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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. 

Key facts about 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.

  1. Photoreceptor cells
  2. Rods
  3. Cones
  4. Clinical notes
  5. Sources
+ Show all

Photoreceptor cells

Photoreceptor layer of retina - histological slide

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.


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.

Rod cell summary
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 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.

Cone cell summary
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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