eli5 :How does your brain communicate with your muscle to know exactly how far to throw a football to hit a target?
In: Biology
I answered similar questions over a year ago. I’ll copy paste and integrate the answers below:
To make any movement, you need to flex different muscles that exert forces at different angles, and at different strengths and for different durations and kinetics. If you want to for example scratch your head, you need to flex your deltoid/shoulder muscle to raise your arm, this needs to start fast but slow down such that it doesn’t overshoot and hit your head, therefore an opposing muscle like the latissimus dorsi needs to begin flexing towards the end of the motion, gradually to make your arm move smoothly. At the same time, somewhere during the motion, you need to flex your elbow with a number of muscles, while turning your forearm outwards (supination). By the time your hand reaches your head, you need to flex your fingers and extended in a specific pattern to scratch your head, you use a large amount of muscles to coordinate that. So one part of the computationally heavy process is coordinating which muscles to flex at which times to get a given motion. But there’s another part, the strength of contraction of each muscle.
Your brain divides each muscle into motor units. Each unit is composed of a neuron and a number of muscle fibers that it innervates. Motor units vary in number (different muscles have different amounts of motor units), size (therefore force), and type of fibers (fast and powerful, slow and weak but fatigue resistant, and medium). Your tendons and muscles and so on have proprioceptors which sense tension. As your brain matures from childhood, it undergoes learning processes much like a computer, it tests different neurons and senses what that corresponds to in terms of tension from your proprioceptors as well as seeing how far the arm went and how much force this corresponds to physically by eye. You also use other senses like mechanoreceptors sensing pressure in the skin like when you grip something. Over the years, your brain calibrates itself (I don’t want to drive you crazy with info, but it also calibrates how faster frequency firing gives more force in a single motor unit). Meaning, it knows which muscles are needed to start a movement, which motor units it should start with for a quick start/acceleration, which units it should recruit to increase force to the requisite level for whatever object you want to lift or move, which other muscles are needed to synergistically stabilize the movement (like shoulder muscles, chest muscles, balancing muscles in torso and legs, etc), and which other muscles to recruit and how (in terms of motor units) to antagonize the movement towards the end so that you don’t overshoot and hurt a join, and also when to stop flexing the initial muscles you started with. This information is stored in your cerebellum, this is where you store how you coordinate all this stuff. But when you want to do the actual action, things often don’t go according to plan, muscles may not be as warmed up, as big as you learned or as weak, or the assessment of needed force may be off, etc, so your brain actually gets live feedback during the action from the proprioceptors, eyes, and mechanoreceptors to ensure you’re following the proper trajectory of motion. Any tiny deviations are quickly computed and the neural output is modified to correct course.
It’s actually insanely complicated, the mathematics involved reach really high derivatives like jerk, snap, crackle and pop. You know, you have for example acceleration being the derivative of velocity (so it’s change in speed, or change in rate of change of position). Jerk would be the rate of change of acceleration (rate of change in rate of change in rate of change in position), and so on. Your brain computes to really high levels of these derivatives to ensure that the muscles move in ultra smooth fashions. All this information of which muscles, how strong, for how long and how (mathematically) is stored in networks of neurons within your cerebellum which is this little brain you have at the bottom back of your skill. If you do an activity many times, you brain stores this information quite strongly (the connections between the neurons in the specific network become really strong making them easy to activate again). That is how muscle memory works, it’s in the brain.
Your brain is an *amazing* computer. It is able, with training, to do advanced calculus on a subconscious level. It plots the trajectory, and sends a signal to your muscles to contract and relax in the correct order and intensity to get the job done.
Now, just starting out, you can barely do it at all. When you are a little kid, you can barely toss a ball. But with every single attempt, every time seeing something fall, every time watching the arc of a ball, your brain is rewriting the pathways it uses to correct for errors. Each time, it gathers more information and adjusts accordingly.
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