BioThalamus

Thalamus facts

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• this page is about dorsal thalamus (DTH=TH/D)
• other parts of thalalus are:
  • ventral thalamus (TH/V) = RN, SVG, ZI
  • epithalamus = habenula, pineal gland

see also:

• Wikipedia

Overview

• thalamus sorts sensory input, helps sort REALITY from FICTION

Structure

• Scholarpedia

inputs

• 90%-95% of synaptic inputs onto the relay cells, arise from:
  • local GABAergic neurons
  • reticular cells
  • interneurons
  • feedback projection from layer 6 of cortex
  • ascending projection from various scattered cell groups in the brainstem reticular formation
• relayed inputs
  • sensory inputs
• modulatory inputs
  • from cerebral cortex
  • limbic pathways make input
  • cerebellar and basal ganglia inputs
  • from reticular thalamic nucleus
  • from various brain stem areas

functions:

• site where sensory inputs can be modulated
• relay for cerebellar and basal ganglia inputs to the cerebral cortex
  • these are feedback pathways, since the cerebellum and basal ganglia respond to outputs from the cerebral cortex

paths

• thalamus nucleus -> cortex -> thalamus nucleus (the same)
  • filtering thalamic inputs to the cerebral cortex

contents

• many inhibitory interneurons
• many neuromodulatory neurotransmitter systems (such as 5HT and NE systems) have terminations within thalamic nuclei
• the relay cell to interneuron ratio is between 3 and 4 to one

Divisions

• anterior nucleus (AN) - association - connections similar to the LD nucleus
• lateral subnuclei
  • reticular thalamic nucleus - nonspecific - brain stem reticular formation, cerebral cortex, thalamus -> inhibitory input to thalamic nuclei (arousal and alertness)
  • ventral tiers subnuclei (total 15 nuclei, project to neocortex)
    • ventral posterior nuclei (VP)
      • ventral posteromedial nuclei (VPM) - ff/relay - trigeminothalamic -> cortex
      • ventral posterolateral nuclei (VPL) - ff/relay - medial lemniscal and spinothalamic connections -> cortex
    • ventral lateral nuclei (VL) - fb/relay - cerebellum/dentate nucleus, basal ganglia -> primary motor, premotor cortex (motor feedback from the cerebellum and basal ganglia to the cerebral cortex)
    • ventral anterior nuclei (VA) - fb/relay - basal ganglia (medial globus pallidus, substantia nigra, parts reticulata) -> premotor cortex, supplementary motor area
  • dorsal tiers subnuclei
    • pulvinar (PV) - association - superior colliculus, association cortex -> secondary visual areas, association areas in parietotemporal region (visual perception and eye movements, probably relating to attention)
    • lateral posterior nuclei (LP) - association - like pulvinar
    • lateral dorsal nuclei (LD) - association - hippocampus -> mamillary bodies -> LD -> posterior cingulate cortex (emotional learning)
• medial subnuclei
  • medial dorsal nucleus (MD)
    • medial subdivision - association - solitary nucleus, substantia nigra reticulata, amygdala and ventral pallidum -> insular cortex, orbital frontal cortex and subcallosal region (autonomic regulation and emotions)
    • lateral subdivision - association - superior colliculus, olfactory cortex and the ventral pallidum -> frontal eye fields, anterior cingulate cortex (controlling eye movements, attending to visual stimuli, emotional tone)
  • midline nuclei - nonspecific
  • intralaminar nuclei
    • central median - fb/relay - (reciprocal connections with the globus pallidus and with the premotor cortex)
    • parafascicular nuclei - nonspecific
• metathalamus (near pulvinar)
  • MGB - medial geniculate body (auditory relay nucleus) - ff/relay - tonotopically auditory afferents from inferior colliculus -> primary auditory cortex
  • LGB - lateral geniculate body (principal visual relay) - ff/relay - retinotopic input -> primary visual cortex

Projection

• each thalamic projection neuron can exist in one of two basic physiological states:
  • "tonic mode"
    • neurons respond like other neurons to depolarization and hyperpolarization
  • "burst mode"
    • oscillatory mode"
    • neurons in this state have an intrinsic rythmicity
    • during sleep, most thalamic neurons are in burst mode
    • neurons cannot communicate specific information
    • if a novel stimulus is presented, the sudden change from burst to tonic mode may be a major factor in alerting the cortex

Functional View

Sensory Relay:

• sensor/retina -> DTH/V/LGB -> PCA/V1 (1-order visual relay)
• sensor/inferior colliculus -> DTH/V/MGB -> PCA/A1,2 (1-order auditory relay)
• BSA/medial lemniscus, ALS, TTT, STT -> DTH/V/VP -> {PCA/S1,2,3 (1-order somatic relay); HCA/insula (1-order taste relay); ACA/M/4 (?)}
• {BSA/anterior olfactory nucleus; SCA (pain)} -> DTH/M/MD -> {HCA/insula (1-order olfactory relay); ACA/PFC (1-order pain relay)}
• {SCA; BSA/olfactory} -> DTH/I/sheet -> (diffuse)
• BSA/SN,SC,PAG,CR -> DTH/V/VM -> ACA,PCA/layer1 (attention)

Motor relay:

• {BSA/CR; BGA/GP,SN} -> DTH/V/VL,VA -> ACA/M,PM,SM
• BGA/GP,SN -> DTH/I/CM -> ACA/M

Association:

• (many) -> DTH/L/PV -> {PCA/occipital,parietal; HCA/temporal}
• limbic/mammillary -> DTH/A/AV,AM,AD -> ACA/CG
• limbic -> DTH/L/LD -> ACA/CG

Axons terminated in Thalamus

• 2 types - R (round) and E (extended), excitatory, using GLU
• R-type terminals are characteristically large (3 nm in diameter), although variable in size and actual shape. They conform to the classical type-2 endings, as described in specific thalamic nuclei. The associated axonal terminations are concentrated in sharply delimited, round arbors and carry of the order of 100 terminals, that typically end on proximal dendrites
• E-type axons have stalked or spinous terminations of classic type-1 corticothalamic endings. Their axonal terminal fields are elongated and quite extended (1–3 mm) and carry between 500 and 1,000 E terminals that typically end on distal dendrites
• in the LGN (and in pulvinar), the driving input from the retina is provided by R-type axon terminals, with type-2 synapses; the input back from cortical area V1 has E-type axon terminals, with type-1 synapses - modulating input, (though there are many more E-type than R-type axons)
• cortical E-type axons derive from medium to small pyramidal cells in the lower cortical layers. They are located in layer 6, and as a rule always have collaterals in the thalamic reticular nucleus
• cortical R-type axons originate from pyramidal cells in cortical layer 5

Axons terminated in Cortex

• projections from thalamus to cortex also fall into two classes
• first type goes mainly into layer 4 or lower layer 3, with a minority also contacting processes in layer 6
  • projection cells in magno- and parvocellular laminae of LGN are prominent examples of such a connection that can very reliably drive cortical cells, despite their small number of synapses.
  • 2.8% of all excitatory synapses on a layer 4C spiny stellate cell originate from magnocellular cells in LGN
• other type projects to layer 1, but not exclusively - modulating connection
  • examples - cells in the interlaminar zones of the LGN that project into the superficial layers of V1
• rules
  • (1) If a cortical area projects to a thalamic region from cortical layer 6, then if there is a reverse projection, it goes mainly into layer 4 or lower layer 3
  • (2) if a cortical area projects to a thalamic region from cortical layer 5, then if there is a reverse projection it avoids layer 4 and often goes mainly to cortical layer 1. These thalamocortical projections are usually much more diffuse than the layer 4 projection.