Explore Module
Basic Level
From physics to physiology: photoreceptors and signal.
On the retina, the inverted image is not a final “snapshot”: it is the physical stimulus that triggers signal transduction. Photoreceptors (rods and cones) contain photopigments that, upon photon absorption, change state and activate a biochemical cascade. The resulting signal is not an “all-or-nothing” action potential, but a graded potential transmitted to bipolar and ganglion cells. The axons of the ganglion cells form the optic nerve, the output channel carrying information to the brain.
Why can we distinguish large shapes in the dark but not colors?
Because rods are more sensitive and active in scotopic (low-light) conditions but cannot detect color; cones, responsible for color and acuity, require higher luminance levels.