A COMPLETE map of all the neurons and their connections in both sexes of an animal has been described for the first time. This “connectome” will not only help us understand how neurons work, but could also improve our understanding of human mental-health problems.
The tiny soil-dwelling nematode worm Caenorhabditis elegans has long been used for research because it has so few neurons. The hermaphrodite version of it, for example, has 302 neurons in its entire nervous system, compared with 86 billion in the human brain alone.
A basic map of these 302 neurons was published in the 1980s, when Nobel-prizewinning biologist Sydney Brenner and his colleagues used an electron microscope to examine minute slices of the hermaphrodite worm, which is essentially female but can produce a limited amount of sperm. “It was a very important piece of work, but it was in pieces, it was incomplete, and it didn’t include the male,” says Scott Emmons at Albert Einstein College of Medicine in New York.
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Since then, Emmons and his colleagues, and other groups, have used similar approaches to note the connections in parts of the male worm. Now Emmons’s team has captured images of the entire male and analysed new and old images from the hermaphrodite. “Electron micrographs are too complex for a computer to analyse. All of the images were examined by a person,” says Emmons, who has been planning the project since 1999.
“It can tell us how a nervous system computes information and translates it into a behaviour”
His team has now been able to describe all of the neurons and their connections in both the male and hermaphrodite worms for the first time. The group has used the data to create a digitised map that shows the neurons’ location and connections, and the strength of those connections (Nature, ).
“This is a very substantial body of work that has been much awaited by the worm community,” says Denise Walker at the MRC Laboratory of Molecular Biology in Cambridge, UK. The original worm connectome has informed all worm neuroscience research since, by providing vital clues as to how neurons work. The new, complete connectome will only increase this power, she says.
Emmons’s team has already used the map to compare hermaphrodite worms with males. The males have 385 neurons, of which 294 are also present in the hermaphrodite worms. However, up to 30 per cent of the connections between them seem to differ in strength. “We don’t know why,” says Emmons. “It was a little surprising to find.”
The worm connectome could help us understand how the brain and nervous system works in other animals, including humans, because many genes overlap between species. “It can tell us how a nervous system computes information and translates it into a behaviour,” says Arantza Barrios at University College London.
Emmons hopes that by studying how the worm’s genes influence its neural connections, we can learn something about human mental-health problems that may be caused by changes to the brain’s wiring.
