Flashcards: Vision (Physiology 2)

Sclera: The tough, outer layer of the eye that maintains its shape and protects internal components.

Vitreous humor: The clear, gel-like substance inside the eye that helps maintain its shape and optical properties.

Cornea: Provides most of the refractive power of the eye.

Lens: Adjusts its shape (accommodation) to focus light onto the retina for clear vision.

Pupil: Regulates the amount of light entering the eye to enhance focus.

Retina: Converts light into electrical signals via photoreceptors (rods and cones).

Optic nerve: Transmits signals from the retina to the brain.

Optic chiasm: The point where some nerve fibers from each eye cross to the opposite side of the brain.

Visual cortex: Processes visual information for perception in the occipital lobe.

Light absorption: Photopigments in the outer segment absorb photons, activating the opsin protein.

Activation of transducin: Light-induced opsin changes activate the G-protein transducin.

Phosphodiesterase activation: Transducin activates phosphodiesterase (PDE), which breaks down cyclic GMP (cGMP).

Closing of ion channels: Reduced cGMP levels cause sodium channels to close, leading to hyperpolarization of the photoreceptor.

Receptor potential: The hyperpolarization alters neurotransmitter release to bipolar cells.

Increased light decreases cGMP levels.

Sodium channels close, leading to hyperpolarization.

Reduced neurotransmitter release occurs at the synaptic terminal.

Decreased light increases cGMP levels.

Sodium channels open, leading to depolarization.

Increased neurotransmitter release occurs at the synaptic terminal.

Pigment epithelium: The outermost layer that provides support and absorbs excess light.

Photoreceptors: Rods and cones that detect light and initiate visual signals.

Interneurons: Bipolar, horizontal, and amacrine cells that process and relay signals.

Ganglion cells: The innermost layer of neurons that transmit signals to the optic nerve.

Vitreous humour: The gel-like substance adjacent to the retina that provides structural support.

Absorbs excess light to prevent scattering.

Provides nutrients to photoreceptors.

Phagocytoses shed photoreceptor outer segments.

Collect visual information from interneurons.

Transmit visual signals to the brain via the optic nerve.

Membrane discs: Found in the outer segment; contain photopigments (opsins) that absorb light.

Outer segment: Houses the membrane discs; responsible for capturing light.

Inner segment: Contains organelles like mitochondria for energy production.

Nucleus: Located in the cell body; controls cellular activities.

“Axon”: A thin extension that connects the cell body to the synaptic terminal.

Synaptic terminal: Releases neurotransmitters to communicate with bipolar cells.

Outer segment: Captures light using photopigments in the membrane discs.

Inner segment: Produces energy and supports phototransduction processes.

Converts electrical signals into neurotransmitter release.

Transmits visual information to bipolar cells for further processing

cGMP: A secondary messenger that maintains the opening of ion channels in the dark.

Opsin: A protein found in photopigments that binds to retinal, enabling the photoreceptor to absorb light.

Retinal: A light-sensitive molecule that binds to opsin to form the photopigment.

Membrane disc in outer segment: Contains photopigments and is where light is absorbed to initiate the transduction process.

Cell membrane: The boundary of the photoreceptor that houses ion channels and receptors.

Na+: Sodium ions that are involved in the dark current by entering the photoreceptor.

Photopigment: The combination of opsin and retinal that absorbs light and begins the phototransduction cascade.

Pigment epithelium: A layer of cells that nourishes the photoreceptors and absorbs excess light.

Ganglion cells: The final output cells that send visual information to the brain via the optic nerve.

Photoreceptors: Rods and cones that detect light and initiate the transduction process.

Interneurons: Cells like horizontal, amacrine, and bipolar cells that process visual information between photoreceptors and ganglion cells.

Bipolar cells: Connect photoreceptors to ganglion cells, transmitting the visual signals.

Central retina (fovea): Contains a high density of cones, with minimal convergence, allowing for sharp, detailed vision.

Peripheral retina: Contains more rods, with significant convergence, leading to less detailed but more sensitive vision in low light.

Central retina: Features tightly packed photoreceptors, resulting in a higher resolution image.

Peripheral retina: Features large gaps in the sampling array, causing a decrease in visual resolution.

Central vision (fovea): The high density of cones and lack of convergence allow for high-resolution, detailed vision.

Peripheral vision: Greater convergence of rods leads to lower resolution but better sensitivity to movement and low-light conditions.

Difference in resolution: Central vision is specialized for tasks like reading and recognizing faces, while peripheral vision detects motion and provides broader visual awareness.

Central vision (foveal vision): It provides the sharpest and most detailed visual information.

High resolution: This is due to the dense concentration of cones and minimal convergence in the central retina.

Specialized tasks: It enables precise activities like reading, recognizing faces, and distinguishing fine details.

Fovea centralis: The center of the retina, where visual acuity is highest due to a high density of cones and minimal convergence.

Macula lutea: The area surrounding the fovea, contributing to sharp central vision.

Retinal blood vessels: Supply oxygen and nutrients to the retina, ensuring proper functioning for detailed vision.

Fovea centralis: A small depression in the retina that contains a high concentration of cones.

Role in central vision: It is responsible for the sharpest visual acuity and is critical for tasks requiring high-detail focus, such as reading.

Macula lutea: The region surrounding the fovea centralis, containing fewer photoreceptors.

Role in vision: It enhances visual acuity by assisting in central vision and focusing on detailed objects, though it has a slightly lower resolution than the fovea.

Retinal blood vessels: Supply oxygen and nutrients to the retina, ensuring its proper functioning.

Impact on vision: Well-functioning blood vessels are essential for maintaining the health of the retina and supporting detailed vision.

Wider field: Peripheral vision covers a larger area of the visual field, allowing us to detect movement and changes in the environment outside the direct line of sight.

Larger coverage: It is responsible for detecting peripheral stimuli and situational awareness rather than detailed focus.

Optical blur: The visual image in peripheral vision is more optically blurred compared to central vision.

Large, widely spaced cones: Cone photoreceptors in the periphery are larger and more spaced out.

More rods present: The periphery contains a higher number of rods, which are specialized for detecting movement and low light rather than detailed color vision.

Convergence onto ganglion cells: Multiple cone signals converge onto a single ganglion cell, reducing spatial resolution.

Increasing convergence with distance from the fovea: As you move further from the fovea, the convergence of signals from cones onto ganglion cells increases, further lowering the resolution of peripheral vision.

Larger cones: In peripheral vision, the cones are larger and more sparsely distributed, which compromises the ability to resolve fine details.

Higher rod concentration: Rods are more numerous in the periphery, supporting better night vision and motion detection but contributing to lower resolution for color and detail.

Signal convergence: The convergence of multiple cones onto a single ganglion cell reduces the resolution, making peripheral vision less sharp.

Decreases with distance: As you move away from the fovea, visual resolution decreases due to increased convergence of signals and a higher rod-to-cone ratio.

Effect on vision: The further from the fovea, the blurrier and coarser the visual image becomes, particularly for fine detail and color discrimination.

Small area: Foveal vision represents a tiny portion of the visual field, specifically the central part where visual acuity is highest.

Specialized function: It is specialized for tasks requiring high detail, like reading, recognizing faces, and distinguishing fine details.

Minimal obstruction: The obstructing layers of the retina are pushed to one side in the fovea, allowing light to directly reach the photoreceptors.

High focus: This arrangement ensures that the visual image is sharply focused and free from distortion or scattering.

Red and green cones: The fovea primarily contains red and green photoreceptors, which are essential for color vision and sharp focus.

Thin and tightly packed: These cones are smaller and packed tightly together, maximizing light reception and clarity.

No convergence: Signals from individual cones are kept separate with no convergence onto a single ganglion cell, ensuring high spatial resolution.

Individual signal transmission: In the fovea, cones transmit signals directly to separate ganglion cells, allowing for high precision.

Tiny receptive fields: The lack of convergence means each ganglion cell receives input from just one cone, which creates very small receptive fields and maximizes resolution.

Centre-surround organization: Receptive fields in retinal ganglion cells (RGCs) and lateral geniculate nucleus (LGN) cells are structured with a central area that responds to light and a surrounding area that inhibits or counteracts this response.

Functional role: This arrangement enhances the contrast between light and dark areas of the image, improving edge detection and spatial resolution.

Higher processing: The visual cortex is capable of more selective responses to stimuli, allowing it to detect specific features such as orientation, movement, and color.

Feature detectors: Neurons in the cortex are tuned to respond to specific features in the visual field, such as lines, edges, or motion, enhancing the brain’s ability to analyze complex visual information.

V1 (Primary visual cortex): This area is responsible for processing basic visual features like orientation, contrast, and spatial frequency.

V2-V4: These areas contribute to more complex visual processing, including color perception (V4) and shape recognition.

MT (Middle temporal area): Specialized for motion detection and processing of dynamic visual stimuli.

Adaptation: Photoreceptors and ganglion cells are designed to detect changes in illumination, which allows for better adaptation to varying light conditions.

Efficiency: The system prioritizes detecting contrasts or differences rather than absolute brightness, which is more useful for interpreting dynamic visual scenes.

Centre-surround structure: These receptive fields have a central region that responds positively to light and a surrounding region that has an inhibitory response.

Encoding contrast: This structure enhances edge detection by highlighting differences in illumination between adjacent regions.

Retinal ganglion cells: Encode spatial differences in light intensity across the visual field, facilitating the detection of objects and their movement.

Parvocellular (P) cells:

Small receptive fields for fine detail and high-resolution vision.

Specialised in detecting color and providing information about stationary objects.

Require a stable image for accurate processing.

Magnocellular (M) cells:

Large receptive fields for coarse resolution and detecting fast movement.

Involved in processing dynamic visual information, such as motion.

Receptive field properties: These are influenced by the convergence of photoreceptor signals and the properties of the retinal circuitry.

Role in vision: P cells are important for color perception and detailed vision, while M cells excel at detecting movement and changes in the environment.

Comparison of cones: Some retinal ganglion cells receive inputs from different cone types, comparing their outputs to detect specific color contrasts.

Color opponency:

Parvocellular cells: Compare red and green cone signals.

Blue-yellow ganglion cells: Compare blue and yellow cone signals.

LGN as a faithful relay: The receptive fields of LGN cells closely resemble those of retinal ganglion cells.

Role in processing: Both retinal ganglion cells and LGN cells play a role in encoding spatial and contrast information, but the LGN is a relay station, sending this information to the visual cortex for further processing.

Cortical receptive fields: These are more complex than those in the retina and LGN. They are capable of responding to more specific visual features, such as orientation, direction of movement, and spatial frequency.

Higher selectivity: Visual cells in the cortex show greater selectivity for features of visual stimuli (e.g., edges, shapes).

Integration of information: Cortical cells integrate information from multiple sources, allowing for the processing of more abstract visual features.

Retina: A thin, layered structure that lines the back of the eye and is responsible for visual processing.

Photoreceptors: Light-sensitive sensory cells; rods for low light levels and cones for daylight vision.

Interneurons and Retinal Ganglion Cells: Interneurons form circuits that process photoreceptor responses, while retinal ganglion cells serve as afferents, transmitting visual information to the brain.

Fovea: A tiny region at the center of the retina, responsible for focusing the point in visual space directly in front of us.

Structural Adaptation: The fovea has a high density of cones, allowing for detailed, sharp vision. It is often used interchangeably with the macula, a larger region distinguished by yellow pigmentation.

Photopigment: A molecule that captures photons of light and triggers changes in a photoreceptor's membrane potential.

Composition: Consists of a protein called opsin and a light-sensitive chromophore, 11-cis retinal.

Receptive Field: The region of the visual image that affects the action potential firing rate of a given cell.

Encoding: It encompasses the specific pattern of light needed in that region to elicit a response from the cell.

Binocular Cell: A cell that receives input from both eyes.

Function: These cells contribute to depth perception by matching receptive fields in the two eye fields.

dLGN: The thalamic nucleus that relays visual information from the retina to the primary visual cortex.

Role: It serves as part of the perceptual pathway for visual processing.

Primary Visual Cortex: Also known as Area 17, it receives input from the retina via the dLGN.

Role: It is responsible for initial processing of visual information, which is then distributed to higher visual areas for more specific processing.

Higher Visual Cortical Areas: At least 20 regions in the primate cortex contribute to vision, with some estimates suggesting up to half of the cortex is involved.

Infero-Temporal Pathway: Supports object identification and detailed vision.

Parietal Pathway: Supports understanding the location, relationships, and movement of objects, helping us interact with the environment.