The topic of this week was the human motor system: the spinal and the brain control of movement. I was a little surprised to learn that taken the brain aside, the spinal cord contains quite complex neural feedback and control systems. If we think about an alpha motor neuron, it only receives input from three different places: sensory information from muscle spindles, input from upper motor neurons from the brain and input from spinal interneurons. The input from spinal interneurons is the largest of these three, and there is quite a lot going on there that I had not come to think of earlier. The crossed extensor reflex, for example: if you suddenly lift up your right leg because you stepped on a lego block, the activation of extensor muscles and inhibition of the flexors on the left side compensates for the withdrawal of antigravity extensor muscles on the right side. This keeps you from falling down. This is something I have always taken for granted, and never stopped to think about what kind of control system lies behind it. Thus, the interneurons in the spinal cord play a very important role in our daily life.
One topic I found truly fascinating from a cell biologist point of view was the different types of muscle cells and neurons that innervate them. The are different types of muscle fibers: slow and two types of fast, fatigue-resistant and fast-fatigable. Each type has an innervating neuron that is the same type, because slow fibers require different signal frequencies from the neuron than fast fibers. It seems that the muscle fibre and the neuron are always a matching pair, so to say. But now we get to the truly interesting part: it is possible for the muscle fiber to change phenotype to match the changed phenotype of the innervating alpha motor neuron. This has been tested in a study where a fast muscle fiber was stripped of it’s original neuron and innervated with a slow alpha motor neuron. The muscle fiber adapted and changed it’s properties to be like a slow muscle fiber, and not just the contraction type but there was also observable changes in the underlying biochemistry of the cell. These studies are something I am going to have to look into more properly, because I am interested to get a more detailed view on the changes of the gene expression in these cells. After all, this is something that might be behind our learning and memory.