Considering the vast number of cells in the nervous system, it is mind-boggling to comprehend how they all ultimately connect to form functioning neural networks. Clusters of neurons in the cerebellum, for example, form connections with populations of cell bodies in the prefrontal cortex. In humans, nearly 90 billion neurons have to establish connections to various destinations across the brain. How they do so remains a mystery.
In the past, these networks have been difficult to observe visually. The histological technique introduced by 19th century researcher Camillo Golgi provided valuable images of neurons, but the number of stained cells was sparse. Staining a few neurons helped scientists view the characteristics of a single neuron, but did not allow them to view complete neural networks. Even if more cells were stained, if they were the same color, the single cells would appear as a blur. Consequently, progress was slow toward producing a histological technique that provided more information about the connections of neurons. Recent technological advancements, however, have moved this field forward by taking advantage of fluorescent proteins found in jellyfish and coral.
When scientists insert these fluorescent protein genes into mouse genes, the mouse genes produce the fluorescent proteins. Working at Harvard University, microbiologist Jeff Lichtman has refined his technique to produce “fluorescing mice.” These mice produce three proteins that can be randomly produced to tag each neuron with a distinctive color from about 100 different potential colors. This technique enables scientists to visualize different neurons in crowded brain regions. When the brains of these genetically engineered, or transgenic, mice, known as Brainbow mice, are processed and viewed with high-resolution specialized microscopes, each neuron can easily be seen even in the midst of all its neighboring neurons.
Although these vivid images are quite impressive, researchers working with this technology currently face the challenge of having too much data. As you might imagine, staining the cell bodies, dendrites, and axons for all the neurons in the brain produces an inordinate amount of data. According to Lichtman, even if he had several dozen electron microscopes scanning brain tissue around the clock, it would take months or years to reconstruct all the connections in just a single mouse brain (Lehrer, 2009).
Even though the prospect of tracing all the neural connections in the human brain seems daunting, the National Institute of Mental Health has invested $40 million in this project, hoping it will reveal clues about various brain functions and mental illnesses. The Human Connectome Project will result in revealing structural and functional maps of the brain’s neural wiring. Thus, there is cautious optimism that this project will reveal important information about the brain, with potential clinical applications such as cures for neurological diseases (Lichtman & Smith, 2008).