thalamus by dr.arshad
DESCRIPTION
TRANSCRIPT
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GETTING TO AND FROM THE CEREBRAL CORTEX
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THALAMUS
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THALAMUS
• 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
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• 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.
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• 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..
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• 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
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• Dorsal tier consists of lateral dorsal, lateral posterior nuclei and pulvinar.
• Lateral posterior nucleus and pulvinar have almost similar connections
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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]
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• 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
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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.
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Midline nuclei
• Rostral continuation of periaqueductal gray matter
• Form interthalamic adhesion [when present]
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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
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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
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• Regulatory inputs contribute to decisions about whether or in what form information leaves a thalamic nucleus
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Sources
• cortical area to which the nucleus projects• thalamic reticular nucleus• diffuse cholinergic, noradrenergic,
serotonergic endings from brainstem reticular formation
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Categories of nuclei depending on pattern of inputs
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
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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
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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.
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• 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.
• .
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• 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
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• 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
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CONNECTIONS AND FUNCTIONS OF THALAMIC NUCLEI
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RETICULAR NUCLEUS
Input Output Functions Collateral branches of thalamocortical and corticothalamic axons
To each thalamic nucleus that sends afferents to reticular nucleus
Inhibitory modulation of thalamocortical transmission
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Intralaminar nuclei
Input Output Functions
Cholinergic and central nuclei of reticular formation,locus coeruleus, collateral branches from spinothalamictracts, 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
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VENTRAL GROUP OF NUCLEI
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Medial geniculate body
Input Output Functions
Inferior colliculus Primary auditory cortex
Auditory pathway [from both ears]
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Lateral geniculate body
Input Output Functions
Ipsilateral halves of both retinas
Primary visual cortex
Visual pathway [from contralateral visual fields]
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Ventral posterolateral
Input Output Functions Contralateral gracile and cuneate nuclei; contralateral dorsal horn of spinal cord
Primary somatosensory area
Somatic sensation [principal pathway, from contralateral body below head]
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Ventral posteromedial
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]
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Ventral lateral [posterior division]
Input Output Functions Contralateral cerebellar nuclei
Primary motor area Cerebellar modulation of commands sent to motor neurons
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Ventral lateral [anterior division]
Input Output Functions Pallidum Premotor and
supplementary motor areas
Planning commands to be sent to motor neutons
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Ventral anterior
Input Output Functions
Pallidum Frontal lobe, including premotor and supplementary motor areas
Motor planning and more complex behavior
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Posterior group
Input Output Functions
Spinothalamic and trigeminothalamic tracts
Insula and nearby temporal and parietal cortex, including second somatosensory srea
Visceral and other responses to somatic sensory stimuli
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LATERAL GROUP OF NUCLEI
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Lateral dorsal
Input Output Functions
Hippocampal formation; pretectal area, superior colliculus
Cingulate gyrus; visual association cortex [occipital,posterior parietal and temporal lobes]
Memory ; interpretation of visual stimuli
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Lateral posterior
Input Output Functions
Superior colliculus Parietal, temporal, and association cortex
Interpretation of visual and other sensory stimuli; formation of complex behavioral responses
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Pulvinar
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
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MEDIAL GROUP OF NUCLEI
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Mediodorsal/dorsomedial
Input Output Functions
Etorhinal cortex, amygdala ,collaterals from spinothalamic tract, pallidum, substantia nigra
Prefrontal cortex Behavioral responses that involve decisions based on prediction and incentives
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‘Midline’ nuclei
Input Output Funtions
Amygdala, hypothalamus
Hippocampal formation and parahippocampal gyrus
Behaviorr;including visceral and emotional responses
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Anterior
Input Output Funtions
Mamillary body Cingulate gyrus Memory
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Thalamic damage
• Vascular accidents• Can involve adjacent structures• Small lesion can lead to large collection of
deficits
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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
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• 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
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• 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
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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
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• Discriminative touch may be abolished• Position sense may be greatly impaired• Sensory ataxia [due to loss of proprioception]
may be present
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• 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]
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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]