Lecture 5 Flashcards
Fundus
The interior surface of the eye, visible through an opthalmoscope, which includes the retina, optic disc, macula, and blood vessels
Optic Disc
The region where the optic nerve exits the eye, containing no photoreceptor cells and creating the ‘‘blind spot’’
-This area is where the blood vessels that feed the retina enter the eye and where the axons of the retinal ganglion cells merge into the optic nerve
Macula
The central area of the retina responsible for detailed central vision, critical for tasks like reading and recognizing faces
-An area that contains a high density of photoreceptors and almost no blood vessels
Fovea
A small, central depression within the macula that contains a high concentration of cone cells, providing the sharpest visual acuity and colour vision
Central Vision
Tasks that require high visual acuity, like reading, are actually limited to a visual angle of 2-3 degrees
-No more than the width of a thumbnail when viewed at arm’s length (57 cm)
-The rest you actually don’t see very well. We think we see everything clear because our eyes are always moving
Visual Acuity
The clarity or sharpness of vision, measured by the ability to discern fine details, typically evaluated using standardized eye charts
Visual Angle
The angle formed by an object at the eye, determined by the object’s size and distance from the observer, used to quantify how large an object appears in the field of view
Ganglion Cells
Neurons located in the retina that receive visual information from bipolar cells and transmit it to the brain via their axons, which form the optic nerve
Bipolar Cells
Intermediate neurons in the retina that connect photoreceptors (rods + cones) to ganglion cells, transmitting visual signals from the outer to the inner retina
Photoreceptors (rods + cones)
Specialized cells in the retina that detect light and convert it into electrical signals
Rods
Sensitive to low light levels and responsible for vision in dim light (scotopic vision)
Cones
Active in bright light and responsible for colour vision and fine detail (photopic vision)
Phototransduction
The process by which photoreceptor cells in the retina convert light into electrical signals that can be processed by the brain
Photopigment
A light-sensitive molecule in photoreceptors composed of opsin and chromophore
Opsin
A protein component of photopigments that determines the wavelength sensitivity of the photoreceptor, enabling colour and light detection
Chromophore
The light-absorbing molecule within photopigments; in mammels, this is 11-cis retinal, which changes shape upon light absorption
Photoisomerization
The process by which the 11-cis retinal chromophore changes shape to all-trans retinal when exposed to light, initiating phototransduction
-This results in the photoreceptor being ‘‘bleached’’, rendering it temporarily unable to absorb more light
Photopigment Regeneration
The process of converting the all-trans retinal back to its 11-cis retinal form, allowing photoreceptors to recover from bleaching and resume light detection
-This occurs in the retinal pigment epithelium
S-cones
Responsible for our perception of the colour blue
M-cones
Responsible for our perception of the colour green
L-cones
Responsible for our perception of the colour red
Melanopsin
Monitors ambient light levels so as to influence our circadian rhythm
The ‘‘Duplex’’ Retina
-Rods function better in low-light conditions, but they become overwhelmed (saturate) in bright environments
-Cones are ineffective in low-light conditions, but they function well in bright environments
-The difference in light intensity between night and day is very large!
Photopic Vision
Vision under well-lit conditions, primarily mediated by cones, enabling colour perception and high visual aquity
Scotopic Vision
Vision in low-light conditions, primarily mediated by rods, providing high sensitivity to light but no colour perception
Dark Adaptation
The process by which the eyes adjust to low-light conditions, involving increased sensitivity of rods and a shift from cone-dominated to rod-dominated vision
-Can take up to 30 mins
3 Mechanisms that allow us to see in dimly-lit situations
- Pupil dilation
- Gain in light sensitivity of photoreceptors
- Normalization through lateral inhibition
Pupil Dilation
Allows the entry of more light
-Although it is the fastest mechanism, its effects are relatively limited
Gain in light sensitivity of photoreceptors
A dark environment will be associated with more regeneration than bleaching, thus rendering photoreceptors more receptive to light
Normalization through lateral inhibition
A neural mechanism in which active neurons suppress the acitivity of neighboring neurons, enhancing contrast and improving the detection of edges and fine details
-E.g. going from outside to inside, big light difference, but we don’t perceive it!
