Transcript
GETTING TO AND FROM THE CEREBRAL CORTEX
THALAMUS
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
• 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 tier
Intralaminar 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 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
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
CONNECTIONS AND FUNCTIONS OF THALAMIC NUCLEI
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
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
VENTRAL GROUP OF NUCLEI
Medial geniculate body
Input Output Functions
Inferior colliculus Primary auditory cortex
Auditory pathway [from both ears]
Lateral geniculate body
Input Output Functions
Ipsilateral halves of both retinas
Primary visual cortex
Visual pathway [from contralateral visual fields]
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]
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]
Ventral lateral [posterior division]
Input Output Functions Contralateral cerebellar nuclei
Primary motor area Cerebellar modulation of commands sent to motor neurons
Ventral lateral [anterior division]
Input Output Functions Pallidum Premotor and
supplementary motor areas
Planning commands to be sent to motor neutons
Ventral anterior
Input Output Functions
Pallidum Frontal lobe, including premotor and supplementary motor areas
Motor planning and more complex behavior
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
LATERAL GROUP OF NUCLEI
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
Lateral posterior
Input Output Functions
Superior colliculus Parietal, temporal, and association cortex
Interpretation of visual and other sensory stimuli; formation of complex behavioral responses
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
MEDIAL GROUP OF NUCLEI
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
‘Midline’ nuclei
Input Output Funtions
Amygdala, hypothalamus
Hippocampal formation and parahippocampal gyrus
Behaviorr;including visceral and emotional responses
Anterior
Input Output Funtions
Mamillary 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]
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