This is the tiniest, most intricate brain reconstruction ever.

 

Have you ever pondered the thoughts of a fruit fly? Maybe it wonders how frequently to fly by your ear or where the best spot is to settle on your slice of watermelon. Although it is still unknown what goes through Drosophila melanogaster's mind, researchers from Johns Hopkins University and the University of Cambridge's Department of Bioengineering and Neuroscience are starting to comprehend the neural pathways that underlie the insect's every action.

In a study published this week in the journal Science, the research team rebuilt the brain of a newborn fruit fly neuron by neuron, as well as any potential connections between those brain cells. It is the first time that scientists have used a fruit fly to create this type of network, which is known as a connectome. According to the paper's authors, the novel neural connectivity map may even provide insight into how we think and act.

OPENING THE BLACK BOX

The fruit fly larva is the fourth creature whose neural network has ever been mapped. Researchers have previously traced the neural connections in the brains of nematodes, tadpole larvae, and worms. Although its appearance would lead you to believe otherwise, it represents the biggest connectome to date. 3,016 neurons are found in the 170 by 100 by 70 micrometer-sized brain of a newborn fruit fly. A synaptic link is made when one neuron sends an electrical impulse to another neuron through the synapse, the area between them. Neurons interact with one another in this way to form complete circuits that include thoughts and behaviours. A meticulous plot of 548,000 synaptic connections was created by the researchers. Understanding how the numerous exact permutations of pathways connecting different neurons confer behaviour is currently a challenge.

According to lead author Marta Zlatic, "all brains are networks of interconnected neurons, but we don't really know what the structure of these networks is. Neuroscientist Zlatic works at the University of Cambridge. Understanding the structure advances our knowledge of how brain function affects behaviour.

 

Brains — drosophila and otherwise — take in sensory information and, in reaction, send signals that translate into motor function and behaviour. The gap between input and output, however, is a mysterious dark box. According to co-author Albert Cardona, a neurobiology researcher at the University of California, San Francisco, "there are neural circuits organised in a very specific way that connect our sensors to our motor systems. We can study behaviour and make some educated guesses about what this black box is actually doing."

THE FRUIT FLY IN US

Several standards are represented by this connectome. The mapping of neural networks in more complicated organisms is one benefit of this. Caenorhabditis elegans, a nematode, has a central nervous system that was traced by researchers in 1986. The connectome of the tadpole larva Ciona intestinalis was then released in 2016. Most recently, in 2020, researchers built a model of the Platynereis dumerilii juvenile annelid worm's brain. However, these are considerably less complex than the brain of the fruit fly embryo.

According to some sources, the baby fruit fly is the first organism whose connectome has been mapped that truly meets the criteria for a brain.

 

According to co-author Joshua "Jovo" Vogelstein, a biomedical researcher at Johns Hopkins University, "it's the first brain," he says. "C. elegans does not actually have a brain that is distinct from the rest of its neural system. Just a bunch of jitters, that's all. In general, the peripheral nervous system mediates sensory input from the outside environment and regulates physiological functions like blood pressure. A connectome, which is bursting with neural circuits and incorporates enormous quantities of information with each decision, is not the same as a system of nerves. Therefore, even though this article describes a connectome from the fourth organism ever, it might be the first one that matters.

The baby fruit fly may actually be the first organism whose connectome has been mapped that truly counts as a brain, depending on who you ask.

Another model organism for examining human circumstances is the fruit fly. Given that fruit flies and humans share about 60% of their genetic makeup, these organisms are useful for studying everything from decision-making to cancer. These neural pathways mirror those of humans, though on a much simpler level, as a result of thousands of years of evolution perfecting the brain across species. According to Zlatic, "they're not at all randomly connected at all, but evolution has carefully shaped them to be suitable for the whole range of different computations and functions." Zlatic also claims that these connections are conserved across species, implying that as brains became more complex, they did so on similar foundations in other species. The 100 billion neurons and billions of synaptic connections in human brains

Thought is one of these processes. Although young fruit flies presumably don't think, Vogelstein claims that their connectome "is kind of like a blueprint" for all of the thinking that adults do. Vogelstein believes connectomes will ultimately shed light on the "mechanisms of enlightenment" that turn humans into reflective beings.

TO MAP A BRAIN

In an electron microscope, which produces high-resolution images of incredibly tiny structures by using electrons as an illumination source, the Cambridge team began by examining the baby fruit fly brain in 2012. Then, they manually traced every single neuron and its synaptic connections to construct the connectome. The team then carefully plotted each of the 3,016 neurons, one neuron per day, using a computer. The task of creating the 548,000 synaptic links then began. It took the crew about seven years to complete.

In order to identify the most significant synaptic connections, Vogelstein started studying the thousands of synaptic connections in 2019. As it turned out, there were several fascinating links. Axon and dendrite are the basic components of a cell. Although axon to dendrite is the typical direction for synaptic connections, these researchers observed some synapses that were oriented differently.

 

Co-author Michael Winding, a research assistant in zoology at the University of Cambridge, describes the connections as being axon-to-axon, dendrite-to-dendrite, or dendrite-to-axon. Additionally, statistical research showed that only about half of the synaptic connections were made using traditional axon-to-dendrite connections. Winding claims that nearly 46% of them were these "noncanonical sorts," whereas he had anticipated that axon-dendrite links would make up the bulk of them.