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Photoreceptors
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Photoreceptors are neurons which function as specialized receptor cells which are located in the retina of the eyeball. They are primarily responsible for the transduction of light stimuli for vision. Photoreceptors lay the foundation for perceiving the world around us thanks to the special architecture of the retina for the transmission of visual information to the brain to be further processed and interpreted into a coherent visual experience.
This article will outline the anatomy and physiology of the different types of photoreceptor cells as well as the events which occur during the phototransduction cascade.
| What are photoreceptors? | Specialized, light-sensitive neurons located in the retina of the eye which are capable of transducing light into electrical signals, primarily for visual perception |
| Types of photoreceptors | Rod cell Cone cell Intrinsically photosensitive retinal ganglion cells |
| Structure of photoreceptor |
Outer segment: contains the light-absorbing materials (photopigments) with the light-sensitive chromophore molecule retinal Inner segment: composed ellipsoid (mitochondria rich) and myoid (site of photopigment production) Cell body: (nuclear region) Axonal process: forms synapses with bipolar/horizontal cells |
| Photopigments | Light-sensitive molecules which consist of an opsin protein bound to a light-absorbing molecule called retinal; it undergoes a chemical change when it absorbs light |
| Rod Cells |
Structure: rod-like elongated cylindrical outer segments, stacks of membrane-bound discs Photopigment: rhodopsin (scotopsin & retinal) Function: peripheral vision, high sensitivity to light, scotopic vision, low visual acuity, vision on a grayscale |
| Cone Cells |
Structure: cone-shaped short outer segments, membrane folds creating sacs Photopigment: iodopsins (red, green, blue; photosins I-III & retinal) Function: visual acuity, photopic vision, color vision |
- Photoreceptor cells
- Rods
- Cones
- Intrinsically photosensitive retinal ganglion cells
- Phototransduction cascade
- Clinical notes
- Sources
Photoreceptor cells
Photoreceptors are a specialised type of neuroepithelial cell/neuron which are capable of absorbing light and converting it into an electrical signal in the initial stages of vision, a process known as phototransduction. Photoreceptors are packed tightly together, allowing a large volume of light to be absorbed across a small area on the retina.
The two main types of photoreceptor cells, rods and cones, consist of the following parts:
- Outer segment: A conical/cylindrical region responsible for photoreception. It contains G-protein coupled receptor proteins (GPCR) known as opsins as well as the light-sensitive chromophore retinal (an oxidized derivative of vitamin A).
- Inner segment: composed of outer ellipsoid and inner myoid parts. It contains several types of organelles such as mitochondria (ellipsoid part) and various protein synthesizing organelles such as Golgi apparatus, rough endoplasmic reticulum, free ribosomes (myoid part). This is where photopigments are produced.
- Cell body (nuclear region): which contains the cell nucleus.
- Axonal process: which forms synapses with bipolar or horizontal cells in the outer plexiform layer of the retina.
According to most recent classifications, rod and cone cells can be considered as unipolar neurons.
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Rods
Rod cells have elongated cylindrical outer segments, resembling rods, with stacks of membrane-bound discs containing the photopigment rhodopsin. Rhodopsin contains two components: a form of the protein opsin, known as scotopsin and the chromophore, retinal. It showcases maximum sensitivity to light at a wavelength of 498nm.
Rods are predominantly located in the periphery of the retina, thus contributing mainly to peripheral vision. Overall, they significantly outnumber cones by a margin of 20:1, except in the region of the fovea centralis of the retina. They are highly sensitive to light, enabling perception of even faint sources of illumination and are responsible for scotopic vision (i.e., seeing in the dark or dim light). However, they have little role in color vision and do not perceive fine details, which is why night vision is largely on a grayscale, with lower visual acuity.
| Shape | Cylindrical |
| Number | High |
| Light sensitivity | High |
| Visual acuity | Low |
| Vision type | Night vision |
| Present at fovea centralis | No |
Cones
Cone cells have cone-shaped outer segments that are short with membrane folds creating sacs that contain three photopigments, collectively known as iodopsins. Similar to rhodospins, they comprise two components: a subgroup of the opsin family known as photopsins which hold the chromophore retinal in place. There are three types of photosin, each of which responds to different wavelengths within the visible spectrum, specifically around the primary colors red (with peak sensitivity at 564 nm (photopsin I)), green (with peak sensitivity at 534 nm (photosin II) and blue (with peak sensitivity at 420 nm (photopsin III)).
Cones are important for visual acuity and contribute to high-resolution vision, allowing us to determine visual details in object shapes and to photopic vision (i.e., seeing in bright conditions). The sensitivity of cone opsins to different wavelengths is important for color vision. The brain compares the activity of the three different cone types and based on their relative activation can extract color information from visual stimuli.
| Shape | Conical |
| Number | Low |
| Light sensitivity | Low |
| Visual acuity | High |
| Vision type | Color vision |
| Present at fovea centralis | Yes |
This gives rise to 3 different types of cone cells: L (long/red), M (medium/green) type, and S (short/blue). Cone density is highest around the fovea centralis (region of most acute vision). Outside of this region, cones are more numerous in the nasal retina compared to the temporal retina, and slightly more numerous inferiorly than superiorly.
If you want to cement your knowledge about rods and cons and integrate it within the wider structure of the retina, practice with the following quiz.
Intrinsically photosensitive retinal ganglion cells
Aside from rod/cone cells, a more recently discovered type of photoreceptor are known as intrinsically photosensitive retinal ganglion cells (ipRGC), also referred to as photosensitive retinal ganglion cells (pRGC) or melanopsin-containing retinal ganglion cells (mRGC). As suggested by their name(s), they are located in the ganglionic layer of the retina and contain the photopigment melanopsin, which is particularly sensitive to short-wavelength (blue) visible light. They are believed to play a role in regulation of circadian rhythm and other nonvisual functions by sending signals to the suprachiasmatic nucleus (SCN) in the brain, the master body clock which controls sleep-wake cycle and other related processes. In addition, ipRGCs are also involved in the regulation of pupil size, release of the hormone melatonin and other physiological responses to light, thereby helping to synchronize biological processes with the photoperiodic (daylight vs. darkness) environment.
Phototransduction cascade
Though structurally and functionally different, rods and cones share a similar transduction process. A light ray, consisting of photons, passes through the cornea and lens before it is focused onto the retina. These photons are absorbed by the photopigments within the photoreceptors (rhodopsin in rods and iodopsins in cones) where they interact with the retinal molecule.
Unlike other sensory systems, where the receptor cell/neuron is ‘off’/inactive until a stimulus turns them ‘on’, photoreceptors are tonically active (i.e., in a state of continuous depolarization), continually releasing neurotransmitters. The reception of light waves interrupts this process, reducing the release of neurotransmitters to other cells of the retina.
Dark (absense of light)
In dark conditions, retinal is found in a ‘bent’ configuration, known as cis-retinal. Specialised sodium/calcium ion channels (regulated by the signalling molecule cycle guanosine monophosphate (cGMP)) are open, allowing a steady inward current, known as ‘dark current’. This results in a depolarization of the membrane depolarization of the neuron cell membrane from a resting membrane potential of -70mV to -40mV. This depolarization opens voltage-gated calcium channels leading to increased intracellular concentration of Ca2+ and subsequent continuous release of the neurotransmitter glutamate to bipolar cells.
Light
When a photon hits a cis-retinal molecule, it is biochemically altered into ‘straight’ form known as trans-retinal; this change of configuration causes the retinal to separate from opsin, which triggers a cascade of events:
- The unbound opsin molecule triggers the activation of a G-protein, known as transducin.
- Transducin then activates the enzyme phosphodiesterase (PDE) which catalyzes the breakdown of cGMP by hydrolysis, resulting in a decrease in the concentration of cGMP in the cytosol.
- The lower amounts of cGMP causes the cGMP-gated sodium and calcium ion channels in the plasma membrane to close, leading to a decrease in the influx of positively charged ions (Na+/Ca2+), which causes the membrane potential to become more negative.
Therefore, the electrical response of the photoreceptors to light stimulation is hyperpolarization and as a result, the release of glutamate by the synaptic/axon terminal to the bipolar cells is reduced.
This decrease in neurotransmitter release signals the presence of a light stimulus and leads to the depolarization of ON bipolar cells and the production of excitatory post-synaptic potentials (EPSPs) as well as the hyperpolarization of OFF bipolar cells and the production of inhibitory post-synaptic potentials (IPSPs), propagating the signal to downstream neurons. This way, ON and OFF bipolar cells contribute to the processing of visual information by encoding alterations in light intensity and spatial contrast. The photoreceptor potentials and the electrical responses of most of the retinal neurons are graded potentials (proportional to the light intensity) and only ganglion cells send action potentials along the optic nerve.
Recovery
During recovery, guanylate cyclase activating protein (GCAP) is activated, as calcium levels are low and they dissociate from GCAP, leading to the activation of guanylate cyclase and restoration of the cell’s cGMP. A protein called arrestin binds to opsin, blocking its ability to activate transducin. Transducin deactivation terminates the phototransduction cascade, allowing cGMP-gated ion channels to open again and restore the ‘dark current’.
Signal amplification
The phototransduction cascade provides enormous signal amplification; a single activated rhodopsin molecule can activate an estimated 800 transducin molecules. Although each transducing molecule activates only one phosphodiesterase molecule, each phosphodiesterase is capable of breaking up to 6 cGMP molecules resulting in the closing of around 200 cGMP-gated ion channels.
Bleaching
As long as the retinal molecule is in a trans- conformation, the opsin cannot respond to further light stimulation, a phenomenon referred to as bleaching. When the light source is removed, new photopigment molecules are synthesized and the photoreceptors recover their sensitivity.
However, the photoreceptors not exposed to the intense light stimuli and remain unbleached, continue to function normally during the recovery period. This contrast in the photoreceptor activity leads to the perception of afterimages as the visual system interprets the inactivity of the bleached photoreceptors as opposing visual information. That is why after a bright flash of light, afterimages are seen in negative, meaning that the afterimage appears as a complementary color to the original stimulus. For example, the negative afterimage of a bright red light will appear in a cyan hue.
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Clinical notes
Vitamin A deficiency
Vitamin A is a fat-soluble vitamin found in dark leafy greens, orange-colored vegetables, milk products, liver and fish. Vitamin A deficiency, though rarely seen in the developed world, is still common in low-income countries among children and pregnant women. It is marked by nyctalopia (night blindness) and followed by xerophthalmia. Vitamin A deficiency initially disrupts the rods' function, but soon also leads to cone degeneration. Symptoms are reversed after Vitamin A consumption. However, if not treated, vitamin A deficiency will lead to serious xerophthalmia and true blindness due to degeneration of the retina.
Macular degeneration
Macular degeneration is often an age related disease in which the photoreceptor cells in the macula are disrupted, resulting in visual defects. Central vision, responsible for high resolution visual to distinguish fine detail, is mostly affected. Macular degeneration can be wet or dry: dry macular degeneration is a slow onset of the disease where gradual loss of central vision occurs. Wet macular degeneration is a neovascular type of the disease that occurs as a result of an abnormal blood vessel in the eye. Symptoms generally consist of an array of visual disruptions, with slower, gradual onset of symptoms in dry macular degeneration. There is currently no treatment for dry macular degeneration, but wet macular degeneration can be treated by a number of pharmacological and surgical methods.
Retinitis pigmentosa
Retinitis pigmentosa is an inherited disease of the retina which is caused by photoreceptor loss and deposition of retinal photopigments across the retina. Typical symptoms of the disease occur due to the loss of both rod and cone photoreceptor cells. These include initial night blindness followed by progressive loss of peripheral vision (causing tunnel vision), photophobia, and eventual vision loss in the long term. There is currently no cure for retinitis pigmentosa, but there is a focus on the delaying of symptom onset by intake of certain supplements such as Vitamin A. In 2011, the Argus II retinal prosthesis or the ‘bionic eye’ became the first approved solution for retinitis pigmentosa, but is only available in a handful of countries.
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