Actuators

Most actuators at present are ion channels, proteins which regulate ion channels or ion pumps. Many are based on visual rhodopsin which is coupled to G protein or microbial rhodopsins that are proton or chloride pumps and it is this which makes them light responsive. These allow control over the same variables that the sensors respond to: intracellular calcium concentration, membrane voltage and secretion of signaling molecules. There are also light-activated G-protein-coupled receptors, second messengers, kinases, and transcription factors. A light-activated glutamate receptor (LiGluR) is a calcium channel and hence allows control over calcium-dependent processes such as secretion.

Actuators are stimulated by LED or laser light delivered via optical fibers.  These can be used in vitro (e.g., in brain slices) or in vivo, implanted into the brain ahead of time:  this allows optogenetic experiments to be done in awake, behaving animals. Actuators available now allow control over processes with a timescale of a few milliseconds to seconds.