6.1.8 Generator Potential

Structure of the Eye

1. Light Pathway:

• Light enters the eye through the pupil.
• The iris muscles control the amount of light entering.
• The lens focuses light onto the retina.

2. Retina:

• Contains photoreceptors: rods and cones.
• The fovea (centre of the retina) has the highest density of cones.
• The optic nerve transmits impulses from photoreceptors to the brain.
• The blind spot is where the optic nerve exits the eye and contains no photoreceptors.

Types of Photoreceptors

Cone Cells (Colour Vision):

• Found in high density in the fovea.
• Contain the pigment iodopsin, which is sensitive to bright light.
• Enable colour vision, with three types of cones sensitive to red, green, or blue light.
• Provide high visual acuity because each cone connects to a single bipolar neurone.

Rod Cells (Low-Light Vision):

• Concentrated in areas outside the fovea.
• Contain the pigment rhodopsin, which is sensitive to dim light.
• Enable monochromatic vision (black and white).
• Provide low visual acuity because multiple rods share the same bipolar neurone, meaning signals are combined.

Generator Potential Formation

1. Light Absorption:

• When light hits the photoreceptor, rhodopsin (in rods) or iodopsin (in cones) absorbs the light.
• This triggers a chemical change in the pigment.

2. Depolarisation:

• The chemical change causes sodium ion channels to close in the photoreceptor membrane.
• This results in hyperpolarisation of the photoreceptor and reduces neurotransmitter release.

3. Bipolar Neurone Activation:

• Reduced neurotransmitter release allows the bipolar neurone to depolarise.
• If the depolarisation reaches the threshold, a generator potential is created.

4. Signal Transmission:

• The generator potential in the bipolar neurone initiates an action potential in the sensory neurone of the optic nerve, transmitting signals to the brain for visual processing.

Key Comparisons Between Rods and Cones

Feature	Rods	Cones
Pigment	Rhodopsin	Iodopsin
Light Sensitivity	Sensitive to dim light (low light)	Sensitive to bright light
Visual Acuity	Low (multiple rods share bipolar cell)	High (one cone per bipolar cell)
Colour Vision	Monochromatic (black and white)	Trichromatic (red, green, blue)
Location	High density outside the fovea	High density in the fovea