The common fruit fly or vinegar fly, Drosophila melanogaster, is a tiny little fly, less than 2.5 mm in length (see below). Nevertheless, its complete central nervous system (CNS) has about 100,000 neurons and perhaps, 80-100 million connections between them.
These 100,000 nerve cells are packed into the brain of this fly, which is about the size of a poppy seed (or ∼8 × 107 μm3). It is worth keeping these numbers in mind when we come to compare it with the human brain.
The first ‘functional connectome’ of an animal ‘brain’ was achieved in 1986 for the much simpler creature: the nematode, Caenorhabditis elegans, which only has about 302 neurons in its nervous system, and some 7,600 synapses. So the connectome for Drosophila is a massive leap forward; from 7,600 connections in 1986, to 20 million connections in 2020. The synapses, or connections between the different brain cells, is what gives the nervous system its complexity. It is said that this is where memory is stored in humans.
The human brain has approximately 100 billion neurons and 1015 connections. That is 1,000,000,000,000,000 connections (apparently it is called a Quadrillion). The step up from 108 to 1015 is enormous, so one would assume that the human connectome is still a long way off. However, given the current rate of progress, I am confident that a complete ‘wiring diagram’ of the human brain, perhaps your brain (!) will happen some day.
The raison dêtre for doing this work, is that once you have a complete synaptic ‘wiring diagram’, it may be possible to determine the precise causes and controls of specific behaviours, for example by experimentally switching on and off individual neurons or neural circuits. Brave New World indeed!
Zheng, Zhihao, et al. “A complete electron microscopy volume of the brain of adult Drosophila melanogaster.” Cell 174.3 (2018): 730-743.
Xu, C. Shan, et al. “A Connectome of the Adult Drosophila Central Brain.” bioRxiv (2020).