Photoreceptors: What They Are & What They Do

Photoreceptors are dedicated cells within the retina that are responsible for specific types of vision.

Photoreceptors allow for color vision and the ability to see at night. They consist of rods and cones, which are responsible for converting light into electrical signs that are decoded and enable visual processing. 

Types of Photoreceptors

There are two primary types of photoreceptors: cones and rods, but these are divided further into different subtypes. 

Different types of cones are responsible for seeing different colors, and more than one type of cone is involved in seeing certain colors. For example, to see yellow, both the medium (green) and long (red) wavelength cones will be activated.

• Short wavelength cones: These cones are responsive to short waves of light, to a maximum of 420 nm. These are sometimes referred to as S cones and also blue cones since they are responsible for seeing blue colors. 
• Medium wavelength cones: These cones are responsive to medium waves of light, to a maximum of 530 nm. These are sometimes referred to as M cones and green cones since they are responsible for seeing green colors. 
• Long wavelength cones: These cones are response to longer wavelengths, to a maximum of 560 nm. These are sometimes referred to as L cones or red cones as they are responsible for seeing red colors. 
• Rods: Rods are responsible for vision at low levels of light, referred to as scotopic vision, so they are responsible for seeing in dimly lit environments. Rods have low spatial acuity and do not facilitate vision or colors. This is why you don’t see colors vividly at night, and oftentimes, things appear in more of a grayscale, or black and white.

There are millions of rods and cones within the eye, but there are far more rods. The human eye has an average of at least 91 million rods versus 4.5 million cones.

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Structure & Function

Photoreceptors transform light into electrical impulses. These electrical impulses are transferred along the optic nerve to the visual cortex. Once there, they are processed by the brain, and this processing allows for certain aspects of vision. 

Specifically, particles of light, called photons, enter the lens and are focused onto the retina. There, rods and cones convert them into electrical and chemical stimuli that are decoded by the central nervous system. 

What Are Photopigments?

Photopigments are found within photoreceptors. They are unsteady pigments within the eye that transform chemically when they are exposed to light. 

If there is a defect in photopigments, color blindness may occur.

Eye Problems & Conditions Related to Photoreceptors

Since photoreceptors are key to color vision and night vision, issues with them can result in reduced acuity in both areas. These vision conditions relate to photoreceptors:  

• Usher syndrome: This is a genetic disease that negatively affects vision and hearing. This condition can result in loss of hearing, including deafness, and retinitis pigmentosa. In some cases, Usher syndrome can cause problems with balance. Early symptoms of the disorder include some loss of peripheral vision and difficulty with seeing at night.
  
  While there is no cure for Usher syndrome, there are potential treatments to manage symptoms and slow progression of vision loss.
  
• Photokeratitis: This is a painful condition that is related to excessive exposure to UV rays. It usually occurs within about 12 hours of prolonged exposure to the sun. It is temporary and can involve a partial or total loss of vision. Symptoms include tearing, pain, and decreased vision.
  
  Though the condition is temporary, persistent cases of photokeratitis can result in scarring and permanent vision loss.
  
• Color blindness: This is an inability to see colors normally. It doesn’t always involve an inability to see all color. Colors may just be perceived differently than they are for most people. This condition is often genetic. 
  
• Retinitis pigmentosa: This is a group of eye problems that affect the retina and the manner in which it responds to light. It is an inherited disorder that can lead to progressive loss of vision. Unfortunately, there is no cure or treatment for RP.

Keeping Photoreceptors Healthy

Vitamin A is essential to the proper functioning of photoreceptors due to its place in rhodopsin, which is found in rods. Inadequate vitamin A consumption can lead to various visual conditions related to photoreceptors, including reduced night vision and color vision.

If you have a vitamin A deficiency, your doctor may recommend oral supplements or eye drops that contain vitamin A as well as increasing your intake of foods that contain vitamin A.

References

1. Photoreceptors. (February 2012). National Library of Medicine.

2. Photoreceptors. (September 2017). American Academy of Ophthalmology.

3. Rods & Cones. Rochester Institute of Technology.

4. Sensors: Natural and Synthetic Sensors. (2019). ScienceDirect.

5. Photoreceptors at a Glance. (November 2015). Journal of Cell Science.

6. Structure of Cone Photoreceptors. (June 2009). Progress in Retinal and Eye Research.

7. Functional Changes Within the Rod Inner Segment Ellipsoid in Wildtype Mice: An Optical Coherence Tomography and Electron Microscopy Study. (July 2022). Investigative Ophthalmology & Visual Science.

8. Retinal Phototransduction. (October 2014). NeuroSciences.

9. Anatomical Distribution of Rods and Cones. Neuroscience.

10. Photoreceptor Disruption and Vision Loss Associated With Central Serous Retinopathy. (October 2017). American Journal of Ophthalmology Case Reports.

11. What Is Photokeratitis — Including Snow Blindness? (May 2022). American Academy of Ophthalmology.

12. How to Assess if Photoreceptors Are Functioning Properly: A New Approach for Diagnosing Eye Diseases. (May 2022). Ophthalmology Times.

13. Retina, Retinol, Retinal and the Natural History of Vitamin A as a Light Sensor. (December 2012). Nutrients.