The retina is the part of the human eye that is sensitive to light.
Through a layer of photoreceptors, it is able to turn the light captured into signals for the brain.
There are two types of photoreceptors: rods and cones.
Rods are responsible for peripheral vision, and are located outside of the central part of the retina. There are some 120 million of them, and they are responsible for night vision, because they are highly sensitive to low-intensity light. They are completely blind to high-intensity light, so they are not important for daytime vision or for visual acuity. Because they are not able to distinguish colours, they produce achromatic vision.
Cones, which vary in number from 6 to 7 million, are responsible for the visual acuity of the human eye (the ability of the eye to resolve and to pick up the minor details on an object) and for distinguishing colours. They are concentrated in the small central part of the retina known as the fovea centralis, measuring 0.3 millimetres across and devoid of rods.
There are three types of cones:
– Red cones, accounting for 64% of the total, also known as L-cones (maximally sensitive to long-wave light).
– Green cones, accounting for 32% of the total, also known as M-cones (maximally sensitive to medium-wave light).
– Blue cones, accounting for 2 – 7% of the total, also known as S-cones (maximally sensitive to short-wave light).
In the previous picture we see the peak of red cone light perception at 565 nm, the peak of green cone at 535 nm and the peak of blue cone at 430 nm. Although each photoreceptor class is most sensitive to wavelengths of light at the peak sensitivity, all photoreceptors also perceive other colours around the peak, and there is overlap with colors perceived by others kinds of cones. Note that the names of the cone types (red, green, and blue) do not mean that those cones ONLY “see” those colors, but they are each sensitive to a wide range of colors/wavelengths. All three cone types are needed to provide our ability to see ALL colors.
The cones are therefore responsible both for visual acuity and for distinguishing colours. Those sensitive to green light and red light are concentrated in the fovea, and are much more numerous, while those sensitive to blue light are located outside the fovea and are a small minority.
In the structure of the photoreceptors, three parts can be identified:
1) an outer segment, characterised by membranous structures (called discs), positioned on which are the pigments that react to the stimulus of the photons (light that comes in “packages” known as quanta).
2) an inner segment, characterised by the presence of the internal organelles such as the mitocondria, Golgi apparatus, etc., indispensable for the cell metabolism and the nucleus.
3) the synaptic ending, which allows for the transmission of the signals from the photoreceptor to the bipolar cells by synaptic biochemical transmission between nerve cells (thanks to molecules called neurotransmitters).
In the picture above, on the right side, we see rods and cones packed together in the human retina. The upper part is the outer segment.
Each rod and each cone (S, M and L) contains a specific pigment-protein compound, called photopigment, found in the outer segment of the photoreceptors.
– The rods have RHODOPSIN
– The S-cones have S-OPSIN
– The M-cones have M-OPSIN
– The L-cones have L-OPSIN
Without these proteins, the cones are unable to capture light.
In the next picture we see the outer segment of a rod, composed of discs:
In each disc there are thousands of OPSIN proteins embedded.
The situation is very similar for cones.
In humans the discs of each cone contain the OPSIN proteins of type S, M and L, exclusively only one type for each cone.
The form of the OPSIN proteins is like a chain that passes for 7 times inside the disc, as we can see in the next picture:
Structural model of rhodopsin showing seven transmembrane components and the attachment site for retinal
There are about 20.000 proteins embedded in each disc and about 100 discs for each cone for a total of about 2 milion OPSIN proteins per cone.
Without these proteins the cone is unable to capture the light because these proteins convert light into chemical signal starting a process called ‘phototransduction‘.
To learn more about photoreceptors and color vision please refer to:
– Debarshi Mustaﬁ, Andreas H. Engel, Krzysztof Palczewski ‘Structure of Cone Photoreceptors – Review.’ Progress in Retinal and Eye Research 28 (2009) 289–302. PMID: 19501669.
– Jay Neitz, Joseph Carroll and Maureen Neitz ‘Color Vision.’ Optics and Photonics News 12(1):26-33, Jan-2001.
– Katherine Mancuso, Matthew C. Mauck, James A. Kuchenbecker, Maureen Neitz, and Jay Neitz, ‘A Multi-Stage Color Model Revisited: Implications for a Gene Therapy Cure for Red-Green Colorblindness’ 2010 R.E. Anderson et al. (eds.), Retinal Degenerative Diseases, Advances in Experimental Medicine and Biology 664. PMID: 20238067.
What happens in people with BCM
People with BCM do not have the M and L opsins, so the retina is able to capture light only with the rods and the blue cones. Therefore, when medium-wave (green) light or long-wave (red) light enters the eye, in BCM sufferers it does not send a message to the brain, causing discomfort (photophobia) instead. The same happens with white light, which is composed of a mixture of red, green and blue light.
Researches are studying L and M photoreceptors in Blue Cone Monochromats in order to see if there are a sufficient number of cones to consider gene therapy as a cure for BCM.
The research study A.V. Cideciyan et al. ‘Human Cone Visual Pigment Deletions Spare Sufficient Photoreceptors to Warrant Gene Therapy’, HUMAN GENE THERAPY 24:993–1006, 2013 PMID: 24067079, has been conducted with the help of BCM Families Foundation and showes that, in some BCM patients with gene deletions, there are sufficient cones with detectable outer segments that were abnormally shortened, to warrant Gene Therapy.