Myelinated inhibitory axons in human neocortex
Numerous myelinated axons traverse the human neocortex. In a previous paper (Micheva et al., 2016) we showed that in mouse many of these axons belong to local inhibitory neurons, the parvalbumin-positive basket cells. Here, using samples of neurosurgically-excised cortex, we confirm the presence of myelinated inhibitory axons in all layers of human neocortex. As in mouse, these axons have distinctive features, including high neurofilament content, short nodes of Ranvier, and high content of myelin basic protein in their myelin sheath. We further show that, consistent with the known high-energy demands of parvalbumin interneurons, the inhibitory myelinated axons have more mitochondria, as well more 2,3-cyclic nucleotide 3-phosphodiesterase (a protein enriched in the myelin cytoplasmic channels thought to provide access for trophic support from oligodendrocytes). The distinctive features of myelinated inhibitory axons in human cortical grey matter may have important implications for neurological disorders that involve pathologies of myelinated axons.