Sensory and Visual System Anatomy and Physiology Practice Test

Cone color vision depends on the photoresponse cascade in cones that uses a G-protein (transducin) to regulate cGMP levels, together with three cone pigments (S, M, L) that peak at different wavelengths. When light hits cone opsins, the activated opsin triggers transducin, which then activates a phosphodiesterase to hydrolyze cGMP. As cGMP falls, the cGMP-gated channels close, reducing the influx of Na+ and Ca2+, causing the cell to hyperpolarize and decrease glutamate release. This chain of events is essential for fast, graded responses to light and sets up a molecular basis for distinguishing wavelengths.

The presence of three distinct cone pigments—short, medium, and long wavelength—allows the retina to compare signals across the three cone types. By analyzing the relative stimulation of each cone type, the brain constructs color perception. That’s why this statement captures both the transducin-based, cGMP-dependent phototransduction in cones and the critical role of the S, M, and L pigments in enabling color vision.

IP3 signaling isn’t part of the cone transduction cascade, cones do rely on G-proteins (transducin), and color vision does not come from a rod pigment.