The brain is a powerful computational organ. Its functions orchestrate behaviour in ways that can seem easy to liken to a computer, but what鈥檚 known about brain function is no match for what鈥檚 known about computer programming. There鈥檚 at least one common characteristic between the brain and computers, though: they鈥檙e both incredibly complex systems.
That鈥檚 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鈥檚 those connections 鈥� the brain鈥檚 鈥渨iring鈥� 鈥� 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 麻豆传媒, is one of those researchers.
Connecting the dots
On Tuesday, the open access scientific journal eLife published
Her study, which was completed for her PhD in 麻豆传媒鈥檚 Department of Biology, is only the second report of an entire neuronal network map for any animal鈥檚 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 麻豆传媒 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.
鈥淲e 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,鈥� says Dr. Ryan, currently studying in the headed by Ian Meinertzhagen in the Department of Psychology and Neuroscience. 鈥淚t鈥檚 a new way to look at the central nervous system.鈥�
The sea squirt, an invertebrate, is a sibling to the ancestor of all vertebrates 鈥� 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鈥檚 larval brain has only 177 neurons 鈥� a stark contrast to the billions of cells in a human鈥檚 brain. Ryan鈥檚 study looked at how the pathways between neurons in its brain could result in its swimming patterns.
鈥淗ere you have a mere 177 neurons in the central nervous system, but there鈥檚 still a huge degree of complexity if you look at their connections,鈥� says Dr. Ryan. 鈥淪o what it鈥檚 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.鈥�
Mountain of data
Dr. Ryan and Dr. Meinertzhage used an electron microscope to take ultra-high resolution images of the brain鈥檚 neurons, their fibre-like connections, and the all-important contacts 鈥� called synapses 鈥� 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鈥檚 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鈥檚 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.
鈥淗aving 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,鈥� says Dr. Ryan. 鈥淚t鈥檚 important not to disregard what we can learn from simpler organisms.鈥�