|Image: A 3D computer image of three neurons, displaying differing myelination. Myelin, a fatty insulation around nerve axon (tail) segments, speeds up neural transmission significantly. Credit Daniel Berger and Giulio Tomassy/Harvard University.|
This year, Harvard Drs. Paola Arlotta, Jeff Lichtman and Giulio Srubek Tomassy discovered something that refutes 160 years of neuroanatomy knowledge: it seems the fatty neural insulator myelin, long thought critical for fast impulse transmission through neural axons, is not ubiquitous.
Classic neuroscience teaches that myelin is distributed along axons in several myelinated segments separated by tiny myelin-free nodes. This was believed to be a universal principle, but the new findings prove the assumption to be wrong - neural axons don't have a universal myelin distribution.
According to Dr. Arlotta, myelin is a somewhat recent evolutionary innovation, and, despite its importance in the brain, the team was surprised to discover that some of our most complex neurons are less well-insulated with myelin than older, more primitive ones. Paradoxically, the further into the most evolved regions of the cerebral cortex that one explores, the less myelin is found. Neurons there have a previously unknown, intermittent myelin distribution - very short myelin-rich regions interspersed with long, myelin-free regions.
Lead author Dr. Tomassy suggests these myelin distributions might enable neurons to branch out and communicate with neighboring neurons. Since mylenated axons can't synapse, it's possible that long myelin gaps serve to synchronize responses across different neurons or to enhance neural communication. It's also possible that intermittent myelin fine-tunes electrical impulses being transmitted along axons, enabling more complex behaviors to emerge.
The findings mean the purpose of myelin is not completely understood. What's more, says Dr. Arlotta, because the most advanced, highly-evolved neurons use less myelin, this might point to our future evolution. Increased processing needs seem to be driving neural diversity and myelin alterations.
Source: Harvard researchers present new view of myelin, press release, April 18, 2014, B. D. Colen, Neuroscience News, Harvard University