|John Dowling (2007), Scholarpedia, 2(12):3487.
|revision #91715 [link to/cite this article]
The retina is a thin (~0.25 mm) layer of neural tissue that lines the back of the eye. It is part of the brain (central nervous system) displaced into the eye during development. In addition to the light-sensitive photoreceptor cells – the rods and cones - the retina contains four basic classes of neurons – horizontal, bipolar, amacrine and ganglion cells – along with one major type of glial cell – the Müller cell. The neurons are organized into three cellular (nuclear) layers which are separated by two synaptic (plexiform layers). Virtually all retinal synapses are found in the plexiform layers, and all visual information passes across at least two synapses, one in the outer plexiform layer and another in the inner plexiform layer, before leaving the eye.
Analysis of the visual image occurs in both plexiform layers. The outer plexiform layer (OPL) separates visual information into ON- and OFF-channels and carries out a spatial type analysis on the visual input. The output neurons of the OPL, the ON- and OFF-bipolar cells demonstrate a center-surround antagonistic receptive field organization. The inner plexiform layer (IPL) is concerned more with the temporal and complex aspects of visual stimuli. Many cells receiving input in the IPL respond with transient responses and are highly sensitive to moving stimuli. The output neurons of the INL – the ganglion cells – carry the visual message to the rest of the brain; some ganglion cell responses reflect more OPL processing, others IPL processing.
Most vertebrate retinas contain two basic classes of photoreceptor cells, rods and cones. Rods mediate dim-light vision, whereas cones function in bright light and are responsible for color vision. The light sensitivity of the photoreceptors results from the presence of visual pigment molecules contained within a specialized region of the cell, called the outer segment. One rod pigment called rhodopsin, and three cone pigments which absorb light maximally in the red, green and blue regions of the spectrum, are in the human retina; non-mammalian species may have 4 or more cone visual pigments.
All visual pigments have a similar chemistry. They consist of two components, retinal (vitamin A aldehyde), termed a chromophore, bound to a protein called opsin. Different visual pigments have different opsins, and this accounts for the variations in their color sensitivity. The light sensitivity of the visual pigments is due to the retinal chromophore. When a visual pigment molecule absorbs a quantum of light, several conformational (shape) changes occur, first in the chromophore and then in the protein (opsin) part of the molecule. The opsin conformational changes lead to a series of intermediates called metarhodopsins ( Figure 2), and one, metarhodopsin II, results in excitation of the photoreceptor cell via a cascade of molecular interactions.
All vertebrate photoreceptors hyperpolarize in response to light (i.e., the inside of the cell becomes more negative). Why photoreceptors hyperpolarize in the light is as follows: In darkness, the membrane of the outer segment is permeable (leaky) to Na+. Because Na+ levels are higher outside the cell than inside, positively charged Na+ ions enter the cell in darkness, causing the cell to be partially depolarized (i.e., the cell’s inside is more positive than is typically the case for neurons at rest). Light decreases the permeability of the outer segment membrane to Na+, thereby decreasing the flow of positive ions into the cell and causing the inside of the cell to become more negative (i.e., to hyperpolarize).
In many primates including humans, some birds and reptiles, a small region of the retina is specialized for high-acuity vision. It is called the fovea and is centrally located (i.e. on the visual axis of the eye). The layers of the retina below the photoreceptors are displaced laterally around the fovea so that light can impinge more directly on the photoreceptors. This results in a retinal thinning centrally, forming the so-called foveal pit. Only cones are present in the central region of the fovea, and the foveal cones are the thinnest and longest photoreceptors in the retina. The rod-free area in humans is about 0.3 mm in diameter and contains approximately 35,000 cones. No blood vessels are found in the human fovea and, furthermore, there are few blue-absorbing cones in the center of the fovea. These specializations serve to improve the visual resolution of the fovea.
Neurons: Cellular and synaptic organization
Although there are just four basic classes of retinal neurons in addition to the photoreceptors, many neuronal types exist in each of these classes. Retinas typically have one type of rod cell and three or four cone types which are maximally responsive to light of different wavelengths (e.g. red, green, blue light). Three to four horizontal cell types exist in most retinas and as many as nine to eleven bipolar cell types. Amacrine cell types tend to be most numerous with retinas having as many as twenty or more. Finally, twelve to fifteen ganglion cell types have been identified in various species. Two kinds of chemical synapses have been identified in all retinas – ribbon synapses made by photoreceptor and bipolar terminal endings, and conventional synapses made by amacrine and horizontal cells ( Figure 3). Ribbon synapses are characterized by a dense ribbon-like structure found presynaptically that serves as a conveyor belt to bring synaptic vesicles to the presynaptic membrane where they bind and release their contents. Conventional synapses are characterized by a cluster of synaptic vesicles apposed to the membrane to which they bind. In addition, cone photoreceptors make an unusual chemical synapse, called a flat or basal junction mainly onto OFF-type bipolar cells, and finally, electrical synapses are seen between many types of retinal neurons.
Figure 4 depicts in a highly simplified way the cellular and synaptic organization of the primate retina. The photoreceptors, rods (R) and cones (C) provide the input to the OPL, contacting horizontal cells (H) via ribbon synapses and bipolar cells via ribbon or flat/basal synapses. One type of horizontal cell is depicted along with four bipolar cell types; flat (OFF-) and invaginating (ON-) midget bipolar cells that contact a single cone (FMB and IMB), diffuse bipolars that contact several cones (IDB), and rod bipolars (RB) that contact only rods. In the IPL the bipolar cells via ribbon synapses contact midget ganglion cells (MG), larger, diffuse ganglion cells (G) and amacrine cells (A). The rod bipolar cells contact amacrine cells that pass rod information onto cone bipolar cell terminals and the large ganglion cells. Other amacrine cells contact midget bipolar terminals, and midget and diffuse ganglion cells. Finally, a specialized type of amacrine cell, called an interplexiform cell (I), receives input in the INL and feeds information back to cells of the OPL ( Figure 4).
Light responses and retinal processing of information
The distal retinal neurons – photoreceptor, horizontal and bipolar cells, respond to light with sustained graded membrane potential (voltage) changes. Most of these cells also respond by hyperpolarizing in response to light stimuli – the membrane potential becomes more negative ( Figure 5). The more proximal retinal neurons respond to light mainly by depolarizing – the membrane potential becomes more positive – and they generate action potentials like most brain neurons. Distal retinal neurons use sustained graded potentials to carry visual signals, probably for two reasons: 1) they have relatively short processes and do not need to carry information long distances; hence, the electrotonic spread of potential along the cell membrane is sufficient to transmit information from one end of the cell to the other; and 2) graded potentials are capable of discriminating a wider range of signals than can all-or-none events (i.e. action potentials).
All photoreceptors hyperpolarize in response to light; via their synapses they cause horizontal and OFF-bipolar cells to hyperpolarize and ON-bipolar cells to depolarize. The horizontal cells act as lateral inhibitory neurons, establishing the classic center-surround receptive field organization first seen prominently in the bipolar cells. The center response reflects the direct photoreceptor-bipolar cell input; the antagonistic surround response reflects inhibitory horizontal cell input from more distal photoreceptors. Two types of amacrine cell responses – transient and sustained – have been observed in most retinas. Transient amacrine cells may respond by depolarizing at the ON-, OFF- or at both the ON- and OFF- of illumination. Sustained amacrine cells may either depolarize or hyperpolarize to light. Action potentials are often observed on the depolarizing responses of amacrine cells.
All ganglion cells generate action potentials and to full-field illumination, three basic types of responses are observed; ON, OFF, and ON-OFF cells. However, by probing the retina with small spots of light, the receptive field organizations of ganglion cells are found to be varied and complex, reflecting excitatory, inhibitory and modulatory interactions occurring in the retina, especially in the inner plexiform layer. Most prominent are the contrast-sensitive ON-center OFF-surround ganglion cells, and OFF-center ON-surround cells ( Figure 6a). A recent study identified ten subtypes of ganglion cell responses in the rabbit retina and correlated these with various morphological ganglion cell types. Each of these ganglion cell types provides higher visual centers with a somewhat different representation of the image falling on the retina. Thus, much visual processing occurs in the retina, the extent of which is almost certainly not fully recognized.
An intriguing type of ganglion cell found in many retinas is a movement-sensitive cell that is directionally selective – it responds well when a spot of light moves across the retina in one direction, but not at all when the spot moves in the opposite direction ( Figure 6b). A special type of amacrine cell – the starburst amacrine cell – is involved in the generation of the directionally-selective responses, and understanding of the synaptic interactions that underlie direction-selectivity in these ganglion cells is well along.
Transmitters, modulators, and receptors
The retina, like other regions of the brain, employs a large number of neuroactive substances (Figure 7). They include both neurotransmitters – substances that act rapidly and directly on the retinal neurons by altering membrane permeability to one or several ions; and neuromodulators – substances that usually act more slowly by altering the neurons via intracellular biochemical cascades. Relatively few substances appear to serve as retinal neurotransmitters; the majority of neuroactive substances released from retinal neurons (mainly from amacrine cells) appear to be neuromodulatory in nature although little is known about the action or role of most of these substances.
Amino acids and acetylcholine are the principal neurotransmitters used in the retina. L-glutamate is used by both photoreceptors and bipolar cells as their excitatory transmitter, whereas acetylcholine is used as an excitatory neurotransmitter in the IPL, principally by starburst amacrine cells. GABA and glycine are the major inhibitory transmitters used in the retina, and it is believed that 80% of the amacrine cells release one or another of these substances. A major area of study is to identify the various receptors/channels present on the neurons that mediate the actions of the neurotransmitters/modulators in the retina. For example, it is clear that ON- and OFF-bipolar cells, and perhaps even the horizontal cells, express different glutamate receptors, and how these receptors/channels give rise to the responses of the various neurons is beginning to be understood. A particularly interesting challenge is to understand how the glutamate released from photoreceptor cells gives rise to the ON-bipolar cell response. Two types of molecule have been implicated – a metabotropic glutamate receptor expressed by rod bipolar cells in all species and a transporter-like receptor expressed by ON-cone bipolar cells in the teleost retina.
A number of substances are believed to function as neuromodulatory agents in the retina, although how they function and their role is poorly understood. The proposed neuromodulatory substances include a number of amines, neuropeptides and other substances such as nitric oxide, retinoic acid, and zinc. About a dozen neuropeptides have been identified in the retina, mainly in amacrine cells and often they are colocalized with GABA. Their roles and the significance of the colocalization with GABA is not known.
Of all the proposed neuromodulatory substances, we know most about the role of dopamine which has been shown to modulate electrical (gap) junctions throughout the retina and the glutamate receptors on horizontal and bipolar cells. Dopamine, through cyclic AMP and protein kinase A (PKA), alters the conductance of many retinal gap junctions, and the sensitivity of certain glutamate receptors to glutamate. Dopamine in the retina is also believed to play a role in establishing retinal circadian rhythmicity, and regulating retinal motor movements in fish and other non-mammalian species.
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Dowling, John E. (1987) The Retina: An Approachable Part of the Brain. Harvard University Press.
Fried, S. I., Münch, T. A. and Werblin, Frank S. (2005) Directional selectivity is formed at multiple levels by laterally offset inhibition in the rabbit retina. Neuron. 46, 117-127.
Kolb, Helga, Ripps, H. and Wu S. (eds.) (2001) Concepts and Challenges in Retinal Biology. Amsterdam:Elsevier.
Polyak, Steven L. (1941) The Retina. Chicago: Chicago University Press.
Rodieck, Robert W. (1998) The First Steps in Seeing. Sunderland, MA: Sinauer Associates.
Roska, Botond, Molnar, A., Werblin, Frank S. (2006) Parallel processing in retinal ganglion cells: How integration of space-time patterns of excitation and inhibition form the spiking output, Journal of Neurophysiology, 95, 3810-3822.
Sterling, P. and Demb, J. B. (2004) Retina. In Shepherd, G. M. (ed) The Synaptic Organization of the Brain. pp. 217-269. Oxford: Oxford University Press.
- Valentino Braitenberg (2007) Brain. Scholarpedia, 2(11):2918.
- Keith Rayner and Monica Castelhano (2007) Eye movements. Scholarpedia, 2(10):3649.
- Eugene Roberts (2007) Gamma-aminobutyric acid. Scholarpedia, 2(10):3356.
John Dowling (2007), Scholarpedia, 2 (12): 3487. Retina ni tabaka la nyembamba (~ 0.25 mm) la tishu ya neva ambayo inashughulikia nyuma ya jicho. Ni sehemu ya ubongo (mfumo wa neva wa kati) iliyohamishwa ndani ya jicho wakati wa ukuzi. Mbali na seli za photoreceptor zenye hisia za mwanga - fimbo na koni - retina ina madarasa manne ya msingi ya neuroni - seli za usawa, bipolar, amacrine na ganglion - pamoja na aina moja kuu ya seli ya glial - seli ya Müller. Neurons ni kupangwa katika tatu seli (nyuklia) tabaka ambayo ni kutenganishwa na mbili synaptic (plexiform tabaka). Synapses zote za retina zinapatikana katika tabaka la plexiform, na habari zote za kuona hupita kupitia angalau synapses mbili, moja katika tabaka la nje la plexiform na nyingine katika tabaka la ndani la plexiform, kabla ya kuondoka jicho. Uchambuzi wa picha ya kuona hutokea katika tabaka zote mbili plexiform. Plexiform tabaka ya nje (OPL) hutenganisha habari ya kuona katika ON na OFF njia na hufanya spatial aina uchambuzi juu ya pembejeo ya kuona. Neurons ya pato ya OPL, seli za ON- na OFF-bipolar zinaonyesha shirika la uwanja wa mapokezi ya kupinga katikati-kuzunguka. Plexiform ya ndani (IPL) ni sehemu ya ndani ya mfumo wa kuona. Chembe nyingi zinazopokea pembejeo katika IPL hujibu kwa majibu ya muda mfupi na zina hisia sana kwa vichocheo vinavyohamia. Neurons ya INL (chembe za ganglion) husafirisha ujumbe wa kuona kwa ubongo wote; majibu ya chembe za ganglion huonyesha usindikaji zaidi wa OPL, wengine usindikaji wa IPL. Retina nyingi za wanyama wenye uti wa mgongo zina aina mbili za msingi za chembe za photoreceptor, fimbo na koni. Vipande vya macho huwezesha kuona katika nuru dhaifu, ilhali vifungo vya macho hufanya kazi katika nuru nyangavu na huongoza katika kuona rangi. Hisia za mwanga za photoreceptors hutokana na uwepo wa molekuli za rangi ya kuona zilizo ndani ya eneo maalum la seli, linaloitwa sehemu ya nje. Rhodopsin ni rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi ya rangi. Rangi zote za kuona zina kemikali zinazofanana. Vitamini A inaitwa retinal (vitamini A aldehyde) na inaitwa chromophore, ambayo imefungwa na protini inayoitwa opsin. Rangi tofauti-tofauti za kuona zina opsin tofauti-tofauti, na hilo husababisha tofauti katika hisia zao za rangi. Uwezo wa rangi za kuona kuonekana kwa mwanga unatokana na kromofora ya retina. Wakati molekuli ya rangi ya kuona inachukua kiasi cha mwanga, mabadiliko kadhaa ya umbo hutokea, kwanza katika chromophore na kisha katika sehemu ya protini (opsin) ya molekuli. Mabadiliko ya muundo wa opsin husababisha mfululizo wa vipengele vya kati vinavyoitwa metarhodopsin (Figure 2), na moja, metarhodopsin II, husababisha msisimko wa seli ya photoreceptor kupitia mfululizo wa mwingiliano wa molekuli. Vipokezi vyote vya picha vya wanyama wenye uti wa mgongo huongeza mwangaza kwa kujibu nuru (yaani, ndani ya chembe inakuwa hasi zaidi). Kwa nini photoreceptors hyperpolarize katika mwanga ni kama ifuatavyo: Katika giza, utando wa sehemu ya nje ni permeable (leaky) kwa Na+. Kwa sababu viwango vya Na + ni juu nje ya seli kuliko ndani, ioni za Na + zenye malipo mazuri huingia seli katika giza, na kusababisha seli kuwa na sehemu ya depolarized (yaani, seli ya ndani ni chanya zaidi kuliko kawaida kwa neurons katika mapumziko). Nuru hupunguza upenyezaji wa membrane ya sehemu ya nje ya Na +, na hivyo kupunguza mtiririko wa ioni chanya ndani ya seli na kusababisha ndani ya seli kuwa hasi zaidi (yaani, hyperpolarize). Katika primates wengi ikiwa ni pamoja na wanadamu, baadhi ya ndege na wanyama wanaotambaa, eneo dogo la retina ni maalumu kwa ajili ya maono ya juu-acuity. Inaitwa fovea na iko katikati (yaani Mfano wa mviringo wa macho (Visual axis of the eye) Sehemu za retina zilizo chini ya photoreceptors zimehamishwa upande mmoja kuzunguka fovea ili mwanga uweze kugonga zaidi moja kwa moja kwenye photoreceptors. Hilo husababisha upungufu wa retina katikati, na kufanyiza kile kinachoitwa shimo la foveal. Kuna konesi katika sehemu ya kati ya fovea, na konesi za foveal ndizo zenye urefu na nyembamba zaidi katika retina. Sehemu ya kibofu cha binadamu ni 0.3mm kwa kipenyo na ina cones 35,000 hivi. Hakuna mishipa ya damu inayopatikana katika fovea ya binadamu na, zaidi ya hayo, kuna koni chache za kunyonya bluu katikati ya fovea. Ubora huo husaidia kuboresha uwezo wa kuona fovea. Neurons: Seli na synaptic shirika Ingawa kuna tu nne msingi madarasa ya retinal neurons pamoja na photoreceptors, aina nyingi za neurons zipo katika kila moja ya madarasa haya. Retina ina aina moja ya seli ya fimbo na aina tatu au nne za koni ambazo zina majibu ya juu kwa mwanga wa urefu tofauti wa mawimbi. Nuru ya rangi ya kijani, rangi ya bluu, rangi ya bluu, rangi ya bluu. Aina tatu hadi nne za chembe za usawa zipo katika retina nyingi na aina tisa hadi kumi na moja za chembe za bipolar. Aina za chembe za amacrine huwa nyingi zaidi na retinas kuwa na kama ishirini au zaidi. Hatimaye, aina kumi na mbili hadi kumi na tano za chembe za ganglion zimetambuliwa katika spishi mbalimbali. Aina mbili za synapses za kemikali zimetambuliwa katika retina zote: synapses za mkanda zilizotengenezwa na photoreceptors na mwisho wa bipolar, na synapses za kawaida zilizotengenezwa na seli za amacrine na za usawa. Synapses ya Ribbon ni muundo wa ribbon-kama-presynaptically ambayo hutumika kama mkanda wa usafirishaji kuleta vesicles synaptic kwa utando presynaptic ambapo wao kuunganisha na kutolewa maudhui yao. Synapses ya kawaida ni sifa na kundi la vesicles synaptic apposed kwa utando ambayo wao kushikamana. Kwa kuongezea, photoreceptors za koni hufanya synapse isiyo ya kawaida ya kemikali, inayoitwa flat au basal junction, hasa kwenye seli za bipolar za aina ya OFF, na hatimaye, synapses za umeme zinaonekana kati ya aina nyingi za neuroni za retina. Picha ya 4 inaonyesha kwa njia rahisi sana utaratibu wa seli na synaptic ya retina ya primate. Photoreceptors (R) na cones (C) hutoa pembejeo kwa OPL, kuwasiliana seli za usawa (H) kupitia synapses ya mkanda na seli za bipolar kupitia synapses ya mkanda au gorofa. Aina moja ya seli ya usawa imeonyeshwa pamoja na aina nne za seli za bipolar: seli za bipolar za gorofa (OFF) na za intraginating (ON) ambazo zinagusa koni moja (FMB na IMB), bipolares zilizosambazwa ambazo zinagusa koni kadhaa (IDB) na bipolares za rod (RB) ambazo zinagusa tu vijiti. Katika IPL, chembe za bipolar kupitia synapses za mkanda huwasiliana na chembe za ganglion ndogo (MG), chembe kubwa za ganglion (G) na chembe za amacrine (A). Chembe za fimbo za bipolar huwasiliana na chembe za amacrine ambazo hupitisha habari za fimbo kwenye viwanja vya chembe za fimbo za bipolar na chembe kubwa za ganglion. Chembe nyingine za amacrine huwasiliana na terminal za bipolar za dwarf, na seli za dwarf na za ganglion zilizosambaa. Kwa mfano, aina ya seli ya amacrine, inayoitwa interplexiform, hupokea habari kutoka kwa INL na kurudisha habari kwa OPL. Majibu ya mwanga na usindikaji wa habari ya retina Neurons distal retinal <unk> photoreceptor, mviringo na bipolar seli, kujibu mwanga na endelevu graded membrane uwezekano (voltage) mabadiliko. Seli hizi pia kujibu kwa hyperpolarizing katika majibu ya stimulations mwanga <unk> membrane uwezekano inakuwa zaidi hasi (Sura ya 5). Neurons zaidi ya karibu ya retina hujibu mwanga hasa kwa kuondoa polarization <unk> uwezekano wa utando unakuwa chanya zaidi <unk> na hutokeza uwezekano wa hatua kama neurons nyingi za ubongo. Neurons distal retinal kutumia kuendelea graded uwezo wa kubeba ishara za kuona, pengine kwa sababu mbili: 1) wana mchakato mfupi na hawana haja ya kubeba habari umbali mrefu; kwa hiyo, electrotonic kuenea ya uwezo pamoja na utando wa seli ni ya kutosha kuhamisha habari kutoka mwisho mmoja wa seli kwa mwingine; na 2) graded uwezo ni uwezo wa kutofautisha mbalimbali pana ya ishara kuliko inaweza wote au hakuna matukio. uwezo wa hatua). Photoreceptors zote hyperpolarize katika kujibu kwa mwanga; kupitia synapses yao wao kusababisha mviringo na OFF-bipolar seli hyperpolarize na ON-bipolar seli depolarize. Chembe za usawa hufanya kazi kama neuroni za kuzuia upande, kuanzisha shirika la kawaida la uwanja wa mapokezi ya katikati-kuzunguka iliyoonekana kwa mara ya kwanza katika seli za bipolar. Jibu la katikati linaonyesha pembejeo ya moja kwa moja ya seli ya photoreceptor-bipolar; majibu ya pande zote ya upinzani yanaonyesha pembejeo ya seli ya usawa ya kuzuia kutoka kwa photoreceptors zaidi ya mbali. Aina mbili za majibu ya seli ya amacrine <unk> ya muda mfupi na ya kudumu <unk> zimegunduliwa katika retina nyingi. Chembe za amacrine za muda mfupi zinaweza kujibu kwa kuondoa polarization kwenye ON-, OFF- au kwenye ON- na OFF- ya taa. Seli za amacrine zilizohifadhiwa zinaweza kuondoa au kuongeza mwangaza kwa mwanga. Uwezo wa hatua mara nyingi huonekana kwenye majibu ya kuondoa polarization ya seli za amacrine. Chembe zote za ganglion hutokeza uwezo wa vitendo na kwa mwangaza wa uwanja kamili, aina tatu za msingi za majibu huonekana; Chembe za ON, OFF, na ON-OFF. Hata hivyo, kwa kuchunguza retina na spots ndogo ya mwanga, mashirika ya shamba la kupokea ya seli za ganglion hupatikana kuwa tofauti na ngumu, ikionyesha mwingiliano wa kuamsha, kuzuia na modulatory unaotokea katika retina, hasa katika safu ya ndani ya plexiform. Kwa mfano, kuna chembe za ganglion za OFF-center na OFF-center, na chembe za OFF-center na OFF-center. Utafiti wa hivi karibuni uligundua aina kumi ndogo za majibu ya chembe za ganglion katika retina ya sungura na kuunganisha hizi na aina mbalimbali za chembe za ganglion za morphological. Kila moja ya aina hizi za chembe za ganglion hutoa vituo vya juu vya kuona na uwakilishi tofauti kidogo wa picha inayowaka kwenye retina. Kwa hiyo, mambo mengi ya kuona hutukia katika retina, na ni hakika kwamba hatujui kiasi gani cha mambo hayo. Aina ya seli ya ganglion inayopatikana katika retina nyingi ni seli inayoweza kuonekana kwa mwendo, ambayo inajibu vizuri wakati madoa ya mwanga yanapotembea kwenye retina kwa mwelekeo mmoja, lakini sio wakati madoa yanapotembea kwa mwelekeo mwingine. Aina maalum ya seli amacrine - seli amacrine starburst - inahusika katika kizazi cha majibu ya kuchagua mwelekeo, na uelewa wa mwingiliano wa synaptic ambayo ni msingi wa kuchagua mwelekeo katika seli hizi za ganglion ni vizuri pamoja. Vipeperushi, Modulators, na Receptors: Retina, kama maeneo mengine ya ubongo, hutumia idadi kubwa ya vitu vya neuroactive. Neurotransmitters ni vitu vinavyofanya kazi kwa haraka na moja kwa moja kwenye neurons za retina kwa kubadilisha upenyezaji wa utando kwa ioni moja au zaidi, na neuromodulators ni vitu vinavyofanya kazi polepole zaidi kwa kubadilisha neurons kupitia cascades za biochemical za intracellular. Kuna vitu vichache vinavyoonekana kutumika kama neurotransmitters ya retina; wengi wa vitu vya neuroactive vilivyotolewa kutoka kwa neurons za retina (hasa kutoka kwa seli za amacrine) zinaonekana kuwa neuromodulatory katika asili ingawa kidogo inajulikana kuhusu hatua au jukumu la wengi wa vitu hivi. Amino asidi na acetylcholine ndizo vitu vikuu vinavyopitisha neva vinavyotumiwa katika retina. L-glutamate hutumiwa na photoreceptors na seli za bipolar kama transmitter yao ya kuamsha, wakati acetylcholine hutumiwa kama neurotransmitter ya kuamsha katika IPL, hasa na seli za amacrine za starburst. GABA na glycine ni vitu muhimu vya kuzuia utoaji wa retina, na inadhaniwa kwamba 80% ya seli za amacrine hutoa moja au nyingine ya vitu hivi. Utafiti wa kina unaonyesha jinsi receptors mbalimbali zinazohusika katika neurons zinazohusika katika neurotransmitters na modulators katika retina. Kwa mfano, ni wazi kwamba seli za bipolar ON na OFF, na labda hata seli za usawa, huonyesha mapokezi tofauti ya glutamate, na jinsi mapokezi haya yanavyozalisha majibu ya neuroni mbalimbali inaanza kueleweka. "Kuchunguza kwa kina jinsi glutamate inayotokana na seli za photoreceptors inavyosababisha ""swali la seli ya ON-bipolar.""" Aina mbili za molekuli zimehusishwa - receptor ya metabotropic glutamate iliyoelezwa na seli za bipolar za fimbo katika spishi zote na receptor kama ya usafirishaji iliyoelezwa na seli za bipolar za ON-cone katika retina ya teleost. Kuna vitu kadhaa vinavyoaminika kufanya kazi kama wakala wa neuromodulator katika retina, ingawa jinsi wanavyofanya kazi na jukumu lao halijulikani vizuri. Vitu vilivyopendekezwa vya neuromodulatory ni pamoja na idadi ya amines, neuropeptides na vitu vingine kama vile nitric oxide, asidi ya retinoic, na zinki. Neuropeptides kadhaa zimegunduliwa katika retina, hasa katika seli za amacrine, na mara nyingi zinashirikiana na GABA. Majukumu yao na umuhimu wa colocalization na GABA haijulikani. Katika vitu vyote vya neuromodulator, tunajua zaidi juu ya jukumu la dopamine, ambayo imeonyeshwa kuhariri viunganishi vya umeme katika retina na mapokezi ya glutamate kwenye seli za usawa na bipolar. Dopamine, kupitia AMP ya mzunguko na protein kinase A (PKA), hubadilisha conductivity ya viungo vingi vya retinal, na unyeti wa receptors fulani za glutamate kwa glutamate. Dopamine katika retina pia inaaminika kuwa na jukumu katika kuanzisha retinal circadian rhythmicity, na kudhibiti retinal motor harakati katika samaki na aina nyingine zisizo za mamalia. Baylor, Denis A. "Mfano wa ""Photoreceptor Signals and Vision"" ni utafiti wa uchunguzi wa macho na sayansi ya kuona." 28:34-49 - Ni nini kilichotokea? Dowling, John E. (1987) Retina: Sehemu ya Ubongo Inayopatikana. Chapa ya Chuo Kikuu cha Harvard. Fried, S.I., Münch, T.A. na Werblin, Frank, S. (2005) Uteuzi wa mwelekeo huundwa katika ngazi nyingi na kizuizi cha upande katika retina ya sungura. Neuron (kiini cha neva). 117-127 na 46 Kolb, Helga, Ripps, H. na Wu S. (Mhariri) (2001) Dhana na changamoto katika Retinal Biolojia. Amsterdam: Elsevier. "Polyuk, Steven L. (1941) ""Kutoka kwa retina.""" Chicago: Chuo Kikuu cha Chicago Press. Rodick, Robert W. (1998), The First Steps in Seeing (Hatua za Kwanza za Kuona). Sunderland, MA: Sinauer Associates. "Kama ilivyoelezwa katika makala ya ""Utaratibu wa Kuunganisha kwa Chembe za Ganglion za Retina: Jinsi ya Kuunganisha Mifumo ya Nafasi-Nguvu ya Kuchochea na Inhibition"" katika jarida la Neurophysiology, 95, 108, 382." Sterling, P. na Demb, J. B. Retina (2004) In, Shepherd G. M. (ed) The Synaptic Organization of the Brain. Ukurasa wa 21692 - Mfano wa Uongozi Oxford: Oxford Chuo Kikuu Press. Valentino Braitenberg (2007) - Mwili wa mtu Scholarpedia, 2 (11): 2918. Keith Rayner na Monica Castelano (2007) "Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia""" - Eugene Roberts (2007) Gamma-aminobutyric asidi. "Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia: ""Schoolpedia"