Photoreceptors are specialized cells in the retina of the eye that respond to light. The two types, rods and cones, react to light differently. While rods are more sensitive to changes in light and dark, and movement in the periphery, cones are more sensitive to detail and the wavelengths of red, green, and blue, allowing color vision in the eye. Each rod and cone has an outer segment filled with stacks of pigment inside a membrane, with the pigment being the substance that allows them to absorb light. Photoreceptors have the unique function of converting light into neural signals, a process called phototransduction. Due to their purpose in the eyes, malfunctions in photoreceptors can result in conditions such as colorblindness.
There are two types of photoreceptor cells: rods and cones.  Both are comprised of the following: an outer segment, filled with circular stacks of membranes containing visual pigment molecules, such as rhodopsin; an inner segment with long, thin mitochondria, ribosomes, and membranes where opsin (a type of such pigment) is assembled and forwarded to the outer segment discs; a cell body containing the cell's nucleus; a synaptic terminal where neurotransmissions are sent to second order neurons. The inner segment is connected to the outer segment with a thin cilium. 
Rods are slim, named after their shape, made up of an inner and outer segment that are usually thinner than the ones in a cone.  The outer segment, like a rod, resembles a cylinder, with pigment-holding discs inside that become completely detached from the membrane and float around freely.  In rods, these discs are closed. Most rods have 1000 discs, with each disc containing 150,000 rhodopsin molecules. So in total, there are 150 million rhodopsin pigment molecules per rod.  Rhodopsin is the only type of light-sensitive pigment in rods. One of the significant locations where rods are more numerous than cones is the peripheral part of the human retina, where their inner segments are measured at 2 microns. Rods can also occupy the room between the cones in the subretinal space of the eye.  All together, the human retina contains about 120 million rods. 
Cones are wider, conical-shaped structures, whose cell bodies are lined in a single row right underneath the outer lining membrane of the eye. Their outer and inner segments stick out into the subretinal space.  In cones, the pigment discs stay attached to the membrane, and are partly open to the surrounding fluid. Their pigment in these outer segments can be one of three different types of opsin, that absorb light in short, medium, and long wavelengths--the basic concept of human color vision.   A key location of cones is in the fovea centralis, the central retina region of the eye, where there are no rods. Here the cones are layered closer together in slanting columns, where most cones have a diameter of 1.5 microns. In total, there are about 6 million cones in the human retina.  
Rods are the photoreceptors most sensitive to changes in light and darkness, as well as shape and movement. In a dimly lit room, the human eye mostly uses rods to see--however, because rods aren't the best for seeing color, the eye cannot detect color well in the dark. Rods require about seven to ten minutes to adjust to the lack of light and become the dominant photoreceptors in the eye. In reverse, when reentering a brightly lit location, the rods also take a few minutes to return to normal because they have become saturated from the sudden amount of brightness.  This describes how rods are accountable for darkness adaption, or scotopic vision. Rods adapt slower than cones, but are one thousand times as sensitive, according to reports able to react to individual photons. Due to the abundance of rods in the peripheral of the eye, dimmer objects are able to be seen in the human peripheral vision and movement is easier to notice. 
While cones are not as sensitive to light, this type of photoreceptor is most sensitive to the colors green, red, and blue. Cones actually only work when in bright light, explaining why the eye cannot detect color very well in the dark.  Cones can be labeled as three categories: "red" cones, "green" cones, and "blue" cones. Of all the cones in eye, 64% can be called "red", 32% called "green", and 2% called blue. Red and green cones are located primarily in the fovea centralis, and blue cones, which have the highest sensitivity, on the outside. Cones are able to adapt very quickly to changes in amount of lights, evident by how fast the eyes adjust to entering a room after coming from a brighter outside. They are the photoreceptors that provide high resolution vision and more able to see fine details. When trying to see an object, the eyes move continuously in order to let its light hit the fovea centralis and its high concentration of cones. 
The closely packed nature of the photopigment discs in photoreceptors is their distinctive characteristic, as it makes them able to absorb a bigger amount of photons, and the amount of photons absorbed affect the photoreceptor's output signal for perceiving color and other qualities of an object.  The outer segments of both rods and cones are continually growing in length due to their base's constant reproduction of new pigment discs. 
The function of photoreceptors is to convert light energy into membrane potential through the process of phototransduction.  When light hits the photopigments in the photoreceptor cells, they trigger a series of chemical reactions.  First, the photon hits a molecule called retinal, the light-absorbing portion of photopigment. This causes part of the molecule's important double bond to break; hydrogen atoms that were on the same side of this double bond switch to be on opposite sides of the bond. The altered photopigment, rhodopsin, now activates trasducin, a G-protein, which in turn activates the enzyme phosphodiesterase in a disc membrane. The enzyme hydrolyzes cGMP, a nucleotide, lowering its concentration in the outer segment of the cell. 
With the lowering of cGMP concentration, the cell's membranes close its ion channels in the surface of the outer segment, effectively hyperpolarizing the cell, which decreases the amount of neurotransmitters, the chemicals that send signals from one neuron to another, released into the synapse at the end of the cell.   This decrease in membrane permeability alters the membrane potential, allowing it to send a nerve signal to the next layer of the retina. These signals eventually make their way up to the brain. Thus, photoreceptor cells change light energy to neural activity the brain can understand, translating light to an eventual registered image. 
Unique Effects in Eyes
Colorblindness, or color deficiency, results from a variety of issues in cones. An individual could not have a particular types of cone in their retina, or one of the cone types could be weak.  Due to abnormal genes, the wrong type of pigment could be produced in the cones, leading them to be sensitive to different wavelengths than usual. People with regular cones and pigment have trichromatic vision--all three cone types work as they should. Anomalous trichromacy is when a person has one or more of the cone pigments not working correctly. The two types of anomalous trichromacy are protanomaly, where red is perceived weakly and hues lean towards green on the color spectrum, and deuternomaly, where green is perceived weakly and hues lean towards red. In both conditions, a person has difficulty determining minute differences between red, orange, yellow, and green. A stronger form of color deficiency is dichromacy, where one cannot tell any difference between red, orange, yellow, and green. The first type of this condition is protanopia, where red's brightness has been lowered so greatly that the color may look gray or black and colors with red cannot be differentiated. The second type is deuteranopia, which is similar in its problem differentiating hues but with not as extreme dimming of color. Tritanopia is a color deficiency to blue, where the person has trouble discriminating blue and yellow.  Other types of color deficiency include monochromacy, where there is a complete lack of color in vision. 
A blind spot, also called a scotoma, is where there are no photoreceptors to take in light and thus produce an image in the brain.   It is located above the optic nerve of the eye, the bundle of nerve fibers that come through the back of the eye and spread out to form the retina layer. The point where it enters, called the optic disc, has no photoreceptors, thus creating a blind spot in a person's vision.  The right eye's blind spot is to the right of the center of vision, and the left eye's is to the left.  The eyes' fields of vision overlap, covering the blind spot as the brain works to "fill in" the gap, thus a person doesn't notice it.  The blind spot is still hard to locate even with one eye closed because of this characteristic of the brain. 
Video showing the difference between rods and cones
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