Thalamus

Anat and Functional Anat with clinical syndromes

1. Thalamus -Dr. Sachin A. Adukia

2. Headings Introduction Anatomy: Gross and Location (with normal MRI sections) Physiology Functional Anatomy Neurobiochemistry Thalamus infarction and Syndromes Theraputic importance

3. Introduction Thalamus (Greek) means “ inner chamber ” or “ meeting place ” Historical interest Earlier – “ Optic Thalamus ” and chamber of vision- 2 nd century AD; Galen. Later - Prefix “optic” was dropped - when discovered that sensory modalities other than vision are also processed in the thalamus

4. Primary Divisions of the Brain

5. 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

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

7. Thalamus- Axial View

8. Thalamus- Sagittal View

9. Thalamus- Coronal View

10. Thalamus- Coronal View 3 rd ventricle

11. Thalamus- Coronal View Internal capsule

12. Thalamus- Coronal View Mediodorsal nucleus Internal capsule

13. Thalamus- Blood Supply Posterior 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.

14. 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

15. 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: Anterior Medial Lateral Intralaminar and reticular Midline Posterior

16. 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.

17. 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.

18. 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

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

20. 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.

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

22. 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

23. Lateral group of Nuclei (Overview)

24. 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

25. 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

26. 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

27. Connections of Ventral Anterior Nuclei INPUTS GABAergic 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)

28. 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

29. 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

30. 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 )

31. 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

32. 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 Nucleus This 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)

33. 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 ).

34. 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

35. 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

36. 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.

37. Thalamic hand with the corresponding MRI

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

39. 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

40. 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

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

42. Thalamic pain syndromes The four types are differentiated from each other on the basis of the presence or absence in each of central (thalamic) pain proprioceptiv e sensations (vibration, touch, joint) exteroceptive sensations (pain and temperature) and abnormalities in somatosensory evoked potentials

43. 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

44. 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

45. 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

46. 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

47. 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