• All sensory pathways relay in thalamus.• Many circuits used by cerebellum, basal nuclei and limbic system involve thalamus.• These utilize more or less separate portions of thalamus, which has been subdivided into a series of nuclei.
• Nuclei can be distinguished from each other by topographical locations within thalamus and by input/output patterns.• Thalamus is divided into medial and lateral nuclear groups by a thin curved sheet of myelinated fibres called internal medullary lamina..
• It splits anteriorly to enclose a group of nuclei, collectively called anterior nucleus, which is close to interventricular foramen• Medial group contains one large nucleus called dosomedial nucleus• Lateral group is subdivided into a dorsal and ventral tier
• Dorsal tier consists of lateral dorsal, lateral posterior nuclei and pulvinar.• Lateral posterior nucleus and pulvinar have almost similar connections
Nuclei of ventral tier• Ventral anterior, ventral lateral- concerned with motor control; are connected to basal nuclei and cerebellum• Ventral posterior is subdivided into ventral posterolateral[ smatosensory input from body] and ventral posteromedial [somatosensory input from head]
• Lateral and medial geniculate nuclei / bodies are considered as posterior extensions of ventral tierIntralaminar nuclei• Embedded in internal medullary lamina• Largest of this group are centromedian and parafascicular nuclei
Reticular nucleus• Lies between lateral thalamic surface and external medullary lamina• Reticular nucleus is developmentally not a part of thalamus.• It has distinct anatomical and physiological properties.• Considered a part of thalamus because of location and extensive involvement in thalamic function.
Midline nuclei• Rostral continuation of periaqueductal gray matter• Form interthalamic adhesion [when present]
Role of thalamic nuclei• Pipelines for flow of information to cerebral cortex• Site where decisions are implemented about which information should reach cerebral cortex for processing• Any particular type of information affected by any thalamic nucleus is a function of its input and output connections
Inputs• Specific - Regulatory• Specific inputs convey information that a given nucleus may pass to cerebral cortex [and for some nuclei to additional sites].• Examples; Medial lemniscus specifically to VPL. Optic tract to LGB
• Regulatory inputs contribute to decisions about whether or in what form information leaves a thalamic nucleus
Sources• cortical area to which the nucleus projects• thalamic reticular nucleus• diffuse cholinergic, noradrenergic, serotonergic endings from brainstem reticular formation
Categories of nuclei depending on pattern of inputsRelay nuclei• receive well defined specific input fibres and project to specific functional areas of cerebral cortex• deliver information from specific functional systems to appropriate cortical areasIntralaminar and midline nuclei seem to have special role in function of basal nuclei and limbic system
Association nuclei• project to association areas of cerebral cortex• receive major inputs from cerebral cortex and subcortical structures• probably important in distribution and gating of information between cortical areas
SCHEME OF THALAMIC ORGANIZATION• Every nucleus of the thalamus except the reticular nucleus sends axons to the cerebral cortex, either to a sharply defined area or diffusely to a large area.• Every part of the cortex receives afferent fibers from the thalamus, probably from at least two nuclei.
• Every thalamocortical projection is faithfully copied by a reciprocal corticothalamic connection.• Thalamic nuclei receive other afferent fibers from subcortical regions.• Probably only one noncortical structure, the striatum , receives afferent fibers from the thalamus.• .
• The thalamocortical and corticothalamic axons give collateral branches to neurons in the reticular nucleus, whose neurons project to and inhibit the other nuclei of the thalamus• No connections exist between the various nuclei of the main mass of the thalamus, although each individual nucleus contains interneurons
• The synapses of the interneurons are inhibitory, and most are dendrodendritic.• Other synapses in the thalamus are excitatory, with glutamate as the transmitter, and so are thalamocortical projections
RETICULAR NUCLEUSInput Output FunctionsCollateral branches To each thalamic Inhibitoryof thalamocortical nucleus that sends modulation ofand corticothalamic afferents to thalamocorticalaxons reticular nucleus transmission
Intralaminar nucleiInput Output FunctionsCholinergic and Extensive cortical Stimulation ofcentral nuclei of projections, cerebral cortex inreticular especially to frontal waking state andformation,locus and parietal lobes; arousal fromcoeruleus, collateral striatum sleep;somaticbranches from sensation, especiallyspinothalamictracts, pain [fromcerebellar nuclei, contralateral headpallidum and body]; control of movement
Ventral posterolateralInput Output FunctionsContralateral gracile Primary Somatic sensationand cuneate nuclei; somatosensory area [principal pathway,contralateral dorsal from contralateralhorn of spinal cord body below head]
Ventral posteromedialInput Output FunctionsContralateral Primary Somatic sensationtrigeminal sensory somatosensory area [principal pathway,nuclei from contralateral side of head: face, mouth, larynx, pharynx, dura mater]
Ventral lateral [posterior division]Input Output FunctionsContralateral Primary motor area Cerebellarcerebellar nuclei modulation of commands sent to motor neurons
Ventral lateral [anterior division]Input Output FunctionsPallidum Premotor and Planning commands supplementary to be sent to motor motor areas neutons
Ventral anteriorInput Output FunctionsPallidum Frontal lobe, Motor planning and including premotor more complex and supplementary behavior motor areas
Posterior groupInput Output FunctionsSpinothalamic and Insula and nearby Visceral and othertrigeminothalamic temporal and responses totracts parietal cortex, somatic sensory including second stimuli somatosensory srea
Mediodorsal/dorsomedialInput Output FunctionsEtorhinal cortex, Prefrontal cortex Behavioralamygdala responses that,collaterals from involve decisionsspinothalamic tract, based on predictionpallidum, substantia and incentivesnigra
‘Midline’ nucleiInput Output FuntionsAmygdala, Hippocampal Behaviorr;includinghypothalamus formation and visceral and parahippocampal emotional gyrus responses
AnteriorInput Output FuntionsMamillary body Cingulate gyrus Memory
Thalamic damage• Vascular accidents• Can involve adjacent structures• Small lesion can lead to large collection of deficits
Damage restricted to posterior thalamus• Paroxysms of intense pain triggered by somatosensory stimuli• Pain may spread to involve entire one- half of the body- analgesic resistant• Abnormal perception of stimuli that do not cause pain
• Intensity and modality may be distorted• May seem unusually uncomfortable or unpleaseant• Similar syndrome can develop in some patients after damage in almost any part of Anterolateral pathway
• This type of pain is called Thalamic pain/central pain• Cause not understood• Lesions causing this pain always involve VPL/VPM nuclei with sparing of spinothalamic and spinoreticulothalamic fibres that end in other thalamic nuclei• May result in imbalanced thalamic activity
Extensive thalamic damage to posterior thalamus• Total/nearly total loss of somatic sensation in contralateral head and body• Gradually – return of some appreciation of painful, thermal and gross tactile stimuli• Functions associated with Medial lemniscus tend to more severely and oermanently impaired
• Discriminative touch may be abolished• Position sense may be greatly impaired• Sensory ataxia [due to loss of proprioception] may be present
• Tahalamic pain+ hemianaesthesia+sensory ataxia contralateral to a posterior thalamic lesion= thalamic syndrome• It is often accompanied by mild and transient paralysis [damage to corticospinal fibres in Internal capsule] and various types of residual involuntary movements [damage to adjacent basal nuclei]
It is often accompanied by• mild and transient paralysis [damage to corticospinal fibres in Internal capsule]• various types of residual involuntary movements [damage to adjacent basal nuclei]