Connectomics: Mapping the Connections of the Nervous System

Michelle Po
14 March 2013

Above: Structural and functional brain imaging analyses, combined with computational analyses, reveal highly connected, centrally located regions of the human cortex that form a “structural core” of the brain (Liza Gross)

Did You Know? A typical neuron is made up of three parts – The cell body contains the neuron’s genetic information. Dendrites sense signals from the environment and other neurons. Axons transfer signals to other neurons.Our brains allow us to move, think, and remember. But have you ever wondered how they accomplish such complex tasks? Scientists have recognized the importance of understanding the brain’s structure. Tens of billions of neurons—specialized cells in the brain—need to be connected to each other in a specific pattern. Scientific interest in brain structure has given birth to a new field of study: “Connectomics.”

The three basic parts of a neuron. Click to enlarge (Quasar Jarosz, adapted by Stan McGraw)

Let’s review the basic components nervous system. The basic functional units are called neurons. They receive and transmit information in the form of chemical and electrical signals. These signals are transferred from one neuron to another at the points of contact between neurons, specialized structures called synapses. The neuronal connectivity pattern of an organism is often referred to as its “wiring pattern,” because neurons in the brain can be compared to wires in an electrical circuit. During the past decade, a new term has emerged to describe an organism’s neuronal wiring pattern: “connectome.”

Did You Know? There are three main types of neurons – Sensory neurons detect environmental cues, like temperature. Motor neurons send signals to muscles. Interneurons relay signals between sensory neurons and motor neurons.Connectomics therefore refers to the application of microscopy and neural imaging techniques to reconstruct an organism’s connectome. Connectomics allows scientists to visualize and understand the structure of an organism’s nervous system.

The mapping of neuronal connections begins by preserving a piece of nervous tissue, like a brain. The sample is then cut into very thin slices, each of which is about 70 nanometers thick (that’s less than 1/10000 of a millimetre!). A very powerful microscope, called an electron microscope, is then used look at each brain slice so that individual neurons, which are typically only one micrometre (1/1000 of a millimetre) thick, can be seen. By tracing neurons and their processes from one section of brain to the next, scientists can build a 3D model of the path that each particular neuron follows through the brain, as well as the connections it makes with neighbouring neurons.

To date, only one organism’s connectome has been completely mapped. With only 302 neurons, the connectome of the round worm Caenorhabditis elegans was mapped in the 1980s. The work was done completely by hand, and took over a decade to complete.

Did You Know? EyeWire is a citizen science project that lets you help map the human connectome by tracing neurons in a block of retinal tissue. A computer program has already begun the work, but it requires human correction.Because human brains have tens of billions of neurons and trillions of synapses, the human connectome will likely also take over a decade to map, even with the help of today’s sophisticated computer software. The US National Institutes of Health (NIH) has awarded $40 million in funding to scientists working toward this goal. The hope is that a complete map of the human brain’s wiring will help scientists understand how a normal brain functions. And it may someday help find treatments for diseases that result from incorrect neuronal wiring.

Learn More!

Connectomics: Tracing the Wires of the Brain (Sebastian Seung, The Dana Foundation)

Other References

Sporns O, Tononi G, Kotter R. 2005. The human connectome: A structural description of the human brain. PLoS Computational Biology. 1:e42. Ware RW. 1975. Three-dimensional reconstruction from serial sections. International Review of Cytology. 40:325-440. White JG, Southgate E, Thomson J. N., Brenner S. 1986. The structure of the nervous system of the nematode Caenorhabditis elegans. Philosophical Transactions of the Royal Society B: Biological. 314:1-340.

Michelle Po

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