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Photoreceptors |
| Answer» Photoreceptors are the cells in the retina that respond to light. Their distinguishing feature is the presence of large amounts of tightly packed membrane that contains the photopigment rhodopsin or a related molecule. The tight packing is needed to achieve a high photopigment density, which allows a large proportion of the light photons that reach the photoreceptor to be absorbed. Photon absorption contributes to the photoreceptor’s output signal.In the retina of vertebrates the rods and cones have photopigment-bearing regions (outer segments) composed of a large number of pancakelike disks. In rods the disks are closed, but in cones the disks are partially open to the surrounding fluid. In a typical rod there are about a thousand disks, and each disk holds about 150,000 rhodopsin molecules, giving a total of 150 million molecules per rod. In most invertebrate photoreceptors the structure is different, with the photopigment borne on regularly arranged microvilli, fingerlike projections with a diameter of about 0.1 μm. This photoreceptor structure is known as a rhabdom. The photopigment packing is less dense in rhabdoms than in vertebrate disks. In both vertebrate photoreceptors and rhabdoms, each photoreceptor cell contains a nucleus, an energy-producing region with mitochondria (in the inner segment in rods and cones), and an axon that conveys electrical signals to the next neurons in the processing chain. In reptiles and birds the receptors may also contain coloured oil droplets that modify the spectrum of the light absorbed by the photopigment, thereby enhancing colour vision. In insects and other invertebrates the receptors may also contain granules of dark pigment that move toward the rhabdom in response to light. They act as a type of pupil, protecting the rhabdom in bright conditions by absorbing light. | |