By: Charles Frye
In addition to constructing a miniature model of the world inside your skull for you to inhabit, the brain is also tasked with generating sequences of actions in the real world – breathe in, breath out; lather, rinse, repeat; stop, drop, and roll. The brain performs these actions using the skeletal muscles, more commonly known as the muscles. When you feel the desire to take a step forward, reach for an object, or scratch an itch, the motor cortex must determine how to tug on these big bundles of springs in order to swing the bones to which they are attached in precisely the correct fashion to produce the desired movement. To gain an appreciation for just how hard this is, check out this compendium of robot fail gifs. Walking isn’t so easy after all!
These commands rely on a well-made interface between the nervous system and the muscles. Each muscle fiber needs to be matched to exactly one neuron, and all of the motor neurons need to be matched to at least one muscle fiber. To complicate matters further, the neurons in question are born inside the spinal cord, while the muscle cells are born far away. In one final twist of complexity, large collections of individual muscle cells combine together, assembling themselves, Voltron-style, into a single, more powerful unit called a muscle fiber, which has many nuclei and many mitochondria.
So how are we to ensure that our motor neurons and our muscle fibers are well matched? One modest proposal is to generate far more neurons than you need, and any that don't manage to find a motor neuron can just be killed. In order to ensure that this diktat is followed, nature adopts a strategy straight out of Saw II: motor neurons are, from the moment they are born, searching frantically for the antidote to a poison that will kill them when a timer runs out. They are, like Biggie Smalls, born ready to die. The antidote is released by muscle fibers, but it is only released in small quantities and to directly-connected neurons.
So, the motor neurons rush out from the spinal cord, making a mad dash for the nearest muscle fiber. Some cells find a partner and begin to form connections, also called neuromuscular junctions, but others are not so lucky.
These unlucky cells are drawn to the “scent” of the antidote as it diffuses away from these immature connections – the table scraps, if you will. In a desperate attempt to survive, these cells become locked in a duel to the death with the original tenants -- whoever can make a stronger connection faster will choke the other one out. When all is said and done, only about half of all the motor neurons will survive to become functional.
If you enjoyed this, check out more explanations of the foundational concepts of neuroscience at Charles' website!