Unlock How Photoreceptors Work to Show Us All We See

All the times our eyes are open, a complex cascade of events goes on to interpret the sights, even if we are unaware of how photoreceptors work. The beauty of a sunset, a disgusting a slimy mold, the face of a loved one.. all these images trigger emotional responses even beyond the sight, but it starts with that configuration of shapes and colors, then interpreted contextually in the brain.

Photoreceptors are in the back of the eye and translate all that we see. As you become aware of how tiny pigments in our eyes turn into electrical signals in the brain that we perceive as vision, you will be astounded at the simple complexity.

Colors and shape create texture and context for our perception of what we see. But we are not only working with the “ROYGBIV” colors of the rainbow, as any digital (or conventional) artist knows. The capabilities of our eyesight go far beyond the seven-color rainbow Newton somewhat arbitrarily differentiated.

Read more about how arbitrary our delineations of color are in my article about Color Perception Across Cultures.

Physically, humans are capable of perceiving up to:

• 500 brightness levels
• 20 saturation levels
• 200 hues
  
  …to create a grand total of 2 million distinct colors. The differences and edges of these colors configured spatially make up the complete description of everything we have ever and will ever see. So let’s go into how, chemically and physically, our eyes make this possible.

The Full Picture of an Eye Shows How Photoreceptors Work

If the eye is a camera, the retina is the film, where the light from the pupil aperture is captured and turns into a picture. The pupil can change from about 2mm to 8mm in size, resulting in up to 16-fold increase in brightness depending on ambient light levels. But with all the conversions to electrical signals involved, the eye is really more like a digital camera than analog. The retina is the back layer of the eye. But even within the retina, there are many layers that house the photoreceptors and enable sight.

These words aren’t that important, but just so you know, in the center of the retina is the macula, and in the center of the macula is called the fovea. We’ll spend most of the discussion of how photoreceptors work in the fovea (the pit), the center of the center of the eye. Photoreceptor cells also go in the category of neuroepithelial cell, epithelial meaning it is a type of skin there in the back of the eye.

Nerve fiber layer	the endings of the nerves that carry the light signals
Ganglion layer	a single cell thick, always firing at rest to about 100 rods and cones each
Amacrine layer	distributes information
Bipolar cell layer	responsible for discerning edges and contrast
Horizontal cell layer	carries information horizontally between rods and cones
Rods & cones	translate the signals using photopigments, before passing to the brain

Did you know? We also have photoreceptors embedded in our skin! (Called melanopsin, and others!) Want to learn about this other “opsin” protein, that is mysteriously called the receptor for “non-image-forming vision”, melanopsin? It’s in the pineal gland, read about melanopsin in Piezoelectricity in the Pineal Gland.

Rods and Cones: The Main Organs of Vision

The photoreceptive components of the eye are its rods and cones, different in their ability to translate the light signal. The signals come together to create the whole picture. During the slight light of dawn and dusk, there are about 100 photons per second that make it into your eye. Both rods and cones respond to single photons, but rods have a greater electrical response per single photon than cones do.

Rods	Cones
100 million rod cells, around the edges of the retina mainly	4-6 million, in the foveal center of the retina
Only one type	Three Types: 64% of these cones respond to red (558-560 nm, Long) 32% respond to green (530-531 nm, Medium) 2% respond to blue (other sources say 10%..) (419-420 nm, Short)
very sensitive to low-light levels and “photobleach,” or saturate	detect color through bright light without bleaching
aid in night vision, identifying black and white hues, associated with “visual purple”	color vision, in pixel arrangement
rhodopsins as the photopigment containing protein molecule	photopsins (or cone opsins) have a bound chromophore as the pigment (which is retinal)
low acuity, high sensitivity, fast temporal response	high acuity, low sensitivity, slow temporal response
disks are closed, one neuron per rod (discrete signals passed to the brain)	disks are partly open to the surrounding extracellular fluids, many cones per neuron (spectral signals are mixed before being passed)

Rods

Rods are our black and white vision, contributing mainly to edges and contrast. Exposed to bright white light, rhodopsin “photobleaches” or fully saturates. This resets for the individual rod after about 30 minutes (in humans), up to one hour. This is like the eyes adapting to dark conditions after being in a well-lit space. Rod-vision is called “scopic.” It tells you the scope and intensity of light, not the color.

Cones

The cones begin to dominate the sight processes at the light level of bright moonlight. The cones lay in the fovea, which literally means pit, in the back of the eyeball. And, with about 150,000 cones per square millimeter, the arrangement and distribution of the three differently color-sensitive cones determines our color vision and peripheral vision.

The ratio of L:M cones is approximately 1.5:1 and the ratio of L and M combined to S is 100:1. So the S cones are in the least supply, and from a 0.34 degree angle at the center of the fovea, there are no S cones at all. In density, there are 150,000 cones per square millimeter, similar to the rods. But the cones occupy a smaller area, so we do have much less of them.The total area of the fovea is approximately 1.2 mm, leading to about 4.5 million cone cells in each eye.

In tetrachromats, which is estimated somewhere between 2 and 10% of women, there is a fourth cone with spectral sensitivity between the M & S cones, at about 496 nm. As a result, tetrachromacy confers more of a propensity to distinguish colors based on the mixing signals of the cones activated.

How Photoreceptors Work based on their Arrangement

Proteins with names ending in -opsin are G protein receptors. This means they differ in a few amino acids to give them their particular responses. Photopsins, the cone -opsins, for example, absorb light selectively at different wavelengths. The -opsins together with a chromophore make up the photopigment. And the specific -opsin in each cone gives it its classification as S, M, or L wave reception.

All cones equally stimulated makes white, but the rods can also see a white in their black or white vision. Many light levels have both rods and cones active. The cones fall off 10-15 degrees from the fovea’s center. Every eye also has a blindspot at about 16 degrees off center, where the cord of the optic nerve passes through. In the higher cone-density area of the fovea, the cones actually look a lot like rods. Their diameter is reduced, an adaptation to pack more into space. The central 300 micrometers of the foveola contain no rods at all.

These are hexagonally packed, which is why the images of the cones in the fovea may remind you of beehives. You can see other examples of hexagonal packing in my hexagonal templates post.

Transduction through Photopigments: Little Circuits for Vision

Here I will give the exact cascade, translating all the weird names of the molecules involved for better understanding for the non-biologists among us. But first, some background on the general state in which all tha light transduction transpires.

Between the ganglionic and horizontal cell layer, our channels are always firing at rest. The continuous firing at rest of retinal nerves contrast to other neurons, which make the distinction in firing by either being on or off. In the relaxed state the nerves in the back of the eye are always letting go and holding on in succession.

Another peculiarity in the eye’s nerve cells is the hyperpolarization state, deciding the transduction then. This tonic activity can distinguish an “on” state from either an increased signal OR a decreased signal with respect to the ever-active baseline. Other sensory neurons relay information when they become depolarized. Between the horizontal layer and the rods and cones, the hyperpolarized state removes the inhibition, facilitating electrical signals to travel.

The photopigment itself resides in the disks and folds of the rods and cones. These are all embedded in the membrane lipid rafts which span the cell’s wall. The precise folding and sequence of these proteins give their exact functions. The functional chromophore in human eyes is retinal, the aldehyde of Vitamin A1.

Rhodopsin

Rhodopsin has seven subunits that lie in the cell membrane. The subunits span the membrane in a circular shape, like columns. There are parts of the rhodopsin protein that also stick out of the membrane. Each rod has about a thousand disks, and each disk has about 150 thousand molecules of rhodopsin. So in total, each rod had about 150 million molecules of rhodopsin.

Photopsin

The opsins. inthe cones are only called photopsin when unbound to retinal. When bound, they are called iodopsins.

When a photon hits the retinal chromophore of a photopigment, the 11 and 12 double carbon bond flips, changing the conformation from what’s known as 11-cis to all-trans. Then a cascade of event occurs in which the sodium channel opens to hyperpolarize the cell. There is another chromophore, the 3-dehydroretinol-poryphyrins. these are 30nm redshifted in peak absorption spectra.

The isomerization (change in shape and bonding) of retinal converts to an electrical signal. The excitation lasts about 1 ms, and activates a heterotrimeric G-protein called guanine. Transducin, the hang-y out part of the opsin in the membrane, changes to phosphodiesterase, which cleaves cGMP(3’5’…) into 5’GMP (reducing concentration of cGMP in the cell.)

Again, during the inactive state, the cell is depolarized with a constant in and out flow of sodium ions.

How Photoreceptors Work Simply Explained

The photoreceptors in the eye are the entire machine-like pathway from the lens to the neurons hooked up through the rods and cones to the brain. The light is organized by many layers of nerves before the rods and cones read out the brightness and color respectively.

The structural differentiation of the rods and cones give the chemical reaction in response to light exposure. The light is mainly processed in the high-surface area foldings of the rods and cones, which contain trans-membrane proteins in the -opsin family. These opsins in turn contain photopigments, the precise location carrying out light transduction, via conformational changes relaying chemical changes relaying electrical signals to the brain.

Related articles:

• Color perception across Cultures
• The Mechanism of Light Translation (Transduction) – The chemical/electrical visual cascade
• Science of color perception
• -Escences phenomena, including Fluorescence, phosphorescence, and iridescence
• Visual interferometry, the time domain to the spectral domain

Sources

Krishnamoorthi, A.; Khosh Abady, K.; Dhankhar, D.; Rentzepis, P.M. Ultrafast Transient Absorption Spectra and Kinetics of Rod and Cone Visual Pigments. Molecules 2023, 28, 5829. https://doi.org/10.3390/molecules28155829

Wang, F.; Fernandez-Gonzalez, P.; Ramon, E.; Gomez-Gutierrez, P.; Morillo, M.; Garriga, P. Effect of Trace Metal Ions on the Conformational Stability of the Visual Photoreceptor Rhodopsin. Int. J. Mol. Sci. 2023, 24, 11231. https://doi.org/10.3390/ijms241311231

Kefalov VJ. Rod and cone visual pigments and phototransduction through pharmacological, genetic, and physiological approaches. J Biol Chem. 2012 Jan 13;287(3):1635-41. doi: 10.1074/jbc.R111.303008. Epub 2011 Nov 10. PMID: 22074928; PMCID: PMC3265844.

Zhou XE, Melcher K, Xu HE. Structure and activation of rhodopsin. Acta Pharmacol Sin. 2012 Mar;33(3):291-9. doi: 10.1038/aps.2011.171. Epub 2012 Jan 23. PMID: 22266727; PMCID: PMC3677203.

https://home.csulb.edu/~cwallis/482/visualsystem/eye.html