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# Retinal processing: ganglion cells, receptive fields and adaptation

- Mechanism
- For completeness

Before the signal leaves the eye, the retina compresses it: ganglion cells report edges, contrast and colour differences rather than raw light.

Updated Nov 13, 2026

Also called: Centre–surround receptive fields, ON and OFF pathways, Midget and parasol cells

**See it in 3D:** [Retina](https://durgical.com/en/anatomy/retina/) · [Optic nerve](https://durgical.com/en/anatomy/optic-nerve/) · [Optic disc](https://durgical.com/en/anatomy/optic-disc/)

## Key points

- Photoreceptors and bipolar cells release glutamate; photoreceptors release most in the dark and less when light hyperpolarizes them.
- ON bipolars (mGluR6, sign-inverting) depolarize in light; OFF bipolars (AMPA/kainate) depolarize in dark: two parallel pathways from the start.
- Horizontal cells create the antagonistic surround (lateral inhibition), which sharpens edges and contrast.
- Ganglion cells are the output neurons that fire action potentials, with a spontaneous rate that light can raise or lower.
- Fovea: about one cone per midget ganglion cell (high acuity); periphery: many rods per ganglion cell (high sensitivity, low acuity).

## What it is

About 100 million photoreceptors converge onto roughly a million ganglion cells, whose axons form the optic nerve. Bipolar, horizontal and amacrine cells shape each ganglion cell's receptive field into a centre and an opposing surround, so the cell reports contrast rather than overall brightness. Different ganglion classes trade detail for sensitivity and carry separate channels for form, motion, colour and the body clock.

## How it works, step by step

- Light hyperpolarizes photoreceptors and reduces glutamate release onto bipolar cells.
- ON-centre cells are excited by light in the centre; OFF-centre cells are excited when the centre darkens.
- Light in the surround, through horizontal cells, opposes the centre response, so even illumination drives the cell little.
- Amacrine cells (GABA, glycine, dopamine, acetylcholine) shape timing, motion direction and the rod pathway.
- Ganglion cells fire spontaneously even in steady light; changes above or below this rate carry the signal in both directions.
- Cone-opponent wiring makes midget and small bistratified cells respond to colour differences.
- Adaptation: neural gain in the retina and pigment bleaching adjust sensitivity across a huge range of light levels.

## Main ganglion cell classes

| Class | Share | Carries | Projects to |
| --- | --- | --- | --- |
| Midget (P) | ≈ 70–80% | Fine detail, red–green | Parvocellular LGN layers |
| Parasol (M) | ≈ 10% | Motion, low contrast, flicker | Magnocellular LGN layers |
| Small bistratified (K) | small | Blue–yellow | Koniocellular LGN layers |
| Intrinsically photosensitive (melanopsin) | ≈ 1–2% | Overall light level | Suprachiasmatic nucleus, pretectum |

## Clinical notes

- Glaucoma kills ganglion cells and their axons at the optic disc, producing arcuate field defects and disc cupping.
- Because the surround cancels uniform light, a large evenly lit area looks less distinct than an edge, the basis of several contrast illusions.

## Memory aid

Centre says yes, surround says no: the retina sends edges, not pixels.

## Test yourself

Think of the answer, then tap the question to see it.

## An ON-centre ganglion cell receives a uniform light covering both centre and surround. What happens to its firing?

It changes little: the excitatory centre and inhibitory surround largely cancel, so the cell signals contrast, not overall brightness.

## Why is peripheral vision sensitive in dim light but poor at detail?

Many rods converge onto each peripheral ganglion cell: their signals sum (sensitivity) but location within the large field is lost (low acuity).

## Which retinal cells provide lateral inhibition for the receptive-field surround?

Horizontal cells, which link many photoreceptors and feed back an opposing signal; amacrine cells add further lateral shaping.

## Why does glaucoma damage the visual field but not colour or acuity early on?

It first kills ganglion cell axons at the disc margins (arcuate bundles), sparing the central papillomacular fibres until late.

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## Connections

## See it in 3D 3

The structures of Durgical's 3D anatomy where it happens.

- [Retina](https://durgical.com/en/anatomy/retina/)
- [Optic nerve](https://durgical.com/en/anatomy/optic-nerve/)
- [Optic disc](https://durgical.com/en/anatomy/optic-disc/)

## Related topics 5

- [The retina and phototransduction](https://durgical.com/en/physiology/the-retina-and-phototransduction/)
- [The visual pathway, pupillary reflexes and eye movements](https://durgical.com/en/physiology/the-visual-pathway-pupillary-reflexes-and-eye-movements/)
- [Sensory receptors, coding and neuronal circuits](https://durgical.com/en/physiology/sensory-receptors-coding-and-neuronal-circuits/)
- [Colour vision and colour blindness](https://durgical.com/en/physiology/colour-vision-and-colour-blindness/)
- [Visual processing in the thalamus and cortex: LGN, V1 and the two streams](https://durgical.com/en/physiology/visual-processing-in-the-thalamus-and-cortex-lgn-v1-and-the-two-streams/)

## Related conditions 1

- [Open-angle glaucoma](https://durgical.com/en/conditions/open-angle-glaucoma/)

## In Biochemistry 1

- [Glutamate (glutamic acid)](https://durgical.com/en/biochemistry/molecules/glutamate/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [Anatomy, Head and Neck: Retina](https://www.ncbi.nlm.nih.gov/books/NBK542332/) (StatPearls (NCBI Bookshelf), 2024); [Open Angle Glaucoma](https://www.ncbi.nlm.nih.gov/books/NBK441887/) (StatPearls (NCBI Bookshelf), 2024); [Neuroanatomy, Cranial Nerve 2 (Optic)](https://www.ncbi.nlm.nih.gov/books/NBK507907/) (StatPearls (NCBI Bookshelf), 2024).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/retinal-processing-ganglion-cells-receptive-fields-and-adaptation/