In various sci fi settings you'll often see soldiers that have been modified beyond their baseline human level. Sometimes they are genetically grown this way, but other times they're modified as adults. The latter is the focus of this question.

If one were to place a custom organ into a person, how does the brain control it in the first place?

Such organs aren't mere replacements to existing body function, but entirely new body parts in this situation. I.e extra glands that secrete a hormone, or help the immune system, or filter air before it gets to the lungs etc.

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    $\begingroup$ Is it an organ mainly controlled by nerves (like muscles) or by hormones (thinking of blood sugar level control)? In the case of nerves, shall we consider the surgeon made the link between the nervous system and the new organ (hopefully so!)? $\endgroup$
    – Tortliena
    Oct 14 at 11:21
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    $\begingroup$ @FIRES_ICE, you've received some interesting answers! However, it's worth pointing out that authors don't explain things like this for a reason. To underscore Starfish Prime's answer, our brains evolved to control our bodies. Period. His suggestions make sense. Or you could use the Marvel Universe's solution (See Ironman 3) where it's declared as a rule of the MCU world that the human brain has evolved with the capacity for an upgrade, and that upgrade is exploited to create the antagonists. How it does that is entirely ignored (mostly because it would be boring). $\endgroup$
    – JBH
    Oct 14 at 16:20
  • $\begingroup$ @Tortliena Ideally it should be both if we're going by the standards of sci fi soldiers. On one end of the spectrum you have drug glands from The Culture series, which is controlled via thought. On the other end you have things like Warhammer where there's entirely custom organs or even extra hearts. $\endgroup$
    Oct 15 at 8:00
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    $\begingroup$ Hi FIRES_ICE! I find your question interesting, as I had never thought about these mechanisms before! Just as feedback though, I think this question could benefit from a bit more context and specifics. What mechanisms do these mods have in your world? Are they mainly biological or technological? What specifically makes the best answer to your question? Generally the more context and specificity you have in your question, the more likely you are to get higher quality answers. Hope this helps for future questions you may ask! $\endgroup$
    – Enthu5ed
    Oct 15 at 12:47

6 Answers 6


Your brain doesn't really have any spare capacity... the jobs of various bits can be shifted around thanks to neuroplasticity, but there aren't idle bits just hang around waiting for something to do.

Peter Watts had augmented humans in Blindsight (full book available for free on the author's webpage, worth a read) make functional tradeoffs. One guy has effectively lost his ability to form facial expressions, but because facial muscles are complex and there's a lot of wetware controlling them, that freed up a lot of brainpower for driving new bits of equipment.

Many things don't actually need to be directed by the brain, though. They can detect and maintain homeostasis itself, like modern-day combinations of blood glucose meters and insulin pumps.

Other things could be driven via manual intervention, eg. by operating an external control system. That's technically brain control, in the same way that when driving something like a backhoe or other complex multijointed arm a human is technically controlling it via their brain and is limited in the other things they can do at the same time, but is then free to use their various limbs for their regular purposes once they've finished operating their tools.

Another alternative might be to augment the brain, but that's something for another question, I think, and has substantial implications for any scifi setting it appears in.

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    $\begingroup$ What would it be like to have custom wetware controlled by the brain? Well, you'll just have to imagine you're Siri Keeton. $\endgroup$
    Oct 15 at 1:23
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    $\begingroup$ I'm not sure about the theory behind this answer. The concept that the brain has nothing extra to give or use is strongly countered by the fact that people who undergo a hemispherectomy can live completely normal lives while still retaining their personality. This more likely implies to me that the brain exists the way that it does as a way of structured redundancy-- thus, there should be plenty of ability for plasticity to learn to use new organs or even limbs. $\endgroup$
    – Onyz
    Oct 17 at 11:55
  • $\begingroup$ @onyz recovery from hemispherectomy is extremely variable, and strongly tied to the age of the patient. Many areas of function can be partially picked up by the remaining hemisphere, but imperfectly, because whilst there is some level of redundancy there is still plenty of specialisation. Humans are highly adaptable though, which can partially make up for these defecits as with other kinds of injury. Remember also that lesser amounts of injury of surgery can cause permanent loss of function. $\endgroup$ Oct 17 at 12:27
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    $\begingroup$ There are quite a few studies that show that the brain can process new inputs. One had the user with a belt of 8 pagers spread evenly around his waist that regularly vibrated the northernmost one. Within a fairly short amount of time (weeks, I think), this new ability allowed the user to be very aware of their exact location in relation to a starting position, regardless of how twisty-turny the path might have been. $\endgroup$ Oct 17 at 18:58
  • $\begingroup$ Many of the tradeoffs aren't very large, though. For example, by implanting small magnets under your fingertips, you gain the ability to feel varying electric fields at the cost of a small amount of touch sensitivity. $\endgroup$
    – Mark
    Oct 17 at 21:55


Humans are exceptionally adept at learning and operating new machinery. Let's get an example that everyone knows, if not has experience with. Computers! They have a multitude of ways to operate them, and we can learn to operate an absolutely incredible amount of different things.

Seeing it in action is easy. Watch someone type in a text program for the first time, then at someone adept at it. Pne is taking an incredible amount of effort, the other isn't even thinking about typing, but is just making the sentences as the words materialise on the screen. In games we have a ton of different movements. 2D, 3D, walking, flying, driving, operating weird machinery. The list is endless.

An interesting study about brain augmentation showed people can learn how to move a mouse on a screen. I cannot remember if it was the hand or arm they moved, but this was translated to the mouse. After some time they had learned to move the mouse without moving the arm or hand. They could thus use the same part of the brain controlling different things.

This means that as long as someone can have feedback on the workings of an organ, it can be learned at used.

Do keep in mind that it isn't 'free'. Brain areas are being used, making the traffic in the brain higher and possibly shutting out other things at moments. But learning and using an extra organ should be less difficult in most cases than learning how to control a character in a game. The functions and translations required fir a gane are generally much higher (movements, interactions, etc through fingers and different kinds of controllers).

Worst case scenario you "just add some extra brain matter".


Already been done

Dr. Miguel Nicholelis had made a Rhesus monkey control a robotic arm in 2003. In 2014 he had a paralyzed person controlling an exoskeleton:

He is currently working on a project that allowed paraplegic Juliano Pinto, a 29-year-old with complete paralysis of the lower trunk to deliver the kickoff at the opening game of the 2014 FIFA World Cup, in Brazi.

Also from the wiki on him:

He and his colleagues at Duke University implanted electrode arrays into a monkey's brain that were able to detect the monkey's motor intent and thus able to control reaching and grasping movements performed by a robotic arm. This was possible by decoding signals of hundreds of neurons recorded in volitional areas of the cerebral cortex while the monkey played with a hand-held joystick to move a shape in a video game. These signals were sent to the robot arm, which then mimicked the monkey's movements and thus controlled the game. After a while the monkey realised that thinking about moving the shape was enough and it no longer needed to move the joystick.

By the way:

A system in which brain signals directly control an artificial actuator is commonly referred to as brain-computer interface ("BCI").

Going further:

On January 15, 2008, Dr. Nicolelis lab saw a monkey implanted with a new BCI successfully control a robot walking on a treadmill in Kyoto, Japan. The monkey could see the robot, named CB, on a screen in front of him, and was rewarded for walking in sync with the robot (which was under the control of the monkey). After an hour the monkey's treadmill was turned off, but he was able to continue to direct the robot to walk normally for another few minutes, indicating that a part of the brain not sufficient to induce a motor response in the monkey had become dedicated to controlling the robot, as if it were an extension of itself.

Just so you don't think this guy is a lone mad scientist doing these things, making monkeys control robotic arms has been fairly commonplace for over a decade now:

Monkey controls robotic arm using brain signals sent over Internet - the work of professors James Biggs and Mandayam Srinivasan

Advanced Robotic Arm Controlled by Monkey’s Thoughts - the work of Dr Andrew Schwartz

In all cases, monkeys (abd one people) learn to map their robotic extensions to their body schema (see Shern Ren Tee's answer)


An augmented appendage (like a power suit or extendable arms or even shooting webs) will be incorporated into the brain's internal body schema. Most people's body schemas already incorporate mobile phones. (Is that making us less smart? You decide.)

A synthetic internal organ will have to be wired up to the nervous system. People don't have existing nervous structures for consciously effecting glandular actions (you can't pause your kidneys), only muscles, so it would be unnecessarily risky to try to create direct nerve-gland controls. Instead, such organs would likely be wired into the autonomic nervous system which handles our unconscious functions.

New "reflexes" could be created to fire off these organs (for example, an artificial detoxifier could be hooked up to the vagus nerve sensation of abdominal discomfort). Organs could also be wired to either the sympathetic or parasympathetic nervous system, depending on whether they should be activated by stress or by rest. This would also allow users some conscious control -- clenching muscles activates the sympathetic nervous system (different muscles trigger different levels of response), and deep breathing activates the parasympathetic nervous system.

EDIT: Every answer here (including mine!) has missed an obvious interface, which is conscious input. Nothing stops an artificial organ from coming with a dial or switch or Bluetooth app. After all, that's already how real-world humans manage prosthetic pancreas replacements -- also known as insulin pumps.

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    $\begingroup$ Starfish Prime's answer didn't miss that method, it's in their fourth paragraph. $\endgroup$ Oct 16 at 18:42

The brain doesn't actually control glands, in the sense that the central nervous system is not the prime mover. Glands react to chemicals in the body, which can include those produced by the brain, but usually are chemicals that are produced by other glands, by-products of chemicals it produced itself, or foreign chemicals/stimuli. A well-designed artificial gland will tap into existing biology accordingly. It just needs access to blood and lymph. This fact didn't stop Iain Banks claiming his Culture people could activate glandular reactions simply by thinking about it, and his books are very popular.

There are reactions the peripheral nervous system triggers, like fight or fight, which as an example is a whole body reaction that cascades into a hundred different things. Overall though, the brain itself is not really responsible to the parts of your body you don't control. This is why a brain dead body can be kept alive and "healthy" on a respirator, but remains totally unresponsive to rousing stimuli (like loud noise, or pain).


Humans have small nerve bundles spread throughout the body which help control "autonomous" functions. Some of these can still function after connection to the brain has been lost. https://www.innerbody.com/anatomy/nervous/lower-torso In order to control a new organ, first add the local nerves but connect them to a nerve bundle that provides the moment to moment control over that organ. Then, add controlling nerves that connect to the brain. In that way, you don't need to use a lot of brain power to control that new organ.


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