---
title: "From Photons to Spikes: the Retina"
description: "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 fr"
canonical: https://lightmysky.com/learn/science/from-photons-to-spikes-the-retina-mt_kviN7WskmP
source: https://lightmysky.com/learn/science/from-photons-to-spikes-the-retina-mt_kviN7WskmP.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: The Human Body · Ages 19 to 20
Page: https://lightmysky.com/learn/science/from-photons-to-spikes-the-retina-mt_kviN7WskmP

## Ready when they can

- 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

## Lesson: Your eye reports edges, not light

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.

**Tip.** Side vision beats central vision at night for one reason: the periphery holds the rods. Look slightly aside to catch a faint star.

**Recap.** Light quiets backwards cells, the middle layers subtract surround from centre, and the brain receives contrast.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Synaptic Transmission, Summation and Inhibition](https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B)
- [Ion Channels: Selectivity, Gating and the Evidence from Single Channels](https://lightmysky.com/learn/science/ion-channels-selectivity-gating-and-the-evidence-from-single-channels-mt_2xPzPeXZZ_)
- [G-Protein-Coupled Receptors and Second Messengers](https://lightmysky.com/learn/science/g-protein-coupled-receptors-and-second-messengers-mt_7ozcs_sUOJ)
- [The Eye as a Receptor Organ](https://lightmysky.com/learn/science/the-eye-as-a-receptor-organ-mt_fslwsM3I7F)

## Opens up

- [Building a Visual Scene: Pathways, Maps and Receptive Fields](https://lightmysky.com/learn/science/building-a-visual-scene-pathways-maps-and-receptive-fields-mt_wOc4YpkdYL)