Muscular movements are controlled by the central nervous system. Right now my brain is actively planning the muscular contractions that will make my fingers exert a force on different buttons on my keyboard, ultimately resulting in the text that you are hopefully enjoying right now. How does this work? Well, there are many unsolved mysteries, but we have also learned quite a lot.
Two areas of the brain, the "supplementary motor area" (SMA) and the "premotor area" (PMA), seems to be involved in planning the exact series of muscle contractions that are required in order to do something such as writing this text. How do we know this? Put a person in an fMRI machine and ask them to imagine doing something, and wallah, SMA and PMA lights up (meaning that more blood is going to these areas, mening that more glucose gets there, meaning more activity in those cells)! In order to actually move you will also have to get your motor cortex involved. It is from this area that axons travel down into the spinal cord and elicits movements. If you stimulate the motor cortex electrically during a surgery (this is sometimes done to see how the body is mapped onto the motor cortex), then the corresponding muscles will contract, perhaps resulting in an arm or a leg flapping out. So when your SMA and PMA have planned the contractions they will communicate with the motor cortex which then sends signals down to the alpha motor neurons in your spinal cord which in turn will release acetylcholine onto the muscles causing them to contract. All this happens within a few milliseconds! (Of course other areas such as the basal ganglia and my own darling, the cerebellum are also involved in movements, this was merely a simplified account.)
Who cares?, where are the applications? Back in 1982 a guy named Georgopoulos measured the activity in the motor cortex of a monkey while it was performing some well defined motor tasks. Georgopoulos found that a certain movement would be associated with a particular pattern of activation in the motor cortex. Thus, merely by looking at the activation in the motor cortex he could predict that the monkey was trying to move say, its left index finger. This could be very useful for people suffering from paralysis. Theoretically it should be possible to measure motor cortex activity and from that see what movements the motor cortex is trying to do. Then, to help a paralysed patient what you need to do is to connect a robotic arm programmed to move in response to neuronal activity. This procedure is no longer science fiction, it is reality. In this article it is described how some surgeons used signals coming from the motor cortex to make a prosthetic arm move, in other words they made a true cyborg out of lucky Mitchell, 24.
Although I am pessimistic about the prospect of creating a brain that is as complex as our own I do think that in the future more and more applications like this one will appear.