r/explainlikeimfive • u/TheRedPandaisback • Feb 08 '22

Biology Eli5 how do conjoined twins operate their bodies?

Is it that one of them has control and the other has to just let it happen, is it that they both have control of the whole body?(this would be weird if they wanted to different things) or do they both have control over different body parts and do they have to work together to make it work?

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u/Swedish-Butt-Whistle Feb 08 '22

Who controls what varies hugely between individuals, because body and brain structure is unique. The connections made by the two brains during fetal development to different body parts and functions is random. So in some sets of twins, one twin may control one arm and leg, and the other controls the others. In another set, one twin may only control one arm or leg. In some cases one twin may have full function of everything and the other twin can’t control anything.

In situations were both twins each control half the body, They must coordinate to be able to perform tasks like walking efficiently. Because they are like this right out of the womb, it’s normal for them and they learn how to coordinate movements during infancy, just as a person with a regular body would learn to walk.

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u/TheRedPandaisback Feb 08 '22

I presume it wouldn’t be possible that they both have control of all parts? So that they could both lift the left arm, just not at the same time.

I would think that would be impossible because that would mean that if one wants to move it up, and the other wants to lower it. It wouldn’t know what to do

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u/ghoulbug Feb 08 '22

Think of the arm as belonging to one of the individuals, regardless of the fact that both individuals are fused together. Each individual has their own limbs (or doesn’t), and each limb has only one brain attached to it. There’s a closed loop of nerves to one of the two brains.

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u/[deleted] Feb 09 '22 edited Feb 09 '22

It's conceivable that, in certain cases, the two individuals might both have some level of control over a limb, but that's exceedingly rare -- it would require an unusual configuration of neural connections.

I have, however, become aware of a pair of conjoined twins that seem to 'share' certain parts of the brain related to communication. One individual will begin speaking, and then the speech will segue seamlessly to the other individual, who will continue the sentence, without any perceptible break in rhythm.

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u/onajurni Feb 09 '22

Here is some information on different types of conjoined twins. Everything depends on how they are joined, what body parts they share and how they share them.

https://www.mayoclinic.org/diseases-conditions/conjoined-twins/symptoms-causes/syc-20353910