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The Man Who Slept for 19 Years
The brain bounces back
The brain is extremely sensitive to damage, but it is also surprisingly robust when it comes to recovery. Stroke victims, for instance, often suffer from partial paralysis or speech problems, but they usually regain some or all of their faculties over time. The speed and extent of recovery will depend on the location and extent of the injury, but the chances of improvement are generally much greater for the young than for the old.
A serious brain injury may condemn millions of neurons to death, but amazingly, new neurons may grow in their place. Recent research has shown that regeneration can occur in the hippocampus, a relatively primitive part of the brain that's associated with learning and memory. Elsewhere, the powers of neuron replacement seem restricted. Even so, the brain still has other strategies to overcome the injury.
When neurons die they release toxins that can paralyze neighbouring and otherwise healthy areas of the brain, exacerbating the effects of the initial injury. The job of cleaning up the toxins falls to the glial cells – the neurons' own support network. Once this mopping up operation is complete, millions of neurons are back in business. The disposal of toxic waste together with the rebuilding of damaged blood supplies can do much to aid the brain's initial period of recovery.
Elsewhere, surviving neurons themselves get in on the act, sprouting new lines of contact to help patch up damaged circuits; while the contacts themselves can become more sensitive to compensate for the loss of synaptic inputs. There is also evidence that the brain has circuits which are ordinarily silent but can be switched on in times of crisis.
Concussion and coma
Head injuries are often accompanied by a loss of consciousness. In mild cases, this means concussion lasting a few minutes or seconds. Concussion is caused by temporary neuronal paralysis, but strictly defined, there is no damage to the brain itself.
At the opposite end of the scale, comas are usually measured in terms of hours, days and weeks. They are typically caused by damage to the brain stem, an arousal centre located at the base of the brain. Injury, which can come from a direct hit or from pressure caused by swelling in other parts of the brain, effectively shuts down consciousness. Since the brain stem is a central hub for neuronal circuits, damage to this area can also have drastic knock-on effects that extend throughout the brain.
Comas are poorly understood and difficult to define. But in general, someone with their eyes closed all the time, who is unable to communicate or respond to instructions, is in a coma. Whether comatose people really are oblivious to the outside world is a moot point. People who have recovered from comas claim they had at least some awareness of their surroundings; they were just unable to demonstrate it. One apocryphal story even tells of a comatose Terry Wallis shaking his head when his family were presented with a massive doctor's bill.
Officially, the end of a coma is signalled by the opening of the eyes. Although it is a good sign that some functionality is returning to the arousal centre of the brain, people can remain trapped in so-called 'vegetative states' for months or even years after they open their eyes for the first time. In truth, people do not suddenly 'wake-up' from a coma; they make a slow and sometimes painful return to consciousness, via incremental improvements to their sense of themselves and their environment.
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