From Photons to Spikes: the Retina
A photoreceptor works backwards from most sensory cells: light closes channels and the cell hyperpolarises. The retina then does real processing before anything leaves the eye, subtracting a centre from its surround so that what the optic nerve carries is contrast rather than brightness.
What a learner can do afterwards
- Describe phototransduction from photon absorption to a change in membrane potential
- Explain centre-surround organisation and what it discards
- Relate the distribution of rods and cones to the difference between night and daylight vision
1 · Read
Photoreceptors break the usual rule: they stay active in the dark and quiet down in light. Darkness keeps them releasing signal steadily, and photons close channels so the cell hyperpolarises, turning more negative instead of firing.
The signal path runs photoreceptors to bipolar cells to ganglion cells, then out along the optic nerve to the brain. Rods are exquisitely sensitive and own dim night vision from the retinal periphery, while cones need brighter light and deliver colour, packed tightest in the fovea for sharp detail. The optic nerve exit has no receptors at all, which leaves a blind spot.
The retina computes before it reports. Each bipolar cell compares a centre patch against its surround ring, with horizontal cells wiring the surround to cancel the centre. Uniform light therefore earns no reply: only contrast and borders get through. What reaches the brain is edges, not raw brightness.
Side vision beats central vision at night for one reason: the periphery holds the rods. Look slightly aside to catch a faint star.
Light quiets backwards cells, the middle layers subtract surround from centre, and the brain receives contrast.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.