Your brain is built of cells called neurons and glia - hundreds of billions of them. Each one of these cells is as complicated as a city.

A typical neuron makes about ten thousand connections to neighboring neurons. Given the billions of neurons, this means there are as many connections in a single cubic centimeter of brain tissue as there are stars in the Milky Way galaxy.

Throughout our lifetimes, we are constantly regenerating new brain cells in the hippocampus, a process called neurogenesis. New stem cells are constantly being born in the hippocampus that ultimately differentiate into fully functional neurons.

The human brain has 100 billion neurons, each neuron connected to 10 thousand other neurons. Sitting on your shoulders is the most complicated object in the known universe.

My brain cells are dying in their trillions.

Brain cells are normally not sensitive to light. So by introducing light-sensitive proteins into specific types of neurons, we can now selectively control that specific type of neuron by shining light in the brain.