

also been reported in infarcts involving the posterolateral and anterolateral thalamic territories: ventral posterior ventral lateral dorsomedial nuclei

Thalamic Acalculia

Infarctions in the left anterolateral thalamic territory supplied by the tuberothalamic artery have been reported to produce acalculia.

The lesion usually involves the ventral lateral and dorsomedial thalamic nuclei

Language Deficits

Dominant hemisphere thalamic lesions may cause a transient deficit in language.

Three types have been described: medial anterolateral lateral.

medial type, involving the dorsomedial and centromedian nuclei:- is characterized by anomia and attentionally induced language impairment.

anterolateral type, involves ventral anterior (anterolateral thalamic territory):- aphasic syndrome resembling transcortical aphasia.

lateral thalamic territory: mild anomia

CLINICAL IMPORTANCE

Discrete lesions in various regions of the thalamus, and, more recently, deep brain stimulation (DBS) through implanted electrodes, are increasingly used for the treatment of: Parkinsonian and essential tremor, dystonia, pain, epilepsy, manifestations of Gilles de la Tourette's syndrome.

Essential tremor can be treated by DBS with electrodes in the ventrolateral nucleus. The ventrolateral nucleus includes the nuclei Ventralis Intermedius (VIM) and ventralis oralis posterior (VOP).

Treatment of the tremor is the most extensively used and best understood DBS thalamic procedure

thalamus

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