Two types of projection neurons in the internal pallidum of primates: single-axon tracing and three-dimensional reconstruction

J Comp Neurol. 2001 Oct 15;439(2):162-75. doi: 10.1002/cne.1340.


The axonal projections of the internal pallidum (GPi) in cynomolgus monkeys (Macaca fascicularis) were studied by labeling small pools of neurons with biotinylated dextran amine. Fifty-two axons were reconstructed entirely from serial sections with a camera lucida. Two types of projection neurons were identified in the GPi on the basis of their target sites. The abundant and centrally located type I neurons gave rise to a long axonal branch that descended directly to the pedunculopontine tegmental nucleus, where it arborized discretely. Other branches ascended to the thalamus and broke into 10-15 thinner collaterals that ran through most of the ventral anterior nucleus, where they terminated as typical plexuses. About half of these axons gave rise to collaterals that arborized in both components of the centre médian/parafascicular thalamic complex. The less numerous and peripherally located type II neurons had an axon that climbed the rostral thalamic pole, coursed along the stria medullaris, and arborized profusely within the lateral habenular nucleus, which stood out as the most densely innervated pallidal target. Some type II axons provided collaterals to the anterior thalamic nuclei. A small proportion of axons of both types had branches that crossed the midline and terminated in contralateral GPi target structures. Three-dimensional reconstruction showed that type I axons arborized principally along the sagittal plane. These data reveal that GPi neurons of type I act through a widely distributed axonal network upon thalamic and brainstem premotor neurons, whereas type II neurons act in a much more focused manner upon lateral habenular neurons.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Action Potentials / physiology*
  • Amidines / pharmacokinetics
  • Animals
  • Axons / physiology
  • Axons / ultrastructure*
  • Biotin / analogs & derivatives*
  • Biotin / pharmacokinetics
  • Calbindins
  • Cell Size / physiology
  • Dendrites / physiology
  • Dendrites / ultrastructure*
  • Dextrans / pharmacokinetics
  • Efferent Pathways / cytology*
  • Efferent Pathways / physiology
  • Electron Transport Complex IV / metabolism
  • Female
  • Fluorescent Dyes / pharmacokinetics
  • Globus Pallidus / cytology*
  • Globus Pallidus / physiology
  • Habenula / cytology
  • Habenula / physiology
  • Image Processing, Computer-Assisted
  • Immunohistochemistry
  • Intralaminar Thalamic Nuclei / cytology
  • Intralaminar Thalamic Nuclei / physiology
  • Macaca fascicularis / anatomy & histology*
  • Macaca fascicularis / physiology
  • Male
  • Molecular Probes / pharmacokinetics
  • S100 Calcium Binding Protein G / metabolism
  • Tegmentum Mesencephali / cytology
  • Tegmentum Mesencephali / physiology
  • Ventral Thalamic Nuclei / cytology
  • Ventral Thalamic Nuclei / physiology


  • Amidines
  • Calbindins
  • Dextrans
  • Fluorescent Dyes
  • Molecular Probes
  • S100 Calcium Binding Protein G
  • biotinylated dextran amine
  • diamidino compound 253-50
  • Biotin
  • Electron Transport Complex IV