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Is a human head transplant possible? 

No, human head transplants are not possible - and they will not be possible for the foreseeable future. Not unless one would want the head to be able to control the body supporting it.
This is what makes it impossible:
The above is a cross section of single spinal nerve bundle under a scanning electron microscope (the colours are computer generated - hence the red at the center, a blood vessel). Every circular shape is the myelin sheath (think of a cable insulation) encasing a neuronal axon (think of a cable wire), originating somewhere in the central nervous system and aiming at a particular sensory or kinetic (muscle) target. And the spinal cord contains a great number of these bundles, with millions of axons.
No light microscope can achieve such magnification, mind you, an electron microscope would be needed. And observing a tissue sample under the electron microscope means that tissue is already dead, its micro-shape imprinted on a thin layer of metal (usually silver) and all traces of the original tissue dissolved before sliding its mold alone under the electron beam. The process of preparing the specimen usually takes several hours, if not days, to complete. In other words, such an image is impossible to generate in a operating theater. And even if it were possible, it would still not matter because neither human nor surgical robot can operate at this scale.
The best a neurosurgeon could do today, in an attempt to connect the two ends of a severed spinal cord, would be try to identify major regions of the severed segments, guesstimate the location of the contained bundles (about as effective as ignoring them) and push the two pieces together. Hoping that the supporting Schwann cells, that generate the myelin and do neuronal repairs in the peripheral nervous system, will try to seal together the opposing axons - even it their are mismatched, incompatible and lack the targeted growth signals to guide their regeneration.
If such a mismatched connection were possible to work, people who have suffered spinal cord trauma would had regained the use of their body a long time ago - and reattaching the two severed ends of the same spinal cord would be a much easier task. We all know that, unfortunately, this has not been the case and spinal cord injuries are not yet repairable.
A spinal cord millions of times more complex than a car but, in order to illustrate this, imagine sawing a car in half - and then simply …duct-taping the two halves together. What are the chances of that car ever working again? That would be a crude approximation as to why we cannot fix spinal cord injuries.
Now, how about if you take two halves from different cars? Because that would be a simplified approximation of the claims of head transplants made by the notorious Italian doctor. Declaring it a “success” after trying it on a cadaver is like gluing together a TOYOTA and a BMW and then concluding your attempt to be successful - without ever bothering to turn your contraption on!
Nowadays is not be very hard to connect the major arteries, reattach the spinal column segments & their ligaments and stitch up the muscles and skin. Supporting the head’s tissues with oxygen, nutrients and blood circulation has been a perilous yet routine operation for some time. That is why, ever since 1959, it has been possible to attach a second head to an animal: this works because the original head is still in place and is the one controlling the body.
The insurmountable obstacle, however, still is making the correct connections to the millions of spinal nerve axons - that will not even match in number, let alone shape, size and location - in order for the head to be able to sense and take control the new body.
Some readers have suggested nanotechnology, and I think it needs to be addressed, even if it is science fiction at the moment. So, even in 30 or 50 years, for nanotechnology to work, it would still demand a number of steep requirements: the nanites will have to be able to recognize and map the different severed neuronal axons, communicate their findings either to a central processing unit or to each other for collective (hive) processing and then grab and move around the matched axons in order to try to seal them together. The issue of filling the gaps between remote matches will have to be addressed, as well as how to triagethe axons and even decide what to do with the surplus. Not two persons will be an exact match, not even monozygotic twins - and that may be only one of our problems.
For example, at the other end of this there will be the problem of excess heat. Having molecular nanites moving around, taking visual readings, sending, receiving and processing information and then pushing and pulling cellular appendices before stitching them together or integrating with them will require energy - and Thermodynamics are a very unforgiving mistress. Human cells have a narrow range of tolerated temperatures. Cool the system too much and the nanites stop functioning; allow it to heat up to much, and the cells we would be trying to connect will be damaged beyond recovery.

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