I thought I recalled the term ‘Myelin Sheath’ from my research into Methylsulfonylmethane (MSM). Just had a quick look on Google and MSM is mooted as being of potential benefit to those suffering from Multiple Sclerosis (which is Myelin Sheath inflammation/damage, albeit brain/spinal cord related). There are also a few more general myelin sheath and MSM related topics.
MSM also has myriad additional benefits for general health as well as many specific to training and exercise. It’s readily available in bulk raw powder form, and inexpensive. Suggested doses are usually between 1g and 3g daily – I believe these are a little on the low side, with 6g daily more realistic (I currently use 8g – 4g twice a day). Being water soluble and non-toxic higher doses are readily tolerated (but it’s wise to build them up over a period of time).
Whilst not a downside per se, note that MSM is not a quick fix, requiring chronic elevated dosing for as much as 6 to 9 months to elicit maximal benefit. So it may not be the immediate answer to your problem but it may be something to consider for longer term management.
PS: As MSM is a lot cheaper than glucosamine (and certainly it's sidekick chondroitin) it leaves funds free to aim at a quality source of methylcobalamin B12.