The Human Retina
Biology: AQA A Level, topic 6.1.7
Rod and Cone Cells
The two main receptors in the human eye are rod and cone cells. Although both of these receptor types respond to photons of light, there are a number of differences:
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The eye
Sensitivity to light
- Rod cells -
- Highly sensitive to light.
- Cone cells -
- Less sensitive to light.
Visual acuity
- Visual acuity is the ability to distinguish between close objects or two points.
- Rod cells -
- Low visual acuity.
- Cone cells -
- High visual acuity.
Number and distribution
- Rod cells -
- Highly numerous.
- Evenly distributed on the retina but absent in the fovea.
- Cone cells -
- Fewer cells than rod cells.
- Distributed mainly at a single point in the retina called the fovea.
Pigment
- Rod cells -
- Use a pigment called Rhodopsin.
- Rhodopsin detects light and dark.
- Rod cells are monochromatic - only detect one wavelength of light.
- Cone cells -
- Use a pigment called Iodopsin.
- Iodopsin detects colour.
- Cone cells are trichromatic - divided into three types and each responds to a different wavelength of light, either red, blue or green.
Converting Light to Electrical Impulses
Rod and cone cells are types of photoreceptor. When light is detected by a photoreceptor cell, a generator potential is created and an impulse travels along a neurone. The steps in this process are:
Detecting light
- Light is absorbed by the pigments in photoreceptor cells:
- Rhodopsin is the pigment in rod cells
- Iodopsin is the pigment in cone cells.
- Absorption of light induces a change in the membrane permeability of the pigments. This causes Na+ ions to flood into the cell and a generator potential is established.
- If the generator potential reaches the threshold, a nerve impulse flows along a bipolar neurone.
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Bipolar neurone
- Each photoreceptor synapses (forms a junction) with a relay neurone called a bipolar neurone.
- Each bipolar neurone synapses with a sensory neurone called a ganglion cell.
- Axons of ganglions leave the eye via the optic nerve to send a signal to the brain.
- An axon is the long, extended cell body of a nerve cell.
Sensitivity to light
- Differences in sensitivity to light are due to differences in how rod and cone cells connect to bipolar neurones.
- Each cone cell synapses with a single bipolar neurone.
- Sufficient light must stimulate the cone cell to generate an action potential in the bipolar neurone.
- Several rod cells synapse with the same bipolar neurone.
- Light stimulating a single rod cell may not be sufficient to generate an action potential in the bipolar neurone.
Spatial summation
- Several rod cells synapse with the same bipolar neurone. This means that the cumulative stimulation of more than one rod cell can create an action potential in the bipolar neurone.
- This is called spatial summation.
- Spatial summation results in retinal convergence. This is the idea that several rod cells generate a signal in a single sensory neurone.
Test yourself on The Human Retina
Which of the following statements about rod and cone cells are true?
Which pigment does rod cells use?
Steps Involved in the Detection of Light
Which of the following statements about bipolar neurones are true?
What does spatial summation result in?