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Oval, nuclear mass Forms 80% 0f diencephalon Anterior extent- interventricular foramen Superiorly- transverse cerebral fissure, floor of 3rd ventricle Inferiorly- hypothalamic sulcus Posteriorly- overlaps midbrain 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 All thalamic nuclei are a mixture of projection neurons, whose axons provide the output of thalamus, and small inhibitory interneurons that use GABA as a neurotransmitter Projection neurons account for 75% or more of the neurons of the most thalamic nuclei, though the relative proportions of projection neurons and interneurons vary in different nuclei Dorsal tier consists of lateral dorsal, lateral posterior nuclei and pulvinar. Lateral posterior nucleus and pulvinar have almost similar connections 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 tier Intralaminar nuclei Embedded in internal medullary lamina Largest of this group are centromedian and parafascicular nuclei • • • • 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. Rostral continuation of periaqueductal gray matter Form interthalamic adhesion [when present] 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 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 cortical area to which the nucleus projects thalamic reticular nucleus diffuse cholinergic, noradrenergic, serotonergic endings from brainstem reticular formation Relay 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 areas Intralaminar and midline nuclei seem to have special role in function of basal nuclei and limbic system 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 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 Input Collateral branches of thalamocortical and corticothalamic axons Output To each thalamic nucleus that sends afferents to reticular nucleus Functions Inhibitory modulation of thalamocortical transmission Input Output Functions Cholinergic and central nuclei of reticular formation,locus coeruleus, collateral branches from spinothalamictra cts, cerebellar nuclei, pallidum Extensive cortical projections, especially to frontal and parietal lobes; striatum Stimulation of cerebral cortex in waking state and arousal from sleep;somatic sensation, especially pain [from contralateral head and body]; control of movement Input Output Functions Inferior colliculus Primary auditory cortex Auditory pathway [from both ears] Input Output Ipsilateral halves Primary visual of both retinas cortex Functions Visual pathway [from contralateral visual fields] Input Contralateral gracile and cuneate nuclei; contralateral dorsal horn of spinal cord Output Primary somatosensory area Functions Somatic sensation [principal pathway, from contralateral body below head] Input Output Functions Contralateral trigeminal sensory nuclei Primary somatosensory area Somatic sensation [principal pathway, from contralateral side of head: face, mouth, larynx, pharynx, dura mater] Input Contralateral cerebellar nuclei Output Primary motor area Functions Cerebellar modulation of commands sent to motor neurons Input Pallidum Output Premotor and supplementary motor areas Functions Planning commands to be sent to motor neutons Input Output Functions Pallidum Frontal lobe, including premotor and supplementary motor areas Motor planning and more complex behavior Input Output Functions Spinothalamic and trigeminothalami c tracts Insula and nearby temporal and parietal cortex, including second somatosensory srea Visceral and other responses to somatic sensory stimuli Input Output Functions Hippocampal formation; pretectal area, superior colliculus Cingulate gyrus; Memory ; visual interpretation of association visual stimuli cortex [occipital,posteri or parietal and temporal lobes] Input Output Functions Superior colliculus Parietal, temporal, and association cortex Interpretation of visual and other sensory stimuli; formation of complex behavioral responses Input Output Functions Pretectal area; primary and all association cortex for vision;retinas Parietal lobe, anterior frontal cortex, cingulate gyrus, amygdala Interpretation of visual and other sensory stimuli, formation of complex behavioral responses Input Output Functions Etorhinal cortex, Prefrontal cortex Behavioral amygdala responses that ,collaterals from involve decisions spinothalamic based on tract, pallidum, prediction and substantia nigra incentives Input Output Funtions Amygdala, hypothalamus Hippocampal formation and parahippocampa l gyrus Behaviorr;includi ng visceral and emotional responses Input Output Funtions Mamillary body Cingulate gyrus Memory Vascular accidents Can involve adjacent structures Small lesion can lead to large collection of deficits 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 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]