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]