6.1.10 The Human Retina

The Retina

The retina is the light-sensitive layer at the back of the eye, responsible for detecting light and converting it into electrical signals sent to the brain via the optic nerve. It contains specialised photoreceptor cells called rods and cones.

Structure and Function of the Retina

1. Photoreceptor Cells:

• Rods:
• Specialised for low-light (scotopic) vision.
• Do not detect colour (monochromatic vision).
• Found mainly in the periphery of the retina.
• Contain the pigment rhodopsin, which breaks down in the presence of light to initiate a nerve impulse.
• Cones:
• Specialised for colour vision and sharp detail (photopic vision).
• Function best in bright light.
• Found mainly in the fovea, the central part of the retina.
• Contain the pigment iodopsin, which is sensitive to different wavelengths of light (red, green, and blue cones).

2. Fovea:

• A small depression in the centre of the retina.
• Contains a high concentration of cones but no rods.
• Responsible for sharp, central vision.

3. Blind Spot:

• The point where the optic nerve leaves the retina.
• Contains no photoreceptor cells, so it cannot detect light.

4. Other Retinal Layers:

• Bipolar cells: Connect photoreceptors to ganglion cells.
• Ganglion cells: Their axons form the optic nerve, carrying signals to the brain.

How the Retina Processes Light

1. Light enters the eye through the pupil and is focused by the cornea and lens onto the retina.
2. Photoreceptor cells absorb light, breaking down their pigments (rhodopsin or iodopsin) and generating a generator potential.
3. The electrical signal is transmitted through bipolar cells to ganglion cells.
4. The optic nerve carries the signal to the visual cortex of the brain for processing.

Comparison of Rods and Cones

Feature	Rods	Cones
Sensitivity	Very sensitive to dim light	Require bright light
Colour Vision	Monochromatic (black and white)	Colour vision (red, green, blue)
Location	Peripheral retina	Central retina (fovea)
Visual Acuity	Low (many rods share one bipolar cell)	High (each cone has its own connection to the brain)

Adaptations of the Retina

• High density of cones in the fovea provides sharp colour vision.
• Peripheral retina with rods detects motion and light in low-light conditions.
• Adaptation to changing light levels:
  • Dark adaptation: Rhodopsin in rods regenerates, allowing better vision in low light.
  • Light adaptation: Cone cells adjust to bright light.