Vision begins when light hits the photoreceptors in the retina. There are two main types of photoreceptors – rods and cones. Rods are sensitive to light intensity but not color. Cones are sensitive to color as well as light intensity. This article will explore the roles of rods and cones in vision, looking at how their differences allow us to see in both bright and dim light.
The Retina and Photoreceptors
The retina is a thin layer of tissue at the back of the eye that contains the photoreceptors. There are around 120 million rods and 6 million cones in the human retina. The rods outweigh the cones by 20 to 1. However, the cones are concentrated in one small area of the retina, called the fovea. This gives us excellent color vision in the center of our field of view.
Rods and cones have different structures that suit their functions. Rods are long and thin so they can detect even tiny amounts of light. Cones are shorter, fatter and more tapered to allow them to distinguish between different wavelengths of light. Rods use a pigment called rhodopsin whereas cones contain pigments called photopsins.
Rods Detect Dim Light
Rods are incredibly sensitive and can respond to a single photon of light. This means they work well in dim conditions. Rods are distributed across the outer retina except for the central fovea. They are responsible for peripheral and night vision.
Rods become saturated in bright light. They cannot distinguish between different wavelengths so do not detect color. We see shades of gray using our rod vision. This is why colors appear faded at nighttime.
Advantages of Rods
- Detect dim light
- Provide peripheral vision
- Essential for vision in darkness
Disadvantages of Rods
- Not sensitive to color
- Get saturated in bright light
Cones Detect Color and Fine Details
Cones require much more light to respond than rods but allow us to see color and fine details. There are three types of cones, each containing a different photopsin pigment. These are sensitive to short (S), medium (M), and long (L) wavelengths of light.
| Cone type | Pigment | Peak sensitivity |
|---|---|---|
| Short wavelength (S) | Cyanolabe | 420 nm – blue |
| Medium wavelength (M) | Chlorolabe | 534 nm – green |
| Long wavelength (L) | Erythrolabe | 564 nm – red |
The cones are concentrated in the fovea centralis. This small central area of the retina contains mostly cones and allows us to see color and fine details when we direct our gaze at something.
Advantages of Cones
- Detect color
- Provide high visual acuity
- Allow detection of fine details
Disadvantages of Cones
- Require bright light
- Do not respond well in darkness
Rhodopsin in Rods Extends Vision in Dim Light
Rods contain a high proportion of rhodopsin pigment which makes them extremely sensitive. Rhodopsin consists of retinal (a form of vitamin A) bound to a protein called opsin. When a photon hits rhodopsin, it changes shape in a process called photoisomerization. This activates the rod cell, allowing an electrical signal to be sent to the brain.
In bright light, rhodopsin breaks down. It takes time for the rods to rebuild rhodopsin stores in dimmer conditions. This is why moving from bright light to darkness causes temporary blindness. As the rods replenish their rhodopsin, vision improves. Rhodopsin allows the 120 million rods in the retina to reliably detect dim light.
Signal Processing in Rods and Cones
When light hits the photoreceptive pigment in rods and cones, it initiates a signaling pathway called phototransduction. Here are the steps involved:
- Photon hits the photopigment, changing its shape
- This activates the G-protein transducin
- Transducin activates PDE enzyme
- PDE breaks down cGMP
- Reduced cGMP causes sodium channels to close
- The cell membrane becomes hyperpolarized
- Hyperpolarization causes less glutamate release
- The decrease in glutamate inhibits downstream bipolar cells
- This change is transmitted to the visual cortex as a light response
Rods and cones signal using the same phototransduction pathway. However, rods are much more sensitive due to having greater amplification at certain steps. This allows them to respond to single photons.
Potential Problems With Rods and Cones
Given the vital roles of rods and cones, it is not surprising that problems with these photoreceptors can lead to vision loss and blindness. Here are some examples:
Retinitis pigmentosa
Retinitis pigmentosa is a genetic disease causing progressive degeneration of the rods. This leads to “tunnel vision” and eventually blindness. It is an incurable condition affecting around 1 in 4,000 people.
Cone dystrophy
Cone dystrophy specifically affects the cones, leading to impaired color vision, light sensitivity, and central vision loss. It can be inherited or caused by factors like medications or infections.
Macular degeneration
Age-related macular degeneration damages the macula and fovea, where cones are concentrated. It causes loss of central vision and is a leading cause of blindness in older adults.
Color blindness
Color blindness is usually an inherited disorder where one or more cone pigments are missing or defective. It affects cone function and makes it hard to distinguish between some colors.
Differences Between Rod and Cone Function
Here is a summary of the key differences between rods and cones:
| Rods | Cones | |
|---|---|---|
| Photopigment | Rhodopsin | Photopsins |
| Light sensitivity | High – respond to single photons | Low – require tens to hundreds of photons |
| Color detection | None | Detect short, medium and long wavelengths |
| Visual acuity | Low – motion and light detection | High – see color and fine detail |
| Distribution in retina | All over except fovea | Concentrated in fovea |
| Active under | Dim light | Bright light |
| Night vision role | Critical | Minimal |
Conclusion
Rods contain rhodopsin, allowing them to detect dim light but not color. They provide peripheral and nighttime vision. Cones contain photopsins tuned to different wavelengths and provide daylight, color vision and visual acuity concentrated in the retinal fovea. The two photoreceptor types are complementary, enabling us to see under both bright and dim conditions.
Problems affecting either the rods or cones can lead to profound vision loss. Understanding the differences between rod and cone structure and function gives insight into conditions like retinitis pigmentosa, macular degeneration and color blindness. While rods and cones have distinct roles, they work together seamlessly through signaling pathways enabling us to perceive our visual world.