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Photoreceptors
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In this article we'll talk about the photoreceptors, the structures responsible for vision. The retina is a membrane containing sensory receptors that lines the internal aspect of the posterior wall of the eyeball, deep to the choroid layer and superficial to the vitreous humor. It is composed of epithelial, glial, and neural cells, which are organised into 10 distinctive layers in which a specialised group of receptors, photoreceptors, can be found.
These photoreceptors are localised around an area near the centre of the retina called the macula, which is the functional centre of the retina. The fovea is located in the centre of the macula. The macula is responsible for high-resolution, colour vision, provided by different types of photoreceptors.
| What are photoreceptor Cells? | Cells located on the retina that are capable of converting light into visual information. |
| Structure of photoreceptor cell | Two types: Rods and Cones Five structural components: outer segment, connecting cilium, inner segment, nuclear region, and synaptic region. |
| Photopigments | Absorb light in the photoreceptor cell. There are four different types in the human retina. |
| Rod Cells | More abundant, cylindrical shaped, high sensitivity to light, function in night vision, low visual acuity, absent at the fovea. |
| Cone Cells | Fewer in number, conical shaped, low sensitivity to light, responsible for colour vision, localised at fovea |
| Macular degeneration | Loss of photoreceptors in retina, often age related, dry and wet types, mostly affects central vision. |
| Retinitis pigmentosa | Photoreceptor loss with photopigment deposits on the retina, inherited disorder, initially night blindness followed by gradual loss of peripheral vision and eventually complete loss of vision. |
This article will outline the typical organisation of a photoreceptor cell, a detailed description of the different types of photoreceptor cells, and an overview of some of the most common disorders affecting photoreceptor cells.
Photoreceptor cells
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Photoreceptors are image forming cells. They are a specialised type of neuroepithelial cell that is capable of absorbing light and converting it into an electrical signal in the initial stages of the vision mechanism, 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.
Photoreceptors in the retina are classified into two groups, named after their physical morphologies. Rod cells are highly sensitive to light and function in nightvision, whereas cone cells are capable of detecting a wide spectrum of light photons and are responsible for colour vision. Rods and cones are structurally compartmentalised. They consist of five principal regions:
- outer segment
- connecting cilium
- inner segment
- nuclear region
- synaptic region
The outer segment is involved in capturing light and converting it to an electrical stimulus, while the connecting cilium connects the outer and inner segments to each other. The inner segment contains metabolic organelles such as mitochondria, lysosomes and endoplasmic reticulum and the nuclear region houses the nucleus of the cell. Finally, the synaptic region functions in the transfer of neurotransmitters, such as glutamate, between the photoreceptor cells and bipolar cells or secondary neurons.
Light is absorbed by rhodopsins in the photoreceptor cells. These are visual pigments consisting of a protein, opsin, that is located across the membrane of the outer segment discs. Human photoreceptors contain 4 types of opsins; one located in rod cells and three in the cone cells.
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Rods
Rods are cylindrical shaped photoreceptors. They are more numerous than cone cells, with an estimated 92 million rod cells located in the human retina. They function best in low intensity light (scotopic) and are thus responsible for vision in dimly lit surroundings, such as at dusk. Rod outer segments are cylindrical in shape, consisting of around 1000 flat, lobulated, membranous discs. The inner segment of the rod cell is divided into an outer mitochondria-rich part and an inner part containing endoplasmic reticulum. The structure of the rod cell is consistent across all areas of the retina.
Rod cells are located across the retina except at the centre of the fovea. The density of rod cells increases as you move away from the fovea, reaching a peak and declining again towards the periphery. Rod vision provides high sensitivity to light, but with relatively low spatial discrimination and no ability to distinguish different wavelengths of light. This is why they are not capable of detecting different colours. Compared to cone cells, rods have poor visual acuity, or ability to distinguish fine detail.
| Shape | Cylindrical |
| Number | High |
| Light sensitivity | High |
| Visual acuity | Low |
| Vision type | Night vision |
| Present at fovea | No |
| Cell types | Single type |
| Photopigment types | Achromatic (one type) |
Cones
Cones are conical shaped cells that operate best in high intensity lighting (photopic) and are responsible for the perception of colour. There are far fewer cone cells in the human retina compared to rod cells, numbering approximately 4.6 million. Cone outer segments are generally shorter than that of rods and, as their name implies, are often conical. As is the case of rod cells, the inner segment of the cone cells have an outer mitochondria-rich part and an inner part containing endoplasmic reticulum.
| Shape | Conical |
| Number | Low |
| Light sensitivity | Low |
| Visual acuity | High |
| Vision type | Color vision |
| Present at fovea | Yes |
| Cell types | 3 types: L, S, M |
| Photopigment types | Chromatic: Red, green, blue |
The structure of the cone cell varies across the retina, and is highly modified towards the fovea compared to those located more peripherally. There are 3 different types of cone cells: L type, S type, and M type. Cones are located throughout the entire retina, but are localised around the fovea. Cone density is higher in the nasal retina compared to the temporal retina, and slightly higher inferiorly than superiorly. The three different opsins located in the cone cells account for their ability to detect different wavelengths of light, producing colour vision. These three opsins are sometimes classified as red (L-cone), blue (S-cone), and green (M-cone). The dense packing of cones at the fovea allows maximal resolution.
If you want to cement your knowledge about rods and cons and integrate it within the structure of the retina, practice the following quizzes.
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Clinical notes
Photoreceptor degeneration is a loss of rods and cones in the retina, which can lead to visual impairment or entire loss of vision. Loss of photoreceptor cells is a major contributor to conditions such as macular degeneration and retinitis pigmentosa.
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 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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