Rods, cones and the mechanism of vision

Two kinds of photoreceptor

The retina holds two kinds of photoreceptor cells.

• The cones work in daylight. This is photopic vision, and it includes colour vision.
• The rods work in twilight. This is scotopic vision.

The rods hold a purple pigment, rhodopsin, also called visual purple. It is made from a derivative of vitamin A.

The human eye has three kinds of cones. Each has a different photopigment, and each responds to red, green or blue light. The sensation of a colour comes from a combination of the three. When the three are stimulated equally, we see white light.

You should find that a light of 550 nm excites both the green and the red cones, and the mixture gives the sense of yellow. The rods have one pigment only, so they cannot tell colours apart. This is why colours fade at night.

Rods and cones in the retina

The numbers of the two kinds of cell differ from place to place. The standard values are about 120 million rods and about 6 million cones in one retina. NCERT does not give the numbers. The cones are crowded in the fovea, where the rods are absent. The rods are most numerous some distance away from the fovea. So to see a faint star, you look a little to one side of it.

The mechanism of vision

Light that enters the eye is focused on the retina by the cornea and the lens. Then:

1. The photopigments of the rods and the cones are made of opsin, a protein, and retinal, an aldehyde of vitamin A.
2. Light makes the retinal separate from the opsin. The structure of the opsin changes.
3. This changes the permeability of the membrane. A potential difference is generated in the photoreceptor cell.
4. The signal passes through the bipolar cells and generates action potentials in the ganglion cells.
5. The optic nerves carry the impulses to the visual cortex of the brain.
6. The brain analyses the impulses, and it recognises the image from earlier memory and experience.

In the light, the rhodopsin splits. In the dark, the retinal and the opsin join again, which takes some minutes. This is why you see poorly when you walk from bright sunlight into a dark room, and better a few minutes later. The pupil also widens in the dark. This is not described in NCERT, and it is not in the NEET syllabus.

Focusing on a near object (not in the NEET syllabus)

A relaxed eye is focused on distant objects. To see a near object, the ciliary muscle contracts, and the lens becomes thicker and more curved. Its power increases. This is accommodation. NCERT does not describe it in this chapter, but A-Level and IB ask it.

A simple model of the eye puts the retina 17 mm behind the lens system. An object at a distance $u$ is focused when the power is $P = \dfrac{1}{u} + \dfrac{1}{0.017}$, in dioptres, with $u$ in metres. For a distant object, $P = 58.8\ \text{D}$. For an object at 25 cm, $P = 62.8\ \text{D}$, so the power has to rise by $4\ \text{D}$. These are standard model values for a student of physics.

Try it in Python

The program works out the power that the eye needs for objects at different distances.

Practise with new numbers

Common mistakes

• Saying that rods give colour vision. The cones give colour vision. The rods work in dim light.
• Saying that the cones work best at night. The cones work in daylight.
• Saying that light only passes through the optic nerve. Light is focused on the retina. The optic nerve carries impulses and not light.
• Saying that the opsin is a vitamin. The opsin is a protein. The retinal comes from vitamin A.
• Saying that the action potentials start in the rods and cones. The photoreceptors make potential differences. The action potentials arise in the ganglion cells.

Remember these

• Cones: daylight and colour. Three kinds, with red, green and blue pigments. Equal stimulation gives white.
• Rods: twilight. Rhodopsin (visual purple), from vitamin A.
• Photopigment = opsin (protein) + retinal (aldehyde of vitamin A). Light separates them and changes the potential.
• Path: photoreceptors, bipolar cells, ganglion cells (action potentials), optic nerve, visual cortex.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which cells work in daylight, and which work in twilight?

2. What is rhodopsin, and from what is it made?

3. What are the two parts of a photopigment?

4. What does light do to a photopigment, and what is the result for the cell?

5. Name, in order, the cells and the nerve that the signal passes from the photoreceptor to the brain.

References

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 36, Sensory Systems, section 36.5, Vision.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 14, The Somatic Nervous System.
• NCERT (2023). Biology, Textbook for Class XI, chapter 18.