title: Axon diameters and conduction velocities in the macaque pyramidal tract.
abstract: Small axons far outnumber larger fibers in the corticospinal tract, but the function of these small axons remains poorly understood. This is because they are difficult to identify, and therefore their physiology remains obscure. To assess the extent of the mismatch between anatomic and physiological measures, we compared conduction time and velocity in a large number of macaque corticospinal neurons with the distribution of axon diameters at the level of the medullary pyramid, using both light and electron microscopy. At the electron microscopic level, a total of 4,172 axons were sampled from 2 adult male macaque monkeys. We confirmed that there were virtually no unmyelinated fibers in the pyramidal tract. About 14% of pyramidal tract axons had a diameter smaller than 0.50 μm (including myelin sheath), most of these remaining undetected using light microscopy, and 52% were smaller than 1 μm. In the electrophysiological study, we determined the distribution of antidromic latencies of pyramidal tract neurons, recorded in primary motor cortex, ventral premotor cortex, and supplementary motor area and identified by pyramidal tract stimulation (799 pyramidal tract neurons, 7 adult awake macaques) or orthodromically from corticospinal axons recorded at the mid-cervical spinal level (192 axons, 5 adult anesthetized macaques). The distribution of antidromic and orthodromic latencies of corticospinal neurons was strongly biased toward those with large, fast-conducting axons. Axons smaller than 3 μm and with a conduction velocity below 18 m/s were grossly underrepresented in our electrophysiological recordings, and those below 1 μm (6 m/s) were probably not represented at all. The identity, location, and function of the majority of corticospinal neurons with small, slowly conducting axons remains unknown.
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, United Kingdom; Research Department of Cell and Developmental Biology, University College London, United Kingdom; FR3636 Centre National de la Recherche Scientifique/Université Paris Descartes and Université Paris Diderot, Sorbonne Paris Cité, France; and.
authors.forename: L
authors.initials: L
authors.lastname: Firmin
authors.affiliation: Research Department of Cell and Developmental Biology, University College London, United Kingdom;
authors.forename: P
authors.initials: P
authors.lastname: Field
authors.affiliation: FR3636 Centre National de la Recherche Scientifique/Université Paris Descartes and Université Paris Diderot, Sorbonne Paris Cité, France; and.
authors.forename: M A
authors.initials: MA
authors.lastname: Maier
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, United Kingdom;
authors.forename: A
authors.initials: A
authors.lastname: Kraskov
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, United Kingdom;
authors.forename: P A
authors.initials: PA
authors.lastname: Kirkwood
authors.affiliation: Department of Physiology, Faculty of Medicine, Kinki University, Osaka, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nakajima
authors.affiliation: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, United Kingdom;
authors.forename: R N
authors.initials: RN
authors.lastname: Lemon
authors.affiliation: Research Department of Cell and Developmental Biology, University College London, United Kingdom; m.glickstein@ucl.ac.uk.
authors.forename: M
authors.initials: M
authors.lastname: Glickstein
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure*; D008252:Macaca fascicularis; D008253:Macaca mulatta; D008297:Male; D009413:Nerve Fibers, Myelinated / Q000502:physiology / Q000648:ultrastructure; D009431:Neural Conduction; D011712:Pyramidal Tracts / Q000502:physiology* / Q000648:ultrastructure; D011930:Reaction Time
keywords: axon; conduction velocity; corticospinal; macaque