Physiology Of Eyes Flashcards
What are the two types of photoreceptors in the human vision system?
Rods and Cones
Which photoreceptors are specialized for night vision?
Rods
Which photoreceptors are specialized for color vision?
Cones
What is the relative abundance of rods compared to cones?
More numerous than cones (20:1)
What causes decreased visual acuity when lost?
Loss of cones
What is the acuity of vision mediated by rods?
Low
What happens to the acuity of vision when rods are lost?
Night blindness and loss of peripheral vision
What are the three types of visual pigments found in cones?
- Blue (419 nm)
- Green (533 nm)
- Red (564 nm)
What is the primary pigment found in rods?
Rhodopsin
At what wavelength does rhodopsin absorb light most effectively?
500 nm
What is the process called where light is converted to visual signals?
Visual transduction
What is the first step in the phototransduction mechanism?
Photon absorption by visual pigment
What occurs to 11-cis retinal during phototransduction?
Isomerizes to all-trans retinal
What is the result of retinal detaching from the opsin protein?
Visual pigment bleaching
What is transported to the retinal pigmented epithelial layer after bleaching?
All-trans retinal
What is all-trans retinal converted to in the retinal pigmented epithelial layer?
Retinol
What is isomerized back to 11-cis retinal after conversion to retinol?
Retinol
What type of cells are involved in the faster pathway for cones?
Müller cells
True or False: Cones are less numerous than rods.
True
Fill in the blank: Loss of rods causes ______ and loss of peripheral vision.
night blindness
What effect does high light intensity have on cones?
Saturate
What is the primary function of rods?
Night vision
What is the effect of damage to cones?
Decreased visual acuity
What type of vision is mediated by cones?
Day vision
What is rhodopsin?
A visual pigment that decomposes by light energy to trigger vision.
Rhodopsin is crucial for vision in low-light conditions.
What happens to rhodopsin when light hits it?
It breaks down into opsin and retinal, triggering vision.
This process is essential for the visual cycle.
How is rhodopsin re-formed in darkness?
Opsin and retinal recombine to restore rhodopsin.
This occurs when there is no light present.
What role does vitamin A play in vision?
It supplies retinal, which is essential for rhodopsin synthesis and night vision.
A deficiency in vitamin A can lead to night blindness.
What is the dark current in photoreceptors?
Photoreceptors are slightly depolarized (~-40 mV) in darkness.
This state allows for a steady influx of Na+ and Ca2+.
What are CGMP-gated cation channels?
Channels that are open in darkness, allowing Na+ and Ca2+ influx.
This influx generates a steady dark current.
What occurs during phototransduction in light?
Photon absorption activates rhodopsin or cone visual pigments.
This initiates a cascade of biochemical events leading to vision.
What is transducin?
A G protein activated by rhodopsin during phototransduction.
Transducin plays a key role in the signaling pathway of vision.
What does transducin activate?
GMP phosphodiesterase, reducing GMP levels.
This reduction leads to changes in photoreceptor activity.
What happens to GMP-gated cation channels when GMP levels decrease?
They close, leading to hyperpolarization of the photoreceptor.
Hyperpolarization is critical for the cessation of glutamate release.
What is the result of hyperpolarization in photoreceptors?
Glutamate release from photoreceptors is reduced.
This reduction occurs without action potential generation.
What occurs in the dark regarding cGMP levels?
High cGMP levels
High cGMP levels lead to open Na+ channels, allowing Na+ to enter the cells (dark current) and causing depolarization.
What happens to neurotransmitter release in the dark?
Neurotransmitter release occurs
This release inhibits bipolar cells, resulting in no signal to the brain.
What is the effect of low cGMP levels in the light?
Closure of Na+ channels
This causes hyperpolarization and subsequent neurotransmitter release.
In the light, what is activated as a result of disinhibition of bipolar cells?
Ganglion cells are activated
This leads to a signal being sent to the brain.
What type of visual field defect occurs with damage to the optic tract?
Contralateral (left) homonymous hemianopia
This means loss of the same side of the visual field in both eyes.
What visual defect is associated with an optic chiasm lesion?
Nonhomonymous bitemporal hemianopia
This results in loss of peripheral vision in both eyes.
What is the visual field deficit resulting from a lesion in the temporal lobe (Meyer’s loop)?
Superior left homonymous quadrantanopia
This results in loss of vision in the upper quadrant of the left visual field.
What is the effect of a lesion in the parietal lobe on visual fields?
Inferior left homonymous quadrantanopia
This results in loss of vision in the lower quadrant of the left visual field.
What is the result of a lesion in both banks of the calcarine fissure?
Total loss of vision in the right eye
This indicates a complete loss of input from the right visual field.
Fill in the blank: Damage to the optic radiations can cause _______.
various types of visual field defects
These can include homonymous hemianopias depending on the location of the lesion.
What is the role of neurotransmitter release in vision?
It regulates the signaling to bipolar and ganglion cells
This process is crucial for transmitting visual information to the brain.
