Proteins in the eye
Iona Twaddell explores the function of some of the proteins that allow you to see the world
Proteins for seeing
At the back of the eye is the retina, a layer of cells that are photosensitive (they respond to light). These photoreceptor cells contain photopigments, light-sensitive molecules that are made up of a protein called opsin and a cofactor that helps it work. (Confusingly, the different photopigments can also be called opsins.)
Photopigments change shape when they detect light, triggering a series of chemical reactions and sending a signal to the visual cortex of the brain telling it how much light there is at each point on the retina. It is this that creates a picture of the outside world.
Different types of photoreceptor contain different kinds of photopigment. There are two main kinds of photoreceptor cells involved in vision: rods and cones. Rods are used to see when it’s dark, and they don’t detect colour. They contain the photopigment rhodopsin.
Cones are used to see when there is bright light. There are three different types of cone, each sensitive to different wavelengths of light, which allow us to see colour. The cone photopigment is called iodopsin and comes in three forms: erythrolabe, which is most sensitive to red light, chlorolabe, which is most sensitive to green light, and cyanolabe, which is most sensitive to blue light.
Spike Walker/Wellcome Images
Proteins for sleeping
There are some light-sensitive proteins in the eye that are not used for seeing. Some cells in the retina contain the photopigment melanopsin; the signals from these cells don’t go to the visual cortex but instead go to areas of the brain that regulate our sleep cycle and pupil size.
Our biological (or circadian) clock determines when we sleep. This is controlled by a brain area called the suprachiasmatic nucleus, or SCN, which is located in the middle of the brain and is the size of a grain of rice.
The cells here undergo a cycle of protein formation and breakdown that lasts around 24 hours, releasing chemicals that make us sleepy at night and awake during the day. This cycle can be modified by input from melanopsin cells, which tell the SCN when it is light (daytime).
One way this is done is through melatonin (not to be confused with melanopsin). Melatonin is a hormone that makes you feel sleepy. It is produced by the pineal gland, connected to the SCN, which receives the light/dark signals from the melanopsin cells in the eye.
So when the melanopsin cells detect light (normally in the daytime), less melatonin is produced, making you feel more awake. When it is dark and melanopsin cells don’t detect light, more melatonin is produced, making you feel tired.
Melanopsin is most sensitive to blue light. This is why it’s a bad idea to be on your phone or computer late at night: these devices give out a lot of blue light, which the melanopsin detects as bright daylight. This tells your body clock that it is daytime and you should be awake, stopping you from getting to sleep.
Why is melanopsin particularly sensitive to blue light? Scientists think that this might be because blue light can reach further in the sea (where photoreceptors first evolved) than other colours. This blue preference was then maintained because of the colour of the sky.
Proteins for pupil size
Melanopsin cells also send their signals to the olivary pretectal nucleus (OPN), a region that is responsible for controlling the pupil of the eye (the black-looking area in its centre), through which light enters. The iris (the colourful bit of the eye) controls the size of the pupil to ensure the optimal amount of light enters the eye.
If a bright light is suddenly shone into the eye, melanopsin-containing cells detect this and trigger the OPN to trigger the iris to reduce the pupil size and so avoid damaging the eye.
This could be why people who are low in melanopsin take a while to react to light. Since they also suffer more often from sleep and mood disorders, the speed at which their pupil shrinks can be used to diagnose seasonal affective disorder, a condition that causes low mood during winter.
Questions for discussion
- If we only have three types of cones, why can we see more than three colours?
- Which other factors, apart from circadian rhythms, affect how sleepy we are?
- Melatonin is a hormone, but it’s not strictly a peptide or steroid hormone. What is it?