The firing of every neuron in an animal鈥檚 body has been recorded, live. The breakthrough in imaging the nervous system of a hydra 鈥� a tiny, transparent creature related to jellyfish 鈥� as it twitches and moves has provided insights into how such simple animals control their behaviour.
Similar techniques might one day help us get a deeper understanding of how our own brains work. 鈥淭his could be important not just for the human brain but for neuroscience in general,鈥� says at Columbia University in New York City.
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Instead of a brain, hydra have the most basic nervous system in nature, a nerve net in which neurons spread throughout its body. Even so, researchers still know almost nothing about how the hydra鈥檚 few thousand neurons interact to create behaviour.
To find out, Yuste and colleague Christophe Dupre genetically modified hydra so that their neurons glowed in the presence of calcium. Since calcium ions rise in concentration when neurons are active and fire a signal, Yuste and Dupre were able to relate behaviour to 聽activity in glowing circuits of neurons.
For example, a circuit that seems to be involved in digestion in the hydra鈥檚 stomach-like cavity became active whenever the animal opened its mouth to feed. This circuit may be an ancestor of our gut nervous system, the pair suggest.
Neural code
A second circuit fires when the hydra contracts its body into a ball to hide from predators. A third seems to sense light and may help let the animal know when to eat 鈥� despite being blind, hydra need light to hunt and they do more of this in the morning.
The team found that no neuron was a member of more than one circuit. This suggests the animal has evolved distinct networks for each reflex 鈥� a primitive arrangement, much less complex than our own interconnected nervous systems.
Nevertheless, the hydra is the first step towards breaking the neural code 鈥� the way that neural activity determines behaviour, says Yuste. 鈥淗ydra have the simplest 鈥榖rain鈥� in the history of the earth, so we might have a shot at understanding those first and then applying those lessons to more complicated brains,鈥� he says.
Yuste hopes that seeing how the circuits work in real time might lead to new insights into the human brain and tell us more about mental illnesses such as schizophrenia, for example. 鈥淲e cannot cure patients until we know how the system works,鈥� he says.
Yuste was one of several neuroscientists, including at Harvard University, who launched the Brain Activity Map Project in 2012. It was a rallying cry to neuroscientists, calling on them to record the activity of every neuron in the human brain. The project forms the central plank of the billion-dollar BRAIN Initiative launched by President Obama鈥檚 administration in 2013.
Aha moment
The hydra is now the first animal to have one of these maps created for the whole body, although the activity of the whole brains of zebrafish have also been mapped in a similar way. The work is an 鈥渁wesome milestone worth celebrating鈥�, says Church. But scaling this up to rodents or primates will be very challenging, he says.
, a neuroscientist at the Duke Institute for Brain Sciences, North Carolina, doubts if the animal will prove useful for understanding ourselves. 鈥淵ou have to ask: is this an animal that鈥檚 going to join the fruit fly, worm and mouse as a model organism to look at in the quest to better understand the nervous system?鈥� he says. 鈥淢y answer would unfortunately be no.鈥�
But Yuste is now collaborating with seven other teams to decipher the hydra鈥檚 neural code. They want to get such a complete understanding of the way its neurons fire that they can use a computational model to predict its behaviour just from its neural activity.
鈥淥ne of our dreams is to get to the point in neuroscience that genetics got to when they figured out the DNA double helix,鈥� says Yuste. While some have suggested that the brain is too complicated for that, Yuste is optimistic. 鈥淚 hope it will happen in our lifetime and it will be an aha moment when the jigsaw puzzle comes together,鈥� he says.
Current Biology
Read more: 鈥�A brief history of the brain鈥�
Our brains followed a twisting path of development through creatures that swam, crawled and walked the earth long before we did. Here are a few of these animals, and how they helped make us what we are.
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