Researchers have, for the first time, mapped the entire nervous system of an animal – the roundworm (Caenorhabditis elegans) – and found marked differences between the male and female of the species.
Led by molecular geneticist Scott Emmons, from the Albert Einstein College of Medicine in New York, US, the team used a range of techniques, including patching together electron microscope images one thousandth the width of a human hair, to create the worm’s wiring diagram.
The result was a tally of 302 neurons in the hermaphrodite/female and 385 neurons in the male, with remarkably close connections between them.
In a case of “two degrees of separation”, a single sensory neuron could reach up to 98% of all other neurons in the network by crossing just two synapses, the chemical junction between the nerve cells. {%recommended 6345%}
There were also notable differences between the sexes.
“In total eight neurons and 16 sex muscles are specific to the hermaphrodite; 91 neurons and 39 sex muscles are specific to the male,” the authors write.
The hermaphrodite neurons control muscles in the uterus and vulva while the male neurons link to “sex muscles” in the tail that are important for copulation. Sex differences did not, however, stop at wiring.
“While the synaptic pathways in the two sexes are substantially similar, a number of the synapses differ in strength, providing a basis for understanding sex-specific behaviours,” says Emmons.
The researchers estimate that 10-30% of connections between neurons vary in strength between the sexes.
The study represents a major advance in the field of “connectomics”, the effort to map the complex neural pathways of organisms with the ultimate aim of modelling the human nervous system, especially the brain, and what can go wrong with it.
Autism, which includes deficits in the ability to perceive emotions, may result from faulty connections in brain pathways that process “social information”, such as facial expressions.
On this view, autism is a “connectopathy”. Psychiatric disorders such as schizophrenia may be caused by similar faulty wiring.
“This hypothesis is strengthened by the finding that several mental disorders are associated with mutations in genes that are thought to determine connectivity,” says Emmons.
“Connectomics has the potential to help us understand the basis of some mental illnesses, possibly suggesting avenues for therapy.”
“The structure of DNA revealed how genes work, and the structure of proteins revealed how enzymes function,” Emmons adds.
“Now, the structure of the nervous system is revealing how animals behave and how neural connections go wrong to cause disease.”
The study is published in the journal Nature.
