(ORDO NEWS) — A detailed map of the connections between the neurons of the Drosophila larvae has been obtained – the largest and most accurate structure of the nervous system of all that exists today.
A complete description of the structure of connections in the brain or the nervous system as a whole is called the connectome. It largely determines the body’s ability to perceive and complex behavior.
Recently, scientists managed to understand the connectome of the Drosophila larva, including the brain and nerve chain, covering a total of 3016 nerve cells and about 548 thousand synapses – the connections between them.
Until now, such work has been done only for very primitive organisms, the nervous system of which includes no more than a few hundred neurons, such as the nematodes C. elegans (302 neurons and 7600 synapses) and sea squirts (177 neurons and 6618 synapses).
Mapping of the adult Drosophila connectome continues, and a couple of years ago, scientists described part of it.
However, the new paper provides the first example of a fully mapped insect nervous system, albeit still an “underdeveloped” larva.
Michael Winding and his colleagues at the University of Cambridge painstakingly examined brain sections of six-hour-old Drosophila larvae under an electron microscope.
The entire structure occupies only 170 x 160 x 70 micrometers of volume. Microscopy data were brought together using software specially developed at Johns Hopkins University.
The computer system automatically recognized different types of neurons and synapses, allowing it to identify neural patterns associated with different functions.
Already the first analysis of the obtained “brain map” brought several unexpected findings. First, scientists noted a large number of zigzag neural pathways that jump from one hemisphere to another.
Secondly, it was found that only about two-thirds of all connections between neurons are classical synapses connecting the axon of the signal-transmitting cell with the dendrite of the receiving cell.
A whole third is accounted for by “non-canonical” interactions: axon with axon, dendrite with dendrite, dendrite with axon. Such variants were known before, but no one thought that they could be so common.
As expected, the Drosophila connectome demonstrates not only a layered structure, including several “levels” of neurons, but also segmentation.
In particular, cells associated with the processing of data of a particular modality (smell, vision, etc.) are combined into separate groups.
In some areas, especially those associated with learning, contacts between nerve cells are greatly enhanced by parallel and feedback connections.
Such a structure is reminiscent of the residual artificial neural networks that scientists are actively developing and applying today.
It is worth noting that it took 14 years to determine the connectome of nematodes, and 12 years for Drosophila.
At the same time, the mouse brain is millions of times larger than that of insects, and the human brain is even orders of magnitude larger and more complex.
When scientists will be able to determine the connectome of such large-scale nervous systems and how long such work will take is unknown.
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