Fig. 2. Myelin plasticity contributes to learning and memory by altering the frequency, phase and amplitude coupling of neural oscillations.

a Conduction latency will have a profound influence on phase and frequency coupling of neural oscillations between distant brain regions that are critical for memory induction and recall. b The frequency of oscillations in local circuits, such as gamma oscillations in cerebral cortex, will be affected by conduction velocity in axons between inhibitor and excitatory neurons. Excitatory-inhibitory coupling between inhibitory interneurons and excitatory pyramidal neurons generate gamma oscillations. The frequency of oscillation is affected by the strength and latency of synaptic transmission, which is influenced by myelination of axons on inhibitory and excitatory neurons. Action potential firing (green) is entrained by the phase and frequency of subthreshold neural oscillations and local field potentials. c Myelinated projecting axons from inhibitory neurons in CA1 of the hippocampus to the subiculum. Myelin sheath thickness and axon diameter are larger in axons from hippocampal inhibitory neurons that project over long distances than CA1 axons from excitatory neurons to provide more rapid conduction velocity to sustain neural oscillations at appropriate phase and frequency between hippocampus and cortical regions participating in memory formation and recall. The apparent loose compaction of myelin is an artifact of fixation necessary for immunocytochemistry. Reprinted from Jinno et al., (2007)⁹⁵ with permission (copyright 2007 Society for Neuroscience). d Scanning electron micrograph of a node of Ranvier on an axon of an interneuron in the cerebral cortex, immunolabeled with myelin basic protein (cyan) and GABA (red). e A large fraction of inhibitory interneurons are myelinated in cerebral cortex. GABA (red), myelin basic protein (white), nuclei stained with DAPI (blue). d, e reprinted from Micheva et al., (2016)¹⁸ with permission.