Rods vs Cones
Photoreceptors are a special type of neurons found in the retina and made up of basic four regions; an outer segment, an inner segment, a cell body, and a synaptic terminal. They are important to convert electromagnetic radiation into neural signals. Normally retina of a human eye contains around 125 million photoreceptors. These photoreceptors can be divided into two types; namely, rods and cones according to their basic differences. These two types of cells basically differ from each other in structure, photochemical molecules, sensitivity, retinal distribution, synaptic connections, and function.
Rods
Rod receptors are the cells which contain long cylindrical outer segments and many disks. A higher number of disks and high concentration of pigment in rods makes them more sensitive to light than cones. So that, under low light conditions or scotopic conditions, only rods contributes to vision. Unlike the cones, these photoreceptors do not mediate color vision.
Cones
Cones are the cells capable of color vision and responsible for high spatial acuity. Unlike the rods, cones do not have individual disks to hold the photochemicals. Photochemicals are in the outer membrane of the cell, and the shape of the cone is determined by folding of the outer membrane. This folding area increases the surface area, which eventually provides more membrane exposure for light absorption. Due to the low concentration of pigments and less amplification in cones, they need more light to produce a proper signal. The cones are divided into three main categories depending on their wavelength specificity; namely, S- cone (short- wavelength sensitive cone), M- cones (middle- wavelength sensitive cone), and L- cone (long- wavelength sensitive cones).
What is the difference between Rods and Cones?
• Rods are rod-shaped, and cones are cone-shaped.
• Rods contain more photopigments, whereas cones contain less.
• Response of rods is slow, whereas that of cones is fast.
• Rods take long integration time while cones take short integration time.
• Cones have less amplification, whereas rods have high amplification due to the single quantum detection in rods.
• Unlike the cones (except S-cones), rods’ response saturates when a small amount of a pigment is bleached.
• Rods are not directionally selective, unlike the cones.
• Cones have lower absolute sensitivity while rods have high sensitivity due to the greater number of disks and higher pigment concentration.
• Degree of spatial integration results low acuity in Rods while, high acuity in cones.
• Rods are achromatic while cones are Chromatic. Hence, cones are important in color vision.
• The scotopic retina uses rods while the photopic retina uses cones.