What are the differences between rod cells and cone cells?

Rod cells are responsible for vision in low light conditions, while cone cells enable colour vision and detail perception in bright light.

Rod cells and cone cells are two types of photoreceptor cells found in the retina of the eye. They play crucial roles in visual perception, but their functions and structures are distinctly different.

Rod cells are more numerous, with around 120 million in the human eye. They are highly sensitive to light, which makes them perfect for night vision, peripheral vision, and motion detection. However, they do not provide colour perception or sharp detail. This is why in low light conditions, we see mostly in shades of grey and our ability to perceive detail is reduced. Rod cells are more elongated and rod-like in shape, hence their name.

On the other hand, cone cells are less numerous, with about 6 million in the human eye. They are less sensitive to light but have the ability to perceive colour and fine detail. This is why in bright light conditions, we can see a wide spectrum of colours and sharp images. Cone cells are responsible for our central vision, which is used for tasks like reading and driving. They are shorter and more conical in shape, hence their name.

Cone cells are further divided into three types based on the wavelengths of light they are sensitive to: short (S), medium (M), and long (L). These correspond to blue, green, and red light, respectively. This is the basis for our trichromatic colour vision.

In terms of distribution, rod cells are absent in the fovea (the centre of the retina) and densely packed in the peripheral retina. In contrast, cone cells are densely packed in the fovea and sparse in the peripheral retina. This distribution reflects their respective roles in vision.

In summary, rod cells and cone cells have different sensitivities to light, shapes, distributions in the retina, and roles in vision. Rod cells are essential for vision in low light, while cone cells enable us to see in colour and perceive fine detail in bright light.