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.
Network anatomy and in vivo physiology of visual cortical neurons.Publicly available at the Open Connectome Project (Mouse V1; http://openconnecto.me/bock11/).
Animal experimentation: The tissue for the array tomography experiments was provided by Dr. Richard Weinberg, University of North Carolina (UNC). All animal procedures were performed according to NIH and UNC guidelines with a protocol (#13-258.0) approved by the UNC Institutional Animal Care and Use Committee. Mice were housed in an approved UNC animal care facility on a 12-hour light/dark cycle with ad libitum food and water access. Immediately before the terminal surgery, mice were transported to the research laboratory, where they were deeply anesthetized with sodium pentobarbital (80 mg/kg ip). JRC Immunohistochemistry: Mice were housed on a 12-hour light/dark cycle with ad libitum food and water access. Experimental procedures were conducted according to the National Institute of Health guidelines for animal research and approved by the Institutional Animal Care and Use Committee at Janelia Farm Research Campus. Approved animal protocol is IACUC 11-71.
- Inna Slutsky, Tel Aviv University, Israel
© 2016, Micheva et al.
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