What is the Difference Between Rod Cells and Cone Cells

The main difference between rod cells and cone cells is that the rod cells are responsible for the vision under low light whereas the cone cells are responsible for the vision under higher light levels. Furthermore, rod cells do not mediate coloured vision while cone cells are responsible for the coloured vision. Moreover, rod cells have a low spatial acuity while the cone cells have a higher spatial acuity.

Rod cells and cone cells are two types of photoreceptor cells in the mammalian retina. The third type of photoreceptor cell is photosensitive retinal ganglion cells.

Key Areas Covered

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

Key Terms

Cone Cells, High Light Vision, Low Light Vision, Mammalian Retina, Photoreceptive Cells, Rod Cells, Vision

What are Rod Cells

Rod cells are a type of photoreceptive cells in the retina responsible for the night vision. Rod cells are long and narrow. They occur at the peripheral portion of the retina; hence, they are involved in the peripheral vision. Rod cells are extremely sensitive to the low levels of light. They can even trigger a single photon. Therefore, they are responsible for the low-light vision (scotopic vision) as well as the night vision. Thus, the loss of rod cells causes night blindness.

Figure 1: Rod and Cone Cells

The only type of photoreceptive pigments that occurs in the rod cells is rhodopsin. Therefore, rod cells can only produce monochromatic vision, which involves in the black and white vision. However, the spatial acuity or the resolution of rod cells is low. This is due to the synapsis of the group of rod cells with a single bipolar cell.

What are Cone Cells

Cone cells are the second type of photoreceptive cells in the retina responsible for the vision during the daytime. They are shorter and wider when compared to rod cells, and the outer membrane has a unique cone shape. Significantly, cone cells are concentrated into the fovea or the central part of the retina. They are sensitiveto  bright light. This means these cells require more photons for vision. Hence, their vision is called high-light vision (photopic vision).

Figure 2: Spectral Absorption Curves for Rod and Cone cells

There are three types of cone cells in the retina. They respond to different wavelengths: long (564–580 nm), medium (534–545 nm), and shorter (420–440 nm). Therefore, the cone cells are called L-cones (sensitive to the red light), M-cones (sensitive to the green light), and S-cones (sensitive to the blue light). They have three different types of photopsins and give trichromatic or colour vision. Moreover, each cone cell connects to an individual bipolar cell, increasing the resolution of the image.

Similarities Between Rod Cells and Cone Cells

• Rod cells and cone cells are two types of photoreceptive cells in the mammalian retina.
• They are a type of modified nerve cells.
• Moreover, both absorb light (photons).
• Because, both contain photoreceptive proteins to absorb light. The light-absorbing pigments occur in the outer segments of the membrane invaginations.
• Besides, their chemical process for the phototransduction is similar.
• Furthermore, they form synapses with bipolar cells.

Difference Between Rod Cells and Cone Cells

Definition

Rod cells are cylindrically-shaped cells in the retina that respond to dim light. Cone cells are cone-shaped, visual receptor cells in the retina that are sensitive to bright light and colour. These definitions explains the main difference between rod cells and cone cells.

Shape of the Outer Segment

As the names suggest, the outer segment of the rod cells is rod-shaped while the outer segment of the cone cells is cone-shaped.

Length

Also, another difference between rod cells and cone cells is that rod cells are comparatively long while cone cells are short.

Types

Besides, only a single type of rod cells occur in the retina while three types of cone cells occur in the retina.

Number of Cells per Retina

Furthermore, the retina contains approximately 90 million rod cells and 6 million cone cells.

Distribution in the Retina

Moreover, rod cells are distributed through the retina while the cone cells mainly occur in the fovea.

Connection with Bipolar Cells

In addition, several rod cells connect to a single bipolar cell while one cone cell connects to one bipolar cell.

Type of Vision

Rod cells are involved in peripheral vision while cone cells can only detect the images in the centre of the retina. Hence, this an important difference between rod cells and cone cells.

Monochromatic/Colored Vision

Another difference between rod cells and cone cells is that rod cells are responsible for monochromatic vision while cone cells are responsible for coloured vision.

Photoreceptive Pigments

Photoreceptive pigments is another difference between rod cells and cone cells. Rod cells contain rhodopsin while cone cells contain photopsin.

Sensitivity

Rod cells are more sensitive and responsible for night vision while cone cells have a poor sensitivity and require bright light for vision. Thus, this is also a major difference between rod cells and cone cells.

Acuity/Resolution

More to above, the rod cells possess a poor acuity while cone cells possess a higher acuity.

Regenerative Power

Above all, the regenerative power of the rod cells is high while the regenerative power of the cone cells is low.

Conclusion

Rod cells are a type of photoreceptive cells in the retina, which are highly sensitive to light. Therefore, they are responsible for the night vision. Only one type of red cells occurs in the retina and they are responsible for the black and white vision. On the other hand, cone cells are the other type of photoreceptive cells in the retina, which are less sensitive to the light. They are responsible for day vision. Also, three types of cone cells occur in the retina; therefore, cone cells are responsible for the colour vision. The main difference between rod cells and cone cells is the structure of cells and the type of vision.

Reference:

1. Purves D, et al., Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Functional Specialization of the Rod and Cone Systems. Available Here

Image Courtesy:

1. 1414 Rods and Cones” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013. (CC BY 3.0) via Commons Wikimedia  
2. “Cone-response-en” By Vectorized version of the GFDL image Cone-response.png uploaded by User:Maxim Razin based on work by w:User:DrBob and w:User:Zeimusu. – After Bowmaker J.K. ; and Dartnall H.J.A., “Visual pigments of rods and cones in a human retina.” J. ; Physiol. 298: pp501-511 (1980). (CC BY-SA 3.0) via Commons Wikimedia