Photoreceptors in The Eye
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of the eye from GCSE. You can test your knowledge on this below.
What is the function of the retina?
It contains light-sensitive receptor cells (rods and cones) that detect light and send electrical impulses to the brain via the optic nerve.
What is the role of the iris in the eye?
It controls the amount of light entering the eye by adjusting the size of the pupil.
How are impulses transmitted from the eye to the brain?
Via the optic nerve, which carries impulses from photoreceptors to the brain.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains photoreceptors in the eye or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Structure and Types of Photoreceptors
- Located in the retina at the back of the eye.
- Two types: rod cells and cone cells that contain light sensitive optical pigments.
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Both synapse with Bipolar Neurones, which connect to the optic nerve. The optical nerve carries electrical impulses to the brain.
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They detect the stimulus, Light, and convert this stimulus into an electrical impulse.
Rod Cells
- Found mainly in the peripheral retina.
- Sensitive to low light intensity.
- Only detect black and white.
- Many rods converge onto a single bipolar neurone- this is called retinal convergence.
- Pigment: rhodopsin – needs a small amount of light to be broken down and generate an action potential.
High Sensitivity (Why?)
- Retinal convergence: Multiple rod cells share one bipolar neurone.
- This allows spatial summation, where impulses from multiple rod cells combine.
- Even at low light intensities, sufficient neurotransmitter can be released to reach threshold and generate an action potential in the bipolar neurone.
- More likely to produce an action potential in dim light.
Low Visual Acuity (Why?)
- Retinal Convergence: Multiple rod cells share one bipolar neurone.
- Multiple rod cells synapse with a single bipolar neurone, meaning the brain only receives one electrical impulse when any of these rod cells are stimulated.
- This means visual acuity, or clarity, will be low. Two dots close together would appear as a single dot.
Cone Cells
- Concentrated in the fovea (central region of the retina).
- Responsible for colour vision as there are three different types of cone cell that respond to different wavelengths of light (red, green and blue cones).
- Require high light intensity to function.
- Pigment: Iodopsin- requires a higher light intensity to be broken down so these cells cannot allow us to see in low light intensities.
- Each cone connects to one bipolar neurone – no retinal convergence.
High Visual Acuity (Why?)
- No Retinal Convergence: Each cone cell sends a separate impulse along a separate bipolar neurone to the brain, giving a detailed image.
- The brain can distinguish between two points close together.
Comparison Table
| Feature | Rod Cells | Cone Cells |
| Light Sensitivity | High (used in dim light). | Low (bright light needed). |
| Visual Acuity | Low. | High. |
| Colour Sensitivity | No (black & white only). | Yes (3 types: R/G/B). |
| Retinal Location | Periphery of retina. | Fovea (central retina). |
| Neurone Connection | Many rods to 1 bipolar neurone. Retinal convergence | 1 cone to 1 bipolar neurone. No retinal conergence. |
| Convergence? | Yes – spatial summation. | No – separate pathways. |
Key Terms
- Photoreceptor: A receptor that detects light (rods and cones in the retina) and converts light energy into an electrical impulse.
- Retinal Convergence: Multiple rod cells synapse with a single bipolar neurone.
- Visual Acuity: Sharpness or clarity of vision.
- Spatial Summation: When several weak stimuli from different sources combine to reach the threshold for an action potential.
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Exam Tip
Avoid using vague terms like "signals" or "messages". Use "action potential" or "electrical impulse".
No answer provided.
Explain why rod cells have high sensitivity to light and cone cells give high visual acuity. (5 marks)
- Several rod cells are connected to one bipolar neurone via retinal convergence.
- This allows spatial summation, where impulses from multiple rod cells combine.
- Ensures threshold is reached even in low light – action potential / electrical impulse is triggered.
- Each cone cell is connected to a single bipolar neurone (no retinal convergence).
- Cone cells send separate impulses/ action potentials to brain (Accept ‘along the optic nerve’).
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!