title: A large fraction of neocortical myelin ensheathes axons of local inhibitory neurons.
abstract: Myelin is best known for its role in increasing the conduction velocity and metabolic efficiency of long-range excitatory axons. Accordingly, the myelin observed in neocortical gray matter is thought to mostly ensheath excitatory axons connecting to subcortical regions and distant cortical areas. Using independent analyses of light and electron microscopy data from mouse neocortex, we show that a surprisingly large fraction of cortical myelin (half the myelin in layer 2/3 and a quarter in layer 4) ensheathes axons of inhibitory neurons, specifically of parvalbumin-positive basket cells. This myelin differs significantly from that of excitatory axons in distribution and protein composition. Myelin on inhibitory axons is unlikely to meaningfully hasten the arrival of spikes at their pre-synaptic terminals, due to the patchy distribution and short path-lengths observed. Our results thus highlight the need for exploring alternative roles for myelin in neocortical circuits.
authors.affiliation: Department of Molecular and Cellular Physiology, Stanford University, Stanford, United States.
authors.forename: Kristina D
authors.initials: KD
authors.lastname: Micheva
authors.affiliation: Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, United States.
authors.forename: Dylan
authors.identifier: 0000-0002-5012-1690
authors.initials: D
authors.lastname: Wolman
authors.affiliation: Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, United States.
authors.forename: Brett D
authors.initials: BD
authors.lastname: Mensh
authors.affiliation: Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, United States.
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Pax
authors.affiliation: Department of Molecular and Cellular Physiology, Stanford University, Stanford, United States.
authors.forename: JoAnn
authors.initials: J
authors.lastname: Buchanan
authors.affiliation: Department of Molecular and Cellular Physiology, Stanford University, Stanford, United States.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Smith
authors.affiliation: Janelia Research Campus, Howard Hughes Medical Institute, Ashburn, United States.
authors.forename: Davi D
authors.identifier: 0000-0002-8218-7926
authors.initials: DD
authors.lastname: Bock
mesh_terms: D000818:Animals; D007395:Interneurons / Q000166:cytology*; D051379:Mice; D009186:Myelin Sheath / Q000737:chemistry* / Q000378:metabolism*; D019579:Neocortex / Q000737:chemistry* / Q000166:cytology*; D011506:Proteins / Q000032:analysis*
chemical_list: D011506:Proteins
keywords: array tomography; basket cells; cortex; electron microscopy; interneurons; mouse; myelin; neuroscience