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2.2.2 Axonal regeneration and rate of elongation
Two major contributions of physical exercise to peripheral nerve regener-
ation are the increase in the number of outgrowing axons and the increase in
the rate of axonal elongation ( English, Cucoranu, et al., 2011; English et al.,
2009; English, Wilhelm, et al., 2011; English, Cucoranu, et al., 2011;
English, Wilhelm, et al., 2011; Sabatier et al., 2008 ). The studies of
Doyle and Roberts (2006) , in the spinal cord, and of Sabatier et al.
(2008) , in the sciatic nerve, are two exemplar studies that demonstrate
the ability of physical exercise in increasing the rate of elongation of reg-
enerating axons. In the latter study, different treadmill running protocols
were used, ranging from continuous 1-h exercise (belt speed: 10 m/min)
to low- and high-intensity interval training, with different number of rep-
etitions (belt speed: 20 m/min) ( Sabatier et al., 2008 ). In general, treadmill
exercise during the initial 2 weeks of recovery accelerated the growth of
axons in the common fibular nerve of the thy-1-YFP-H mice. Continuous
1-h treadmill running and high-intensity interval training both were associ-
ated with axonal profiles that were twice the length of those observed in
unexercised animals ( Sabatier et al., 2008 ). In another study, mild-intensity
continuous (1 h, 10 m/min) or interval (20 m/min, 2 min exercise, 5 min
rest) treadmill exercise for 2 weeks immediately following sciatic nerve tran-
section and repair increased the number of regenerating motoneurons, com-
pared to unexercised animals ( English et al., 2009 ). Importantly, the effect of
treadmill running in stimulating regeneration of axotomized motoneurons
was of similar magnitude of that seen with electrical stimulation or treatment
with the glycosaminoglycans-degrading enzyme chondroitinase ABC, but
avoiding the undesired axonal misrouting ( English et al., 2009; Sabatier
et al., 2008 ). Treadmill walking/running also raises the number of myelin-
ated nerve fibers ( Asensio-Pinilla et al., 2009; Ilha et al., 2008; Udina,
Puigdemasa, & Navarro, 2011 ) and, in some cases, improves myelin sheath
thickness of regenerated peripheral nerves ( Ilha et al., 2008 ).
The effect of treadmill running in increasing the elongation rate of reg-
enerating axons relies on expression of BDNF ( Molteni, Zheng, Ying,
G´mez-Pinilla, & Twiss, 2004; Wilhelm et al., 2012 ) and NT-4/5
( English, Cucoranu, et al., 2011; English, Wilhelm, et al., 2011 ) in axons
and Schwann cells. Mice of the thy-YFP-H lineage, lacking BDNF in their
YFP þ neurons, show defective ability to regenerate their axons though
nerve grafts harvested fromwild-type counterparts or frommice also lacking
the expression of BDNF by Schwann cells ( Wilhelm et al., 2012 ). Treadmill
running was unable to correct the abnormal axonal growth of neurons not
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