ELM 14 Sensory Pathways - Vision Flashcards
What are the properties of light that affect vision?
Light is both an electromagnetic wave and a collection of photons. The amplitude of the wave corresponds to the intensity of light, while the wavelength determines its color. Humans can only perceive a narrow range of wavelengths as visible light, with variations leading to differences in color perception among individuals.
Describe the structure of the vertebrate eye.
The vertebrate eye includes structures such as the cornea, lens, iris, and retina. The cornea, a curved transparent layer, and the lens, a crystalline structure, focus incoming light onto the retina. The iris regulates the size of the pupil, controlling the amount of light entering the eye. Fluid-filled chambers called the aqueous humor and vitreous humor help maintain the shape of the eye and allow light to pass through.
How does the eye adjust to focus on objects at different distances?
The lens of the eye can adjust its shape and strength, a process known as accommodation, to focus on objects at varying distances. While the cornea provides the initial focusing power, the lens provides fine-tuning to ensure clear vision, particularly for objects that are closer or farther away.
What are common imperfections of the vertebrate eye, and how are they corrected?
Common imperfections of the vertebrate eye include myopia (short-sightedness) and hyperopia (far-sightedness), which result from errors in the focusing of light. Myopia occurs when the focal point of light is too soon, corrected with a concave lens, while hyperopia is the opposite, corrected with a convex lens. Astigmatism, caused by irregularities in the cornea or lens, can also affect vision.
What is the function of the retina in vision?
The retina, located at the back of the eye, contains photoreceptor cells that respond to light. Although light must pass through other retinal cells before reaching the photoreceptors, this arrangement provides structural support and allows for the regeneration of retinal cells. The retina enables the conversion of light stimuli into neural signals that are processed by the brain, facilitating vision.
What is an oddity about the structure of the retina?
The retina is inside-out, with some retinal ganglion cells (RGCs) capable of responding to light stimuli. Photoreceptor cells, including rods and cones, absorb photons of light, and the signals are transmitted through the various layers of the retina to the ganglion cells, which convert them into action potentials and convey them to the brain via the optic nerve.
Describe the pathway of visual signals through the retina.
Visual signals are transmitted from rods and cones to bipolar cells and then to ganglion cells in the retina. This pathway involves excitatory neurotransmission using glutamate. Additionally, the retina is structured to achieve noise suppression by combining signals from multiple sources, with horizontal and amacrine cells serving as inhibitory neurons to reduce background noise and enhance signaling.
What is the role of the Retinal Pigment Epithelium (RPE)?
The Retinal Pigment Epithelium (RPE) is a pigmented layer located at the back of the retina. It serves several functions, including making the pupil appear dark, recycling retinaldehyde to maintain the function of rods and cones, and helping these photoreceptor cells cope with oxidative stress. Additionally, the tapetum lucidum, a layer behind the RPE, aids in reflecting light within the eye, enhancing the ability to detect light, particularly useful for nocturnal animals.
Where are cones most concentrated, and what is their function?
Cones are most concentrated in the fovea, which is the area of the retina associated with maximum visual acuity. Cones are responsible for color vision and function best in bright light conditions. However, the fovea is less sensitive in low light conditions and is essentially blind at night.
What are the differences between rods and cones in terms of sensitivity and function?
Rods are specialized for night vision and peripheral vision, being approximately 1000 times more sensitive to light than cones. They are primarily responsible for vision in low-light conditions. In contrast, cones are responsible for color vision and function best in bright light conditions.
Where are the cell bodies of rods and cones located?
he cell bodies of rods and cones are found in the outer nuclear layer of the retina.
What is opsin, and how does it contribute to the sensitivity of photoreceptor cells?
Opsin is a G protein-coupled receptor (GPCR) with seven transmembrane domains found in photoreceptor cells. Different types of opsins are present in rods, cones, and melanopsin-containing retinal ganglion cells (RGCs), with a total of five opsins in these cells. Opsins maximize the sensitivity of these cells to light, with rods having a higher surface area for opsin protein, making them more sensitive to light. Opsin is embedded in the lipid bilayers of photoreceptor cells and binds to retinal, a derivative of Vitamin A.
What is retinal, and how does it contribute to the functioning of photoreceptor cells?
Retinal is a Vitamin A derivative found in photoreceptor cells. It absorbs light and undergoes a conformational change, a process known as bleaching. This change in conformation renders retinal unable to absorb another photon of light, leading to a temporary decrease in sensitivity and potentially causing a white spot in vision, especially in response to bright light. Retinal takes time to regenerate back to its normal state after bleaching.
Explain the phenomenon by which photoreceptors detect darkness.
In darkness, photoreceptors are depolarized with Na+ channels open. When exposed to light, these channels close, leading to hyperpolarization of the photoreceptors. Surprisingly, the presence of a stimulant also leads to hyperpolarization, as the enzymes responsible for activating the Na+ channels degrade in the presence of light. Thus, the absence of light causes the photoreceptors to be more depolarized, signaling darkness.
Describe the process of phototransduction in photoreceptor cells.
Phototransduction begins when retinal, a component of opsins, absorbs light and undergoes a conformational change. This leads to the closure of sodium (Na+) channels and hyperpolarization of the photoreceptor cell. The steps involved in phototransduction are as follows: 1. A photon of light is absorbed. 2. This causes a conformational change in the rhodopsin molecule. 3. Transducin, a G-protein, is activated. 4. Transducin activates phosphodiesterase. 5. Phosphodiesterase reduces the levels of cyclic guanosine monophosphate (cGMP). 6. The reduction in cGMP leads to the closure of Na+ channels.
