Neuron maps unveiled for mouse and fly
Anyone who has ever tried to swat a fly will know that they have sharp vision. However, they also make good models for studying the visual circuitry of neurons, as shown in two papers published yesterday in Nature. These were published alongside another study that has produced a reconstruction of the neurons of the inner layer of the mouse retina.
These neuron circuits are incredibly complex; in the human brain there are approximately 100 billion neurons with 1000 connections each. Focussing on the retina allows scientists to reduce the complexity of the problem, but even the inner layer of the mouse retina contains 950 neurons that are hard to piece together. The researchers, led by Winfried Denk, sliced thin sections of the mouse retina and imaged each slice before reconstructing the neurons and their connections. The nature of this reconstruction makes it difficult for computers to solve and so the researchers have needed human input and even employed crowd-sourcing to rebuild a portion of the mouse retina.
A similar approach was taken by Dmitri Chklovskii’s lab with the fruit fly to determine the ‘connectome’, the total connections made between neurones. These maps of connections allow researchers to defines the circuits responsible for different activities and definitively classify the types of cells present.
In the second fruit fly study, Alexander Borst and colleagues deciphered the mechanism for cells detecting motion. They found that two cell types, T4 and T5, are specialised for detecting light and dark respectively. Each of these cell types are then further split into four variations that individually detect motion going up, down, forward and backward.
These studies lay the groundwork for bigger experiments in the future. The €1 billion Human Brain Project and $3 billion BRAIN initiative are seeking to improve our understanding of the human brain by reconstructing its connectome and simulating its behaviour with a supercomputer. These are both long-term projects and will require years of gradually building the sophistication of our questions and techniques, while progressing through increasingly complex animals from nematode worms to primates.