Photoreceptors are polarized neurons that contain specialized, antenna-like structures for receiving light signals and transmitting those signals into electrochemical impulses. The light-receiving elements of the cell are contained in the outer segment, while the synaptic terminal connects with bipolar cells, which in turn connect to retinal ganglion cells en route to the brain. There are two basic types of photoreceptors, rods and cones. Depicted here is a rod photoreceptor.
The outer segments of photoreceptors contain proteins called opsins, which in turn house chromophores. Through a process known as phototransduction, chromophores undergo isomerization when they absorb light, forcing a structural change in the opsin. This event triggers a cascade of chemical amplifications that lead the photoreceptor to transmit a signal. The system must then be restored in order to participate in another light detection cycle.
In humans, rod-type photoreceptors in the retina detect low-level, gray-scale light, such as is required for peripheral or night vision. Cone-type photoreceptors detect more intense light necessary for visual acuity during the day. Cones are also responsible for color vision and they accomplish this via the specialization of three cone sub-types ('Long' (red), 'Medium' (green), and 'Short' (blue)), each containing distinct opsins. Signals from the three cone sub-types are ultimately merged and interpreted upstream to convey a final color perception.
The human retina has approximately six million cones and one-hundred twenty million rods. The distribution of these cells is uneven. With the exception of the center, where they are less prominent, rods are spread evenly across the retina. Conversely, cones are concentrated in the center in the macula, and particularly within a sub-structure of the macula known as the fovea. These regions convey the highest degree of visual acuity and thus are most important for our visual function.