8.8—Rod cells and stimulus detection

Rod cells are photoreceptors specialised for low-light detection. Absorbed light changes the photopigment and alters ion-channel activity, which changes neurotransmitter release onto the next neurone.

Rods are sensitive but do not provide colour discrimination; their convergence improves sensitivity while reducing spatial resolution. The retina therefore transforms light into a pattern of neural signals.

In dim light, rods can detect movement at the edge of vision, whereas bright-light colour and fine detail depend more on cone pathways.

A rod cell does not send light itself and “more rods” does not guarantee sharper vision. Keep photoreceptor transduction separate from CNS interpretation.