Cones

Cones are photoreceptors that work in bright light and give detailed, colour vision. There are about 6 million in each human retina, mostly concentrated in the fovea.Cones contain a light-sensitive pigment called iodopsin. Like rhodopsin, it consists of retinal joined to an opsin protein, but the opsin is different. Iodopsin is less sensitive than rhodopsin: it needs a higher light intensity to break down. This is why cones do not work in dim light, and why colours seem to disappear at night.Cones respond to light in the same way as rods. Light breaks down the pigment, Na⁺ channels close, the cone hyperpolarises and stops releasing inhibitory neurotransmitter, so the bipolar neurone it synapses with is stimulated.The key difference is in how cones are connected. Each cone in the fovea usually synapses with its own bipolar neurone, which connects to its own ganglion cell. There is no retinal convergence. This has two consequences:• High visual acuity. Each cone sends a separate signal to the brain, so two points of light falling on neighbouring cones are seen as two separate points.
• Low sensitivity. There is no summation from several cones, so more light is needed to stimulate the bipolar neurone.There are three types of cone, each with a different form of iodopsin, which allows colour vision.Comparison: rods give high sensitivity, low acuity and black and white vision in dim light; cones give low sensitivity, high acuity and colour vision in bright light. To see fine detail, you look straight at an object so its image falls on the fovea.