We have mentioned in the previous blog [link part 5 the retina and the vitreous] that the retina is one very complicated structure indeed. To understand, or at least to try to understand, how the dysfunctions of specific retinal structures translate into the symptoms that the affected patients experience, it is essential that you understand the basic structure of the retina.
Regions of the Retina
In a nutshell, speaking of retinal function, two distinct regions exist. A peripheral retina that is abundant with rods and almost devoid of cones, and the macula (with the fovea) with a large number of cones with no rods. In general, there are about 120 million rods in human retina and 6 million cones, of which virtually all are concentrated in the fovea of the macula.
Macula (with the fovea)
The fovea with its high density of cones is responsible for high visual acuity. It only sees two central degrees of the visual field, which is approximately the size of a bigger coin at arm’s length. Even though it only comprises less than 1% of the whole retinal size, the information from that little area takes up at least 50% of the brain visual cortex. It is this tiny area that enables us to read, to recognize faces, watch television, drive the car, recognize colours or do virtually everything that requires us to see very accurately. The key are the photoreceptors, namely the cones. Humans have three different types of cones: blue, green and red cones. The secret is that each type of those cones has got the proteins that differ slightly in the absorbance of light wavelengths. The blue cones have the maximum absorbance at 420 nm, green at 534 nm and red at 564 nm. The result is that we can percept literally every colour we can imagine. It is basically the same as your computer screen, which can project any colour you want from small red, green and blue dots (RGB display). What is also worth mentioning, is that the cones are much less sensitive to light than are the rods, which we will cover later. Consequently, in bad light environments, first things you would notice are very much decreased visual acuity and fading of the colour intensities.
Problems of Macula
Some people, as you probably know, cannot percept certain colours. Well, it is because the proteins in certain types of cones are affected so that they cannot sense the light as they should. In fact, certain types of colour blindness are very frequent diseases indeed, with prevalence of 1-5% of, mostly male, population. Why males? Well, these disorders are usually inherited linked on the X chromosome, which makes males far more susceptible to get the disease than females. Another disease that you have probably heard of is the age-related macular degeneration. It is a disease that affect the macula of the elderly. As a consequence, central vision in highly impacted which, in late stages of the disease disables the affected people to read, to recognise faces etc. They perceive the changes as a constant black blind spot right in the middle of the visual field that would not go away. Thankfully, there are certain medications that can halt the progression or even improve the sight in those suffering.
The peripheral retina – land of rods
The peripheral retina on the other hand, is as already mentioned, abundant with rods and almost devoid of cones. The rods are much more sensitive to light but only enable us to percept the light without colours. A single photon can trigger them, which means our eyes are able to spot a single candle 3 km (2 miles) away! Apart from night vision, the rods are also responsible for peripheral vision. This enables us to function in space, to walk around without hitting anything and so on and so forth. A disease that represents just how important the rods are is the disease called retinitis pigmentosa. In this disease only rods are affected, which means that with progression, people suffering will experience night blindness with tunnel vision. Basically, all aspects of vision that are dependent of rods will be affected, with the remaining small tunnel-like vision in the centre of the visual field – the aspect of vision that cones are responsible for. As of today, very few treatment options for retinitis pigmentosa or similar retinal degenerations, for that matter, exist. Nonetheless, the ongoing research with gene therapies and implantable devices are promising, so we can expect that in a few decades there will be way more treatment options than there are today.
So far, we have followed the light during our current blog series. However, in the retina, as you now know, the light impulses were converted into the electrical ones. Therefore, in our next blogs, we will focus on the nerves that transfer those impulses from the photoreceptors to the brain. I assure you it will be very interesting! Stay tuned and subscribe, so that you do not miss it!