History

Most neuroanatomists would agree that understanding how the brain is connected to itself and the body is of paramount importance. As such, it is of equal importance to have a way to visualize and study the connections among neurons. Neuronal tracing methods offer an unprecedented view into the morphology and connectivity of neural networks. Depending on the tracer used, this can be limited to a single neuron or can progress trans-synaptically to adjacent neurons. After the tracer has spread sufficiently, the extent may be measured either by fluorescence (for dyes) or by immunohistochemistry (for biological tracers). An important innovation in this field is the use of neurotropic viruses as tracers. These not only spread throughout the initial site of infection, but can jump across synapses. The use of a virus provides a self-replicating tracer. This can allow for the elucidation of neural microcircuitry to an extent that was previously unobtainable. This has significant implications for the real world. If we can better understand what parts of the brain are intimately connected, we can predict the effect of localized brain injury. For example, if a patient has a stroke in the amygdala, primarily responsible for emotion, the patient might also have trouble learning to perform certain tasks because the amygdala is highly interconnected with the orbitofrontal cortex, responsible for reward learning. As always, the first step to solving a problem is fully understanding it, so if we are to have any hope of fixing brain injury, we must first understand its extent and complexity.