Brussels, 02 Aug 2006
Researchers from the UK and Germany have thrown new light onto our understanding of the factors controlling the organisation and layout of brains.
Previously, it had been thought that to function at its most effective, a nervous system should have mainly very short nerve fibre connections between nerve cells. This theory was based on the idea that creating and maintaining neural connections carries a significant metabolic cost, which should be reduced if possible.
The researchers carried out sophisticated computer analyses of anatomical studies of the brains of the nematode worm Caenorhabditis elegans, and the macaque. Their findings are published in PLoS Computational Biology.
To their surprise, they found that both species had a surprisingly large number of long-distance connections, to the point that if wired optimally, the total wiring length could be reduced by up to 50 per cent. This raised the question of why the nervous systems of these two very different species were apparently wired so inefficiently.
They found that minimal network wiring significantly increased the average path length (in terms of the number of connection steps) between distant points on the networks. According to the paper, maintaining a number of long fibres provides the networks with advantages which make up for the cost of maintaining them.
Firstly, a signal does not have to pass through as many steps as with short fibres where the intermediate nodes may introduce interfering signals. Second, the speed of signal processing, and ultimately behavioural decisions arising from the signal, are increased as transmission delays are cut. Third, long-distance connections enable neighbouring as well as distant regions to receive the signal at the same time, thereby facilitating synchronous information processing. Finally, long distance fibres increase the reliability of the system, as with each node in the pathway, there is a risk of the information in the signal being completely or partially lost.
Lead researcher Dr Marcus Kaiser of Newcastle University likens the system to a train journey between Newcastle (which is in the far north of England) and London. 'You would get to London much more quickly and easily if you took a direct train there,' he explained. 'However, if you had to make the journey via Durham, Leeds and Stevenage, changing trains each time, then it will take you longer to get there, and there is the possibility you would miss a connection at some point. It's the same in the human brain.'
'Many people have suggested that the brain is like a computer and that for optimum effectiveness it should have mainly short connections between the nerve cells,' added Dr Claus Hilgetag of the International University Bremen. 'Our research suggests that a combination of different lengths of neural projections is essential.'
Although the research is unlikely to lead immediately to clinical treatments, the scientists believe it could contribute to our understanding of diseases like Alzheimer's and autism. Brain scans of patients with these conditions have shown that they lack certain long-distance neural connections.