Difference Between Rods and Cones

Main Difference – Rods vs Cones

Rods and cones are the two types of photoreceptor cells in the vertebrate retina. About 120 million rod cells and 6 million of cones can be found in the retina. The main difference between rods and cones is that rods are very sensitive to the light and can be used for vision under low light conditions (scotopic vision) whereas cones are not very sensitive to light and can be used in high light conditions (photopic vision). Rods confer monochromatic vision whereas cones confer color vision. Rods contain one type of photoreceptor cells while cones contain three types of photoreceptor cells, each detecting blue, red, and green light.

Key Areas Covered

1. What are Rods
      – Definition, Structure, Function
2. What are Cones
      – Definition, Structure, Function
3. What are the Similarities Between Rods and Cones
      – Outline of Common Features
4. What is the Difference Between Rods and Cones
      – Comparison of Key Differences

Key Terms: Cones, Fovea Centralis, Photopic Vision, Photoreceptor Cells, Retina, Rods, Scotopic Vision

What are Rods

Rods are rod-shaped, light-sensitive cells on most of the peripheral parts of the retina in the vertebrate eye. About 120 million of rods are found in the retina and they are very sensitive to the light. The vision gained by rods is called scotopic vision. Since rods are sensitive to the scattered light, they provide the vision at night. However, rods are not sensitive to colors. Thereby, they provide a monochromatic vision. Rods mainly occur in the peripheral regions of the retina and the fovea centralis, which is the center region of the retina is free with rods. The structure of a rod cell is shown in figure 1.

Figure 1: Rod Cell

Rhodopsin is the type of visual pigments present in rods. All the membrane stacks of the rod cells contain rhodopsin. Therefore, only one type of rods can be identified in the retina. The light response of the rod cells sharply peaks at the blue color.

What are Cones

Cones are a type of photoreceptors in the retina, which are responsible for the color vision at daylight. Cones are less in number when compared to rods. About 6 million cones can be identified in the retina. Cones are not very sensitive to light. The vision of cones is gained under high light conditions. The type of vision gained by cones is called photopic vision. Fovea centralis consists of very thin, densely-packed cones. The diameter of the fovea centralis is 0.3 mm. The structure of a cone cell is shown in figure 2.

Figure 2: Cone Cell

Three types of cone cells can be identified in the retina with different color receptions: red, blue, and green. 67% of the cones are red; 32% are green and 2% are blue. Rods show a fast response to light. They can perceive rapid changes of the stimuli than the. On that account, fovea centralis contains the highest number of cones in the retina, and has the highest visual acuity of the eye.

Similarities Between Rods and Cones

Both rods and cones are the photoreceptor cells in the vertebrae retina.
Both rods and cones contain visual pigments.
Both rods and cones are types of secondary exteroreceptor cells.
When both types of the cells are active, the vision is mesopic.

Definition

Rods: Rods is a rod-shaped, light-sensitive cell which lies on most peripheral parts of the retina in the vertebrate eye.

Rods: Cones is a type of photoreceptor in the retina which is responsible for the color vision at the daylight.

Visual Pigments

Rods: Rods contain rhodopsin as the visual pigment.

Rods: Cones contain iodopsin as the visual pigment.

Outer Segment

Rods: The outer segment of the rods is cylindrical.

Rods: The outer segment of the cones is conical.

Inner End

Rods: The inner end of the rods contains a small knob.

Cones: The inner end of the cones is branched.

Type of Cells

Rods: Rods consist of a single type of cells.

Cones: Cones consist of three types of cells.

Vision

Rods: Rods lead to monochromatic vision.

Cones: Cones lead to color vision.

Connection to Bipolar Neuron

Rods: Several rods are connected to one cell.

Cones: One cone cell is connected to another cone cell.

Light Condition

Rods: Rods can be used for the vision under low light conditions (scotopic vision).

Cones: Cones can be used for the vision under high light conditions (photopic vision).

Light Sensitivity

Rods: Rods are very light sensitive.

Cones: Cones are not very light sensitive.

Scattered Light

Rods: Rods are sensitive to scattered light.

Cones: Cones are not sensitive to scattered light.

Visual Acuity

Rods: Rods provide less visual acuity.

Cones: Cones provide high visual acuity

Location

Rods: Rods are located in the periphery of the retina.

Cones: Cones are located in the center of the retina.

In Fovea

Rods: Rods are absent in the fovea.

Cones: Cones are present in fovea.

Response to Light

Rods: Rods have a slower response to light.

Cones: Cones have a fast response to light.

Number of Cells

Rods: About 12 million rod cells are present in the retina.

Cones: About 6 million cone cells are present in the retina.

Number of Pigments per Cell

Rods: Rods contain more pigments. Therefore, they require less light to detect images.

Cones: Cones contain lesser pigments than rods. Therefore, they require more light to detect images.

Membrane-enclosed Disks

Rods: The stacks of the membrane-enclosed disks are not attached directly to the rods.

Cones: The disks are attached to the outer membrane.

Insufficiency of the Pigment

Rods: Insufficiency of the rhodopsin causes night blindness.

Cones: Insufficiency of the iodopsin causes color blindness.

Conclusion

Rods and cones are two types of photoreceptors of the retina in the vertebrate eye. Rods are very light sensitive but, cones are not very light sensitive. Therefore, rods may provide vision at night as well. However, cones provide vision at daylight. Rods provide monochromatic vision. Since cones occur in three types, they provide color vision. The main difference between rods and cones is the sensitivity to the light by each type of photoreceptor cells under different light conditions.

Reference:

1. “Rod.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 28 Apr. 2017, Available here.
2. Purves, Dale. “Cones and Color Vision.” Neuroscience. 2nd edition., U.S. National Library of Medicine, 1 Jan. 1970, Available here.

Image Courtesy:

1. “Rod&Cone” By Kosigrim at English Wikipedia – Own work (Public Domain) via Commons Wikimedia
2. “Cone cell en” By Ivo Kruusamägi – Own work (CC BY-SA 3.0) via Commons Wikimedia