{1090} revision 17 modified: 02-22-2012 15:48 gmt

PMID-7711769[0] Functional anatomy of the basal ganglia. I. The cortico-basal ganglia-thalamo-cortical loop.

  • Pallidal and nigral neurons have wide dendritic arborizations at right angles to the unbranched incoming striatal axons, leading to (hypothetically) a confulence of information from distinct functional striatal territories on many neurons and to extreme reception convergence [242].
    • This pattern suggests that projections arising from very small areas of the cortex may extend through very large regions of the striatum, particularly along the rostrocaudal plane.
    • Individual striatal neurons receive relatively few synapses from restricted cortical areas; this makes it difficult to conceive how the cortico-striatal projection system could convey information in a highly specific manner; specificity does not exist at a cellular level.
  • Cortex to striatum:
    • Virtually all cortical functional areas contribute, at varying degrees, to the cortico-striatal projection, inputs from the sensorimotor cortex being particularly extensive and those from the visual cortex much less so.
    • Cortico-sriatal projection originates from neurons located in both supragranular (layers I-III) and infragranular (V,VI) cortical layers.
    • Cortical neurons project ipsilaterally or contralaterally, but not usually bilaterally.
    • Cortical cells arborize on restricted, topologically defined domains in the striatum.
    • Restricted cortical regions project to parasagitally elongated domains in the caudate nucleus.
      • this seems to be a general feature. see B and C below.
      • Reminds me of the cerebellum.
    • non-adjacent cortical areas (prefrontal and pareital cortices)project to adjacent striatal territories.
    • The association, sensorimotor, and limbic cortical areas project in a segregated manner onto threes distinct striatal regions referred to as the associative, sensorimotor, and limbic striatal territories.
    • In this view, cortical information is not directly transposed at striatal level, but is integrated and transformed into strict associative, sensorimotor, and limbic functional modalities.
  • Convergence and divergence:
    • There is a vast reduction in the number of neurons from the cortex to the striatum.
    • This has led many to infer overlap or convergence.
    • Actual projection is patchy -- divisions of striosomes and extrastriosomal matrix -- with the individual axons sending out further sub-patches.
      • This degree of segregation breaks down for sensorimotor territory.
    • cortico-striatal neurons in infragranular layers project principally to striosomes while those in supragranular layers send their axons to the matrix. things are tightly organized.
  • The output cells of the matrix are grouped in clusters in relation to the different projection systems that lead from the striatum to the GPe and GPi. These are called 'matrisomes'.
    • These might be a way of bringing into proximity different cortical signals so they can be recombined in novel ways.
    • That said, there was substantial topographical overlap of the frontal eye field and the supplementary eye field, and though these are closely interdigitated they do not mix.
  • Medium spiny neurons:
    • The primary projection neurons of the striatum.
    • GABA. Plus substance P, enkephalin, dynorphin and neurotensin. (!)
      • The coexistence of GABA with a given peptide in a spiny neuron is in correlation with it's target site.
      • At that time they didn't know what the peptides did.
    • Axon emits several collaterals:
      • Local axonal arborizations restricted tot he dendritic domain of its cell of origin or a nearby cell -- inluding an 'autonapse' or of nearby projection neurons.
      • Less common axonal arborization goes far beyond and often does not overlap the dendritic domain of the cell of origin.
    • Projected to by the cortex, thalamus, and the SNc.
    • Usually silent, except with cortical / thalamic input.
  • Interneurons in the striatum are non-spiny.
    • Less than 2% (of entire striatal population, not just interneurons) them are huge, cholinergic cells.
      • These form symmetric synapses on virtually all parts of MSN.
    • Medium, 1% of population, have short axons and are GABA ergic.
    • Second medium, nitrous oxide signaling interneurons.
    • SNc efferents synapes ontot the base of the spines, but only on MSN that have cortical afferents.
    • Thalamic input synapse onto morphologically distinct type of MSN.
    • Destruction of the dopaminergic nicgro-striatal pathway results in a decrease in levels of mRNA for substance P and increase in mRNA for enkephalin.
  • Striatal MSN projections:
    • Relatively discrete in cats and monkeys; highly collateralized in rats, where many neurons project to GPe, GPi, SN, or some pair.
  • Fibers from the associative territory massively invade the whole extent of SNr, without clear territorial demarcation.
    • Meanwhile, inputs from the limbic striatal territory appears to be widely distributed in the substantia nigra & VTA.
  • Most authors think that the distinction between the GPi and SNr is artificial -- they are split by the internal capsule.
    • However, GPi is mostly sensorimotor, while SNr is associative.
  • Projections from striatum to pallidus * SNr very organized and layered.
    • Pictures. read the paper. words do not do this justice.
    • For example, injections of anterograde tracers in various sectors of the striatum produce elongated, longitudinally oriented terminal fields that cover nearly the entire rostrocaudal extent of the substantia nigra.
    • "The dorsal climbing fibers and the corresponding wooly fibers from replicable modular units whose boundaries do not respect the limit between SNc and SNr compartments. ... They are distrinuted along the rostrocaudal extent of the substantia nigra according to a remarkably precise and constant sequence.
  • As in [1]: striatal and subthalamic terminals converge onto the same pallidal neurons within these regions of overlap, possibly in register with those from the striatum.
    • The striato-pallidal fibers and striato-nigral fibers arborize at least twice in the target structures, suggesting there are multiple copies of the same information to distinct subsets of pallidal/nigral populations.
      • Meanwhile, GPi/SNr axons are highly collateralized and not strictly confined to disctinct subnuclei.
      • That is, output is both convergent and divergent.
      • There are several multi-laminar models of the SNr [54] or the globus pallidus [243].
  • Regarding information funneling due to the very large dendritic fields of pallidal neurons:
    • anterograde double-labeling experiments in the squirrel monkey clearly indicate that neighboring striatal cell populations do not have overlapping terminal fields in the GP or SN.
      • Axons from adjacent striatal cell populations produce two sets of terminal fields that interdigitate but never mix.
      • cortical information is conveyed and integrated along multiple, segregated channels.
  • Output of GPi/SNr = VA, VL thalamus, both ipsi and contralateral.
    • Lesser: pedunculopontine tegmental nucleus & centromedian thalamus, superior colliculus.
    • Highly collateralized output.
    • Lamellar distribution of cells that share similar functional characteristics.
    • Synapse almost exclusively on thalamic projection neurons.
    • Centromedian nucleus: no projection to the cortex; rather projects to the striatum, hence is involved in regulation.
    • Pedunculopontine nucleus: mostly re-afferent back to the BG!
      • innervation of the SNc, subthalamic nucleus, and the pallidum. [95,149,186-188,202,207,215,263,277].
      • Acetylcholine output.
      • Deep cerebellar nuclei project to the pedunculopontine nuclei in primates.
  • GPe: efferent fibers from large terminal boutons that make synapses mostly of the symmetrical type with proximal dendrites and soma of GPi/SNr neurons. These GABA synapses may be of ultimate importance in regulating activity.
    • Also projects to the reticulothalamic region, which supplies GABA synapses to the rest of the thalamus, hence GPe can disinhibit most of the thalamus. Such complexity.

____References____

[0] Parent A, Hazrati LN, Functional anatomy of the basal ganglia. I. The cortico-basal ganglia-thalamo-cortical loop.Brain Res Brain Res Rev 20:1, 91-127 (1995 Jan)
[1] Parent A, Hazrati LN, Functional anatomy of the basal ganglia. II. The place of subthalamic nucleus and external pallidum in basal ganglia circuitry.Brain Res Brain Res Rev 20:1, 128-54 (1995 Jan)