--- title: "From Photons to Spikes: the Retina" description: "Light activates a chemical pathway in rods and cones that closes cGMP-gated channels and hyperpolarises the cell. Retinal circuits process these graded signals before ganglion cells send spikes throug" 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # From Photons to Spikes: the Retina Light activates a chemical pathway in rods and cones that closes cGMP-gated channels and hyperpolarises the cell. Retinal circuits process these graded signals before ganglion cells send spikes through the optic nerve. Opposing centre and surround effects make many cells sensitive to local contrast, while retinal output also carries light-level information. 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 how opposing centre and surround effects can reduce responses to uniform illumination and favour local contrast - Relate the distribution of rods and cones to the difference between night and daylight vision ## Lesson: How the retina turns light into signals In darkness, cGMP keeps channels open in rods and cones so sodium and calcium enter. Potassium also leaves. This inward current keeps the membrane relatively depolarised and supports steady glutamate release. Light closes many cGMP-gated channels. Inward current falls while potassium continues to leave, so the cell hyperpolarises and releases less glutamate. Rods and cones signal with graded voltage changes, not optic-nerve spikes. In a rod, a photon changes the shape of retinal within the pigment rhodopsin. This activates rhodopsin, then the G protein transducin. Transducin activates phosphodiesterase (PDE), which breaks down cGMP. Lower cGMP closes cGMP-gated channels and reduces inward current. Blocking PDE activation prevents this pathway's usual light-driven fall in cGMP. *(drawing: Follow the chemical relay from photon absorption to a change in membrane voltage.)* The cornea and lens focus light on the retina, as in the eye lesson. In a direct cone pathway, a cone signals to a bipolar cell, then a ganglion cell. Ganglion cells generate action potentials. Their axons form the optic nerve and carry these spikes to the brain. At the optic disc, where these axons leave the eye, there are no photoreceptors. This creates a blind spot. A receptive field is the retinal area where light can change a cell's response. ON and OFF bipolar cells have different glutamate receptors. When light increases only in the field's centre, the photoreceptors there release less glutamate. An ON bipolar cell depolarises in response. An OFF bipolar cell hyperpolarises. The same decrease in transmitter has opposite effects in these two pathways. Many bipolar and ganglion cells have receptive fields with a centre and an opposing surround. In an ON-centre bipolar cell, centre light promotes depolarisation, while surround light opposes it. Lighting both regions can produce a smaller response than centre-only light. Cancellation need not be complete. This favours local contrast without removing all light-level information or stopping every ganglion cell from firing. Horizontal cells collect signals from nearby photoreceptors. Their feedback to photoreceptors helps create the opposing surround effect in many bipolar cells. This lateral route changes how centre signals reach a bipolar cell. Ganglion-cell axons, rather than horizontal cells, carry spikes through the optic nerve. Rods are more sensitive to dim light than cones and support vision after dark adaptation. The foveal centre has densely packed cones and no rods. Rods are abundant outside it. Looking slightly beside a faint star moves its image onto rod-containing retina, which can make it easier to see. In brighter light, cones support colour and sharp foveal detail. Peripheral vision is not better for every task. **Recap.** Light lowers cGMP and reduces photoreceptor glutamate release. Retinal circuits shape contrast signals. Ganglion cells send spikes. Rods support dim vision; cones support colour and detail. *Lesson, questions, worked explanations and revised flashcards written for this dataset. Biology for AP Courses and Introduction to Behavioral Neuroscience were consulted for factual background.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [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_) - [The Eye as a Receptor Organ](https://lightmysky.com/learn/science/the-eye-as-a-receptor-organ-mt_fslwsM3I7F) ## Optional connections - [Synaptic Transmission, Summation and Inhibition](https://lightmysky.com/learn/science/synaptic-transmission-summation-and-inhibition-mt_0uM8xRV92B) - [G-Protein-Coupled Receptors and Second Messengers](https://lightmysky.com/learn/science/g-protein-coupled-receptors-and-second-messengers-mt_7ozcs_sUOJ) ## 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)