Trophic Effects of Exercise on Motor Neurons: Potential Involvement of Satellite Cell-Derived IGF-I and Other Skeletal Muscle-Derived Growth Factors

This article [Gardiner et al., 2006: (http://0-jap.physiology.org.library.pcc.edu/cgi/content/full/101/4/1228)(http://www.ncbi.nlm.nih.gov/pubmed/16778002?dopt=Abstract)] is really interesting, and the authors discuss research that provides evidence for trophic actions or the retrograde axonal transport of growth factors, produced locally in the skeletal muscles in response to exercise, on the motor neurons that innervate skeletal muscles. Insulin-like growth factor-I (IGF-I), for example, is produced by satellite cells in response to strength training and can act on motor neurons innervating the muscles [Neff et al., 1993: (http://www.ncbi.nlm.nih.gov/pubmed/8301266)]. A growth factor like IGF-I can modify the release of other growth factors in the spinal cord, such as by undergoing retrograde axonal transport or by increasing the activities of enzymes that are components of intracellular signalling cascades. IGF-I could modify gene transcription in the cell body of a motor neuron, for example, by activating various mitogen-activated protein kinase cascades near the axon terminal of the motor neuron. The IGF-I might then be degraded before it undergoes axonal transport, etc. Gardiner et al. (2006) discuss research showing that exercise increases BDNF expression (mRNA content) in the lumbar spinal cord in animals and also can increase the axonal diameters of motor neurons. In my opinion, resistance exercise (strength training) provides a much more potent stimulus for these types of trophic effects on the brain and spinal cord, and there's been a great deal of research on these types of effects of strength training or other forms of exercise. One problem is that it seems difficult to set up experiments to allow animals to do "strength training." One can make them run at a high intensity on some kind of treadmill, etc., but I would think it would be difficult to really reproduce the effects of resistance exercise in animal experiments. I also think there's a lack of recognition of the kinds of mechanisms that could come into play in response to resistance exercise. Growth factors are produced by satellite cells in the skeletal muscles, in response to some types of exercise (especially resistance exercise), and those growth factors could conceivably be transported along both the afferent sensory fibers innervating the skeletal muscles and the motor neurons themselves. The exercise-induced increases in the firing rates of afferent sensory neurons could also provide trophic effects in the spinal cord, etc.