Lectures 4: Vision Flashcards
Function of Lens
(how does this change when its thick vs thin)
Adjusting refraction to focus images within an accomodation range Lens is thicker for nearby images (most refractive power) Lens is thinner for far away images (less refractive power)
Function of Cornea
Specialized transparent tissue that permits light rays to enter the eye. Important for refraction of light.
Function of Iris
Contains two sets of muscles that allow the size of the pupil to be adjusted
Function of Pupil
opening in the center of the iris, size of which is controlled by neural input to change the amount of incoming light
Function of Retina
Photoreceptors in the retina are perhaps the best understood of all sensory cells! Part of the CNS! • Five different types of neurons 1. Photoreceptors 2. Bipolar Cells 3. Ganglion Cells 4. Horizontal Cells 5. Amacrine Cells
Function of Fovea
highest optical resolution (most cones), also the area of least image distortion at the center of the retina. 1. It brings the high resolution areas of vision onto objects of interest (Chapter 20) 2. It prevents sensory adaptation at the level of the sensory receptor
Function of Optic Disc
a blind spot where the ganglion cell axons leave the eye via the optic nerve
Function of Pigmented Epithelium
Plays two important roles in support of photoreceptors 1.Removing expended membranous disks from the tip of the outer segment 2. Regenerate photopigment molecules after light exposure • Also absorbs light and provides nutritive and metabolic support • Largely believed these two critical roles of the pigment epithelium account for our ‘backwards’ retina!
Name the types of neurons that make up the retina
- Photoreceptors 2. Bipolar Cells 3. Ganglion Cells 4. Horizontal Cells 5. Amacrine Cells
dentify differences between rods and cones
Rods: BW, highly sensitive to light, more photopigment (capture more light), low temporal resolution, slow response, long integration time. More sensitive to scattered light. Low acuity, achromatic. Cones: lower sensitivity, specialized for day vision, less photopigment, lower amplification, high temporal resolution, fast response, short integration time, most sensitive to direct axial rays. High acuity in fovea, chromatic (three types of distinct pigments)
Outline how cones are the basis of color vision
Each of 3 photopigments sensitive to different wavelengths • Short wavelengths (blues) • Medium (greens) • Long (Reds) Individual cones are ‘color blind’ as well! • Response is a reflection of the number of photons they capture, regardless of vibrational energy • Can’t tell the difference between exposure to many photons of less- optimal wave length, or a few photons of optimal wave length • Ambiguity is solved by • Comparing activity in different classes of cones • Based on individual ganglion cells and higher levels in the visual system • Can be modeled in terms of several different post-receptoral processes called channels
If rods and cones don’t make action potentials, what do they do?
• Rods and cones DO NOT fire action potentials! • Respond with graded changes in membrane potential • Ganglion cells transmit information via action potentials
Describe how cGMP is involved in phototransduction.
Key molecule in the cascade that leads to change in ionic fluxes across plasma membrane • Nucleotide cyclic guanosine 3’-5’ monophosphate (cGMP) • Acts as a second messenger in rods • Carries information through the cytoplasm connecting the photoreceptor discs where light is absorbed to the plasma membrane • Controls ionic fluxes by opening a specialized series of ion channels • C-GMP-gated ion channels • Allow an inward current carried mostly by NA+ ions that enter the cell • In the dark, cGMP is relatively high, keeping cGMP channels in an open state • Keeping cells relatively depolarized
What are the 3 steps of phototransduction?
- Light activates pigment molecules in the photoreceptors 2. Activation of pigment molecules reduces cytoplasmic concentration of c-GMP 3. the reduction in c-GMP concentration closes c-GMP gated ion channels, hyperpolarizing the photoreceptors.
What is the difference between rods and cones?
Rods
- More photopigment
- Capture more light
- Amplify signals more than cones do
- Single photon can evoke electrical response in a rod
- Highly convergent
- Many rods have synapses on the same bipolar cell
- Signals from rods are pooled in the bipolar cell and reinforce one another
Cones
• Three types, each with a pigment sensitive to different parts of the light spectrum
• Brain obtains color information by comparing the responses from the three types of cones (more about this later!)
• Are outnumbered by rods ~20 to 1
- Still have better spatial resolution
- Neighboring rods converge onto a single bipolar cell, so differences in the responses of the rod are averaged out in the interneuron
- Cones are concentrated in the fovea, where the visual image is less distorted
Describe the pathway for Reduction in cyclic GMP concentration closes cGMP-gated Ion Channels
Cell hyperpolarizes when exposed to light
Photoactivation of a single rhodopsin molecule can lead to the hydrolosis of more than 105 cGMP molecules per second
This turns off the cGMP activated ion channels (leads to the hyperpolarization) because K+ will naturally exit the cell
What are the 3 photopigments?
Each of 3 photopigments sensitive to different wavelengths • Short wavelengths (blues)
• Medium (greens)
• Long (Reds)
What is the receptive field organization of visual ganglion cells?
Have a center surround receptive field organization
How do off-center, on surround visual ganglion cells respond to light in different locations?
Opposite of on-center, off-surround.
