what are the cones in the retina sensitive to
Cone cell Cone cells, or cones, are one of three types of photoreceptor cells in the retina of mammalian eyes (e.g. the human eye). They are responsible for color vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light. Cone cells are densely packed in the fovea centralis, a 0.3Â mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. There are about six to seven million cones in a human eye and are most concentrated towards the macula.[1] The commonly cited figure of six million cone cells in the human eye was found by Osterberg in 1935.[2] Oyster's textbook (1999)[3] cites work by Curcio et al. (1990) indicating an average close to 4.5 million cone cells and 90 million rod cells in the human retina.[4]
Rod cell When light hits photoreceptive pigments within the photoreceptor cell, the pigment changes shape. The pigment, called rhodopsin (conopsin is found in cone cells) comprises a large protein called opsin (situated in the plasma membrane), attached to which is a covalently bound prosthetic group: an organic molecule called retinal (a derivative of vitamin A). The retinal exists in the 11-cis-retinal form when in the dark, and stimulation by light causes its structure to change to all-trans-retinal. This structural change causes an increased affinity for the regulatory protein called transducin (a type of G protein). Upon binding to rhodopsin, the alpha subunit of the G protein replaces a molecule of GDP with a molecule of GTP and becomes activated. This replacement causes the alpha subunit of the G protein to dissociate from the beta and gamma subunits of the G protein. As a result, the alpha subunit is now free to bind to the cGMP phosphodiesterase (an effector protein).[8] The alpha subunit interacts with the inhibitory PDE gamma subunits and prevents them from blocking catalytic sites on the alpha and beta subunits of PDE, leading to the activation of cGMP phosphodiesterase, which hydrolyzes cGMP (the second messenger), breaking it down into 5'-GMP.[9] Reduction in cGMP allows the ion channels to close, preventing the influx of positive ions, hyperpolarizing the cell, and stopping the release of the neurotransmitter glutamate (Kandel et al., 2000). Though cone cells primarily use the neurotransmitter substance acetylcholine, rod cells use a variety. The entire process by which light initiates a sensory response is called visual phototransduction.