A neuron expresses the light-gated cation channel channelrhodopsin-2 (green dots on the cell body) in its cell membrane (1). The neuron is illuminated by a brief pulse of blue light a few milliseconds long, which opens the channelrhodopsin-2 molecules (2), allowing positively charged ions to enter the cells, and causing the neuron to fire an electrical pulse (3). A neural network containing different kinds of cells (pyramidal cell, basket cell, etc.), with the basket cells (small star-shaped cells) selectively sensitized to light activation. When blue light hits the neural network, the basket cells fire electrical pulses (white highlights), while the surrounding neurons are not directly affected by the light (4). The basket cells, once activated, can, however, modulate the activity in the rest of the network.