19.9
La Retina
La retina es una capa de tejido nervioso en la parte posterior del ojo que transduce la luz en señales neuronales. Este proceso, llamado fototransducc…
- [Profesor] Detrás de la retina, hay bastones y conos,
células fotorreceptoras que transducen luz
en señales neuronales.
Tienen segmentos externos con moléculas fotopigmentadas
que absorben la luz, generando cambios electroquímicos
que bajan su tasa de liberación del neurotransmisor
en comparación a cuando están en la oscuridad.
Los bastones son muy sensibles a la luz
y más que nada se usan en condiciones de baja luminosidad.
Por el contrario, los conos son los responsables
de gran parte de la visión diurna.
Están compactados densamente en la fóvea,
una pequeña depresión cerca del centro de la retina
que brinda la agudeza visual mayor.
Los humanos poseen tres tipos de conos:
azul, verde y rojo,
correspondientes a las longitudes de onda de la luz
absorbidas al máximo por sus fotopigmentos.
Por lo tanto, la activación relativa
de los distintos tipos de conos codifican el color.
La información de la luz de los bastones y los conos
se transmite a las células bipolares.
Las células horizontales median las interacciones
entre los fotorreceptores y las células bipolares,
ayudando en el procesamiento de la información visual.
Luego, las células bipolares
envían la información a las células ganglionares.
Otro grupo, las células amacrinas,
se conectan con la sinapsis entre estas neuronas
ayudándolas a analizar aún más el estímulo.
Por último, la información visual
se envía mediante los axones de las células ganglionares,
que componen el nervio óptico,
hacia el cerebro para un nivel mayor de procesamiento.
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Q1: What are rods and cones and what do they do in the retina?
Rods and cones are photoreceptor cells in the back of the retina that convert light into neural signals through phototransduction. Rods are highly sensitive and function in low-light conditions, while cones are less sensitive and enable most daytime vision. Cones also provide color information and are densely concentrated in the fovea, the retina's central region responsible for sharp visual acuity.
Q2: How do photopigments in photoreceptors respond to light?
Photopigment molecules in the outer segments of rods and cones absorb light, triggering electrochemical changes that hyperpolarize the photoreceptor cell. This hyperpolarization decreases neurotransmitter release compared to darkness. Unlike most sensory neurons, photoreceptors reduce their signaling when stimulated by light rather than increasing it.
Q3: Why are there three types of cones in human vision?
Humans have three cone types—blue, green, and red—each containing photopigments that maximally absorb different light wavelengths. The relative activation of these three cone types encodes color information. Although all opsin varieties are present in each cone, different opsins predominate in each type, enabling color discrimination across the visible spectrum.
Q4: What is the fovea and why is it important for vision?
The fovea is a small depression near the center of the retina where cones are densely packed and rods are very few. This region provides the greatest visual acuity, allowing the eye to focus sharply on objects directly in view. The high concentration of cones in the fovea enables detailed daytime vision and color perception in the area of central focus.
Q5: How do bipolar cells, horizontal cells, and amacrine cells process visual information?
Photoreceptors transmit light information to bipolar cells, which relay signals to ganglion cells. Horizontal cells mediate interactions between photoreceptors and bipolar cells, while amacrine cells connect to synapses between bipolar and ganglion cells. Together, these cells integrate information across the retina and enable initial visual processing, such as contrast detection under varying light conditions.
Q6: What role does the optic nerve play in transmitting visual information?
The optic nerve is composed of axons from ganglion cells and glial cells located at the back of the eye. Visual information travels through these axons to the brain for higher-level processing and interpretation. The optic nerve serves as the final output pathway from the retina, carrying all processed visual signals to the central nervous system.
Q7: How does the retina fit into the broader sensory system?
The retina is a layer of nervous tissue that functions as part of the visual sensory system, converting light stimuli into neural signals. Like other sensory systems, the retina detects environmental stimuli and initiates signal processing before transmission to the brain. Understanding the retina's structure and function provides insight into how sensory system and perception work together to create vision.