What are the two types of photoreceptor found in the retina?
Rods (low-light vision)
Cones (acute colour vision)
What happens to the membrane potential of a photoreceptor when light strikes it?
Which type of channel is open in the dark and closed in the light?
cGMP-gated Na+ channel
Why do cells tend to have a negative resting potential?
Action of NaKATPase: Na+ actively transported out so resting membrane potential is closer to conductance of K+ (-90mV)
What causes the photoreceptors to hyperpolarise when struck by light?
Closure of cGMP-Na+ channel
i.e "dark current" is turned off
Cell's rmp was previously between sodium and potassium
Now potassium only > more negative
Which protein is the pigment in photoreceptors?
What are the rho and opsin parts of rhodopsin?
Rho = retinal (Vitamin A)
dopsin = opsin (G protein coupled receptor)
What happens when rhodopsin is struck by light?
Opsin activates cGMP
cGMP activates Na+ channel, causing it to CLOSE
Hyperpolarisation due to reduced Na+
What happens to glutamate release when photoreceptors are:
a) Higher glutamate secretion
b) Lower glutamate secretion
What is visual acuity?
Ability to distinguish two nearby points
i.e how blurry your vision is
What determines your visual acuity?
Which type of photoreceptor is concerned with
a) acute, colour vision
b) low light vision?
Why is low-light vision less acute than colour vision?
Rods are more WIDELY spaced at the peripheries of the retina
Cones are more DENSELY packed at the fovea
Why is low-light vision more sensitive than colour vision?
Ganglia pick up impulses from a GREATER number of rods than they do cones
Multiple rods to a ganglia, few cones to a ganglia
So all the information is added together - your vision is more sensitive but less acute
Acute, colour vision depends on the ___ of cones.
Low light vision depends on the ___ of rods connected to each retinal ganglion cell.
Why can we see more than one colour?
Different CONES for different wavelengths of light
Nerve fibres from the (nasal / temporal) side of the each retina cross over where?
Optic ___ travel to the optic ___ and then continue on as optic ___.
Because the (nasal / temporal) optic nerves cross over at the optic chiasm, the brain processes light from the left and right at the opposite visual cortex.
LEFT VISUAL FIELD > RIGHT VISUAL CORTEX
and vice versa
Information from the superior visual field is processed by the ___ visual cortex.
inferior visual cortex
and vice versa
Which layer of the visual cortex receives all the information from the left and right visual fields?
In layer 4C of the visual cortex, light from each eye is processed (individually / collectively).
Past layer 4C of the visual cortex, information from each eye is processed (individually / collectively).
no distinction anymore
What is Hebb's postulate?
Cells that fire together, wire together
i.e input from right and left visual fields is required for optic pathway to develop properly, otherwise you get stunted development and vision problems