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 Output Functions

Collateral branches of thalamocortical and corticothalamic axons

To each thalamic nucleus that sends afferents to reticular nucleus

Inhibitory modulation of thalamocortical transmission

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

Input Output Functions

Inferior colliculus

Primary auditory cortex

Auditory pathway [from both ears]

Input Output Functions

Ipsilateral halves of both retinas

Primary visual cortex

Visual pathway [from contralateral visual fields]

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]

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 Output Functions

Contralateral cerebellar nuclei

Primary motor area

Cerebellar modulation of commands sent to motor neurons

Input Output Functions

Pallidum Premotor and supplementary motor areas

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 trigeminothalamic 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; visual association cortex [occipital,posterior parietal and temporal lobes]

Memory ; interpretation of visual stimuli

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, amygdala ,collaterals from spinothalamic tract, pallidum, substantia nigra

Prefrontal cortex Behavioral responses that involve decisions based on prediction and incentives

Input Output Funtions

Amygdala, hypothalamus

Hippocampal formation and parahippocampal gyrus

Behaviorr;including 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]