The brain is a complex organ of highly intertwined cellular connections, which receive, relay, and manipulate the plethora of input which our body and the world around us generate. It is characterized by a high degree of cellular heterogeneity and rapid electrical functioning. Given these key characteristics, it has long been speculated that any broadly acceptable tool for manipulating neural activity would have to be mediated by light, whose velocity and spectral diversity would pair perfectly to the core properties of the ‘neural side’ of the brain (see section on Neurons and Astrocytes).
Optogenetics refers to the use of genetic manipulations to render cells sensitive to light. Specifically, these molecules that were discovered in several different organisms, are seven-transmembrane proteins which act to transport ions across a cell’s membrane upon illumination. Depending on the ionic species carried, and the direction of the movement, some variants cause a negative potential to develop and lead to ‘silencing’ of activity, while others lead to relative positive potential to develop and cause an ‘activation’ of neural activity. Though opsins were first discovered in the 1970s, with the showing that bacteriorhodopsin pumps protons out cells in response to green light, it was not until 2005 that opsins were successfully expressed in mammalian neurons, where they were used to manipulate neuronal activity with a high degree of spatial and temporal precision.