Keith Schacht’s Weblog

A complete map of all neurons and the connections in a fruit fly (via)

The brain contains 10^5 neurons and 10^8 synapses that enable a fly to see, smell, hear, walk and fly. Flies engage in dynamic social interactions18, navigate over distances19 and form long-term memories20. Portions of fly brains have been reconstructed from electron microscopy images, which have sufficient resolution to reveal the fine branches of neurons and the synapses that connect them. The resulting wiring diagrams of neural circuits have provided crucial insights into how the brain generates social21,22, memory-related23 or navigation24 behaviours.

I wonder how long it will be before we can run a complete simulation of this brain on silicon? Maybe there is missing information still such as the weight between neural connections.

Ah, indeed they mention:

Finally, the connectome now enables brain simulations—partial connectomes of the early visual system of the fly3 had already inspired simulations of visual processing85. This effort has now been extended to leverage the full connectome34,39 and to—for example—predict taste responses of neurons34. These simulations assume that that physiological connection strength is proportional to anatomical synapse count, either globally34, or for synapses sharing the same presynaptic and postsynaptic cell types85, and have inferred connection signs (excitatory versus inhibitory) from neurotransmitter identity as predicted from electron microscopy images10 or from transcriptomics99.

Fascinating! And there’s more, which I don’t quite understand:

Whereas the simulations mentioned above were based on point neuron models, future simulations could utilize multicompartmental neuron models constructed using the synapse locations and reconstructed neuronal morphologies provided by FlyWire, as well as emerging data about ion channels and receptors from transcriptomics and proteomics.

A couple other points that stood out:

The completeness of the FlyWire brain connectome enables tracing complete pathways from sensory inputs to motor outputs. We demonstrate this capability by examining circuits that emanate from the ocellar ganglion and leveraging cell-type information. In addition to the large compound eyes, flying insects have smaller visual sensory organs89, including the three ocelli on the dorsal surface of the head cuticle (Fig. 7a).

We use the term ‘connectome’ to mean a neuronal wiring diagram of an entire nervous system, or at least an entire brain98. This is in keeping with the intent of the original definition14, which emphasized comprehensiveness. Similarly, the term ‘genome’ refers to the entire DNA sequence of an organism, or at least the entirety of genes. Our neuronal wiring diagram of a whole fly brain arguably crosses the threshold for being called a connectome

I wonder if humans who choose to be frozen upon death preserve enough neural structure that their brains could be mapped?