Because only genetically targeted cells express the light-sensitive channel, a light pulse can selectively control one cell type in a densely mixed brain region, leaving neighboring cells untouched.
dV/dt = -V/tau + opsin_expr * light_power
spike when V > threshold, then reset
- Targeted neurons — cells engineered to express a light-gated ion channel (e.g. ChR2) under a cell-type-specific promoter.
- Opsin expression — how much channelrhodopsin protein each targeted cell actually produces.
- Light pulse power — blue-light irradiance delivered via optical fiber, driving photocurrent into the neuron.
- Firing threshold — the membrane depolarization level a neuron must cross to fire an action potential.
Optogenetics let neuroscientists causally test circuit hypotheses (e.g. 'this hypothalamic circuit drives aggression') by switching specific neurons on or off with millisecond precision — something electrical stimulation could never do selectively.