troy. finally she went to the city of the nymphs ( astu numpheon ) on samos, where she lived many years, prophesying from the cave of the nymphs there [ probably the spiliani cave ]. later prophetesses were called sibyls ( sibullai ) after her. her name phoito reflects her parentage, for it is equipotent with audeessa ( speaking mortal speech ) and kouros ( lad ), that is, kirke and glaukos. she was called herophile ( beloved of hera ) on samos ( where the goddess is especially honored ) because that name is equal to gathelos aitherios ( ethereal ), and she lived for a thousand years because herophile aitheria is equipotent with aion ( aeon ). she was called deiphobe because that name is equal to skotia ( darkness ). she was known by this name as a priestess of both apollo and trioditis, a name for artemis as the threefold goddess of the road. this is all i will say about this sibulla for now. after leaving kirke, godlike glaukos came in human form to iason ( jason ) and used his arts to construct the ship argo ; he himself became an argonaut, in fact, the ship ' s first steersman. he traveled with iason and the other argonauts in quest of the golden fleece of kolkhis ( colchis ), into aia [ that is, gaia, the earth ], his family ' s origin, whose king, called aietes, was blood - brother of kirke and pasiphae and father of medeia ( medea ). ( he is a king of the underworld like aides [ hades ]. ) in the battle between iason and the turrhenoi ( etruscans ), glaukos protected himself by jumping into the sea and taking on his fish form ; he was the only argonaut to escape unscathed. having revealed himself as a sea lord, he subsequently helped the argonauts in many ways. all this took place in the generation before the trojan war. although he never forgot skulla, great - hearted glaukos had many wives. for example, he loved ariadne when she was on dia, but she and dionysos preferred each other, and glaukos


before the trojan war. although he never forgot skulla, great - hearted glaukos had many wives. for example, he loved ariadne when she was on dia, but she and dionysos preferred each other, and glaukos had to give her up. gathelos also went to egypt, where he used his craft and bravery to help the pharaoh to defeat the ethiopians. in gratitude the pharaoh offered him his daughter skotia ( darkness ) as wife, and they were married. they lived happily in egypt until the pharaoh [ perhaps akhenaten, 1367 - 1350 bce ] introduced a new religion that was hostile to the practices of druids ( druidon ). therefore, mighty gathelos took skotia his queen and a large number of followers to seek the land that had been prophesied for them. ( we know that anthedon gathelos was destined for skotia, because gathelos and skotia together equal anthedon. ) first they went to gotthia, where carthage was later built. then gathelos led them on to found a colony in galicia in spain, which is called brigantia ( near modern la coruna ) after the goddess brigintis ( brighid ), and from there they went to ierne ( ireland ). here gathelos was crowned king and his followers took the name skotioi ( dark ones, scots ) after the name of their queen. some say that the entire gaelic clan ( clan - na - gael ) is named for gaodhal glas ( gathelos glaukos ), who led them into these places. eventually the skotioi came to the land of hyperboreans, to caledonia in the northernmost parts of albion ( england ), where they settled. they founded the kingdom of skotia, also named for gathelos ' queen. in time, glaukos returned to live in the sea, where he was most at home, and spawned the race of glaukidai ( descendants of glaukos ), or hoi glaukoi ( the blue men ), who live in caves beneath the waves. although there are now glaukidai in many parts of the world, their largest number are in scotland, where glaukos went after leaving the mediterranean. the highlanders call them the " blue men " ( na fir ghorm ) because, like glaukos, the blue men have glossy bluish


the world, their largest number are in scotland, where glaukos went after leaving the mediterranean. the highlanders call them the " blue men " ( na fir ghorm ) because, like glaukos, the blue men have glossy bluish ( gorm ) skin, a long gray ( glas ) face with curly green hair and beard. their eyes tend to be small, their noses flat, and mouths large. their arms are long and their legs are like fish - tails. they are the size of a full - grown man, and very strong. some say that the glaukidai are gathelos ' descendants by kirke, for the plural glaukidai is equipotent with gathelos and kirke [ i. e. glaukidai is numerically equal to the sum of gaqhlos and kirkh ], which is confirmed by the singular glaukides ( descendant of glaukos ), which is equipotent with aiaie ( she who dwells in aiaia ) and antitheos ( godlike, an epithet of glaukos ), and also equipotent with doloessa leaina ( the lioness who speaks with mortals ), that is, kirke. people who have seen the blue men know that they are not the same as the selkies ( seal - people ), but they often cooperate and accompany each other. since most of the uneducated folk don ' t know the origin of the blue men, they say that they are fallen angels that were not so guilty as to be thrown into hell, or that they were people suffering under some punishment or spell. most of the blue men live in caves under the waters of the minch, the channel through the " charmed islands " ( na h - eileinean ) - lewis and the shiants - of the hebrides ; indeed this channel is called the stream of the blue men ( sruth nam fear gorm ). it is also called the " current of destruction, " because they say the blue men stir up the waves by their incessant swimming. sailors often observe blue men floating from the waist up in the water and know that storms often follow their appearance. the blue men may attack ships or sailors who have mistreated the selkies and other seafolk. sometimes an attack can be averted by engaging the chieftain of the band of blue men in a rhyming contest ; if the chieftain is sufficiently impressed by


may attack ships or sailors who have mistreated the selkies and other seafolk. sometimes an attack can be averted by engaging the chieftain of the band of blue men in a rhyming contest ; if the chieftain is sufficiently impressed by the captain ' s wits, he will leave the ship alone. although innocent sailors have little to fear, they nevertheless often sail around the shiant islands to avoid the blue men ' s stream. they sing : " oh, weary on the blue men, their anger and their wiles! the blue men can help mortals in many ways. therefore, during hallowtide ( the samhain season ), the people light a candle by the sea and pour a libation of new ale into the water, while they pray that the blue men will leave seaweed on the beach, which they use for fertilizer. after the ritual, the candle is carefully extinguished. the whole day long, the whole night long, they ' re splashing round the isles ; they ' ll follow every fisher - ah! they ' ll haunt the fisher ' s dream - where billows toss, oh, who would cross the blue men ' s stream? " like their great - hearted ancestor, the blue men have a lusty disposition, and often seek love from mortal women. ( there are blue women - na te ghorm, hai glaukai - as well, who are not so often seen, but are just as famous for their amorous inclinations. ) the dark - complexioned offspring of these unions often have webs between their fingers, which become horny crusts after the midwives cut them. by this evidence we know that there is considerable blue man blood flowing in highland veins. in fourteenth - century kintail ( ross - shire, scotland ), hugh fraser, later the laird of lovat, had a daughter kerling, who was very attractive, and independent in thought as well as action. she was a devotee of the goddess dana ( danu ) of the three ways, which was their name for artemis, and was well - schooled in magic. for her lover she had chosen her father ' s barn - man ( skallag ), who was very handsome, but she knew she would not be permitted to marry below her station. so she disguised her love and bided her time, until she was able to leave home, at which time she boldly told this barn - man her intentions, and married him, much to the dismay of her


permitted to marry below her station. so she disguised her love and bided her time, until she was able to leave home, at which time she boldly told this barn - man her intentions, and married him, much to the dismay of her relatives and the gentry. however, this man had married her for her wealth and didn ' t love her, so she found little satisfaction in her husband and decided to find herself a leannan sith ( fairy lover ). early one morning kerling went down to the shore by the sound of sleat and climbed a rock at the high - tide mark. then she shed exactly seven tears into the sea and sang : hear me, blue man, hear my call! when the first light of dawn turned the waters gray, she saw an attractive blue man emerge from the waves and swim toward her. when he reached her rock, he asked, " what is thy will with me, fair dame? " and she replied that she wanted a lover. he was agreeable and told her that he would come at the seventh stream ( spring tide ), which is the time when blue men can take on human form and live above the waves. seven tears i have let fall. salty ocean, salty tears : come to me, i have no fears! stormy ocean, stormy hearts : we shall play our destined parts! he came that first time and many times thereafter, and often they were seen together after their trysts, so he became known to the townspeople, who called him gille gorm ( blue lad, also gilligorm etc. ) from his complexion, which was not entirely hidden by his human form. nevertheless, he was considered very handsome ; oral tradition described gille - gorm : he was the tallest of them all, many in our clan, from that day to this, have denied this liaison, which they consider shameful, but the truth has never been suppressed completely. eventually kerling had a child from gille - gorm, and as a result even today a few clan members have the blood of the blue boy in their veins. her father hugh, lord lovat, never forgave her. the broadest in the shoulders too, with feet as small as they could be. ( gillegorm ' s ancestry is shown by his name in greek glaukokouros - blue - lad, which is equipotent with leaina - the lioness, that is, kirke - and megaletor - the great - hearted one, that


ancestry is shown by his name in greek glaukokouros - blue - lad, which is equipotent with leaina - the lioness, that is, kirke - and megaletor - the great - hearted one, that is, glaukos. ) during their years together, gille - gorm taught kerling the glaukou tekhne ( art of the blue man ), which he had learned from his ancestors, and she taught him the arts she had learned as a priestess of dana. eilean donnan castle ( on st. donnan island ) had been built in 1266, under order of king alexander iii, by a son who had been given the second sight by his father, the chief on the island, for he gave his son his first drink out of a raven ' s skull. this castle became a key factor in the conflict between the lord of the isles and the earl of ross. acting under orders from thomas randolph, earl of moray and guardian of the kingdom, earl hugh of ross, captured the castle and later apprehended coinneach ( kenneth ) na sroine, the constable of the castle, imprisoning him at inverness. therefore the allied clans chose gille - gorm to be constable of eilean donnan castle and thus over chief of the allies of the west. later, while william, sixth earl of ross, was away, gille - gorm led his troop of " mountain men " on a diversionary invasion. first they sacked and looted the territory around tain and chanonry, then they proceeded southeast to the muir that lies between kessock ferry and munlochy bay. they said they would burn inverness to the ground unless the provost would free coinneach and pay a ransom. the provost said he would consider their demands and sent barrels of whisky to gille - gorm ' s soldiers as a show of good faith. in the meantime, kerling ' s vengeful father, hugh fraser, who was the crown ' s lieutenant in the north, and his vassals joined the troops of earl william of ross, coming from inverness, at kessock ferry. in the morning they attacked at drumderfit ( blair na coi ), where gille - gorm ' s band was camped. under this onslaught by much superior numbers ( and, some say, because of the effects of the whiskey! ), gille - gorm ' s troops were forced to retreat


as she did the incantation called am mothan [ the mothan ] : and protect thee coming or departing ; by thy heart be mapon macc matrona ; may the lady brigit flow upon thee - o may wise brigintis always bathe thee! now i pick the sacred mothan, she sewed the herb into crotair ' s vest under his left arm, in accord with her craft and thus protected him in his many travels. as did mapon macc matrona, in the holy name of dagda, and of brigit, lugh and danu. though in field of furious fighting, where no bounds are known to anger, be thou happiness and comfort, mighty morrighan ' s protection! crotair was schooled at beauly priory where he took orders. later the hunchback of beauly priory returned to kintail where he was pastor of kilichoinen in glenelg and of kilmore in sleat. further, because crotair was of the celtic church he took no vow of celibacy and so he fathered a son, who was called gille fhinnein [ servant of st. finnan ]. and so was born clan mac gill ' innein ( the sons of the servant of st. finnan ). ( by naming themselves after this saint - a common practice in the 14th century - they hoped to disguise their descent from gille - gorm, the blue man ; other descendants kept the name macgilligorm. ) chief gille - gorm was buried under a large stone cairn, which was visible as late as 1785 ( the largest among the cairns of drumderfit raised to the slain ). a wooden idol of the chief was erected near the house of drumderfit, the estate of some of his descendants, the lobans of drumderfit in easter ross ; it perished when a band of munros and sutherlands destroyed the estate during the jacobite uprising of 1715. now we say : bonnie boy blue, come blow your horn, the wolf ' s in the meadow and prowls through the corn. but where is the brave boy who guards his sheep? he ' s under a hillock, fast asleep. will you awake him? no, not i, for if i do, the wolf will die! send comments about this page


mit professor ’ s book digs into the eclectic, textually linked reading choices of people in medieval london. imagine a future in which wireless power transfer is feasible : cell phones, household robots, mp3 players, laptop computers and other portable electronics capable of charging themselves without ever being plugged in, freeing us from that final, ubiquitous power wire. some of these devices might not even need their bulky batteries to operate. a team from mit ' s department of physics, department of electrical engineering and computer science, and institute for soldier nanotechnologies ( isn ) has experimentally demonstrated an important step toward accomplishing this vision of the future. the team members are andre kurs, aristeidis karalis, robert moffatt, prof. peter fisher, and prof. john joannopoulos ( francis wright davis chair and director of isn ), led by prof. marin soljacic. realizing their recent theoretical prediction, they were able to light a 60w light bulb from a power source seven feet ( more than two meters ) away ; there was no physical connection between the source and the appliance. the mit team refers to its concept as " witricity " ( as in wireless electricity ). the work will be reported in the june 7 issue of science express, the advance online publication of the journal science. the story starts one late night a few years ago, with soljacic ( pronounced soul - ya - cheech ) standing in his pajamas, staring at his cell phone on the kitchen counter. " it was probably the sixth time that month that i was awakened by my cell phone beeping to let me know that i had forgotten to charge it. it occurred to me that it would be so great if the thing took care of its own charging. " to make this possible, one would have to have a way to transmit power wirelessly, so soljacic started thinking about which physical phenomena could help make this wish a reality. various methods of transmitting power wirelessly have been known for centuries. perhaps the best known example is electromagnetic radiation, such as radio waves. while such radiation is excellent for wireless transmission of information, it is not feasible to use it for power transmission. since radiation spreads in all directions, a vast majority of power would end up being wasted into free space. one can envision using directed electromagnetic radiation, such as lasers, but this is not very practical and can even be dangerous. it requires an uninterrupted line of sight between the source and the device, as well as a sophisticated tracking


. one can envision using directed electromagnetic radiation, such as lasers, but this is not very practical and can even be dangerous. it requires an uninterrupted line of sight between the source and the device, as well as a sophisticated tracking mechanism when the device is mobile. the key : magnetically coupled resonance in contrast, witricity is based on using coupled resonant objects. two resonant objects of the same resonant frequency tend to exchange energy efficiently, while interacting weakly with extraneous off - resonant objects. a child on a swing is a good example of this. a swing is a type of mechanical resonance, so only when the child pumps her legs at the natural frequency of the swing is she able to impart substantial energy. another example involves acoustic resonances : imagine a room with 100 identical wine glasses, each filled with wine up to a different level, so they all have different resonant frequencies. if an opera singer sings a sufficiently loud single note inside the room, a glass of the corresponding frequency might accumulate sufficient energy to even explode, while not influencing the other glasses. in any system of coupled resonators there often exists a so - called " strongly coupled " regime of operation. if one ensures to operate in that regime in a given system, the energy transfer can be very efficient. while these considerations are universal, applying to all kinds of resonances ( e. g., acoustic, mechanical, electromagnetic, etc. ), the mit team focused on one particular type : magnetically coupled resonators. the team explored a system of two electromagnetic resonators coupled mostly through their magnetic fields ; they were able to identify the strongly coupled regime in this system, even when the distance between them was several times larger than the sizes of the resonant objects. this way, efficient power transfer was enabled. magnetic coupling is particularly suitable for everyday applications because most common materials interact only very weakly with magnetic fields, so interactions with extraneous environmental objects are suppressed even further. " the fact that magnetic fields interact so weakly with biological organisms is also important for safety considerations, " kurs, a graduate student in physics, points out. the investigated design consists of two copper coils, each a self - resonant system. one of the coils, attached to the power source, is the sending unit. instead of irradiating the environment with electromagnetic waves, it fills the space around it with a non - radiative magnetic field oscillating at mhz frequencies.


of the coils, attached to the power source, is the sending unit. instead of irradiating the environment with electromagnetic waves, it fills the space around it with a non - radiative magnetic field oscillating at mhz frequencies. the non - radiative field mediates the power exchange with the other coil ( the receiving unit ), which is specially designed to resonate with the field. the resonant nature of the process ensures the strong interaction between the sending unit and the receiving unit, while the interaction with the rest of the environment is weak. moffatt, an mit undergraduate in physics, explains : " the crucial advantage of using the non - radiative field lies in the fact that most of the power not picked up by the receiving coil remains bound to the vicinity of the sending unit, instead of being radiated into the environment and lost. " with such a design, power transfer has a limited range, and the range would be shorter for smaller - size receivers. still, for laptop - sized coils, power levels more than sufficient to run a laptop can be transferred over room - sized distances nearly omni - directionally and efficiently, irrespective of the geometry of the surrounding space, even when environmental objects completely obstruct the line - of - sight between the two coils. fisher points out : " as long as the laptop is in a room equipped with a source of such wireless power, it would charge automatically, without having to be plugged in. in fact, it would not even need a battery to operate inside of such a room. " in the long run, this could reduce our society ' s dependence on batteries, which are currently heavy and expensive. at first glance, such a power transfer is reminiscent of relatively commonplace magnetic induction, such as is used in power transformers, which contain coils that transmit power to each other over very short distances. an electric current running in a sending coil induces another current in a receiving coil. the two coils are very close, but they do not touch. however, this behavior changes dramatically when the distance between the coils is increased. as karalis, a graduate student in electrical engineering and computer science, points out, " here is where the magic of the resonant coupling comes about. the usual non - resonant magnetic induction would be almost 1 million times less efficient in this particular system. " old physics, new demand witricity is rooted in such well - known laws of physics that it makes one wonder why


comes about. the usual non - resonant magnetic induction would be almost 1 million times less efficient in this particular system. " old physics, new demand witricity is rooted in such well - known laws of physics that it makes one wonder why no one thought of it before. " in the past, there was no great demand for such a system, so people did not have a strong motivation to look into it, " points out joannopoulos, adding, " over the past several years, portable electronic devices, such as laptops, cell phones, ipods and even household robots have become widespread, all of which require batteries that need to be recharged often. " as for what the future holds, soljacic adds, " once, when my son was about three years old, we visited his grandparents ' house. they had a 20 - year - old phone and my son picked up the handset, asking, ' dad, why is this phone attached with a cord to the wall? ' that is the mindset of a child growing up in a wireless world. my best response was, ' it is strange and awkward, isn ' t it? hopefully, we will be getting rid of some more wires, and also batteries, soon. ' " this work was funded by the army research office ( institute for soldier nanotechnologies ), national science foundation ( center for materials science and engineering ), and the department of energy.


the james webb space telescope has to be kept at bitterly cold temperatures for its sensitive instruments to work. each step of the way, engineers must counter webb ' s ordinary, everyday heat production with a method of dropping the temperature ever lower why an infrared telescope? the webb telescope sees in infrared light, a wavelength that warm objects emit in large amounts. we look for infrared light for several reasons. first, the light from the most distant objects in the universe — objects that existed just 200 million years after the big bang — has been stretched as it travels through the expanding universe. the stretching changes the light, moving it from the visible range into invisible infrared. second, infrared light can pierce the dust that blocks visible light emissions, allowing us to peer through clouds of dust to the warm objects within. finally, warm objects that don ' t emit their own visible light, such as planets, do emit infrared, making them viewable with the right tools. but infrared light also comes with an inherent complication — it ' s emitted by everything made up of atoms. when the atoms in an object collide or vibrate, their electrons are bumped up to a higher energy level. as those electrons return to their normal energy level, they give off radiation. much of this radiation escapes as infrared. warm objects, which have a great number of excited electrons, emit lots of infrared. the more infrared there is nearby, the harder it is to pick out distant sources of infrared. if webb were on earth, it would be constantly awash in the planet ' s own infrared haze. we avoid that by deploying the telescope in space. but even that doesn ' t solve the problem entirely, because webb itself — being made, after all, of atoms — will emit infrared radiation. the solution : keep the webb telescope as cold as possible, so the infrared produced by its own excited atoms doesn ' t overwhelm the distant emissions it ' s designed to detect. back to top webb ' s distant orbit performs two functions. it places the telescope in a frigid environment, and makes it possible to position the telescope so it always has the sun, earth and moon on the same side, behind its sunshield. the second sun - earth lagrange point, one of five points in the solar system where gravitational forces allow an object to remain in a fixed position relative to earth, is 940, 000 miles away from earth. as the earth orbits the sun, its gravity will drag webb along behind it, keeping it in this


in the solar system where gravitational forces allow an object to remain in a fixed position relative to earth, is 940, 000 miles away from earth. as the earth orbits the sun, its gravity will drag webb along behind it, keeping it in this optimal location. it will take a full month for webb to reach its new home beyond the moon. back to top the need to keep webb cold affects its design dramatically. for instance, the telescope ' s odd structure is the result of it not being enclosed in a tube. telescopes are usually enclosed in a tube or a dome to block out nearby, unwanted light and radiation. but webb needs to be open to space in order to keep itself cool enough for its infrared detectors to work properly. a tube would both radiate infrared and trap too much heat inside the telescope, while the open design allows the heat to pour harmlessly off into space. so rather than use a tube, webb stops unwanted visible and infrared light with its giant sunshield and “ baffles, ” small, dark barriers that block light at strategically placed points. the open design is the only way to cool webb to the right temperatures and still maintain its large size. if the webb telescope relied mainly on coolant like other infrared telescopes, it would need many tons of coolant and its life would be significantly shorter, for once the coolant ran out the telescope would be unusable. back to top divide and conquer webb has two sections — a hot side and a cold side, divided by its sunshield. on the hot side of the sunshield, the region exposed to sunlight, parts of webb will reach temperatures near boiling, as high as 358 kelvin ( 185 degrees fahrenheit, or 85 degrees celsius ). this is where webb ' s ambient - temperature equipment, like its solar panel, antennae, computer, gyroscopes and navigational jets are kept. the warm side of the telescope is, overall, where the electronics and navigational system resides. on the cold side, webb will be about 40 kelvin ( - 388 degrees fahrenheit, - 233 degrees celsius ). in contrast, the coldest temperature ever recorded on earth, at the russian vostok station in antarctica, was minus - 129 degrees fahrenheit — far too toasty for webb ' s needs. the cold side of the sunshield is where the science happens, and it contains the parts of webb most sensitive to infrared radiation : its microshutter array


- 129 degrees fahrenheit — far too toasty for webb ' s needs. the cold side of the sunshield is where the science happens, and it contains the parts of webb most sensitive to infrared radiation : its microshutter array, mirror and mirror actuators, filter wheels, and, of course, the infrared detectors. back to top webb ' s mirror will reach about 40 - 50 kelvin ( - 390 degrees fahrenheit, - 230 degrees celsius ) once it ' s deployed. the mirror is made of polished beryllium, a metal that has the benefit of contracting less in extreme cold than the glass that typically makes up a telescope mirror. a mirror that contracts dramatically produces a deformed image. engineers design the webb mirror by running computer models that show exactly how the beryllium will deform as it cools. the engineers then design the mirrors so that when they deform, they deform into the right shape, and test and test until they get them perfect. without the chill of space, these incorrectly shaped mirrors would be useless to the telescope. back to top chilling the instruments but even more dramatic cooling goes on around the camera and spectrograph in the mid - infrared instrument ( miri ). miri sees farther into the infrared than the other instruments, and thus has to be kept spectacularly cold. webb boasts a two - stage cryocooler that works like the world ' s most effective refrigerator, pumping a warmth - absorbing gas through the instrument. the first stage brings miri ' s temperature down to 18 kelvin, and the second stage brings the miri detectors to 6 kelvin — that ' s just six degrees above absolute zero, the theoretical temperature at which all motion freezes, even the movement of atoms. the infrared detectors in webb ' s other instruments need temperatures of about 40 kelvin ( - 375 degrees fahrenheit or - 233. 3 degrees celsius ) to operate correctly. the detectors themselves give off heat when they are in use, so operators will " read " the detectors continuously rather than periodically, to always keep them at a stable temperature. this allows them to tell the difference between the internal infrared of the detectors and the infrared of distant objects. back to top webb is protected from the heat and light of the sun, moon and earth by its huge sunshield. the sunshield, large enough for the telescope to remain protected even as it tilts in different directions, consists of five layers, each about the thickness


from the heat and light of the sun, moon and earth by its huge sunshield. the sunshield, large enough for the telescope to remain protected even as it tilts in different directions, consists of five layers, each about the thickness of plastic wrap. the layers do not touch, because that would allow for the transfer of heat from one layer to the next, and coatings of aluminum help further cut down on warmth transmission. the layers are made of a material called kapton, also used in spacesuits and circuitry, which has the convenient properties of being heat resistant and strong. the sunshield is also coated on the sunward side with silicon, which reflects and radiates away the incoming sunlight. because the telescope is so far away from earth, it lacks the protection of our planet ' s magnetic field. this means it will be bombarded by high - energy cosmic rays, which can interfere with signals or even build up enough of an electrical charge to cause small lightning strikes on the telescope. the sunshield ' s coatings of aluminum and a grid of conducting strips on the sunshield layers control the electricity and prevent voltage arcs from damaging the telescope. with heat banished, pushed away and driven out to the fullest extent possible, webb will wait in its silent, chilly expanse of space for the faint, distant signals from the early universe. as it gazes into the darkness with cold clarity, the infrared universe will reveal itself to us, its invisible light weaving images that human eyes will see for the first time. back to top


ido perlman, helga kolb and ralph nelson horizontal cells are the interneurons of distal vertebrate retina. they provide the pathways for both local and long range interactions between photoreceptors. these interactions are called feedback signals. feedback signals adjust the gain of photoreceptor synaptic output, both as seen in the horizontal cells themselves and in the adjacent, proximally projecting bipolar cells. in forming a distal, lateral synaptic network, horizontal cells enrich not only their own physiology but that of presynaptic and postsynaptic partners. they generate spatial opponency in both cones and bipolar cells. they also generate color opponency in cones, bipolar cells, and chromatic horizontal - cell subtypes. the actions of horizontal cells propagate forward in the retina, can be identified in the physiology of retinal ganglion cells, and in visual perception. horizontal cell actions are in turn tuned by neuromodulators released by other retinal neurons. the first intracellular, light - evoked responses recorded in the vertebrate retina were slow, negative going changes in membrane potential that lasted for as long as the light stimulus was present ( fig. 1 ). before the morphological source of these responses became clear, they were called simply s - potentials ( 1 ). as shown in figure 1, s - potentials are membrane hyperpolarizations that last for as long as the light stimulus is present. the graded character of the s - potential is evident in figure 1a. the brighter the stimulus, the larger the amplitude of the s - potential until a saturation level is reached. in figure 1b, the duration of a light stimulus of fixed intensity is altered in order to examine the temporal properties of s - potentials. for long stimuli, the s - potential only changes in duration but the amplitude remains constant ( 2 leftmost responses in fig. 1b ). further reduction in the stimulus duration causes a decrease in amplitude ( fig. 1b ). this illustrates the temporal summation of the s - potential, following bloch ’ s law. up to certain stimulus durations, in this case about 10 msec, the amplitude is directly related to the quantal content of the stimulus ( quantal flux multiplied by duration ), while for stimuli of longer duration, the amplitude is related to the quantal flux, that is, the rate of quantal absorption by photoreceptors ( 1 ). in s -


the stimulus ( quantal flux multiplied by duration ), while for stimuli of longer duration, the amplitude is related to the quantal flux, that is, the rate of quantal absorption by photoreceptors ( 1 ). in s - potentials this ‘ critical duration ’ varies considerably with stimulus conditions ( 2 ). it is likely that s - potentials were named in honor of their discoverer, gunnar svaetichin, although ‘ slow potential ’ is another common interpretation. figure 1. the s - potential of the fish retina. light stimuli of fixed duration and different irradiances ( a ) and light stimuli of fixed irradiance but different durations ( b ) were used to elicit these potentials. lower traces in a and b record light - stimulus duration and provide 100 msec ‘ tick - marks ’. the illustrations are from svaetichin, 1953 ( 1 ). s - potentials puzzled neurophysiologists of the 1950s when they were first described. at that time, neurons were thought only to be depolarized by synaptic inputs ( inside becoming more positive relative to outside ), thus having their inside - negative resting membrane potentials become reduced. this depolarization was thought of as excitation, and if the excitation was large enough, action potentials, or nerve spikes, were generated to transfer signals down the length of the nerve - cell axon. s - potentials, however, were not depolarized by light, but rather hyperpolarized, and did not fire action potentials, even with the brightest light stimuli. at first, the cell type of origin for s - potentials was not really known other than that microelectrode tips were somewhere in the outer retina. in fact, initially svaetichin thought s - potentials arose from cones ( 1 ). however, later intracellular marking techniques, in which dyes were injected from the electrode tips into the cytoplasm of the recorded neuron, revealed that horizontal cells, second order neurons postsynaptic to cones, were the source of the s - potentials ( 3, 4 ). since being first described in fish retinas, s - potentials have been recorded from retinal horizontal cells in all vertebrate classes. these include both cold - blooded vertebrates ( 4 - 13 ), household mammals ( 14 - 20 ) and primates ( 21 - 26 ). horizontal cells have now been studied


from retinal horizontal cells in all vertebrate classes. these include both cold - blooded vertebrates ( 4 - 13 ), household mammals ( 14 - 20 ) and primates ( 21 - 26 ). horizontal cells have now been studied by numerous investigators using anatomical, biochemical, pharmacological and electrophysiological techniques. in this chapter, we shall try to summarize our current knowledge of horizontal cells in the vertebrate retina. gunnar svaetichin ( 1915 - 1981 ) mastered high impedance microelectrode techniques while studying neurophysiology in the karolinska institute with ragnar granit ( nobel prize, 1967 ). these enabled intracellular recording of light responses from retinal interneurons, a field that he founded. it was an imaginative area to pursue in the 1940 ’ s. he is the discoverer of s - potentials, now known to be retinal horizontal cell responses, and discovered both color opponent and non opponent types. the discovery of neural color opponency ranks with the most significant findings in color vision in the 20th century ( 292 ). the later decades of his career were spent at the venezuelan institute for scientific research. 3. morphology and circuitry. horizontal cells are second order neurons contacting photoreceptors directly all across the plane of the retina ’ s outer plexiform layer. these cell types were first described as huge brick - like structures occupying much of the inner nuclear layer in the fish retina ( 27, 28 ). early interpretations assumed these cells to be glial cells, mainly because they gave rise to hyperpolarizing slow responses to light ( s - potentials ) rather than true depolarizing spikes like ‘ real neurons ’. now we know horizontal cells to be true neurons that make true, if unique, synapses and exhibit most of the structural and ultrastructural characteristics of neurons. through golgi staining, intracellular marking and immunostaining techniques we have learned that horizontal cells can adapt various morphologies in the retinas of different vertebrate species ( 6, 29 - 44 ). selective rod and cone contacts of horizontal cells there are two morphological types of horizontal cell in the majority of terrestrial vertebrate retinas, b - type cells with axons and a - type cells that are axonless ( fig. 2a ). in mammals the dendrites of both b - and a - types contact cones,


majority of terrestrial vertebrate retinas, b - type cells with axons and a - type cells that are axonless ( fig. 2a ). in mammals the dendrites of both b - and a - types contact cones, while the axon terminals of b - type cells contact rods ( gray area, fig. 1a ). in birds and reptiles, horizontal cells come in further axonless subtypes with color specific cone connections ( fig. 2a, axonless ). figure 2a shows the morphologies of these different types of horizontal cells in different species as indicated. also shown are the spectral types of cone ( red, blue and green dots ) that connect with each type. figure 2a. the morphologies of different types of horizontal cell and their known connectivities with rod and cone photoreceptors. the cells are drawn from golgi stained or neurobiotin filled retinal cells and viewed in flat whole - mounted preparations. b - type cells with axons, and a - type axonless cells exist in most vertebrate retinas. the physiological type that is associated with each morphological type is indicated to the right of the figure, i. e luminosity, blue - yellow and color - opponent types. red -, green - and blue - dots represent inputs from red -, green -, and blue cones respectively. black squares or shaded gray areas represent rod inputs. the turtle h1 cell is from ammermuller and kolb, 1995 ( 84 ) ; h2 - h4 are from kolb et al., 1988 ( 293 ). the cat b - type is from nelson et al., 1975 ( 17 ) ; the a - type is from kolb, 1974 ( 294 ). primate h1 and h2 types are from helga kolb not elsewhere published. the horse axonless horizontal cell is from sandmann et al., 1996 ( 40 ). in fish retinas, short - axon or axonless horizontal cells ( type h4 ) contact only rods ( 45, 46 ), while the dendrites of all axon bearing cells contact only cones ( fig. 2b ). the axons of fish horizontal cells do not contact photoreceptors. there are multiple types of axon bearing, cone - contacting, horizontal cells in fish ( h1, h2, h3 ), each with different patterns of cone selectivity. in the zebrafish example (


photoreceptors. there are multiple types of axon bearing, cone - contacting, horizontal cells in fish ( h1, h2, h3 ), each with different patterns of cone selectivity. in the zebrafish example ( fig. 2b ), most are excited by 2 of the 4 cone types and avoid excitatory input from other types. the exception is the h3 uv triphasic type that appears to be selectively excited by either uv or blue cones. h2 and h3 types are color opponent in wavelength physiology, but h1 is not. figure 2b. cone contacts of h1, h2 and h3 horizontal cell types in zebrafish retina ( a, b, c respectively ). these neurons were back - labeled with dii through crystals placed on their axons. dendritic tips and spines contact cones, sometimes in clusters called ‘ rosettes ’. d. zebrafish cones form a regular mosaic of red ( r ), green ( g ), blue ( b ) and ultraviolet ( uv ) types. the position of horizontal - cell contacts within the mosaic identifies the cone color type that is contacted. e. summary of cone contacts for h1, h2 and h3 types. black arrows are contacts proposed for feed - forward excitation, while gray arrows are contacts proposed for feedback inhibition. except for the uv triphasic h3 cell, all horizontal cells are excited by 2 cone types. the figure is from li, et al, 2009 ( 37 ), with physiological identifications based on connaughton and nelson, 2010 ( 6 ). mammalian and reptilian b - type horizontal cells code changes in brightness but do not discriminate color. they regulate adaptational and spatial responses of cones and bipolar cells. these are luminosity type horizontal cells whose physiology will be discussed later. these b - type cells can often be identified with antibodies against calcium binding proteins ( 47 - 49 ) and some contain the neurotransmitter gaba ( 47, 50, 51 ). in turtle retina, acetylcholine has been located in h1 type horizontal cell dendrites where they invaginate the cone pedicles ( 47 ). h1 type cells of turtles are morphologically analogous to mammalian b - type cells. the dendrites of b - type horizontal cells in mammalian retinas are bushy and contact all cones within the dendritic field. the axon emerges from one of these den


turtles are morphologically analogous to mammalian b - type cells. the dendrites of b - type horizontal cells in mammalian retinas are bushy and contact all cones within the dendritic field. the axon emerges from one of these dendrites. at its distal end, an axon terminal sprouts to collect signals from large numbers of rods at some distance from the cone - contacting dendritic field. the length and thinness of the axon is believed to electrically isolate one portion of the cell from the other, thereby, separating a cone - photoreceptor - involved compartment of the cell from a rod - photoreceptor - involved compartment ( 17 ). in species where there are few rods ( fig. 2a, turtle ) the b - type axon terminal makes sparse contacts with both red cones and the few rods present in this retina ( 36, 52 ). nonetheless soma - dendritic and axon terminal compartments maintain physiological separation, as evidenced by different summation areas ( 53 ). the type i horizontal cell of pigeon retina is axon bearing and also appears analogous to mammalian b - type cells. similar to turtle, the axon terminal contacts a mixture of rods and cones, while the dendrites contact cones ( 35 ). a - type horizontal cells in mammals are purely cone connecting ( most species have only green and blue cones ). they have no connections with rod photoreceptors at all. the h2 cell of the trichromatic primate retina ( humans and old world monkeys ) is a kind of a - type cell. even the h2 cell, where one or more dendrites are elongated and axon - like, contacts only cones. interestingly enough, these longer dendrites contact principally blue cones. this may be a general theme for mammalian axonless horizontal cells that is only now being realized. in certain big cats ( not yet confirmed in domestic cat ), the elongated dendritic tips of a - type cells contact blue cones, while the dendrites closer to the cell body are in contact with both red cones and blue cones ( 54 ). in horses the a - type cell is apparently only connected to blue cones ( fig. 2a ) ( 38, 40 ). in reptiles, and birds with well developed color vision, typically there are two axonless horizontal cell types. each of these connects selectively with a cone type. in turtles, birds, and fish, species with excellent color vision, type


). in reptiles, and birds with well developed color vision, typically there are two axonless horizontal cell types. each of these connects selectively with a cone type. in turtles, birds, and fish, species with excellent color vision, type h2 and type h3 horizontal cells are large, stellate cells. in birds and turtles these cells lack axons, but in fish h2 and h3 are axon - bearing. these cells contact cones selectively and sense color ( fig. 2a, axonless, color - opponent types of birds and reptiles ; fig. 2b, h2 and h3 axon bearing types of fish ). h2 and h3 respond to some wavelengths with hyperpolarization, and to other wavelengths with depolarization. thus, in turtle ( pentachromats ) and fish ( trichromats or tetrachromats ), h2 horizontal cells connect to green and blue cones and h3 to blue cones ( fig. 2a, fig. 2b ), or to blue cones and ultraviolet cones ( 6, 7, 36, 37, 41, 43, 52, 55, 56 ). in the turtle and zebrafish, ultraviolet - sensitive cones appear to connect only with h3 type horizontal cells, sharing this type with blue cones. some chromatic horizontal cells can be recognized in immunohistochemistry. the c - type horizontal cells of the turtle can be labeled with antibodies to nitric oxide synthase and calcium binding proteins ( 47 ) ; the former suggests a role for no in their functioning ( see section on neuromodulation ). horizontal cells always interact with photoreceptor terminals at what are known as ‘ lateral elements ’ post - synaptic to photoreceptor ribbons ( fig. 3a, le, arrow ). in fish retinas, the horizontal cell connections with cones at the lateral elements are characterized by minute projections called spinules ( fig. 3b ). spinules are dynamic and change shape with the level of illumination. spinule formation is stimulated by background illumination, while darkness causes retraction ( 57, 58 ). these plastic spinules are known to contain the calcium binding proteins caldendrin and camkii ( 59 ). spinules are under an endogenous circadian control but need centrifugal control from the brain via fmr - amide - like and gnrh - like hormone releasing hormones ( 60, 61 ) acting upon the dopa


). spinules are under an endogenous circadian control but need centrifugal control from the brain via fmr - amide - like and gnrh - like hormone releasing hormones ( 60, 61 ) acting upon the dopaminergic interplexiform cell system in the retina ( 62, 63 ). these spinules appear also to be under the local control of the visual - cycle byproduct retinoic acid ( 64, 65 ). figure 3. a ) electron micrograph of cat outer plexiform layer ( opl ) shows an a - type horizontal cell dendritic terminal entering an overlying cone pedicle to form a lateral element ( le ) at a ribbon triad. b ) light adapted fish retina exhibits spinules ( spin ) on horizontal cell lateral elements in cone pedicles at synaptic ribbon ( sr ) synapses. c ) high magnification electron micrograph of a gap junction between two a - type horizontal cell dendrites in the cat retina. the latest information concerning gap junction connexins in zebrafish retina shows that d ) cx 55. 5 forms a hemi - gap junction between hc dendrites and cone pedicles at the ribbon synapse ( arrow ), while e ) complete gap junctions between hc dendrites are made by cx 52. 6. the illustrations in a and c are from kolb ( 1977 ) ( 103 ). the illustration in b is from from dejuan and garcia, ( 2001 ) ( 62 ). both d and e are taken from shields et al., ( 2007 ) ( 72 ). gap - junction contacts and connexins retinal horizontal cells in all species are interconnected to homologous neighbors, by areas of gap junctions. these occur between dendrites, between axon terminals, and occasionally between dendrites and cell bodies. gap junctions are very selective, occurring only between cells of the same type ( that is h1 to h1 etc ). this exclusivity even extends to axon terminal regions and dendritic regions of b - type horizontal cells, that is, gap junctions connect dendrite to dendrite, and axon terminal to axon terminal, but not axon terminal to dendrite ( 66, 67 ). the gap junctions can be extremely large, appearing as plaques ( fig. 3c ). first described in fish retinas by yamada and ishi


axon terminal, but not axon terminal to dendrite ( 66, 67 ). the gap junctions can be extremely large, appearing as plaques ( fig. 3c ). first described in fish retinas by yamada and ishikawa ( 28 ), gap junctions were considered “ fused membrane structures ” specialized for electrical transmission. procion yellow dye diffusion between morphologically similar horizontal cells was noted by kaneko ( 66 ) and indicated that the electrical junctions could pass small molecules freely through the so - called s - space of horizontal cells ( 10 ). gap junctions are formed at closely opposed plasma membranes ( 2 - 4 nm gap ) from two horizontal cell processes ( fig. 3c ). each of the interacting cells supplies connexons or hemichannels to complete the gap junction contact. these channels can consist of homomeric or heteromeric connexin subunits, thus, potentially allowing a huge variety of gap junctions with slightly different properties to exist between various neurons. each connexon channel contains six connexins ( cx ) surrounding the channel pore. in fish and turtle retinas cx43 and cx26 were originally thought to comprise the horizontal cell gap junction connexins ( 68, 69 ). however more recently connexin 57 has been demonstrated to be expressed at the protein level and to mediate dendritic gap junctions between mouse b - type horizontal cells. the number of junctions appears to increase with light adaptation ( 70 ). in a study on rabbit retina where, unlike mouse, both a - and b - type horizontal cells occur, o ’ brien and colleagues have found that cx50 is the connexin between a - type horizontal - cell dendrites ( 71 ). in zebrafish multiple connexins are expressed, in particular cx52. 6 that participates in dendro - dendritic junctions ( fig. 3e ) and cx55. 5 that forms hemi - gap - junction channels with cones at lateral elements ( fig. 3d ) ( 72 ). in zebrafish with cx55. 5 knockout, horizontal - cell color opponency is reduced ( 73 ). this connexin is important for feedback modulation of cone synapses. in addition to cx55. 5, another connexin ( 52. 9 ) is also expressed in zebrafish horizontal cells. it is found both in lateral elements where it may participate in feedback, and in gap junctions between horizontal cell dendrites within the


cx55. 5, another connexin ( 52. 9 ) is also expressed in zebrafish horizontal cells. it is found both in lateral elements where it may participate in feedback, and in gap junctions between horizontal cell dendrites within the outer plexiform layer ( 73 ). horizontal cell gap junctions pass not only ions but small cytoplasmic molecules and tracer dyes of less than 500 dalton size ( 74 ). gap junctions enlarge horizontal cell receptive fields ( see section on gap junction physiology ), and are modulated by retinal neurotransmitters or humoral factors ( see section on neuromodulation ). 4. physiological types. the vertebrate retina contains a mosaic of rod - and cone - photoreceptors that serve for dark - and light - adapted vision respectively. multiple cone types, each selectively expressing a visual pigment tuned to a specific region of the visible spectrum, provide vertebrates with the opportunity to discriminate colors. cone types are defined by the spectral preference of their visual pigment ; e. g. short - wavelength ( sw ) cones, medium - wavelength ( mw ) cones and long - wavelength ( lw ) cones, also referred to as blue / uv cones, green cones and red cones respectively. most mammals are dichromats ( 2 cone types ) ; however, old world monkeys and humans are trichromats ( 3 different cone types ), and many non - mammalian species are also trichromats or even tetrachromats ( 4 different cone types ). in the retinas of birds, fish and reptiles, a class of sw cones that are sensitive to ultraviolet light ( uv cones ) can be found ( 27, 75 - 79 ), in addition to the lw, mw, and typical sw cones ( red, green and blue cones respectively ). since rods and cones of different spectral types are directly connected to horizontal cells, it is of interest to explore how horizontal cells integrate and process this richness of spectral information. physiologists recognize two major types of horizontal cell, luminosity and chromaticity. these can be distinguished by photoresponses to lights of different wavelength. the luminosity ( l - type ) horizontal cells always respond with hyperpolarization to light stimuli of any wavelength within the visible range of the spectrum, while the chromaticity ( c - type ) horizontal cells respond with different polarity to light stimuli of different wavelengths ( 80


type ) horizontal cells always respond with hyperpolarization to light stimuli of any wavelength within the visible range of the spectrum, while the chromaticity ( c - type ) horizontal cells respond with different polarity to light stimuli of different wavelengths ( 80 ). l - type horizontal cells luminosity ( l - type ) horizontal cells are found in every retina that has been studied, non - mammalian vertebrates, mammals and primates. figure 4 shows the photoresponses of luminosity - type horizontal cells from toad ( a ), rabbit ( b ) and monkey ( c ) retinas. regardless of the wavelength, these cells respond with membrane hyperpolarization that depends only on the brightness and duration of the stimulus. in some species, two classes of l - type horizontal cells can be distinguished both by anatomical structure and physiological properties. in the turtle retina, the l1 and l2 types are similar in spectral sensitivity but differ in receptive field size and in photoresponse kinetics to red and green light ( 53, 81 ). anatomically, these two l - type horizontal cell physiologies are respectively the axon terminal and the soma of the turtle h1 horizontal cell ( fig. 2a ). figure 4. photoresponses recorded from luminosity - type horizontal cells of toad, rabbit and monkey retinas. in each case, photoresponses were elicited by light stimuli of different intensities. stimulus intensity is given in log units relative to the most intense stimulus available for the particular recording system. the illustration in a is from normann and pochobradsky, 1976 ( 295 ) ; in b from bloomfield, 1992 ( 296 ), and in c from dacheux and raviola, 1990 ( 23 ). in cat and rabbit retinas, a - type and b - type cells ( fig. 2a ) are both luminosity types. they differ in morphology and in connectivity with rods and cones but respond only with hyperpolarizations to light stimuli ( 14, 15, 17 ). differences in a - and b - type spectral properties have not yet been reported. like turtle retinas, the retinas of cyprinids and old world monkeys have both red and green cones. in these species some l - type horizontal cells favor green cone signals, while others favor red cone signals. in cyprinids, the former is called an l1 type, and the latter an l2


monkeys have both red and green cones. in these species some l - type horizontal cells favor green cone signals, while others favor red cone signals. in cyprinids, the former is called an l1 type, and the latter an l2 type ( 82 ). in old world primates both red and green favoring l - type signals arise from the h1 morphological type. rather than two discrete types, there is a distribution in the balance of red cone and green cone signals for h1 cells ( 21, 24 ). in zebrafish also, there is a distribution in red - green balance on the h1 cell, yielding at the extremes l1 green - peaking types, and l2 red - peaking types ( 6 ). figure 5. color opponency of s - potentials in the fish retina. a. responses were recorded as a function of wavelength covering the entire visible spectrum using light stimuli of different wavelength but similar quantal content. b. the responses that were elicited by selected light stimuli, numbered in a from - 4 to + 4, are shown at greater time resolution. the longer time marks on the rectangular stimulus traces ( upper traces in b ) are 100 msec, the shorter, 10 msec. the illustration is from svaetichin and macnichol, 1959 ( 80 ). figure 6. wavelength dependency of the photoresponses from an l - type horizontal cell, a red / green c - type horizontal cell, and a yellow / blue c - type horizontal cell in the retina of the turtle mauremys caspica. for each wavelength, several intensities are nested. the illustration is from twig et al., 2003 ( 92 ). c - type horizontal cells in 1959, svaetichin and macnichol ( 80 ) first reported on extreme wavelength - dependency of some s - potentials ( fig. 5 ). as shown in the figure, the photoresponses of this s - unit reversed in polarity at a wavelength of about 560 nm. photoresponses to light stimuli of longer wavelength depolarized the cell membrane, while stimuli of shorter wavelengths hyperpolarized the cell membrane. svaetichin and macnichol called these ‘ color opponent ’ s - units chromaticity or c - type horizontal cells. c - type horizontal cells have been extensively studied in turtle, amphibian and fish retinas ( 7, 9, 55, 83 - 94 ). horizontal cells are named


’ s - units chromaticity or c - type horizontal cells. c - type horizontal cells have been extensively studied in turtle, amphibian and fish retinas ( 7, 9, 55, 83 - 94 ). horizontal cells are named by the number of wavelengths at which response polarity reverses ; no reversal – monophasic ( or l - type ) cells, one reversal – biphasic c - type cells, two reversals – triphasic c - type cells, three reversals – tetraphasic c - type cells. wavelengths at which response polarity reverses are called ‘ null wavelengths ’. in fish retinas, three types of chromaticity horizontal cells have been described ; biphasic, triphasic and tetraphasic ( 6, 86, 95, 96 ). in the bowfin retina, the null wavelength for the biphasic cells is around 640 nm, while null wavelengths of the triphasic cells are ; in the regions of 500 - 530 nm and 650 - 670 nm respectively ( 95 ). the biphasic and triphasic c - type horizontal cells of the fish retina are identified morphologically as h2 and h3 types respectively ( fig. 2b ). in zebrafish the h3 morphological type has taken on as many as 3 physiological identities : the blue triphasic, analogous to previous findings in fish, a uv triphasic type, and a tetraphasic type ( 6 ). in the turtle retina, two classes of biphasic c - type horizontal cells have been identified ( 7, 85, 91 ). typical photoresponses of l - type and c - type horizontal cells in the turtle mauremys caspica to light stimuli of different wavelength and intensity are shown in figure 6. for each wavelength, a series of photoresponses to different intensities is shown. the l - type horizontal cell ( 1st row in fig. 6 ) responds to all stimuli with graded hyperpolarizations regardless of wavelength. the red / green biphasic c - type horizontal cell ( 2nd row, fig 6 ) responds with graded depolarizations to red light stimuli and with graded hyperpolarizations to yellow, green and blue stimuli. the yellow / blue biphasic horizontal cell ( 3rd row, fig. 6 ), responds with depolarizations to red and yellow light stimuli and with hyperpolarizations to blue light stimuli. in order to


and blue stimuli. the yellow / blue biphasic horizontal cell ( 3rd row, fig. 6 ), responds with depolarizations to red and yellow light stimuli and with hyperpolarizations to blue light stimuli. in order to define the spectral properties of the horizontal cells, monochromatic light stimuli of dim intensities are used to elicit small amplitude ( < 1mv ) photoresponses. these photoresponses are within the linear range of the cells and can be used to calculate light sensitivities. sensitivity is defined as the volts of evoked response per quantum of light and is calculated by dividing the voltage response by the number of quanta in the stimulus. the relationship between light sensitivity and wavelength is the action spectrum of the cell and describes the threshold spectral properties. figure 7 shows the mean action spectra of 8 l - types, 7 red / green types and 6 yellow / blue types of horizontal cell ( a, b and c respectively ) from the turtle mauremys caspica. these spectra clearly demonstrate that the l - type horizontal cells are most sensitive to long - wavelength stimuli, with the major excitatory input coming from red cones ( 7, 85 ). these cells receive additional excitatory input from green cones and to a lesser extent from blue cones ( 85 ). the red / green c - type horizontal cells are characterized by reversal of response polarity around 600 nm ( fig. 7b ). these cells are excited by green and blue cones and inhibited by red cones ( 7, 85 ). the yellow / blue c - type horizontal cells are inhibited by green and red cones and excited by blue cones ( 7, 85 ) and interestingly, also by uv cones ( 27, 97 ). their photoresponses reverse in polarity around 540 nm ( fig. 7c ). figure 7. action spectra of 8 l - type ( a ), 7 red / green c - type ( b ) and 6 yellow / blue c - type ( c ) horizontal cells in the turtle retina ( mauremys caspica ). sensitivity data were calculated from photoresponses of small amplitude ( < 1 mv ) that were elicited by dim light stimuli illuminating the entire receptive field of the studied cells. in order to compare the different action spectra, the spectrum of each cell was normalized to the peak sensitivity. in ( a ), all responses were of hyperpolarizing polarity. in ( b ) and


at different wavelengths are fit by a spectral model that takes into account the shapes of zebrafish cone pigments ( 6, 98 ), l - type responses to blue and uv stimuli ( figs. 8e, 8f ) are shown to result from stimulation of the short - wavelength absorbance limbs of red and green cones. l1 types show a balance of red and green cone signals and l2 cells hyperpolarize predominantly in response to red cone stimulation ( 6 ). figure 8. color physiology of zebrafish horizontal cells. a. tetraphasic type is depolarized by ultraviolet ( uv - 330nm ) hyperpolarized by blue ( 410nm ) depolarized by blue - green ( 490nm ) and hyperpolarized both by yellow ( 570nm ) and red ( 650nm ) wavelengths. b. uv triphasic cell hyperpolarizes at full amplitude for both irradiances of the uv 330nm stimuli but is depolarized by the dimmer of the blue 410nm stimuli and hyperpolarized by the brighter 410nm stimulus. for 490nm the cell depolarizes, but at longer wavelengths the cell hyperpolarizes. the blue triphasic cell ( c ) hyperpolarizes fully for both 410nm blue stimuli, but responds only weakly to 330nm uv stimuli. depolarization occurs in the blue - green ( 490nm ) and yellow ( 570nm ) but the cell hyperpolarizes for red stimuli ( 650nm ). the biphasic cell ( d ) is hyperpolarized for short - wavelengths ( 330nm, 410nm, and 490nm ) but is depolarized by long wavelengths ( 570nm, 650nm ). two l - type cells, l1 ( e ) and l2 ( f ) hyperpolarize at all wavelengths, but the amplitude ratio at 490nm and 570nm favors 570nm for the l2 - type, and is more evenly balanced for the l1 - type. all amplitudes are given in mv. numbers to the right of each trace are stimulus irradiance, given in log ( - 2 · s - 1 ). illustrations are from connaughton and nelson, 2010 ( 6 ). mammals lack c - type horizontal cells given the diversity of horizontal - cell physiology in lower vertebrates, where dendritic contacts with cones


2 · s - 1 ). illustrations are from connaughton and nelson, 2010 ( 6 ). mammals lack c - type horizontal cells given the diversity of horizontal - cell physiology in lower vertebrates, where dendritic contacts with cones containing different visual pigments frequently produce chromatically opponent responses, it is surprising that in mammalian retinas, whose horizontal - cell dendrites also contact cones with different visual pigments, only luminosity - type horizontal cell responses can be recorded ( 18, 19, 22, 99, 100 ). for example, both cat a - and b - horizontal cells are luminosity types ( fig. 9 ), that are dominated by red - cone input, although low - level synergistic input from blue cones can be detected using specific spectral stimulating and adapting conditions ( 100 ). in both a - and b - type cells, rod signals ( fig. 9 ) are intermixed with cone signals ( 16, 17 ). figure. 9. rod - cone mixing in horizontal cells of the cat retina. responses to light stimuli of different intensities were recorded from the cell bodies of a - type and b - type horizontal cells and from the axon terminal of the b - type horizontal cell. brighter stimuli produce greater amplitudes from the cones, and longer ‘ rod after - effects ’ from the saturated rod signals. the illustration is adapted from nelson et al., 1975 ( 17 ). the failure of mammalian horizontal cells to be color opponent may correlate with the absence of a horizontal cell that avoids contact with long - wavelength cones. the biphasic responses in zebrafish retinas are thought to arise from an h2 type horizontal cell that avoids contact with red cones, while selectively contacting green, blue, and uv cones. excitatory input arises from the blue and green cone contacts, and the red inhibitory input arises from feedback contacts from red - cone dominated h1, l - type horizontal cells, onto blue cones ( fig. 2b, panel e ). in the h2 cell there is no red hyperpolarizing signal to mask the h1 red - inhibitory feedback onto the blue cones. this allows the h2 cell to show the color opponent feedback signal ( 37 ). in primate retina, despite a full complement of red, green and blue cone types, horizontal cells also only occur as luminosity types ( 22, 23 ). h1 cells receive synergistic signals from long - and medium


). in primate retina, despite a full complement of red, green and blue cone types, horizontal cells also only occur as luminosity types ( 22, 23 ). h1 cells receive synergistic signals from long - and medium - wavelength cones ( red and green cones ), while h2 cells receive synergistic input from all three spectral types of cones ; long - medium - and short - wavelength ( red, green and blue cones ) as shown in figure 10. anatomically h1 type horizontal cells tend to avoid the pedicles of blue cones ( fig. 10, neurobiotin - injected h1 cell ) ( 22, 101 ) and are therefore not responsive to blue - cone isolating stimuli ( fig. 10e ). on the other hand, large numbers of h2 dendrites contact blue cone pedicles ( fig. 10, neurobiotin - injected h2 cells, outlined clusters ) ( 22, 101, 102 ). the h2 cell is indeed very sensitive to the blue end of the spectrum ( fig. 10j ). yet, the responses of both horizontal cell types are only of hyperpolarizing polarity to all wavelengths ( 22 ). thus, it appears that subsets of mammalian and primate l - type horizontal cells are devoted primarily to processing of either red green and blue signals or red and green signals, but spectral opponency is not part of the processing regime. figure 10. connectivity and physiology of horizontal cells in the monkey retina. above : intracellular staining and chromatic responses of h1 cells. these cells avoid dendritic contact with s - cones ( blue outlined white circles ). there is no response to s - cone ( blue - cone ) isolating stimuli ( trace e ). below : intracellular staining and chromatic responses of h2 cells. these cells send many dendrites to s - cones ( blue outlined cones ) as well as to red and green cones. the response to s - cone isolating stimuli is large ( j ). the illustration is from dacey, 1996 ( 22 ). 5. rod and cone contributions and passive electrical models. mixing of rod and cone signals steinberg ( 19, 99 ) was the first to distinguish separate rod and cone signals in s - potentials from a mammalian retina. he made intracellular recordings from horizontal cells in the cat retina and saw the typical graded hyperpolarizing responses that depended upon the intensity of the light stimulus. however, unlike


and cone signals in s - potentials from a mammalian retina. he made intracellular recordings from horizontal cells in the cat retina and saw the typical graded hyperpolarizing responses that depended upon the intensity of the light stimulus. however, unlike the responses seen in non - mammalian retinas, steinberg ’ s s - potential recordings revealed much more distinct rod and cone contributions to the response. a very slow phase of membrane repolarization seen after termination of the light stimulus was identified as the rod contribution. steinberg called this the ‘ rod after effect ’ ( fig. 9 ). the difference in offset kinetics for rod and cone signal components provides a convenient assay for separating rod and cone inputs to s - potentials in rod - dominated mammalian retinas ( 16 ). in figure 9, typical mixed rod and cone signals of all three horizontal cell structures in cats are shown ( 17 ). in a - and b - type horizontal cell bodies, rod and cone signals are about equal in amplitude, whereas in the axon terminal of the b - type cell, the slowly recovering response characteristic of rod signals occurs. this is interpreted to indicate that rod and cone signals are almost equally mixed in cat horizontal cell bodies, but only rod signals are seen in the axon terminals. the ‘ rod after effect ’ grows increasingly prolonged with bright light stimuli ( fig. 9, bottom ). further evidence for the mixing of rod and cone inputs to cat horizontal cells is seen during light adaptation. when retinas are light adapted by steady background lights, rod signals saturate and rod contributions to the horizontal cell photoresponses, including rod after - effects, vanish. in contrast, the cones adapt to the brighter conditions and their input remains robust. these properties of rod and cone inputs are easily seen in the photoresponses of mammalian horizontal cells such as cats and rabbits ( 15, 17, 19 ). the photoresponses of the rod - dominated b - type horizontal cell axon terminals are virtually abolished by light adaptation. in the horizontal cell bodies ( a and b types ), which receive mixed rod and cone inputs, light adaptation selectively reduces the rod contribution but large cone signals remain and therefore, these horizontal cell elements continue to respond well in the presence of background lights ( 16 ). axon terminals of the axon - bearing b - type horizontal cells in the cat receive an excitatory feed - forward input only from rods, but the cell bodies of a - type and b - type cells contact only cones


( 16 ). axon terminals of the axon - bearing b - type horizontal cells in the cat receive an excitatory feed - forward input only from rods, but the cell bodies of a - type and b - type cells contact only cones. so where does the large rod component of the s - potential in the cell bodies of cat horizontal cell bodies come from? electron microscope observations show gap junctions linking rod and cone photoreceptors in the mammalian outer plexiform layer ( 103, 104 ). these electrical synapses introduce rod signals into cones and thence into horizontal cells. this rod / cone mixing in the photoreceptors is now considered one of the pathways whereby rod driven inputs pass to horizontal cells and to all subsequent second and third order cells in the retina ( 105 - 108 ). in other species, similar characteristics of rod - cone mixing can be found in horizontal cells. in the rabbit retina, as in the cat retina, rod - dominated input to the axon terminal is identified by waveform, sensitivity and the presence of after - potentials ( 15, 109 ). also in primate retina, the axon terminals of the axon - bearing h1 - type horizontal cells produce rod signals while the cell bodies produce cone or mixed rod - cone signals ( 22, 23, 26 ). similar to other mammalian retinas, the somata of primate h1 - type horizontal cells receive rod input indirectly via gap junctions between rods and cones ( 26 ). horizontal cells of reptiles and amphibians, like mammals, mix rod and cone signals. in the retina of the turtle chelydra serpentina, the photoresponses of h1 axon terminals contain slow, low - amplitude components that are contributed by rod photoreceptors, despite the major excitatory input being from long - wavelength sensitive cones. the cell bodies of the same h1 horizontal cells receive input only from long - and medium - wavelength sensitive cones ( 110 ). in xenopus, horizontal cell dendrites contact both rods and cones ( 111 ). similar to mammals, waveforms characteristic of rods and cones, including rod after - effects are recorded in these cells with light stimulation ( 112 ). the amphibian circuitry, with direct rod contacts onto horizontal cell dendrites, differs from mammals, where only cones are contacted, and rod signals enter cones directly through gap junctions. no rod signals have been reported in the purely cone - contacting horizontal cells of fish ( fig


with direct rod contacts onto horizontal cell dendrites, differs from mammals, where only cones are contacted, and rod signals enter cones directly through gap junctions. no rod signals have been reported in the purely cone - contacting horizontal cells of fish ( fig. 2b, fig. 8 ). in fish rod signals are restricted to h4, a special rod - connected horizontal cell type ( 45, 46 ). passive electrical models all vertebrate retinas contain at least one horizontal cell type that is an axon - bearing cell ( fig. 2a, b - types ). the axon terminals and somata of these horizontal cell types are connected by a long thin axon, but in many cases each end behaves as an isolated physiological unit. in the turtle pseudemys scripta elegans, both axon terminals and somata receive excitatory input mainly from red cones but differ in spatial properties and response kinetics for red and green stimuli ( 53, 81 ). in another turtle species, chelydra serpentina, the axon terminal receives excitatory input from red cones and rods while the cell body dendrites contact red and green cones ( 110 ). in domestic mammals ( cat, rabbit ) and in primates ( monkey ), the dendrites of the cell body receive input from cones while the terminals of the axon contact thousands of rods. no evidence has been found to indicate any synaptic or electrical interaction between the axon terminal and the soma of the same cell in any of these cases. where horizontal cell types have the dendritic portion of the cell much distant from the axonal ending ( fig. 2a, b - types ), calculations based on the anatomical dimensions and geometry of the neuron, and ohmic linear properties of cell membrane and cytoplasm, indicate that the axons are too long and thin to allow significant passive electrotonic spread of signals from one end of the cell to the other ( 17, 26, 36, 110 ). such models tell us that signals in cell bodies and axon terminals of horizontal cells reflect only the local synaptic inputs. indeed, the two portions of the same cell act as independent units ( 17 ), in clear violation of neuron theory. the exception to this rule of physiological separation between cell body and axon terminal appears to be the cone - connected horizontal cells in fish. the cone responses of axon terminals in these cells are similar to the cell bodies ( 3 ). as there are


the exception to this rule of physiological separation between cell body and axon terminal appears to be the cone - connected horizontal cells in fish. the cone responses of axon terminals in these cells are similar to the cell bodies ( 3 ). as there are no cone contacts from / to the axon terminals in fish, it appears likely that signals are passively conducted from the cell body. the high impedance of the simple unbranched axon terminal may be permissive to the passive propagation of signals through the axon. 6. ionic conductances. ionotropic glutamate - gated channels photoreceptors excite horizontal cells through sign - conserving synapses. in darkness, photoreceptors continuously release an excitatory neurotransmitter that opens horizontal - cell cationic channels with a depolarized reversal potential. therefore, the horizontal cells are maintained at a relatively depolarized potential in darkness. when the photoreceptors are hyperpolarized by a light stimulus, neurotransmitter release is reduced, the post - synaptic ligand - gated channels close and the horizontal cells hyperpolarize. neurotransmitter release from photoreceptor terminals is calcium dependent. exposing the retina to ions that interfere with calcium influx through voltage - dependent calcium channels blocks neurotransmitter release from the photoreceptors, and consequently horizontal cells hyperpolarize ( 113 - 115 ). early experiments in intact retinas pointed to the excitatory amino acids l - glutamate and l - aspartate as putative candidates for the photoreceptor neurotransmitter ( 116 - 120 ). exposing the retina to either l - glutamate or l - aspartate caused depolarization of horizontal cells and loss of photoresponses as shown in figure 11a for horizontal cells in the intact rabbit retina ( 116 ). these observations are consistent with the notions that an excitatory amino acid is continuously released by the photoreceptors in the dark, and that light - induced hyperpolarization of horizontal cells reflects the reuptake of the amino acid from the synaptic cleft. accordingly, exogenously applied neurotransmitter saturates the receptor sites in the horizontal cells causing further depolarization. since the horizontal cells are saturated with the exogenously applied ne


from the synaptic cleft. accordingly, exogenously applied neurotransmitter saturates the receptor sites in the horizontal cells causing further depolarization. since the horizontal cells are saturated with the exogenously applied neurotransmitter, modulation of endogenous transmitter release by light stimuli has little effect on membrane potential and the photoresponses are eliminated. in the experiment described in figure 11a, solutions containing 15 mm l - aspartate or l - glutamate were needed to induce depolarization of rabbit horizontal cells and loss of photoresponses. these and other studies raised two major questions. ( 1 ) why were relatively high concentrations ( in the millimolar range ) of excitatory amino acids needed to exert the effects shown in figure 11a? ( 2 ) which of the two excitatory amino acids was the photoreceptor neurotransmitter? figure 11. a. l - aspartate and l - glutamate depolarize horizontal cell membrane potential and abolish horizontal cell photoresponses in intact rabbit retina. these photoresponses were elicited by diffuse light stimuli. the illustration is from bloomfield and dowling, 1985 ( 116 ). b. l - glutamate depolarizes the membrane potential of a horizontal cell isolated from the carp retina, while l - aspartate and d - glutamate exert no effect. each application pipette was filled with 500 µm of drug. current pulses measure changes in membrane resistance. the illustration is from lasater and dowling, 1982 ( 125 ). the first question was answered by the identification of efficient uptake systems for glutamate and aspartate by excitatory amino acid transporters ( eaats ) ( 121, 122 ). eaats efficiently remove excitatory amino acids from extracellular space, thereby greatly reducing the concentration of the exogenously applied aspartate or glutamate reaching the cone - to - horizontal - cell synapses. retinal glutamate transporters are reviewed elsewhere in webvision. when non - transportable analogues of excitatory amino acids are applied, considerably lower concentrations are needed to exert effects on horizontal cells ( 109, 123 ). this is illustrated in figure 12 where the effect of 10 mm l - glutamate ( a ) is compared to that of 0. 1 mm kainic


considerably lower concentrations are needed to exert effects on horizontal cells ( 109, 123 ). this is illustrated in figure 12 where the effect of 10 mm l - glutamate ( a ) is compared to that of 0. 1 mm kainic acid ( b ). kainic acid, an agonist for ampa - kainate - type glutamate receptors in vertebrate horizontal cells, but not a substrate for eaats, exerts effects similar to l - glutamate at a concentration lower by 100 fold. in both cases horizontal cells are depolarized and photoresponses lost. in further support of the excitatory amino acid receptor hypothesis, kynurenic acid, a non - selective antagonist of excitatory amino acid receptors ( 124 ) hyperpolarizes horizontal cells, also with loss of photoresponse ( fig, 12 c ). figure 12. the effects of l - glutamate ( a ), kainic acid ( b ) and kynurenic acid ( c ) upon the membrane potential and photoresponses of l - type horizontal cells in the retina of the turtle mauremys caspica. the illustrations are from perlman. the exact identity of the excitatory amino acid was debated for more than a decade. a variety of experiments in intact retinas were designed to decide between l - glutamate and l - aspartate as candidates for photoreceptor neurotransmitter, but with conflicting conclusions. it was not until the development of the isolated cell preparation that a clear answer to this question was obtained. lasater and dowling ( 125 ), in their pioneering work, isolated horizontal cells from the carp retina and recorded the membrane potentials with sharp microelectrodes. since these horizontal cells in culture were separated from all other influences of retinal circuitry, including photoreceptor stimulation, they were characterized by hyperpolarized resting potentials ( close to the potassium equilibrium potential ). application of low doses of l - glutamate, but not of l - aspartate or d - glutamate, induced depolarization, indicating that selective receptors for l - glutamate existed in the horizontal cell membrane ( fig. 11b ). with this experimental approach, a low concentration of l - glutamate was sufficient to induce effects, because in this preparation, removal of extracellular l - glutama


in addition to ampa receptors in horizontal cells cultured from human retina ( 137 ). metabotropic glutamate effects glutamate also influences horizontal cells through metabotropic pathways. unlike ionotropic ampa - glutamate responses, metabotropic actions do not spread globally through membrane voltage changes within the horizontal cell network, but remain local, influencing only nearby cytoplasmic chemistry. dl - ap4, a glutamate analogue selective for type iii metabotropic glutamate receptors, suppresses nearby voltage sensitive potassium channels ( 138 ). similarly, group i and group iii metabotropic glutamate receptors increase the amplitudes of nearby voltage - gated calcium currents in horizontal cells ( 139 ). glutamate provides further metabotropic regulation of calcium and potassium channels through modulation of cytoplasmic ph ( 140 ). nmda receptors on catfish horizontal cells have been observed to down - regulate voltage dependent sodium and calcium channel activity through a calmodulin - dependent mechanism, also a type of metabotropic effect ( 141 ). furthermore sodium entry through stimulated ampa - glutamate channels stimulates local activity of na + k + atpase. this in turn appears important in setting membrane potential ( 142 ). thus, in addition to causing the light response itself, photoreceptor glutamate regulates a variety of cellular and membrane functions. voltage - and time - dependent conductances the photoresponses of horizontal cells from cold - blooded, as well as warm - blooded mammalian retinas, are shaped by voltage - and time - dependent ionic channels located within the plasma membrane ( 143 - 146 ). five types of ionic currents ; a sodium current, a calcium current and three types of potassium current have been identified in isolated horizontal cells of goldfish ( 147 ), catfish ( 148 ), white perch ( 143 ), skate ( 149 ), turtle ( 150 ), rabbit ( 145 ) and cat ( 146 ). figure 13. voltage - and time - dependent ionic conductances in isolated horizontal cells. a. current responses to different voltage steps from a holding potential at - 60mv in horizontal cells from the cat retina, from ueda et al., 1992 ( 146 ). b. current responses and i - v curves from 4 types of horizontal cells that were isolated from the retina of the white perch, from lasater, 1986 ( 143 ). voltage steps evoke membrane


et al., 1992 ( 146 ). b. current responses and i - v curves from 4 types of horizontal cells that were isolated from the retina of the white perch, from lasater, 1986 ( 143 ). voltage steps evoke membrane current responses for two types of horizontal cells isolated from the cat retina ( fig. 13a ) and four types of horizontal cells isolated from the retina of the white perch ( fig. 13b ). in cat horizontal cells, the inward sodium current is clearly evident during the first 10 ms of the current responses while at the steady state, a sustained outward potassium current is seen ( fig. 13a ). fish horizontal cells exhibit a more complex array of currents as evident from the current waveforms and the i - v curves for transient and sustained portions of the response to electrical stimulation ( fig. 13b ). basically all four types of fish horizontal cells exhibit similar ionic currents but they differ in relative magnitude. the sodium current is inward going and is seen during the initial 10ms of the current pulses that are recorded when the horizontal cells are depolarized from a negative holding potential ( about - 70mv ) to around 0mv. it is very similar to the regenerative sodium current typical of spiking neurons. the physiological role of this current in horizontal cells is not known since these cells do not exhibit action potentials. three types of potassium channel have been identified by their voltage - and time - dependency and by their sensitivity to specific blockers. the inwardly rectifying channels conduct an inward current when the cells are hyperpolarized below potassium equilibrium potential. in some species, extracellular sodium ions influence the reversal potential of these channels making the currents functional at physiological potentials ( 151 ). the sustained outward potassium current ( delayed rectifier ) is activated at potentials more depolarized than the resting, dark potential of the horizontal cells. the outward transient potassium current ( ia ), is activated upon fast depolarization from negative potentials. in the intact turtle retina, the outward transient potassium current is activated upon termination of a bright light stimulus, speeding the recovery of the l - type horizontal cell from the depolarizing overshoot at light offset back to the dark level. this effect allows the l - type horizontal cells to follow fast flickering stimuli and thus, improves the frequency response curve of the cells ( 152 ). l - type calcium responses are a prominent feature of isolated horizontal cells. these channels inactivate only slowly resulting in


allows the l - type horizontal cells to follow fast flickering stimuli and thus, improves the frequency response curve of the cells ( 152 ). l - type calcium responses are a prominent feature of isolated horizontal cells. these channels inactivate only slowly resulting in long duration spikes and persistent depolarized states ( 142, 146, 148, 153 ). outward potassium currents appear in themselves too weak to inactivate calcium currents in these cells ( 154 ), however glutamate activation of the electrogenic na / k atpase appears sufficient to restore hyperpolarized membrane potentials ( 142 ). calcium currents may sometimes contribute to oscillations in the light responses of horizontal cells ( 155 ). gaba receptors and transporters some, but not all, horizontal - cell types accumulate gaba ( 51 ). a likely target of horizontal - cell gaba is other horizontal cells. kamermans suggested gaba as mediator of positive feedback between these neurons ( 156 ). in many species gabaa and / or gabac receptors are located on horizontal cells. qian and dowling discovered retinal gabac receptors. they were localized on the membranes of isolated rod horizontal cells of white perch ( 157 ). retinal gabac receptors are reviewed elsewhere in webvision. currents from gabaa receptors are found in dissociated rabbit horizontal cells ( 158 ). both gabaa and gabac receptor currents as well as gaba transporter currents occur in isolated catfish horizontal cells ( 159, 160 ). isolated horizontal cells from skate ( 161 ) or zebrafish ( 162 ) show no evidence of gaba receptor physiology, but do possess gaba transporter physiology. horizontal - cell gaba circuits are species specific, nonetheless gaba would appear to be an important signaling molecule within the vertebrate outer plexiform layer, and gaba roles include signaling between horizontal cells. 7. gap junctions and spatial characteristics in all vertebrate retinas including mammals, horizontal cells are characterized by large - surface - area gap junctions ( fig. 3c ) between each others ’ dendrites ( 163, 164 ). these junctions allow lateral flow of small molecules and ions within the horizontal cell network. gap - junction - permeant tracers such as lucifer yellow or neurobiotin that are injected into one horizontal cell in the layer spread to hundreds of neighboring cells, forming striking images of the interconnected horizontal cell network ( figs. 14, 15a ) ( 14, 66, 67


as lucifer yellow or neurobiotin that are injected into one horizontal cell in the layer spread to hundreds of neighboring cells, forming striking images of the interconnected horizontal cell network ( figs. 14, 15a ) ( 14, 66, 67, 84, 165, 166 ). gap junctions are formed only between neighboring horizontal cells of the same physiological type as shown in figures 14 and 15a. networks of turtle h2 cells ( fig. 15a, left panel ) and of rabbit a - type cells ( fig. 15a, right panel ) are revealed by neurobiotin injection into one cell of the network. gap - junction connexons between rabbit a - type horizontal cells accommodate molecules as large as 500 daltons ( 74 ). the permeance of the gap junctions between horizontal cells and therefore, dye coupling, is influenced by the intracellular and extracellular milieu and by chemicals such as dopamine, retinoic acid, nitric oxide or hydrogen ions that are released by other retinal cells ( see section on neuromodulators ). figure 14. rabbit horizontal cell network revealed by dye injection. lucifer dye, injected into one cell, spreads via the gap junctions linking a - type horizontal cells to reveal the centrally injected cell and hundreds of neighboring cells. the illustration is from mills and massey, 1994 ( 67 ). receptive field properties the gap junctions between horizontal cells are highly permeable to small ions and therefore, serve as low - resistance pathways for electrical signals to spread laterally within the horizontal cell layer. thus, horizontal cells receive excitatory input directly from photoreceptors via chemical synapses and also indirectly via gap junctions with neighboring horizontal cells. the physiological consequence of the gap junctions between horizontal cells is to create a very large receptive field that spreads out to distant retinal regions, regions beyond the extent of direct dendritic contact with photoreceptors ( 10, 66 ). the receptive fields of horizontal cells are measured by several spatial protocols. the most common are ( 1 ) examining responses to a series of concentric spots of increasing diameter but fixed irradiance, or ( 2 ) examining responses to long narrow slits of light presented at different positions across the retina. in either case response amplitudes can be modeled, at least to first order, by differential equations describing current flow in a resistive plane ( 16, 167, 168 ), or equivalently by discrete passive networks of interconnected resistors ( 53 ).


either case response amplitudes can be modeled, at least to first order, by differential equations describing current flow in a resistive plane ( 16, 167, 168 ), or equivalently by discrete passive networks of interconnected resistors ( 53 ). an experiment in which an l - type horizontal cell in the retina of the turtle pseudemys scripta elegans was stimulated with a series of concentric spots of fixed intensity is shown in figure 15b. the first photoresponse to the left was elicited with a stimulus of super - saturating intensity that illuminated the entire receptive field of the cell. this is the maximal photoresponse. a series of 6 photoresponses were then elicited by sub - saturating light stimuli with different stimulus diameters, as illustrated diagrammatically above each response. the photoresponse amplitude gradually decreases as the size of the illuminated retinal area is reduced ( fig. 15b ; graph ). this reduction of amplitude is not due to diminution of photoreceptor synaptic input to the recorded cell, as even the small spot stimuli are larger than the dendritic extent of the cell. thus, it is suggested that the low amplitude of the responses to small spot light stimuli are due to shunting of the local photoreceptor input into neighboring horizontal cells through gap junctions. with full - field illumination, all the horizontal cells are evenly illuminated, equipotential, and therefore, no current flows between them. with small spot illumination, the membrane potential of non - illuminated horizontal cells is different from that of illuminated cells causing electrical currents to flow between them and thus, to shunt the responses of the illuminated cells. figure 15. a. the syncytia of h2 cells in turtle retina ( left ) and a - type horizontal cells in the rabbit ( right ) as revealed by neurobiotin injection into one cell in each syncytium. the illustration in turtle retina is from ammermuller et al, 1996 ( 84 ) ; the rabbit retina illustration is from mills and massey, 1994 ( 67 ). b. the size of the receptive field of a turtle l - type horizontal cell as determined from the photoresponses that were elicited by concentric light stimuli of different diameters ( left ). the peak amplitude, normalized to the response to full field illumination ( 2nd response from the left ), is plotted as a function of spot


photoresponses that were elicited by concentric light stimuli of different diameters ( left ). the peak amplitude, normalized to the response to full field illumination ( 2nd response from the left ), is plotted as a function of spot radius in order to derive the length constant ( right ). the illustration is from perlman and ammermuller, 1994 ( 176 ). the receptive field of a cat a - type horizontal cell, mapped with a narrow slit of light, is shown in figure 16. exponential decay of response amplitude flanks both sides of the maximal response, which is elicited when the narrow slit is centered on the studied cell. note that this maximal slit response is always smaller than the one elicited by a large light stimulus of the same intensity that covers the entire receptive field of the studied cell ( vmax ). the red receptive field ( fig. 16a ) measures the spatial extent of cat red cone signals, while the blue receptive field ( fig. 16b ) measures the spatial extent of rod signals, both signal components occur in the same horizontal cell. these signals share a common center, but differ in space constant. figure 16. a - type horizontal - cell receptive field in cat retina is mapped with a long narrow slit ( 100 x 5000 µm ). a. the wavelength is 647nm ( red ) that stimulates a mixture of cat red cone and rod responses. b. the wavelength is 441nm ( blue ) and dim enough to stimulate only rods. rod and cone receptive fields share a common center, but differ in size. the continuous curves are generated by the conductive sheet model, with a space constant of 250 μm in a. and 200 μm in b. the figure is from nelson, 1977 ( 16 ). the dimension of the horizontal cell receptive field can be quantified. using variable diameter spots, one common procedure is to find the minimum spot diameter that elicits a maximal response. another measure is the receptive - field length constant lambda ( λ ). for models based on electrical current flow in the horizontal cell layer, lambda is defined as the slit displacement required to reduce response amplitude by 1 / e. ( 16, 167, 169, 170 ). some other definitions of lambda based on empirical curve fitting of data have also been suggested ( 10, 171 ). for models based on electrical current flow in the horizontal - cell syncytium, receptive field size is directly related to lambda,