20.5: Photoreceptors and Visual Pathways
At the molecular level, visual signals trigger transformations in photopigment molecules, resulting in changes in the photoreceptor cell's membrane potential. The photon's energy level is denoted by its wavelength, with each specific wavelength of visible light associated with a distinct color. The spectral range of visible light, classified as electromagnetic radiation, spans from 380 to 720 nm. Electromagnetic radiation wavelengths exceeding 720 nm fall under the infrared category, whereas those below 380 nm are classified as ultraviolet radiation. Blue light corresponds to a wavelength of 380 nm, while dark red light corresponds to a wavelength of 720 nm. Other colors lie at varying points within this wavelength spectrum, from red to blue.
Opsin pigments, in fact, are transmembrane proteins integrated with a cofactor named retinal. This retinal is a constituent of vitamin A and a hydrocarbon molecule. The significant biochemical alteration in the extensive hydrocarbon chain of the retinal molecule is triggered when a photon impacts it. This specific process, known as photoisomerization, transitions some of the double-bonded carbons inside the chain from a cis to a trans configuration owing to the photon interaction. Before the photon interaction, the flexible double-bonded carbons of the retinal are in the cis conformation, leading to the formation of a molecule known as 11-cis-retinal. The double-bonded carbons assume the trans-conformation when a photon impacts the molecule, forming an all-trans-retinal characterized by a straight hydrocarbon chain.
The visual transduction process within the retina commences with the alteration in the retinal structure in photoreceptors. This leads to the activation of retinal and opsin proteins, which stimulate a G protein. The activated G protein then modifies the photoreceptor cell's membrane potential, causing a decrease in the release of neurotransmitters into the retina's outer synaptic layer. This state continues until the retinal molecule reverts to its original shape, the 11-cis-retinal form - a process referred to as bleaching. If a substantial amount of photopigments undergo bleaching, the retina transmits data as if contrasting visual inputs are being received. Afterimages, typically observed as negative-type images, are a common occurrence following exposure to an intense flash of light. A series of enzymatic alterations facilitate the photoisomerization reversal process, thus enabling the reactivation of the retinal in response to additional light energy.
Opsins exhibit specific sensitivity to particular light wavelengths. The rod photopigment, rhodopsin, exhibits peak sensitivity to light that has a wavelength of 498 nm. On the other hand, three color opsins are optimally responsive to wavelengths of 564 nm, 534 nm, and 420 nm, which approximately align with the primary colors—red, green, and blue. Rhodopsin found in rods demonstrates a higher sensitivity to light than cone opsins; this means that rods contribute to vision under dim light conditions while cones contribute under brighter conditions. In normal sunlight, rhodopsin is continuously bleached, and cones remain active. Conversely, in a dimly lit room, the light intensity is insufficient to stimulate cone opsins, making vision entirely reliant on rods. In fact, rods have such a high sensitivity to light that a solitary photon can trigger an action potential in a rod's corresponding RGC.
Cone opsins, differentiated by their sensitivity to distinct light wavelengths, furnish the ability to perceive color. By analyzing the responses of the three unique cone types, our brain distills color data from what we see. Consider, for instance, a bright blue light with a wavelength near 450 nm. This would cause minimal stimulation of the "red" cones, slight activation of the "green" cones, and significant stimulation of the "blue" cones. The brain computes this differential activation of the cones and interprets the color as blue. However, under dim light conditions, cones are ineffectual, and rods, which cannot discern color, dominate. As a result, our vision in low light is essentially monochromatic, meaning everything appears in varying shades of gray in a dark room.
Some common eye disorders:
Color blindness, clinically known as achromatopsia, is a condition characterized by a deficiency in distinguishing colors. This disorder usually results from an inherited defect in the retina's cones (light-sensitive cells). Symptoms may include difficulty distinguishing between colors or shades of colors.
Night blindness, medically referred to as nyctalopia or hemeralopia, is a disorder that affects an individual's ability to see in low light or at night. Causes can range from vitamin A deficiency to underlying diseases such as retinitis pigmentosa. Individuals with this disorder experience difficulties with night-time vision or adjusting to dim lighting.
Cataracts, a common eye disorder especially among older adults, are characterized by clouding of the normally transparent eye lens. This can result in blurred vision, similar to looking through a fogged-up window. Most cataracts develop slowly over time and can eventually interfere with vision.
Glaucoma is another severe eye condition where the optic nerve, which sends images to the brain, gets damaged due to increased pressure in the eye. It can lead to vision loss if left untreated. The most common type of glaucoma, open-angle glaucoma, often has no symptoms other than gradual vision loss.
