Axons are usually surrounded by a fatty substance called myelin. Myelin allows action potentials to propagate quickly from one neuron to another, allowing for rapid signal transmission. It is a critical part of the nervous system and it's absence has devastating effects, as seen in conditions such as multiple sclerosis.
Researchers have found that patients with low levels of folate in their blood are more likely to develop hearing loss (Gok 2004; Lasisi 2010; Gopinath 2010), and that low vitamin B12 levels are associated with hearing loss (Gok 2004) and tinnitus (Shemesh 1993).
Magnesium is known to help expand blood vessels and improve circulation; it also helps control the release of glutamate, one of the major contributors to noise-induced hearing loss (Le Prell 2011). Animal studies have found that magnesium deficiency increases the risk of noise-induced hearing loss (Sendowski 2006b; Scheibe 2002).