In April this year, Italian neurosurgeon Canavero announced that the world’s first human head transplant would be completed within two years, and a team led by Chinese doctor Ren Xiaoping was ready to join hands with Canavero, optimistically estimating that the operation would be performed in December 2017 at the affiliated hospital of Harbin Medical University. For a while, many media outlets caught wind of the operation, creating a considerable ripple in the community. However, regardless of traditional medical ethics and social ethics issues for the moment, just in terms of the level of technology and medical conditions in today’s world, is it really possible to exchange human heads? To clarify this issue, let’s start with the structure of the human nervous system. The miraculous nervous system The nervous system is the main functional regulatory system in the human body, which controls and regulates the activities of other systems, making the organism an organic whole. The nervous system is divided into the central nervous system and the peripheral nervous system. The brain, located in the cranial cavity, the spinal cord and the optic nerve belong to the former, while the nerve components emanating from the brain and spinal cord belong to the latter. The brain is the “commander-in-chief” of the human body and receives information from the whole body through the spinal cord, which continues in the brain, and then issues orders to the whole body through the spinal cord after analysis and processing. When the spinal cord is diseased or damaged, the pathway for transmitting various information is interrupted, and various body sensations cannot be transmitted to the brain, so the human body loses sensation; at the same time, the brain cannot transmit the orders governing the movement of the limbs, so the limbs are paralyzed. There is a significant difference between the central nervous system of higher vertebrates and lower vertebrates. In lower vertebrates (e.g., fish and reptiles), when the central nerve is severed, it can regenerate spontaneously, just like a cut and growing leek, and continue to innervate the corresponding target organ, restoring it to full normal function. In contrast, in mammals of higher vertebrates, such as rats, rabbits, monkeys and humans, the central nerve has completely lost its ability to spontaneously regenerate and repair after injury, and only the peripheral nerve partially retains this ability to regenerate after injury. Therefore, doctors are able to use the regenerative function of the peripheral nerves to perform amputation or transplantation surgery for patients, but severe central nervous system (including the optic nerve) injuries or diseases can only be reduced to incurable diseases. The secret of the non-regenerative central nerve The central nerve cannot be regenerated, and the connection between the brain and the spinal cord cannot be established after head replacement, which is the fundamental reason why human beings cannot carry out “head replacement surgery” so far. Under a microscope, a nerve is like an insulated wire, with a wire-like protrusion of nerve cells in the middle called an axon, surrounded by a myelin sheath that resembles insulating rubber. The myelin sheath is made up of glial cells, but the myelin sheaths of peripheral and central nerves are derived from two distinct types of glial cells, Schwann’s cells and oligodendrocytes. The Schwann cells that envelop the peripheral nerves are very competent and secrete many chemicals necessary for nerve development, growth and regeneration. Immediately after peripheral nerve injury, Schwann’s cells mobilize urgently, secreting nutrients needed for nerve repair and proliferating to form strips that guide the peripheral nerve to regrow toward the target tissue. In contrast, the central nerve is not so fortunate, because the myelin sheath composed of oligodendrocytes does not provide sufficient nutrients, but rather adds to the problem by secreting many chemicals that strongly inhibit nerve regeneration. More than a hundred years ago, Ramon Caja, a Spanish neuroanatomist who won the Nobel Prize in Medicine, first described the fundamental differences between the peripheral and central nerves in terms of their ability to regenerate. He hypothesized that if peripheral nerves were transplanted into the central nervous system, the damaged central nerves should be able to regenerate in the good microenvironment of the peripheral nerves. Due to the limitations of research conditions and tools at that time, Kaha’s idea could not be confirmed. However, just as the ancient people fantasized about running to the moon, people have been dreaming about head transplantation. In the 1970s, Robert White, an American neurosurgeon, was initially successful. After long and tireless efforts, he invented advanced medical equipment that could lower the temperature of the blood circulation in the head ready for head replacement to 10 degrees Celsius, so that the monkey brain did not die during the head replacement surgery that interrupted blood circulation for more than an hour. The most sensational head transplant was performed by Dr. White two years ago. The monkeys that had their heads replaced not only survived awake for a period of time under the conditions of a foreign heart blood supply, but were also able to see, hear, blink, taste and smell because these functions were innervated by the cranial nerves themselves. Dr. White’s head transplantation simply docked the allografted monkey head to the torso, fixed the spinal column with a metal piece, anastomosed the blood vessels between the head and body to restore blood circulation to the brain, and ensured that the new body would not reject the grafted new head due to an immune response or that the monkey head would not reject its new body for a short period of time after the successful surgery. Since the up-and-down connection between the brain and the spinal cord is never restored, it is simply impossible for the body to move after the head replacement. As Dr. White himself pointed out, “The notion that the transplanted head is truly connected to the transplanted body is purely misleading …… The purpose of my surgery was not to restore the one who received the surgery to a normal person who could use his brain to direct the activities of his body. My purpose is to extend life, because the brain represents human life.” Professor Jiu Jin, a neuroscientist and academician of the Chinese Academy of Sciences, said that, based on the traditional concept that the mind represents human life, it would be more accurate to say “change the head for the body” than “change the body for the head,” although “change “is mutual. Head change may not be a dream In the early 1980s, with the continuous improvement and perfection of neuroscience research technology and means, Canadian scientist Albert Oguayo will take a section of peripheral nerve from the hind limbs of rats implanted in the spinal cord, successfully induced the long-distance regeneration of the injured central nerve in the peripheral nerve, for the first time confirmed the foresight of Kaha more than half a century ago. This major breakthrough made it possible for humans to finally overcome the persistent disease of the central nervous system and rekindled people’s enthusiasm for exploring head replacement. For more than two decades, with the unremitting efforts of neuroscientists around the world, encouraging progress has been made in the study of the extremely complex central nervous system regeneration. In the last five years, scientists have finally found some way to allow rats with completely severed spinal cords to walk with a limp on their once-paralyzed hind limbs. The significance of these findings is that the upper and lower connections between the damaged spinal cord and the brain can finally be rebuilt, paraplegics are expected to leave their wheelchairs and walk on their feet again, and the dream of head replacement is theoretically possible.