Diversity of Cortico-descending Projections: Histological and Diffusion MRI Characterization in the Monkey

Giorgio M. Innocenti*, Roberto Caminiti, Eric M. Rouiller, Graham Knott, Tim B. Dyrby, Maxime Descoteaux, Jean Philippe Thiran

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


The axonal composition of cortical projections originating in premotor, supplementary motor (SMA), primary motor (a4), somatosensory and parietal areas and descending towards the brain stem and spinal cord was characterized in the monkey with histological tract tracing, electron microscopy (EM) and diffusion MRI (dMRI). These 3 approaches provided complementary information. Histology provided accurate assessment of axonal diameters and size of synaptic boutons. dMRI revealed the topography of the projections (tractography), notably in the internal capsule. From measurements of axon diameters axonal conduction velocities were computed. Each area communicates with different diameter axons and this generates a hierarchy of conduction delays in this order: a4 (the shortest), SMA, premotor (F7), parietal, somatosensory, premotor F4 (the longest). We provide new interpretations for i) the well-known different anatomical and electrophysiological estimates of conduction velocity; ii) why conduction delays are probably an essential component of the cortical motor command; and iii) how histological and dMRI tractography can be integrated.

Original languageEnglish
JournalCerebral Cortex
Issue number2
Pages (from-to)788-801
Publication statusPublished - 1 Feb 2019


  • Axon diameters
  • Conduction velocity
  • Corticofugal projections
  • Diffusion tractography
  • Histology


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