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{{DISPLAYTITLE:El ojo y su retina}}[[File:Eye-diagram.png |thumb|right|450px|<ol><div style="float: left; width: 25%;"><li>cuerpo vítreo</li><li>''ora serrata''</li><li>músculo ciliar</li><li>zónula ciliar</li><li>canal de Schlemm</li><li>pupila</li><li>cámara anterior</li><li>córnea</li><li>iris</li><li>corteza del cristalino</li></div><div style="float:left; width:36%"><li>núcleo del cristalino</li><li>proceso ciliar</li><li>conjuntiva</li><li>músculo oblicuo inferior</li><li>músculo recto inferior</li><li>músculo recto medio</li><li>arterias y venas retinianas</li><li>papila óptica</li><li>duramadre</li><li>arteria central de la retina</li></div><div style="float:right; width:33%"><li>vena central de la retina</li><li>nervio óptico</li><li>vena vorticosa</li><li>vaina bulbar</li><li>mácula</li><li>fóvea</li><li>esclerótica</li><li>coroides</li><li>músculo recto superior</li><li>retina</li></div></ol><ref>Diagrama del ojo commons.wikimedia.org/wiki/File:Eye-diagram_no_circles_border.svg </ref>]] El ojo es el órgano de la vista, que nos permite percibir nuestro entorno en una escala superior comparada con los demás sentidos. Los ojos captan los fotones del ambiente y los convierten en impulsos eléctricos, posteriormente integrados en imágenes visuales. Parte del proceso ocurre en el ojo, pero la mayoría de las etapas tienen lugar en la corteza visual, en el encéfalo. ==Anatomía del ojo== ==== La córnea ==== <p>La córnea es la superficie traslúcida del ojo que recubre a la pupila y el iris. Además de brindar protección, es la primera superficie de refracción por la que la luz llega a la retina. La córnea carece de irrigación sanguínea en condiciones normales, de modo que el oxígeno necesario lo obtiene del aire. Cabe resaltar que ante la ausencia prolongada de oxígeno, como en el empleo excesivo de lentes de contacto, el organismo trata de compensarla con angiogénesis corneal. Esto influye en la posible aparición de una respuesta inmunológica perjudicial (enrojecimiento, dolor, hinchazón y, al final, lesión corneal irremediable).</p> <p>La córnea tiene cinco capas: epitelio, membrana de Bowman, estroma, membrana de Descemet y endotelio. El epitelio y la membrana de Bowman protegen los demás componentes de la córnea. El estroma representa 90 % del grosor total en tanto sea transparente. La membrana de Descemet y el endotelio regular el flujo de líquido y nutrientes entre la córnea y el resto del ojo. <ref>Binder, P. S. et al. (July 1991) High-voltage electron microscopy of normal human cornea. Invest. Ophthalmol. Vis. Sci. <strong>32</strong> (8): 2234-43</ref></p> <p>En 2013 se reportó la posible existencia de una sexta capa, con la denominación propuesta de membrana de Dua,<ref>Dua, Harminder S. et al. (September 2013) Human Corneal Anatomy Redefined: A Novel Pre-Descemet's Layer (Dua's Layer) Ophthalmology <strong>120</strong> (9): 1778-1785 doi: 10.1016/j.ophtha.2013.01.018</ref>, aunque algunos investigadores han tomado esta aseveración «con escepticismo» y alegan que no es una capa nueva, sino parte del estroma,<ref name=McKee01a>McKee, Hamish D. et al. (mayo de 2014, publicado en línea en febrero de 2014) Re: Dua et al.: Human corneal anatomy redefined: a novel pre-Descemet layer (Dua's layer) (Ophthalmology 2013;120:1778–85) Ophthalmology <strong>121</strong> (5): e24-e24 doi: 10.1016/j.ophtha.2013.12.021 «Resumen: Leímos con escepticismo la reciente declaración del descubrimiento de Dua y cols. de una nueva capa corneal. Se sabe que queda estroma residual antes de la capa de Descemet una vez hecha la neumodisección. Su investigación de este estroma pre-Descemet corrobora que es estroma, no una nueva capa corneal».</ref> y también han debatido en nombre propuesto<ref name=McKee02a>McKee, Hamish D. et al. ANZ Cornea Meeting 2014 Abstracts, página 3 «La “capa de Dua” es el ya descrito estroma pre-Descemet […] los epónimos médicos se han creado tradicionalmente como reconocimiento de colegas a los aportes y hallazgos de alguien. Dua ha sacado ventaja creando su propio epónimo, antes de que siquiera sus declaraciones hayan pasado la prueba de la investigación minuciosa y el escrutinio, aunado a la reciente tendencia de evitar los epónimos médicos (y cuando ya están en uso, hacerlo en su forma no posesiva [en inglés]). Si se prefiere el epónimo del estroma remanente del estroma pre-Descemet una vez hecha la neumodisección, “estroma de Feizi” es lo más adecuado».</ref><ref name=Steinert01a>Steinert, Roger (Medscape, 01 October 2014) A Controversy in Cornea «[…] el supuesto descubrimiento de una nueva capa en la córnea […] todos sabemos que actualmente se censura el uso de epónimos para nombrar tejidos. Es más adecuado y útil el empleo de términos anatómicos o físicos que sean acordes».</ref></p> ==== El iris y la pupila ==== [[File:Eye dilate.gif|right|300px]] El iris es la zona del ojo entre la córnea y el cristalino. En el centro se encuentra la pupila, una apertura que permite la entrada de luz. Los músculos del iris contraen el iris ante luz brillante y lo dilatan en condiciones de poca luz. La concentración de melanina es el factor preponderante que define el color del iris. En la ausencia relativa de melanina, el color del iris suele ser azul o verde. Por otra parte, con mayor cantidad de melanina, es más probable que el iris sea café o negro.<ref>"iris." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Web. 16 Jun. 2014.</ref> ==== El cristalino ==== El cristalino es una estructura posterior al iris que concentra la luz hacia la retina. El cristalino en el adulto carece de irrigación sanguínea y se nutre mediante el humor acuoso, aunque en la fase de desarrollo, el cristalino en formación obtiene sus nutrientes a través de la arteria hialoidea, que degenera post-parto. Los músculos ciliares deforman el cristalino para que enfoque la luz que ingresa a la retina, de modo que el ojo puede enfocar objetos a diferentes distancias.<ref>"lens." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Internet. 16 de junio. 2014.</ref> ==== El humor vítreo ==== El humor vítreo es un líquido espeso, gelatinoso, que da forma al ojo, pues llena el espacio entre el cristalino y la retina y el disco óptico. Constituye casi 80 % del volumen del ojo y se compone de 98 % agua.<ref>"human eye." Encyclopaedia Britannica. Encyclopaedia Britannica Online Academic Edition. Encyclopædia Britannica Inc., 2014. Internet. 16 de junio de 2014.</ref> ==== Esclerótica ==== La esclerótica se conoce más como «lo blanco del ojo». Es una capa fibrosa que se transparenta en la zona anterior del ojo y forma la córnea. El ojo humano es único dado que el iris es del tamaño ideal para que se pueda observar su posición respecto de la esclerótica, con lo que se puede determinar hacia dónde mira alguien, con el desarrollo de una comunicación no verbal. == Anatomía de la retina humana == [[File:Cells of the Retina.jpg|thumb|right|400px|Tipos de neuronas presentes en la retina de los mamíferos.<ref name="Masland2001">Masland, R. The fundamental plan of the retina (2001). Nature Neuroscience <strong>4</strong> (9): 877-886</ref>]] La retina es una capa tisular fotosensible que cubre la superficie interna del ojo. La luz del campo visual atraviesa el ojo y se proyecta a la retina para crear una imagen. Posteriormente las neuronas retinianas detectan dicha imagen, lo que inicia una cascada de procesos bioquímicos y eléctricos que llegan a la corteza visual del encéfalo pasando por el nervio óptico. Estas señales bioquímicas y eléctricas son la base de la visión. <!--[[File:NormalRetina.gif | thumb | right | 300px | Diagrama que ilustra la posición de la copa y la papila ópticas, la mácula, la fovea, venas y arterias.<ref name="Kolb"></ref>]] ==== Copa y disco óptico ==== ==== Mácula and fóvea ==== ==== Las diez capas histológicas de la retina ==== --> === Células de la retina === ====[[Photoreceptors/es | Fotorreceptores]]==== <gallery mode=packed heights=250px> File:Cone cell en.png|Anatomía de una neurona tipo cono File:Rod.png|Anatomía de una neurona tipo bastón<ref> ''Human Physiology and Mechanisms of Disease'' de Arthur C. Guyton (1992) p. 373</ref> </gallery> Los fotorreceptores agrupan dos clases de neuronas: conos y bastones. Los bastones se concentran en las zonas más externas de la retina y sirven para la visión periférica. Son más sensibles a la luz que los conos y se encargan, casi por completo, de la visión nocturna (llamada visión escotópica). Los conos se acomodan en el centro de la retina, es decir, en la fóvea. Los conos se encargan de la visión a color, llamada fotópica. Los mamíferos suelen tener dos o tres distintos tipos de conos, pues se deben comparar al menos dos señales de salida de clases de conos diferentes para precisar la longitud de onda de un estímulo (p. ej., su color). Estas neuronas fotorreceptoras poseen proteínas llamadas opsinas en sus porciones más externas (cilios modificados) que se unen a un cromóforo que proviene de la vitamina A, con lo que estas células pueden transformar la luz recibida en impulsos eléctricos. Se puede definir la variedad de longitudes de onda dependiendo de la estructura de las opsinas que posee una célula.<ref>Plachetzki, D.; Fong, C.; Oakley, T. (2010). "The evolution of phototransduction from an ancestral cyclic nucleotide gated pathway". Proceedings. Biological sciences / the Royal Society 277 (1690): 1963–1969. doi:10.1098/rspb.2009.1797. PMC 2880087. PMID 20219739.</ref> ====[[Célula horizontal]]==== [[File:Horizontal_Cells.png|300 px|thumb|center|Células horizontales en la retina]] Horizontal cells are thought to exist in two types, each with a distinct shape, which together provide feedback to all photoreceptor cells. Despite the number of cells with which they form [[Synapse|synapses]], horizontal cells represent a relatively small population of the retina’s cells (less than 5% of cells of the inner nuclear layer). The specific reason for the existence of the two classes of horizontal cells is not yet known; it potentially involves detection of color differences in the red-green system. ====[[Amacrine Cell]]==== [[File:StarburstMarathonCell.png|300 px|thumb|center|A [[Starburst Amacrine Cell | starburst amacrine cell]] reconstructed in EyeWire]] Amacrine cells appear to allow for ganglion cells to send temporally correlated signals to the brain: input to two separate ganglion cells from the same amacrine cell will tend to make those ganglion cells send signals at the same time. The amacrine cells whose behaviors are well understood have been shown to have very specific functions. ====[[Bipolar Cell]]==== [[File:114BPCells.jpg|300 px|thumb|center|A reconstruction of 114 rod bipolar nerve cells from a piece of mouse retina. The dense bundles (top) are dendrites, and the sparser processes below are [[Axon|axons]] (credit: MPI for Medical Research).]] Bipolar cells connect photoreceptors and ganglion cells. Their function is to transmit signals from photoreceptors to ganglion cells, either directly or indirectly. Bipolar cells get their name from their shape — they have a central cell body from which two different sets of neurites ([[Axon | axons]] or [[Dendrite | dendrites]]) extend. They can make connections with either rods or cones (but not both simultaneously), and they also form connections with horizontal cells. Unlike most neurons, which communicate with one another using [[Axon#Action_Potential|action potentials]], bipolar cells “talk” with other cells using graded potentials. ====[[Ganglion Cell]]==== [[File:Ganglion Cell.png|300 px|thumb|center|A ganglion cell reconstructed in EyeWire]] Ganglion cells are the output cells of the retina. Their [[Axon|axons]] leave the eye and travel through the optic nerve to the brain, sending the processed visual stimulus to the lateral geniculate nucleus, forming [[Synapse|synapses]] onto neurons that project to the primary visual cortex, where the stimulus can be further interpreted. ===Wiring it all together=== [[File:Simple Diagram of the Organization of the Retina.jpeg|thumb|left|300px|<ref name="Kolb">Kolb, Helga, Nelson, Ralph, Fernandez, Eduardo, Jones, Bryan, The Organization of the Retina and Visual System, Simple Anatomy of the Retina. http://webvision.med.utah.edu/book/part-i-foundations/simple-anatomy-of-the-retina/</ref>]] Understanding the functions of the individual cells is the first step to understanding vision, but it does not begin to explain the complex processes that occur in our eyes providing us with the sense. Today, scientists are studying [[Retinal Connectomics| retinal connectomics]] to better understand vision. But, mapping the full [[Connectomics: An Introduction |connectome]] is too difficult a process for the technology currently available, instead, scientists map partial connectomes. By looking at the [[Synapse | connections]] between the neurons, scientists can understand how an input to one neuron can affect one that's connected to it. Scientists look at connections between multiple neurons, treating them as a system, and try to understand how an input affects the output. While scientists study systems of just a few neurons at a time, there is a basic understanding of how the retina processes information as a whole.<ref name="Masland2001" /> ====A simplification of the process==== To explain the function of the retina in the most basic way, four of the major types of retinal neurons will be considered, the photoreceptors, bipolar cells, amacrine cells, and ganglion cells. This wiki page will continue to expand the model to include more factors from the retinal neurons. The first step to processing visual information is the collection of photons by the photoreceptors. The two types of photoreceptors, rods and cones, while serving different purposes, are similar in function. In general, the rods and cones form synapses with the bipolar cells. While the cones can often be the only cone to form a synapse with a bipolar cell, many rods tend to form synapses with the same bipolar cell. Because multiple rods share a single bipolar cell, the sharpness of the image is traded off for sensitivity of the signal. The bipolar cells then form synapses with amacrine cells. The amacrine cells provide signal feedback and transmit the signal to the ganglion cells. The ganglion cells are the final step of the process in the processing of light in the retina. It is with the ganglion cells that the signals from the cones are compared to process what color something is. The ganglion cells transfer the signal out of the retina to the brain.<ref name="Masland2001" /> ====Horizontal Cell Feedback==== While horizontal cells represent a small portion of the retina, they do form many synapses with the photoreceptors. One of the leading theories on the function of horizontal cells is to increase the contrast between the light and dark regions. The horizontal cells will inhibit the signal of an excited cone and the cones surrounding it. While this dampens the signal, it also eliminates the signal from surrounding cones that aren't excited, so only the ganglion cells associated with the excited cone will be excited, rather than all the ganglion cells of the cone and its neighbors. The rods receive feedback from specialized horizontal cells, which also give feedback to cones, in such a way that the two feedback systems are kept separate.<ref name="Masland2001" /> ====Amacrine Cell Feedback==== While most cells in the retina are flexible in their function, amacrine cells are highly specialized to do specific tasks. Scientists have proposed that amacrine cells make ganglion cells fire in correlation with one another, which could increase the amount of information that can be transferred by the optic nerve.<ref name="Masland2001" /> ==Current Research== According to many neuroscience textbooks, retinal ganglion cells can be categorized into two different types according to a property that is known as their receptive field. Neurons with receptive fields have been found in the auditory (hearing) system, the somatosensory (feeling) system and the visual system, and the receptive field of a particular neuron can generally be defined as a region of space in which the presence of a stimulus will alter the firing of that neuron. In the visual system, a receptive field of a particular retinal ganglion cell is defined as the region of the photoreceptor cell layer in the retina that alters the firing (signal-sending) of that ganglion cell when it is stimulated with light. According to textbook accounts, retinal ganglion cells either have ON-center, OFF-surround or OFF-center, ON-surround receptive field. An ON-center, OFF-surround ganglion cell will send a signal when the center of its receptive field detects light, but will be inhibited from firing when the area surrounding the center (the surround) of its receptive field detects light; OFF-center, ON-surround cells have the exact opposite response to light stimulation. [[File:Receptive_field.png|center|thumb|300px|How an ON center ganglion cell responds differently than an OFF center ganglion cell<ref name="rfwiki">http://en.wikipedia.org/wiki/Receptive_field</ref>]] Within the last decade, it has become increasingly clear that the notion that only two types of receptive fields exist in photoreceptors is a gross oversimplification. Scientists now know that ganglion cells come in at least 15 or 20 types, each of which has a distinct shape and physiological function, and which correspondingly has connections with different types of cells in the rest of the retina. At the Max Planck Institute (MPI) for Medical Research in Heidelberg, Germany, a dataset was obtained from a mouse retina in order to investigate this diversity in retinal ganglion cells – by applying two imaging techniques one after the other (two-photon microscopy (2P) and [[Serial block-face scanning electron microscopy (SBFSEM)|serial block-face scanning electron microscopy (SBFSEM)]]), scientists have been able to obtain images that show both neural activity and connectivity in retinal ganglion cells. However, the images are very difficult to analyze and interpret, and doing so is a very time-consuming process. Computer scientists at MIT are working on developing software to help with retinal image analysis, but computational analysis is currently much less accurate and reliable than that performed by humans. ==Navigating the Jungle== The ultimate goal motivating the research on the retina that is being done at places like MPI and MIT is to use 2P and SBFSEM images in order to identify specific cell types within the broad classes of retinal cells that were described earlier, and further to understand connectivity between these cells. Only once the different cell types have been comprehensively catalogued will researchers be able to investigate their specific functions. '''¡Aquí es donde te involucras! ''' In order to fully understand retinal computation, it is necessary to map all of the connections that converge onto ganglion cells, as this diversity of connections generates the diversity of visual signals that are sent to the brain. The challenge now is to refine the coarse knowledge about retinal connectivity in order to gain a much more in-depth understanding of the specific functions of each and every cell type in the retina. Currently, scientists think that there are at least between fifty and sixty types, so there is much work to be done! ==Referencias== <references />
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