Rods and Cones – a placement at the UCL Institute of Ophthalmology

Lucky recently undertook a placement at UCL’s Institute of Ophthalmology. Here she explains a little of what she learnt about vision…

The retina is the part of the eye that senses light. It consists of two types of photoreceptor cells: rods and cones. Rod cells work in low light levels, and as their name implies, they are rod shaped. This type of photoreceptor cell is responsible for what we would call night vision; they allow us to see in the dark. Rods use a chemical called rhodopsin to absorb photons. Rhodopsin molecules split into a retinal and an opsin molecule when they absorb photons. Rhodopsin then reforms from these molecules at a constant rate but this process is very slow. This is the reason why our eyes take time to adjust to the dark. When the light is on and there are high light levels, rhodopsin molecules are broken down so when the light is switched off and there is less intense light we can’t really see until rhodopsin starts to reform. Gradually our night vision, in a sense, activates. Rod cells are unable to perceive colour, unlike cone cells and so our night vision is black and white.

Cone cells are responsible for our ability to perceive colour. The cone cells of our retina function at higher light levels than rods and have a cone-like shape. There are three types of cone cells, all of which respond to different wavelengths. S-cones respond to short wavelengths and peak at a bluish colour; M-cones to medium wavelengths and peak at green; L-cones to long wavelengths and peak at red.

Our colour vision or perception of colour is based on the wavelengths of light that objects reflect. Take the example of a plant leaf; the cells of a plant leaf contain the green pigment chlorophyll. As a result, plant leaves are green and so absorb other colours of the spectrum such as blue and red, reflecting green visible light into our colour perceiving cone cells. Green light has a medium range wavelength so it is responded to by M-cone cells. Your brain then interprets signals it receives from your cone cells. People without all three cone types functioning correctly have colour blindness. So, if a person’s L-cone isn’t functioning properly they won’t be able to see the colour red properly as L-cones respond to long wavelengths like red light.

By Lucky G