Rods and cones differ in all, Except:
Correct Answer: Signal transduction
Description: Ans is 'a' i.e. signal transduction "The process of phototransduction in cones is virtually the same as in rods: the only major difference is in the types of opsin in the membranous disks of the cone outer segments." -Neuroscience By Mark F. Bear, Barry W. Connors,Michael A. Paradiso 3/e p296The retina is organized in 10 layers and contains rods and cones, the photoreceptor cells along with 4 other types of neurons- bipolar cells, ganglion cells, horizontal cells and amacrine cells.The rods and cones synapse with bipolar cells, and the bipolar cells synapse with ganglion cells. The axons of ganglion cells converge and leave the eye as the optic nerve.Differences between rods and conesThe two photoreceptors differ from each other in several ways. Rods and cones differ in structure, photochemical molecules, sensitivity, retinal distribution, synaptic connections, and function.The total number of rods in the human retina (approx. 90 million) far exceeds the number of cones (roughly 4.5 million). As a result, the density of rods is much greater than cones throughout most of the retina. However, this relationship changes dramatically in the fovea, a highly specialized region of the central retina that measures about 1.2 millimeters in diameter. In the fovea, cone density increases almost 200-fold, reaching, at its center, the highest receptor packing density anywhere in the retina. The increased density of cones in the fovea is accompanied by a sharp decline in the density of rods. In fact, the central 300 pm of the fovea, called the foveola, is totally rod-free. The extremely high density of cone receptors in the fovea, and the one-to- one relationship with bipolar cells and retinal ganglion cells (see later), endows this region (and the cone system generally) with the capacity to mediate high visual acuity. As cone density declines with eccentricity and the degree of convergence onto retinal ganglion cells increases, acuity is markedly reducedThe two types of receptors differ in their size and their disk morphology. The outer segments of rods contain approximately 1000 to 2000 identical disks. The disks in rods contain more photopigment (rhodopsin) and consequently capture more photons of light than cones.There is also a higher amplification in the rods than in the cones. This means that, in rods, more cGMP gets hydrolyzed for each absorbed photon of light. This amplification leads to a longer integration and a more sensitive cell. The high amplification and long integration leads to high sensitivity to scattered light. The high sensitivity is the reason that rods are effective in weak light. The rods are thus responsible for vision when the light is dim (scotopic vision), whereas the cones are responsible in good light (photopic vision).In contrast, the cones do not have individual disks to hold the photochemical used in the transduction process. The photochemical in cones are in the outer membrane of the cell and the shape of the cone is determined by the folding of the outer membrane. The cones require more light because they have fewer photochemical and less amplification.The opsin molecules in the photochemical differ in the two receptors. All rods have the same opsin molecule. Because the opsin molecule determines the particular wavelength that can be absorbed most readily by the retinal, all the rods are similar in their response to light. The rods have an absorption peak near 500 nm.Each cone, on the other hand, has one of three different opsin molecules. Thus there are three different types of cones- red cone, blue cone and the green cone. The wavelength absorption characteristic of each cone depends on which opsin is bound to the retinal. The opsin determines the cone-wavelength absorption sensitivity. This means that cones differ from each other as well as from the rods in their absorption characteristics. Thus the cones are responsible for color vision.Other important difference between rods and cones concern their connections with other neurons in the retina.Properties of Rod and Cone SystemsRodsConesCommentMore photopigmentLess photopigment Slow response: long integration timeFast response: short integration time High amplificationLess amplification Saturating Response (by 6% bleached)Non-saturating response (except S- cones)The rods' response saturates when only a small amount of the pigment is bleached (the absorption of a photon by a pigment molecule is known as bleaching the pigment).Night visionDay vision Highly convergent retinal pathwaysLess convergent retinal pathwaysSpatial integrationHigh sensitivityLower absolute sensitivity Low acuityHigh acuityResults from degree of spatial integrationAchromatic: one type of pigmentChromatic: three types of pigmentColor vision results from comparisons between cone responsesNotably, many rods are connected to one bipolar cell- i.e. there is a high degree of convergence. For the cones, on the other hand, there is typically much less convergence, with a few cones connected to one bipolar cell. This explains more sensitivity of the rods over cones. The difference in convergence means that the cones provide information with a higher spatial resolution than the rods. Signal transduction in the RetinaThe mechanism of signal transduction or phototransduction i.e. the process by which light is converted into electrical signals, is almost similar in both the rods and cones. But both the photoreceptors have differences with other sensory receptors.The photoreceptors do not behave like other receptors when exposed to their stimulus: they are hyperpolarized instead of depolarizedThe photoreceptors are in a depolarized state in the dark. This is probably caused by Na+ channels that are open under such conditions. Unlike other receptors, the photoreceptors (and the bipolar cells) do not produce action potentials but only graded changes of the membrane potential (k/a local graded potential). Because the distance is very short from the outer segment-where the membrane potential changes are produced-to the synapses between the photoreceptors and the bipolar cells, even small fluctuations of the membrane potential cause alterations of the transmitter release from the photoreceptors. There is thus no need for the production of action potentials, which are important when signals are to be propagated over long distances.Light falling on the photoreceptor cells alters the configuration of the retinene (from 11-cis configuration to 11- Trans' isomer). This in turn alters the configuration of the opsin, and the opsin change activates the associated G protein. This ultimately leads to closure of the Na+ channels.Closure of the Na+ channels hyperpolarizes the cell. The hyperpolarization reduces the release of synaptic transmitter, and this generates a signal in the bipolar cells that ultimately leads to action potentials in ganglion cells.* In contrast to the photoreceptors and the bipolar, the ganglion cells produce action potentials (conducted in the optic nerve to the higher visual centers).
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