Final answer:
Retinal cells become terminally differentiated and cease to divide, with their lifespan managed by replacement from stem cells. Rods are responsible for peripheral and low-light vision, and exhibit tonic activity, adjusting to light changes to encode visual signals.
Step-by-step explanation:
Retinal Cell Differentiation and Lifespan
Retinal cells stop differentiating once they reach a stage referred to as terminally differentiated. Although most terminally differentiated cells do not divide again and have a finite lifespan, they are continuously replaced by new cells that are produced by stem cells, similar to red blood cells that are replaced by reticulocytes from bone marrow.
The retina contains specialized photoreceptor cells, notably rods and cones, which are crucial for visual processing. Rods are more sensitive to light and are essential for peripheral and low-light vision, while cones provide color vision and are responsible for high-resolution vision. Rods do not contribute to color vision, which is why in very dark conditions, humans perceive images in shades of gray.
These photoreceptors undergo tonic activity and are always slightly active, even in the absence of light. They adjust their activity based on light stimulation, which either increases the baseline firing rate or decreases it. This modulated activity is crucial for the visual system to encode visual signals for the brain, to sharpen edges, enhance contrast, and facilitate the distinction between light and dark regions in our vision.