7 1 Photoreception Photoreception is the ability to detect electromagnetic radiation or photons Electromagnetic radiation like sound travels in waves that differ in amplitude and frequency As you have seen in chemoreception and mechanoreception the range of stimuli that an animal can detect depends on its sensory receptor cells and the specialized protein receptors in their membranes Similarly the range of frequencies that an animal can detect depends on the membrane properties of its photoreceptors Detection of light leads to the sense of sight as well as entrainment of circadian rhythms As you will see the color an animal perceives depends on what photoreceptor is activated thus color is not an inherent trait of our environment If an animal has different photoreceptors it will perceive that the same environment looks very different A small portion of the electromagnetic spectrum is referred to as visible light waves whose wavelengths are 400 700 nanometers As you can see in the graph below these wavelengths of light travel well in water attenuate much less This is reflective of photoreception having evolved before animals transitioned from water to land Light of a single wavelength is referred to as monochromatic as it is detected by the visual system as being a single pure color Sunlight is polychromatic and this mixture of wavelengths is perceived by most animals as white Light waves of higher frequency contain more energy and can damage cells and tissues such as UV light and X rays Some animals have photoreceptors that can detect UV light allowing them to see colors that humans cannot Light waves of lower frequency contain less energy Those below the visible spectrum are infrared because they contain less energy than red light Although some animals have photoreceptors that can detect near infrared 700 1200 nm infrared light 1200 nm contains too little energy to activate photoreceptors so animals cannot see it Instead animals perceive low energy infrared light as heat Unicellular animals such as Protists can detect light using a photo sensitive patch of membrane that forms an eyespot Multicellular animals have dedicated sensory receptor cells that detect visible light and UV and near infrared light in some animals called photoreceptors The organization of these cells varies from single cells to complex organs called eyes Some eyes can form images in addition to simply detecting light these organs have evolved convergently independent origin and have a variety of anatomical organizations There are two major types of photoreceptor cells ciliary photoreceptors which have a single cilium plural cilia with a highly folded membrane that forms disks and rhabdomeric photoreceptors whose apical surface is covered with many microvilli Both types of photoreceptors have folded membranes which increase their surface area In these membrane folds are the photopigments the molecules that absorb photon energy and allow the cell to detect light The phylogenetic tree below shows several major taxonomic groups and an illustration of what their photoreceptors look like All vertebrate photoreceptors are ciliary while invertebrate taxa vary as to which photoreceptor type they have The cilia of the ciliary photoreceptors make the cells elongated forming an outer segment whose disks detect light and an inner segment which includes basic neuronal structures like the soma and synaptic terminal There are two types of ciliary photoreceptors found in vertebrates which are named based on the shape of their outer segment and express different photopigments Rod photoreceptors are more elongated and sensitive to low levels of light Cone photoreceptors are conical and only active in brighter light The intensity amplitude and color frequency of light that each photoreceptor is activated by are determined by the photopigment in the outer segment which vary between taxa allowing different animals to detect different wavelengths of light and see different colors Photopigment the molecules in the photoreceptor membrane that are sensitive to light consist of two parts that are covalently bound together The chromophore which is a derivative of vitamin A 11 cis retinal in mammals absorbs light and uses the energy to change conformation to all trans retinal in mammals This is bound to the opsin which is a GPCR that determines what wavelength of light is absorbed and what amplitude the pigment is sensitive to When the chromophore absorbs light and changes conformation this changes the opsin conformation and activates a second messenger cascade leading to a change in photoreceptor membrane potential Photoreceptors vary in the wavelength of light they respond to by synthesizing different chromophores and opsin proteins Over 1 000 different opsins have been identified but most vertebrates have less than ten For example most mammals have one opsin in rods and one of two opsins in cones Humans have three different cone opsins Each opsin protein absorbs a different range of wavelengths which allows the chromophore to isomerize to a different range of light wavelengths The relative absorbance of each of the four opsins in humans one in rods three in cones is shown below with the color we perceive in the background Some of these ranges overlap but the cones and the opsins they express are named for the wavelengths that they absorb best blue green and red Note that the rod opsin corresponds to a greenish blue color but recall that they operate in only low levels of light At these levels there is not enough light to convey color information thus rods allow us to perceive shades of light and dark but not color What is the molecular mechanism of signal transduction How does retinal isomerization lead to a change in photoreceptor membrane permeability In rhabdomeric photoreceptors an IP3 mediated cascade opens TRP channels depolarizing the membrane In ciliary photoreceptors a cGMP mediated cascade closes Na channels hyperpolarizing the membrane The photoreceptor either increases or decreases Glu release depending on the direction of voltage change onto the next neuron In the dark ciliary photoreceptors have cGMP gated Na channels that are open so their resting membrane potential is unusually high 35 mV In the dark these photoreceptors release Glu onto the next cell When a photoreceptor is stimulated by a bright light high intensity this will cause many chromophores to isomerize which will cause a greater hyperpolarization causing
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