Spinal nerves tone: the spinal nerves run through the spinal column and move all over the body. They have to maintain proper tone to get what is called the General Adaptive Potential -- this is the way we interpret stress and adapt to it. As we age, we lose tone normally and also when over-stressed. Which is why yoga places so much emphasis on maintaining spinal nerve tone -- I guess that is why they say forward bends indicate how supple and toned your spinal nerves are. So, yes, stretches that challenge and tone the spine, including in yoga, is most important if you want to be able to allow the body to revert to normal levels of functioning (called homeostasis).