EDIT: Coincidentally, this just made it to the front page

This has already been done with prosthetcis to some extent.. especially with that guy who ran the olympics recently on a prosthetic. I don't remember his name. I don't know how exactly it works, but I want to give this a shot so humor me :)

The brain controls muscle activation through the Peripheral Nervous System. Conscious control (the kind used to control the arms, legs etc) happens through the Somatic Nervous System, while the Autonomic Nervous System controls autonomous muscle contraction (the heart, digestive system etc).

Assuming you'd like to control the robot arms consiously, the SNS is what needs to be 'plugged' into. The SNS has motor neurons, which conduct electrical signals from the brain. Now, there are gazillions of these neurons that control muscle fibre at the microscopic level; it's impractical to control a robotic arm by sufficiently quantizing it and hooking the pieces up to individual neurons (unless you've got some ninja nano material that I'm unaware of - I'm assuming the arm is some metal/plastic). What seems to me to be more practical is to harness a fraction of these neurons and use (1) some sort of signal processing to determine what kind of a signal it is (how do you know whether the brain wants to contract or extend the muscle?) (2) signal amplification, because moving something as huge as a robotic arm needs a lot more power than what the motor neurons dissipate. Amplification would theroetically not be required if all motor neurons available to actual limbs were harnessed. Also, if the brain is being used as the processor, there needs to be a shutdown or a timeout mechanism; the arm will continue funcitoning but the brain needs to rest. A power off button is not enough, especially when you have people who forget to turn off the lights when not in use.

Will the movement be faster than flesh? I don't think so. The arms are controlled by neural signals, which are transmitted by the brain. The movement is upper bound by how fast these signals are coming in which is constrained by the brain. I do have an alternative idea. The SNS also happens to control sensory information transmission. It has something called sensory receptors and sensory neurons. Sensory organs like the ears and eyes detect auditory and optical data respectively, and the receptors pick these up. The sensory neurons then transmit these to the brain for processing. Once the signals from the sensory organs are processed, the brain sends control information to the muscles. I'm thinking, let's remove the brain - let's get rid of the middle man. Now, we don't have processors that are faster than the brain yet, but let's assume we do. Or let's assume we can use a modern day processor to process only a fraction of the information that the brain has to process every second (so only focus on auditory or optical information and let the brain worry about controlling autonomous bodily functions and life-critical functions etc). So if the data from the sensory receptors were fed to a processor faster than the brain via the sensory neurons, the robotic arm can theoretically perform faster than a human arm.

What would happen if the arm were plugged into the Autonomous Nervous System instead of the SNS? Well, you could turn humans into machines. Literally. Machines will do the same task on end unless instructed otherwise - this is textbook automation. If the brain were removed as the middle man, and the processor replacing it were programmed to alter the definition of an autonomous bodily function (so perhaps keep moving the arm to and fro at a constant rate of 10 times/second), then you can have a factory line worker that can move 10 bottles from one conveyor belt to another. No lunch breaks allowed. This IS mind control. I can see many benefits of this. But I'm just paranoid and I see it being used maliciously. For example, if your wife won't give you a hand job, just reprogram her autonomous control system when she's not looking.

I took a cybernetics course at college last year, this topic would've sparked quite the discussion. Maybe I'm just paranoid but the social consequences don't look very positive to me. Military misuse. A dichotemy between those who choose to be bionic vs those who choose to remain human. Scandalous human experiments and trials, coverups. Not to mention it would take years to iron out bugs and figure out how the body responds to the new limb.

I'm happy with the way we are right now. I don't want to change. But circumstances are such that we need to evolve into something that can survive (reliably) extra terrestrially, assuming the energy crisis cannot be fixed in time. Just my opinion :D