Once it was realized that it was impossible for new cells to grow following brain cell destruction, attention was directed towards trying to discover how the surrounding cells, unaffected by the injury, respond. This was because it was thought that these were the cells responsible for any restoration of function that did occur.
It was through the work of Lui and Chambers in 1958 that the first evidence of brain cell adaptation was seen under the microscope. The introduction of the electron microscope around this time enabled the scientists to study in finer detail the minute world of the nerve connections in the brain.
After creating injury in the brain of a rat, Lui and Chambers discovered that the undamaged axons (the transmitting 'wires' coming from the nerve cell that relays messages to other cells) in the vicinity of the damaged area attempted to rewire what had been lost through injury. They did this by sending out new connections, or 'sprouts', as Lui and Chambers described them. They called this phenomenon 'collateral sprouting', and it was thought that this was the means by which the brain was attempting to compensate for its inability to grow new brain cells.
This startling discovery naturally created a great deal of excitement in the scientific world. Additional studies were done to test Lui and Chambers' work, and these reinforced the original findings. The fact that collateral sprouting does occur is now beyond dispute. But just when and why it occurs, and in what parts of the brain, is still not clearly understood.
Some studies show that collateral sprouting can occur to a much greater extent than was first realized. For instance, a 1976 study by Matthews showed that after an injury destroyed all but 14 per cent of the connections in a specific area of a rat's brain, up to 80 per cent of the original connections were restored in 280 days presumably by the process of collateral sprouting.
There is still a great deal of dispute as to how much function can be derived from the new connections established by the collateral sprouting. Some studies suggest that no functional use can be achieved by these new connections, while others even feel that the new pathways can have an adverse effect in some situations. There are others who are just as adamant that the rewiring results in permanent functional connections.
Putting aside for the moment the argument concerning its effectiveness, just the fact that collateral sprouting does occur is indisputable evidence that the brain attempts, at a cellular level, to restore function following injury.
According to Wall, there are other ways by which the remaining brain cells may attempt to compensate after injury. For instance, he says that nerve cells show a type of homeostasis - an inbuilt system designed to maintain the proper equilibrium within the brain. By this means, the remaining nerve cells can become more excitable, thereby increasing their ability to receive information. St James-Roberts refers to this process as 'denervation supersensitivity'.
Wall proposes another type of cellular adaptation, which is really a form of redundancy: 'there are large numbers of normally ineffective nerve connections which may become active if the dominant inputs are put out of action. It is proposed that the connections laid down in the embryo are more diffuse than those actually used in the adult brain. The stages of maturation partly involve destruction of the "incorrect" connections, and partly their suppression. If some nervous connections are destroyed in the adult, suppressed connections may become de-repressed. This process is not necessarily a good thing, for the substituted connections may bring in nonsense information which the recovering nervous system cannot handle'.
Wall clearly summarizes how collateral sprouting and redundancy contribute to the recovery of function: 'Sprouting and the unmasking of ineffective connections offers the possibility of new connections after brain damage, but we need to know much more about these processes so that we can guide them to useful ends rather than towards further disorganization'.