Muscle Sprouts and Fibers
SPROUTS AND FIBERS: The “Diet” of Polio Recovery
“As polio survivors recovered strength after the initial onslaught, their individual muscles fibers grew and were on average twice the size of fibers in those who didn't have polio. Fibers also looked abnormal, appearing "moth eaten" and showing damage typically found only in heavy weight lifters. What's more, thanks to axonal sprouting, the remaining motor neurons turn on about 16 times more muscle fibers than in someone who didn't have polio.” From The Polio Paradox
Sprouting isn’t unique to polio survivors. It occurs in motor neuron diseases and in normal aging; as motor neurons die their compatriots sprout to take over the newly orphaned muscle fibers. Unfortunately, older motor neurons sprouting to "adopt" orphaned muscle fibers comes at a cost.
As polio survivors age, their reduced number of remaining, damaged motor neurons are metabolically stressed as they stimulate more and more muscle fibers. A handful of autopsies have been performed on polio survivors who died more than 40 years after having had polio and who reported muscle weakness later in life. Those polio survivors' motor neurons were found to be smaller in size and their sprouts were thinner than normal. This is no surprise. Neurons in older non-polio survivors have been found to be less able to make sprouts and, when they do, the sprouts are thinner and are not covered with myelin, the fatty insulation separating one sprout from another. What's more, fewer muscle fibers are reconnected to the new sprouts and less acetylcholine (the chemical that "tells" muscle fibers to contract) is released.
However, even if sprouts were insulated, fat and fully functioning, the muscle fibers they are supposed to turn on are not. Everyone who gets older, not just polio survivors, loses muscle fibers. Remaining fibers atrophy, get smaller in size. And aging muscle is not as pliable or as able to contract quickly as does younger muscle. These changes cause a loss of muscle strength, reduced muscle contraction speed and decreased muscle endurance as everyone ages. These effects of aging can further overwork polio survivors' decreasing number of damaged motor neurons and reduce polio survivors’ muscle strength.
Given this description of the state of muscle fibers and motor neurons, is it any wonder that overexertion