<P> There are major functional differences between the rods and cones . Rods are extremely sensitive, and can be triggered by a single photon . At very low light levels, visual experience is based solely on the rod signal . This explains why colors cannot be seen at low light levels: only one type of photoreceptor cell is active . </P> <P> Cones require significantly brighter light (i.e., a larger numbers of photons) in order to produce a signal . In humans, there are three different types of cone cell, distinguished by their pattern of response to different wavelengths of light . Color experience is calculated from these three distinct signals, perhaps via an opponent process . The three types of cone cell respond (roughly) to light of short, medium, and long wavelengths . Note that, due to the principle of univariance, the firing of the cell depends upon only the number of photons absorbed . The different responses of the three types of cone cells are determined by the likelihoods that their respective photoreceptor proteins will absorb photons of different wavelengths . So, for example, an L cone cell contains a photoreceptor protein that more readily absorbs long wavelengths of light (i.e., more "red"). Light of a shorter wavelength can also produce the same response, but it must be much brighter to do so . </P> <P> The human retina contains about 120 million rod cells, and 6 million cone cells . The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal . Certain owls, such as the tawny owl, have a tremendous number of rods in their retinae . In addition, there are about 2.4 million to 3 million ganglion cells in the human visual system, 1 to 2% of them photosensitive . The axons of ganglion cells form the two optic nerves . </P> <P> The pineal and parapineal glands are photoreceptive in non-mammalian vertebrates, but not in mammals . Birds have photoactive cerebrospinal fluid (CSF) - contacting neurons within the paraventricular organ that respond to light in the absence of input from the eyes or neurotransmitters . Invertebrate photoreceptors in organisms such as insects and molluscs are different in both their morphological organization and their underlying biochemical pathways . Described here are human photoreceptors . </P>

How many rods are in the human eye