Wiring the Brain: How Viruses Can Be Engineered to Trace Neural Connections

Every stroke rehab researcher should be using something like this to prove how their therapy works. Then we could have a factual discussion on exactly what goes on and how to make it better.
http://bigthink.com/ideas/40335

It is hard to overstate the complexity of the brain. Not only are there tens of billions of individual nerve cells, or neurons, which make literally trillions ofconnections between each other. The complexity really lies in the fact that there are hundreds or maybe even thousands of different types of neurons, which are arranged in highly-organised patterns, and which connect to each other in very specific ways. Now, a new technique is emerging that enables neuroscientists to trace these kinds of connections by enlisting help from an unlikely source – viruses.

To understand why this approach is so exciting, we need to consider the scope of the problem it can help solve. This begins with individual neurons themselves. Neurons are polarised – they have an end for inputs and an end for outputs. Each of these may be branched to give thousands of independent sites of input and output. For any given neuron, there are other neurons that connect to it (information flows from all those neurons into our subject neuron) and other neurons that it connects to (information flows from our subject neuron out to all these neurons).

But neurons are not all the same. The most obvious and perhaps most important difference between neurons is that some are excitatory and some inhibitory. When an excitatory neuron is activated, it releases neurotransmitter at the connections it makes with its output neurons – this neurotransmitter tends to make those other cells electrically active. The exact opposite happens when an inhibitory cell is activated – it releases a different neurotransmitter onto its target neurons, which makes them less electrically active.

Neurons That Fire Together Wire Together

At any given moment, each single neuron is integrating the incoming information from typically hundreds of excitatory and inhibitory neurons – the balance between these and their precise timing will determine whether it becomes active enough to “fire” a signal to its own output neurons. (Without the inhibitory neurons, any electrical activity would rapidly spread through the entire brain in an epileptic-like firestorm).