When we learned the simple overview of the nervous system in grade school, we were taught that the brain sent signals to muscles to contract and that is how we moved. And by brain, we assumed the thinking part up high in the head. But it cannot be so.
A little deer is born and in a very short time is standing and in a little longer is taking its first wobbly step. Within a couple of days it is running and frolicking. Deer are not that special; other animals ‘learn’ to get around very quickly too. Even humans babies, if they are held upright with their feet touching a surface will walk along that surface. In a sense, the spinal cord knows how to walk by lifting and moving forward alternate legs. It does not know how to walk well, but the basics are there. Human babies are slower at managing to get around because they are born at a less developed stage and walking on two legs rather than four is trickier. In all sorts of observations and experiments there is evidence that the ability to walk is innate in the spinal cord and does not require the brain.
The spinal cord has some primitive control modules or muscle synergies. Muscle synergies are present in a number of natural behaviors; they are low-level control networks found in the brain stem and spinal cord that coordinate a group of muscles. They make common movements easier to order up. We have the ‘intent to go over there’ and without any more conscious thought we do it in an automatic way. Now if we had to trigger individual muscles in the right time sequence, it would likely take many hours to get not very far with a number of falls along the way. One could say that we would ‘get the hang of it’ as we did it. But that is saying we would make parts of it automatic (create modules and synergies).
This modularization of motor control is layered. The simplest control is in the spinal cord, but it is modified and adapted to conditions by the brain stem and especially the cerebellum. The cerebellum gets instructions from other parts of the brain and finally these modules within modules are able to execute the simple ‘intention to go over there’.
The synergies in a baby’s spinal cord are an ancient set that is similar of all mammals (probably all land vertebrates). The muscles work in a rhythm where each event triggers the next in a circle. There are two primitives that are involved in human walking that we are born with. One is to bend the leg so that the foot leaves the ground and moves forward then goes back down and straightens. Two is a forward push against the ground by the straight leg. These two complexes of muscle contractions and relaxations are wired so that their action in one leg inhibits their action in the other. When the left leg does one, the right leg cannot do one but can do two. And when the left leg does two, the right cannot do two but can do one. They are also wired so that in each leg it is the end of one that triggers the start of two and the end of two triggers the start of one. It is the same in four legged animals except there is another set of inhibitions between the front and hind legs. At this level it is not very adaptive and can only react to sensory information that comes through the spinal cord from the muscles, joints and skin. Babies cannot use this facility to get around because they do not have the strength to maintain the posture needed with such a large heavy head on such a little body, and more importantly, the spinal cord has no information from the ears about balance. Balance is very important for bipedal walking. The baby must create two other synergies: to react to balance information and to use the hips, back and arms to keep the center of gravity over the legs. In the meantime, when they don’t have the strength, they can crawl using the 4 legged modules.
The cerebellum and brain stem add the control of balance and of pace (there are relative changes to the timing of events when the whole process is sped up). They can correct for uneven ground. They can keep the direction of motion toward a target. But the coordination control of the lower brain is not just direct signals to muscles but uses the synergies built into the spinal cord. And it is much more complex than the action in the spinal cord. In fact, the cerebellum has more neurons that the whole rest of the brain. It manages the modules, timing, adjustments to modules, effects from sensory input and feedback and commands from higher levels of the brain, then packages it all for execution. Another great trick of the cerebellum is to do two things at the same time, say walk and throw a ball. Both may be deep seated modules but there are adjustment to be made where they interfere with one another.
The point I am making here is that although movement seems so easy for us to execute, that is because it is not arranged consciously, or even largely in the cerebral hemispheres. It is modularized so that a simple request in the cerebral cortex goes through layers of calculation and fine-tuning to become individual signals to individual muscles. It is synergy/modularization that gives us this flexible but easy to use system. We are surprised that it is easier to create a program to play chess in the abstract (and win) than it is to program a robot to physically move the pieces and operate the time clock in a game. When we do not understand how something is done, it appears easy. It is a common trap.