BioThalamus

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Thalamus facts

Home -> BiologicalLifeResearch -> BiologicalHierarchyFull -> bioThalamus

  • 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:

Overview

  • thalamus sorts sensory input, helps sort REALITY from FICTION

Structure

637px-Constudthal.gif

ThalamusFigure4.gif

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

MamBSubAsr.gif

400px-ThalamusFigure5_new.gif

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.
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