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.

The ideal way to study the property of different types of neurons is to control individual types of cells independently and see what happens when you alter one type of cell. Optogenetics helps to realize this goal.

My brain cells are dying in their trillions.

I like nonsense; it wakes up the brain cells.

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.

In regular cells, like in bone and blood, the cells divide. But brain cells do not normally divide. The great majority will not divide. The neurons you're born with are pretty much what you have at end of life.