Retina and visual pathway Flashcards
What are the four main sections of the photoreceptor?
Inner segment
Outer segment
Cell body
Synaptic terminal
What are the two types of photoreceptor?
Rods and cones
What structures must light pass through before reaching the photoreceptors?
Ganglion and bipolar cells
In which direction is the pathway of signal transmission in this diagram?
Photoreceptors to bipolar cells to ganglion cells
Ganglion cell axons go to forebrain
What happens to the resting membrane potential of the vertebrate photoreceptors (Vm) with light exposure?
Vm goes from depolarised (~-20mV) and hyperpolarises
What is the mechanism that allows us to percieve objects in the visual field?
In the dark, the membrane potential is between that of Na+ and K+
When there is a light signal, a cGMP-gated Na+ channel is closed, stopping influx of Na+ and moving the membrane potential closer to that of K+
This causes hyperpolarisation
How does light cause closure of the cGMP-gated Na+ channel?
Rhodopsin is a visual pigment molecule consisting of retinal and opsin, a G-protein coupled receptor
Light converts the retinal from it’s cis form to all-trans-retinal, an active form
All-trans-retinal activates a cascade which decreases cGMP, closing the Na+ channel and causing membrane hyperpolarisation
What kind of vision do rods allow?
Seeing in dim light
What kind of vision do cones allow?
Seeing in normal daylight
What does high convergence of rods mean?
Large spacing between rods e.g. the normal spacing between rods in the periphery
What does increasing convergence in the rod system mean for visual acuity?
Increased sensitivity of rods
Decreased visual acuity
What are the characteristics of rods?
Achromatic
Peripheral retina
High convergence
High light sensitivity
Low visual acuity
What are the characteristics of cones?
Chromatic
Found in the fovea
Low convergence
Low light sensitivity
High visual acuity
Which part of the visual cortex are objects detected in the lower visual field processed?
Lower part of the primary visual cortex
Where are objects detected in the right visual field processed?
In the left primary cortex
Where in the visual pathway is affected if bitemporal hemanopia defect is noticed?
Optic chiasm
Where in the visual field is affected if a homonymous hemianopia defect is seen and there is no sparing of the macula?
Optic tract or radiation
Where in the visual pathway is affected if a quadrantanopia defect is seen?
Optic tract or radiation
