What sea squirts can tell us about the brain

- December 6, 2016

The tadpole-like larva of a sea squirt (top image), with two views of the neurons in its central nervous system. (Provided photo)
The tadpole-like larva of a sea squirt (top image), with two views of the neurons in its central nervous system. (Provided photo)

The brain is a powerful computational organ. Its functions orchestrate behaviour in ways that can seem easy to liken to a computer, but what’s known about brain function is no match for what’s known about computer programming. There’s at least one common characteristic between the brain and computers, though: they’re both incredibly complex systems.

That’s why an emerging field called connectomics is sparking an intense debate among neuroscientists, engineers and computer scientists. Connectomics is the current effort to comprehensively map connections between the neurons of a brain or brain region.

Given the immense computational power needed to map hundreds or billions of neurons in a brain, opponents of connectomics suggest the field is too descriptive and its overall objective too unattainable. Proponents argue that only connectomics will enable experts to test how the brain really works. The supporters suggest it’s those connections — the brain’s “wiring” — that make us who we are. Luckily, recent advances in technology have allowed researchers to gain momentum in the field.

Kerrianne Ryan, a postdoctoral fellow at Dalhousie, is one of those researchers.

Connecting the dots

On Tuesday, the open access scientific journal eLife published Dr. Ryan’s study examining all the neuronal networks in a tadpole larva of the common sea squirt.

Her study, which was completed for her PhD in Dalhousie’s Department of Biology, is only the second report of an entire neuronal network map for any animal’s central nervous system. The first, derived from work on a nematode worm, has always been thought unique and not closely related to the brains of other animal groups. Coined a connectome, the map obtained at Dalhousie is particularly interesting to evolutionary biologists. This is because the brain of a larval sea squirt, while far smaller and simpler, is distantly related to our own.

“We were mapping a brain, at a very detailed level and in a comprehensive way, to gain insight into how brains work and might have evolved,” says Dr. Ryan, currently studying in the Laboratory of Invertebrate Neurobiology headed by Ian Meinertzhagen in the Department of Psychology and Neuroscience. “It’s a new way to look at the central nervous system.”

The sea squirt, an invertebrate, is a sibling to the ancestor of all vertebrates — making it distantly related to humans. It releases thousands of identical larvae, each less than 1mm long. The tadpoles then swim freely in the sea, equipped with all the basic sensory and motor systems that also form the central nervous system in humans. But the sea squirt’s larval brain has only 177 neurons — a stark contrast to the billions of cells in a human’s brain. Ryan’s study looked at how the pathways between neurons in its brain could result in its swimming patterns.

“Here you have a mere 177 neurons in the central nervous system, but there’s still a huge degree of complexity if you look at their connections,” says Dr. Ryan. “So what it’s telling us is that the networks and their level of integration are really fundamental to understanding how brains work, even a brain of such tiny size.”

Mountain of data

Dr. Ryan and Dr. Meinertzhage used an electron microscope to take ultra-high resolution images of the brain’s neurons, their fibre-like connections, and the all-important contacts — called synapses — between one neuron and another. It took years to complete this for one small larva, not to mention huge computer memory and storage capacities to process and store the resulting data.

While the tools and technology developed to support connectomics are advancing quickly, such studies still require people to manually (and tediously) check each of the synaptic connections to derive a complete data set. In this case, there were more than 6,600 to go through and annotate.

Needless to say, researchers are still far from deciphering the connectome of a human’s central nervous system. However, Dr. Ryan suggests, that as technology becomes more automated, the holy grail of unlocking the circuits of the human brain could one day become a reality.

First, though, methods need to prove successful in species like the sea squirt before moving on to the brains of species like fruit flies, zebrafish, and mice. Dr. Ryan’s study has shown that this is possible, and provides examples of circuits that represent an ancestral stage in the evolution of those in the brains of vertebrates.

“Having a second connectome out there means more data that better helps us move forward in understanding the integrative pathways of brains in more complex creatures,” says Dr. Ryan. “It’s important not to disregard what we can learn from simpler organisms.”


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