Misplaced Pages

#520479

47-648: The term planet was applied originally only to the five planets known (i.e., visible to the naked eye ) and excluded the Earth . The term was later generalized, particularly during the Middle Ages , to include the sun and the moon (sometimes referred to as "lights"), making a total of seven planets. The seven days of the week of the Hindu calendar also corresponds with the seven classical planets and related day names of European culture and are named accordingly in most languages of

94-412: A pigment molecule called retinal . In rod cells, these together are called rhodopsin . In cone cells, there are different types of opsins that combine with retinal to form pigments called photopsins . Three different classes of photopsins in the cones react to different ranges of light frequency, a selectivity that allows the visual system to transduce color . The function of the photoreceptor cell

141-399: A different number of cone classes, ranging from monochromats to pentachromats . The path of a visual signal is described by the phototransduction cascade , the mechanism by which the energy of a photon signals a mechanism in the cell that leads to its electrical polarization. This polarization ultimately leads to either the transmittance or inhibition of a neural signal that will be fed to

188-531: A new avenue to explore in trying to find treatments for blindness. ipRGCs were only definitively detected ipRGCs in humans during landmark experiments in 2007 on rodless, coneless humans. As had been found in other mammals, the identity of the non-rod non-cone photoreceptor in humans was found to be a ganglion cell in the inner retina. The researchers had tracked down patients with rare diseases wiping out classic rod and cone photoreceptor function but preserving ganglion cell function. Despite having no rods or cones

235-453: A new record as the farthest object that can be seen from Earth with the naked eye. It occurred about 7.5 billion years ago, the light taking that long to reach Earth. Many other things can be estimated without an instrument. If an arm is stretched the span of the hand corresponds to an angle of 18 to 20°. The distance of a person, just covered up by the outstretched thumbnail, is about 100 meters. The vertical can be estimated to about 2° and, in

282-405: A role in the entrainment of the circadian rhythm and the pupillary reflex . Each photoreceptor absorbs light according to its spectral sensitivity (absorptance), which is determined by the photoreceptor proteins expressed in that cell. Humans have three classes of cones (L, M, S) that each differ in spectral sensitivity and 'prefer' photons of different wavelengths (see graph). For example,

329-414: A rudimentary visual pathway enabling conscious sight and brightness detection. Classic photoreceptors (rods and cones) also feed into the novel visual system, which may contribute to color constancy. ipRGCs could be instrumental in understanding many diseases including major causes of blindness worldwide like glaucoma, a disease that affects ganglion cells, and the study of the receptor offered potential as

376-479: A viewing distance of 16" = ~ 400 mm, which is considered a normal reading distance in the US, the smallest object resolution will be ~ 0.116 mm. For inspection purposes laboratories use a viewing distance of 200–250 mm, which gives the smallest size of the object recognizable to the naked eye of ~0.058–0.072 mm (58–72 micrometers). The accuracy of a measurement ranges from 0.1 to 0.3 mm and depends on

423-496: Is a S cone. These events take place at different time periods for different species and include a complex pattern of activities that bring about a spectrum of phenotypes. If these regulatory networks are disrupted, retinitis pigmentosa , macular degeneration or other visual deficits may result. Intrinsically photosensitive retinal ganglion cells (ipRGCs) are a subset (≈1–3%) of retinal ganglion cells , unlike other retinal ganglion cells, are intrinsically photosensitive due to

470-540: Is a difficult averted vision object and only visible at all if it is higher than 50° in the sky. The globular clusters M 3 in Canes Venatici and M 92 in Hercules are also visible with the naked eye under such conditions. Under really dark sky conditions, however, M33 is easy to see, even in direct vision. Many other Messier objects are also visible under such conditions. The most distant objects that have been seen by

517-564: Is absent, stars as faint as +8 might be visible. The angular resolution of the naked eye is about 1 ′ ; however, some people have sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented. Uranus and Vesta had most probably been seen but could not be recognized as planets because they appear so faint even at maximum brightness; Uranus's magnitude varies from +5.3 to +5.9 , and Vesta's from +5.2 to +8.5 (so that it

SECTION 10

#1732844403521

564-482: Is being released to bipolar cells in the dark. Absorption of a photon will hyperpolarize the photoreceptor and therefore result in the release of less glutamate at the presynaptic terminal to the bipolar cell. Every rod or cone photoreceptor releases the same neurotransmitter, glutamate. However, the effect of glutamate differs in the bipolar cells, depending upon the type of receptor imbedded in that cell's membrane . When glutamate binds to an ionotropic receptor ,

611-594: Is capable of visual phototransduction . The great biological importance of photoreceptors is that they convert light (visible electromagnetic radiation ) into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons , triggering a change in the cell's membrane potential . There are currently three known types of photoreceptor cells in mammalian eyes: rods , cones , and intrinsically photosensitive retinal ganglion cells . The two classic photoreceptor cells are rods and cones, each contributing information used by

658-760: Is much more strongly affected by light pollution than is that of planets and stars. Under typical dark conditions only a few such objects are visible. These include the Pleiades , h/χ Persei , the Andromeda Galaxy , the Carina Nebula , the Orion Nebula , Omega Centauri , 47 Tucanae , the Ptolemy Cluster Messier 7 near the tail of Scorpius and the globular cluster M13 in Hercules . The Triangulum Galaxy (M33)

705-413: Is only visible near its opposition dates). Uranus, when discovered in 1781, was the first planet discovered using technology (a telescope ) rather than being spotted by the naked eye. Theoretically, in a typical dark sky, the dark adapted human eye would see the about 5,600 stars brighter than +6 while in perfect dark sky conditions about 45,000 stars brighter than +8 might be visible. In practice,

752-446: Is possible. This represents only 200 meters at the moon's distance of 385,000 km. Observing a nearby small object without a magnifying glass or a microscope , the size of the object depends on the viewing distance. Under normal lighting conditions (light source ~ 1000 lumens at height 600–700 mm, viewing angle ~ 35 degrees) the angular size recognized by naked eye will be round 1 arc minute = 1/60 degrees = 0.0003 radians. At

799-415: Is proportional only to the number of photons absorbed. The photoreceptors can not measure the wavelength of light that it absorbs and therefore does not detect color on its own. Rather, it is the ratios of responses of the three types of cone cells that can estimate wavelength, and therefore enable color vision . Rod and cone photoreceptors are found on the outermost layer of the retina ; they both have

846-501: Is strongly affected by light pollution . Even a few hundred kilometers away from a metropolitan area where the sky can appear to be very dark, it is still the residual light pollution that sets the limit on the visibility of faint objects. For most people, these are likely to be the best observing conditions within their reach. Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6 . Under perfect dark sky conditions where all light pollution

893-420: Is to convert the light information of the photon into a form of information communicable to the nervous system and readily usable to the organism: This conversion is called signal transduction . The opsin found in the intrinsically photosensitive ganglion cells of the retina is called melanopsin . These cells are involved in various reflexive responses of the brain and body to the presence of (day)light, such as

940-506: The Indian subcontinent . Most Hindu temples around the world have a designated place dedicated to the worship of the navagraha. Muthuswami Dikshitar (1776–1835), a Carnatic music composer from southern India, composed the Navagraha Kritis in praise of the nine grahas . Each song is a prayer to one of the nine planets. The Sahitya (lyrics) of the songs reflect a profound knowledge of

987-408: The mantra and jyotisha sastras . This mythology -related article is a stub . You can help Misplaced Pages by expanding it . Naked eye Naked eye , also called bare eye or unaided eye , is the practice of engaging in visual perception unaided by a magnifying , light-collecting optical instrument , such as a telescope or microscope , or eye protection . In astronomy ,

SECTION 20

#1732844403521

1034-646: The phases of Venus , among other things. Meteor showers are better observed by naked eye than with binoculars. Such showers include the Perseids (10–12 August) and the December Geminids . Some 100 satellites per night, the International Space Station and the Milky Way are other popular objects visible to the naked eye. On 19 March 2008, a major gamma-ray burst (GRB) known as GRB 080319B , set

1081-399: The sodium-potassium pump . Finally, closest to the brain (and farthest from the field of view) is the outer segment, the part of the photoreceptor that absorbs light . Outer segments are actually modified cilia that contain disks filled with opsin , the molecule that absorbs photons, as well as voltage-gated sodium channels . The membranous photoreceptor protein opsin contains

1128-426: The visual system to form an image of the environment, sight . Rods primarily mediate scotopic vision (dim conditions) whereas cones primarily mediate photopic vision (bright conditions), but the processes in each that supports phototransduction is similar. The intrinsically photosensitive retinal ganglion cells were discovered during the 1990s. These cells are thought not to contribute to sight directly, but have

1175-600: The Naked eye only if Neptune is at its maximum brightness (magnitude +7.8). The Sun and the Moon—the remaining noticeable naked-eye objects of the solar system—are sometimes added to make seven "planets". During daylight only the Moon and Sun are obvious naked eye objects, but in many cases Venus can be spotted in daylight and in rarer cases Jupiter . Close to sunset and sunrise, bright stars like Sirius or even Canopus can be spotted with

1222-412: The air. This is of importance in meteorology and for the " seeing " of astronomy. Light pollution is a significant problem for amateur astronomers but becomes less late at night when many lights are shut off. Air dust can be seen even far away from a city by its "light dome". Photoreceptor cell A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that

1269-415: The atmospheric extinction and dust reduces this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution can be as low as 2 , as few as 50 stars are visible. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars. The visibility of diffuse objects such as star clusters and galaxies

1316-471: The bipolar cell will depolarize (and therefore will hyperpolarize with light as less glutamate is released). On the other hand, binding of glutamate to a metabotropic receptor results in a hyperpolarization, so this bipolar cell will depolarize to light as less glutamate is released. In essence, this property allows for one population of bipolar cells that gets excited by light and another population that gets inhibited by it, even though all photoreceptors show

1363-423: The bipolar cells, which transmit then to the retinal ganglion cells. Retinal ganglion cell axons collectively form the optic nerve , via which they project to the brain. The rod and cone photoreceptors signal their absorption of photons via a decrease in the release of the neurotransmitter glutamate to bipolar cells at its axon terminal. Since the photoreceptor is depolarized in the dark, a high amount of glutamate

1410-410: The brain via the optic nerve . The steps that apply to the phototransduction pathway from vertebrate rod/cone photoreceptors are: Unlike most sensory receptor cells, photoreceptors actually become hyperpolarized when stimulated; and conversely are depolarized when not stimulated. This means that glutamate is released continuously when the cell is unstimulated, and stimulus causes release to stop. In

1457-404: The dark, cells have a relatively high concentration of cyclic guanosine 3'-5' monophosphate (cGMP), which opens cGMP-gated ion channels . These channels are nonspecific, allowing movement of both sodium and calcium ions when open. The movement of these positively charged ions into the cell (driven by their respective electrochemical gradient ) depolarizes the membrane, and leads to the release of

Navagraha - Misplaced Pages Continue

1504-775: The default photoreceptor program; however, differential transcriptional activity can bring about rod or M cone generation. L cones are present in primates, however there is not much known for their developmental program due to use of rodents in research. There are five steps to developing photoreceptors: proliferation of multi-potent retinal progenitor cells (RPCs); restriction of competence of RPCs; cell fate specification; photoreceptor gene expression; and lastly axonal growth, synapse formation and outer segment growth. Early Notch signaling maintains progenitor cycling. Photoreceptor precursors come about through inhibition of Notch signaling and increased activity of various factors including achaete-scute homologue 1. OTX2 activity commits cells to

1551-455: The experience of the observer. The latter figure is the usual positional accuracy of faint details in maps and technical plans. A clean atmosphere is indicated by the fact that the Milky Way is visible. Comparing the zenith with the horizon shows how the "blue quality" is degraded depending on the amount of air pollution and dust. The twinkling of a star is an indication of the turbulence of

1598-439: The eye. The distribution of cone classes (L, M, S) are also nonhomogenous, with no S-cones in the fovea, and the ratio of L-cones to M-cones differing between individuals. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal . Certain owls, such as the nocturnal tawny owl , have a tremendous number of rods in their retinae. Other vertebrates will also have

1645-465: The naked eye are nearby bright galaxies such as Centaurus A , Bode's Galaxy , Sculptor Galaxy , and Messier 83 . Five planets can be recognized as planets from Earth with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn. Under typical dark sky conditions Uranus (magnitude +5.8) can be seen as well with averted vision, as can the asteroid Vesta at its brighter oppositions. Under perfect dark sky conditions Neptune may be visible to

1692-402: The naked eye as long as one knows the exact position in which to look. Historically, the zenith of naked-eye astronomy was the work of Tycho Brahe (1546–1601). He built an extensive observatory to make precise measurements of the heavens without any instruments for magnification. In 1610, Galileo Galilei pointed a telescope towards the sky. He immediately discovered the moons of Jupiter and

1739-482: The naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions , passing comets , meteor showers , and the brightest asteroids , including 4 Vesta . Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye. Some basic properties of the human eye are: Visual perception allows a person to gain much information about their surroundings: The visibility of astronomical objects

1786-463: The neurotransmitter glutamate . Unstimulated (in the dark), cyclic-nucleotide gated channels in the outer segment are open because cyclic GMP (cGMP) is bound to them. Hence, positively charged ions (namely sodium ions ) enter the photoreceptor, depolarizing it to about −40 mV ( resting potential in other nerve cells is usually −65 mV). This depolarization current is often known as dark current. The photoreceptors ( rods and cones ) transmit to

1833-492: The northern hemisphere, observing the Pole Star and using a protractor can give the observer's geographic latitude , up to 1 degree of accuracy. The Babylonians , Mayans , ancient Egyptians , ancient Indians , and Chinese measured all the basics of their respective time and calendar systems by naked eye: In a similar manner star occultations by the moon can be observed. By using a digital clock an accuracy of 0.2 second

1880-574: The patients continued to exhibit circadian photoentrainment, circadian behavioural patterns, melanopsin suppression, and pupil reactions, with peak spectral sensitivities to environmental and experimental light matching that for the melanopsin photopigment. Their brains could also associate vision with light of this frequency. Rod and cone photoreceptors are common to almost all vertebrates. The pineal and parapineal glands are photoreceptive in non-mammalian vertebrates, but not in mammals. Birds have photoactive cerebrospinal fluid (CSF)-contacting neurons within

1927-466: The peak wavelength of the S-cone's spectral sensitivity is approximately 420 nm (nanometers, a measure of wavelength), so it is more likely to absorb a photon at 420 nm than at any other wavelength. Light of a longer wavelength can also produce the same response from an S-cone, but it would have to be brighter to do so. In accordance with the principle of univariance , a photoreceptor's output signal

Navagraha - Misplaced Pages Continue

1974-399: The photoreceptor fate. CRX further defines the photoreceptor specific panel of genes being expressed. NRL expression leads to the rod fate. NR2E3 further restricts cells to the rod fate by repressing cone genes. RORbeta is needed for both rod and cone development. TRbeta2 mediates the M cone fate. If any of the previously mentioned factors' functions are ablated, the default photoreceptor

2021-400: The presence of melanopsin , a light-sensitive protein. Therefore they constitute a third class of photoreceptors, in addition to rod and cone cells . In humans the ipRGCs contribute to non-image-forming functions like circadian rhythms, behavior and pupillary light reflex . Peak spectral sensitivity of the receptor is between 460 and 482 nm. However, they may also contribute to

2068-413: The regulation of circadian rhythms , pupillary reflex and other non-visual responses to light. Melanopsin functionally resembles invertebrate opsins. Most vertebrate photoreceptors are located in the retina. The distribution of rods and cones (and classes thereof) in the retina is called the retinal mosaic . Each human retina has approximately 6 million cones and 120 million rods. At the "center" of

2115-406: The retina (the point directly behind the lens) lies the fovea (or fovea centralis), which contains only cone cells; and is the region capable of producing the highest visual acuity or highest resolution . Across the rest of the retina, rods and cones are intermingled. No photoreceptors are found at the blind spot , the area where ganglion cell fibers are collected into the optic nerve and leave

2162-427: The same basic structure. Closest to the visual field (and farthest from the brain) is the axon terminal, which releases a neurotransmitter called glutamate to bipolar cells . Farther back is the cell body , which contains the cell's organelles . Farther back still is the inner segment, a specialized part of the cell full of mitochondria . The chief function of the inner segment is to provide ATP (energy) for

2209-799: The same response to light. This complexity becomes both important and necessary for detecting color , contrast , edges , etc. Phototransduction in rods and cones is somewhat unusual in that the stimulus (in this case, light) reduces the cell's response or firing rate, different from most other sensory systems in which a stimulus increases the cell's response or firing rate. This difference has important functional consequences: Comparison of human rod and cone cells, from Eric Kandel et al. in Principles of Neural Science . The key events mediating rod versus S cone versus M cone differentiation are induced by several transcription factors, including RORbeta, OTX2, NRL, CRX, NR2E3 and TRbeta2. The S cone fate represents

#520479