Introduction The term thalamus derives from a Greek wordthat means “inner chamber” or “meeting place”. Paired, oval, nuclear mass of ~ 30 × 20 × 20mm The largest component (80%) of thediencephalon The term diencephalon includes the followingstructures:• epithalamus,• thalamus (including the metathalamus),• hypothalamus, and• subthalamus.
Relations of Thalamus Forms lat wall of 3rd ventricle Separated from hypothalamusby hypothalamic sulcus. connected to opposite thalamus• interthalamic adhesion(massa intermedia)ThalamusHypothalamusHypothalamus sulcus
Massa intermediaMedial - #rd VentricleDorsal – Lateral VentricleLateral – Internal capsuleThalamusVentral – Sub thalamus &hupothalamus
Classification of Thalamic NucleiNEUROANATOMIC CLASSIFICATION Divided by internal medullary lamina into three nucleargroups:• Lateral group of nuclei• Medial group of nuclei• Anterior group of nuclei
Classification of Thalamic NucleiMedial group of nuclei• Centromedian nucleus• Dorsomedial nucleus• Midline nucleusAnterior Group of nuclei• Lying between the bifurcation of internal medullarylamina
Classification of Thalamic NucleiOTHER NUCLEI Midline nuclei• Scattered cells between medial part of the thalamus andependyma of third ventricle Intralaminar nuclei• Cluster of nerve cell embedded in the internal medullary lamina• Centromedian nucleus• Parafascicular nucleus• Limitans nucleus• Paracentral nucleus and• Central lateral nucleus Reticular nucleus• Sheet-like layer of neurons partially covering the thalamus• Only thalamic nucleus with no projections to the cortex
Connections of Thalamus Every thalamic nucleus (except the reticularnucleus) sends axons to specific parts of thecerebral cortex and every part of the cerebralcortex sends reciprocal fibers back to the thalamicnuclei. Information received by the thalamus is alwaysshared with the cerebral cortex and that the cortexand thalamus can modify each others activities.
Connections of Thalamus ANTERIOR GROUP OF NUCLEI• Afferent fibres – From mammillary body of hypothalamusvia mammilothalamic tract (Vicq d’Azyr)• Efferent fibre – To cingulate gyrus of cerebral cortex• also receives significant input from the hippocampalformation of the cerebral cortex (subiculum andpresubiculum) via the fornix.• Function served:• The anterior nuclear group of the thalamus is part of the limbicsystem, which is concerned with emotional behavior and memorymechanisms.• Discrete damage to the mamillothalamic tract has been associatedwith deficits in a specific type of memory, episodic long-termmemory, with relative sparing of short-term memory andintellectual capacities.
Connections of ThalamusVENTRAL NUCLEAR SUBGROUP This subgroup includes the ventral anterior, ventrallateral, and ventral posterior nuclei.
Connections of Thalamus Ventral anterior nucleus• Afferent fibres – Reticular formation,globus pallidus(via the thalamic fasciculus), substantia nigrapars reticulata, intralaminar thalamic nuclei,premotorand prefrontal cortices (areas 6 and 8).• Efferent fibres - Reticular formation, substantia nigra,corpus striatum, premotor cortex, other thalamicnuclei• Function served:• the ventral anterior nucleus is a major relay station in themotor pathways from the basal ganglia to the cerebral cortex.As such, it is involved in the regulation of movement.
Connections of Thalamus The medial (magnocellular) part of the ventral anterior nucleus isconcerned with control of voluntary eye, head, and neckmovements. The lateral (parvicellular) part of the nucleus is concerned withcontrol of body and limb movements. Lesions in this nucleus and adjacent areas of the thalamus havebeen placed surgically (thalamotomy) to relieve disorders ofmovement, especially parkinsonism
Connections of ThalamusSchematic diagram showing the major connections of the ventral anteriornucleus of the thalamus.
Connections of Thalamus Ventral lateral nucleus• Afferent fibres – dentate nucleus of cerebellum (visdentatothalamic tract),globus pallidus internalsegment.• Efferent fibres - primary motor cortex in theprecentral gyrus, pre motor area 6.• Function served :• the ventral lateral nucleus, like the ventral anterior nucleus,is a major relay station in the motor system linking thecerebellum, the basal ganglia, and the cerebral cortex.
Connections of ThalamusSchematic diagram showing the major afferent and efferentconnections of the nucleus ventralis lateralis of the thalamus.
Connections of Thalamus Ventral posterior nucleus• Afferent fibres - trigeminal lemniscus and taste fibersto VPM, the medial lemniscus and spinothalamictracts to VPL.• Efferent fibres - Primary somatic sensory (areas 3, 1,and 2 in post central gyrus) cortex via posterior limbof internal capsule.• Function served:• Relay somatosensory impulse (touch pressure , pain,proprioception, temperature and kinesthetic) from trunk andlimb• Relay sensory impulse from face
Schematic diagram showing the major afferent and efferent connectionsof the ventral posterior lateral and ventral posterior medial nuclei of thethalamus
Connections of Thalamus Dorsomedial nucleus• Afferent fibres - Prefrontal cortex, hypothalamus,other thalamic nuclei• Efferent fibres - Prefrontal cortex (area 8,9,10 and 11)hypothalamus, other thalamic nuclei• Function served:• The dorsomedial nucleus belongs to a neural systemconcerned with affective behavior, decision making andjudgment, memory, and the integration of somatic andvisceral activity.• The reciprocal connections between the prefrontal cortex andthe dorsomedial nucleus can be interrupted surgically torelieve severe anxiety states and other psychiatric disorders.
Connections of ThalamusSchematic diagram showing the major afferent and efferent connections of thedorsomedial nucleus of the thalamus
Connections of Thalamus Intralaminal nuclei• Afferent fibres - Reticular formation of the brain stem,Spinothalamic and trigeminal lemniscus, Globuspallidus , cerebral cortex (In contrast to other thalamicnuclei, the connections between the intralaminar nuclei andcerebral cortex are not reciprocal)• Efferent fibres - To cerebral cortex via other thalamicnuclei, no direct cortical connections for theintralaminar nuclei (exception intralaminar nucleicentrolateral to the primary visual cortex area 17),corpus striatum.• Function served:• Influences levels of consciousness and alertness• Integrates somatic and visceral sensory impulse beforeprojecting to cortex
Connections of ThalamusSchematic diagram showing the major afferent and efferent connectionsof the intralaminar nuclei of the thalamus
Connections of Thalamus Midline nuclei• Afferent fibres – spinothalamic, trigemono thalamictract, medial lemniscus,reticular formation,otherthalamic nuclei, hypothalamus.• Efferent fibres - Hypothalamus , neocortex,basalganglia, other thalamic nuclei• Function served:• poorly developed in humans• Centre for integrating crude visceral and somatic sensation.
Connections of Thalamus Reticular nucleus• Afferent fibres - collaterals of corticothalamicprojections, and thalamocortical projections of otherthalamic nuclei• Efferent fibres – other thalamic nuclei (unique amongthalamic nuclei in that its axons do not leave the thalamus)• Function served:• Based on its connections, the reticular nucleus plays a role inintegrating and gating activities of thalamic nuclei
Connections of Thalamus Medial geniculate nucleus• Afferent fibre :Inferior colliculus, lateral lemniscusfrom both ears but predominantly the contralateralear• Efferent fibre - the auditory radiation of the internalcapsule to the primary auditory cortex in temporallobe (areas 41 and 42)• Function served:• Hearing
Connections of Thalamus Lateral geniculate nucleus• Afferent fibre -the optic tract conveying impulsesfrom both retinae (the inflow from each retina projects ondifferent laminae (ipsilateral retina to laminae II, III, and V;contralateral retina to laminae I, IV, and VI).• Efferent fibre - Optic radiation to visual cortex ofoccipital lobe• Function served:• vision
Neurotransmitters and Neuropeptides in Thalamus The following neurotransmitters have been identified inthe thalamus:• GABA is the inhibitory neurotransmitter in terminals from theglobus pallidus, in local circuit neurons, and in projectionneurons of the reticular nucleus and lateral geniculate nucleus;and• glutamate and aspartate are the excitatory neurotransmitters incorticothalamic and cerebellar terminals and in thalamocorticalprojection neurons.• Several neuropeptides have been identified like substance P,somatostatin, neuropeptide Y, enkephalin, and cholecystokinin
Functions of Thalamus Serves as a great sensory relay station andintegrating centre for most inputs to cerebralcortex. Responsible for maintaining conscios andalerting response of RAS. Responsible for subcortical perception ofsensation to some extent Concerned with control of muscular ,movementvia its various connections with basal ganglia,cerebellum and motor cortex Integrating centre for sleep- Electricalactivation of thalamic nuclei induces sleep.
Thalamus applied aspectsThalamic syndrome Also known as "Dejerine-Roussy disease", after Josephjules dejerine & Gustave Roussy Caused due to infarct in geniculothalamic(thalamogeniculate, posterolateral) artery, a branch ofthe posterior cerebral artery. Thalamic structures involved by the infarct are theposteroventral and the posterolateral nuclei. The clinical hallmark is a pansensory loss contralateralto the lesion, paresthesia, and thalamic pain. Transient hemiparesis, homonymous hemianopsia,hemiataxia, tremor, choreiform movements, and spatialneglect, all contralateral to the lesion in the thalamus
Thalamus applied aspects The contrlateral hand is flexed and pronated at thewrist and metacarpo-phalangeal joints and extended atthe interphalangeal joints. The fingers may be abducted.The thumb is either abducted or pushed against thepalm.(Thalamic hand) Cutaneous stimuli trigger paroxysmal exacerbations ofthe pain that outlast the stimulus. Because theperception of “epicritic” pain (from a pinprick) isreduced on the painful areas, this symptom is known asanesthesia dolorosa, or painful anesthesia
Thalamus applied aspects The Alien Hand Syndrome• The alien hand syndrome is defined as unwilled,uncontrollable movements of an upper limb togetherwith failure to recognize ownership of a limb in theabsence of visual cues.• The syndrome was first described by Goldstein in1908• Most cases are associated with lesions in the corpuscallosum and frontal area but are also seen in infarctsinvolving the posterolateral and anterolateralthalamic territories
Surgical Relief of Pain by ThalamicCauterization• The intralaminar nuclei of the thalamus are known totake part in the relay of pain to the cerebral cortex.• Cauterization of these nuclei has been shown torelieve severe and intractable pain associated withterminal cancer.