The Evolution of Myelin: Theories and Application to Human Disease

Abstract

Myelin, once thought of as a simple insulating sheath, is now known to be a complex, dynamic structure. It has multiple functions in addition to increasing conduction velocity, including reducing the energetic cost of action potentials, saving space, and metabolic functions. Myelin is also notable for likely having arisen independently at least three times over the course of evolutionary history. This article reviews the available evidence about the evolution of myelin and proposes a hypothesis of how it arose in vertebrates. It then discusses the evolutionary trade-offs associated with myelination and suggests a possible animal model for further study of this phenomenon. Finally, it briefly covers the neural regulation of myelination before discussing possible roles of myelin in human social cognition and evolution, and the relevance of this to human disease.

Experimental and Computational Studies on the Basic Transmission Properties of Electromagnetic Waves in Softmaterial Waveguides

“…Our results are consistent with previous evidences that the conduction velocity was positively related to the thickness of the myelin sheath 42 . However, some other literatures demonstrated that the conduction velocity in myelinated axons reached the maximum when the ratio of internal diameter to external diameter tend to 0.6 43 , 44 , while some scientists reported it was sensitive to the internode length, i.e., the distance between nodes of Ranvier in myelinated axon 42 , 45 . The validity of softmaterial waveguide discussed in this work and the proposed soliton-like EM pulse model 14 – 16 may also help to the “saltatory conduction” in myelinated axons 41 , 46 – 48 .…”

The variability of MR axon radii estimates in the human white matter

“…Second, it has been hypothesized that the large‐radius axons of long‐range neurons are essential to maintaining neural synchrony (Buzsáki, Logothetis, & Singer, 2013). However, large‐radii axons come at a disproportionate cost in terms of energy use and spatial constraints (Knowles, 2017; Perge et al, 2009). Histological studies have extensively reported axon radii to be in the range 0.25–1 μm for human brain (Aboitiz et al, 1992; Caminiti, Ghaziri, Galuske, Hof, & Innocenti, 2009; Liewald, Miller, Logothetis, Wagner, & Schüz, 2014; Tang, Nyengaard, Pakkenberg, & Gundersen, 1997), with only 1% of all axons having a radius greater than 1.5 μm (Caminiti et al, 2009).…”

The variability of MR axon radii estimates in the human brain

“…Second, it has been hypothesized that the large-radius axons of long-range neurons are essential to maintaining neural synchrony (Buzsáki et al, 2013). However, large-radii axons come at a disproportionate price in terms of energy use and spatial constraints (Perge et al, 2009;Knowles, 2017). Histological studies have extensively reported axon radii to be in the range 0.25 − 1 µm for human brain (Aboitiz et al, 1992;Caminiti et al, 2009;Liewald et al, 2014;Tang et al, 1997), with only 1% of all axons having a radius larger than 1.5 µm (Caminiti et al, 2009).…”