L11-19 Flashcards
which cell leaves the eye?
Retinal Ganglion cells
Where are the photoreceptors?
in the retina
where does the optic nerve cross?
the Optic Chiasm
where is the Lateral Geniculate Nucleus (LGN)?
Thalamus
where does LGN project to?
layer 4 V1
What is the LGN segregated besed on?
segregated by eye of origin
where does the secondary visual pathway run?
Superior colliculus->pulvinar nucleus in thalamus->V1
explain the layout of the retina
BACK TO FRONT: Photoreceptor (amacrine cell) Bipolar cell (horizontal cell) RGC
which part of the retina has the highest visual acuity? and why?
Fovea
highest density of cone receptors
how much of V1 does the fovea represent
> 50%
what are saccades?
tiny, very fast eye movements that scan the environment to get the entire visual field in the centre of your vision
Describe the journey of light in the eye
- Light enters through the cornea (transparent outer membrane)
- passes through lens and vitreous
- forms an image on the retina
- reaches the photoreceptor
how many and what are segments of the photoreceptor
2 segments
Inner segment: cell body and nucleus
outer segment- stacks of membranes (the famous bits)
how many types of rods are there?
1 : black and white
How many types of cones are there?
3:
Short (S) Blue
Medium (M) Green
Long (L) Red
ratio of rods to cones
100 million: 5 million
what is the Resting potential of photoreceptors?
-40mV- depolarised at rest- off response glutamate released, Na+ channels activated
what happens to photoreceptors when light hits them
Hyperpolarisation proportional to stimulus. reduced glutamate release.
what is the difference between the two classes of bipolar cells?
reaction to glutamate:
off bipolar cells: glutamate gated cation channels gives classical EPSP from Na+ influx. excitatory receptor. in light reduced glu release: stop firing
On bipolar cells: GPCR inhibitory receptor. in dark reduced glu release, less inhibition; fires more
horizontal cells
gabaergic
responsible for lateral inhibition
release gaba onto photoreceptors inverting the synapse
what is a receptive field?
area of the retina that has a change in membrane potential when stimulated.
explain RGC receptive fields
synapses of bipolar and horizontal cells in retina are arranged so RGC have opposing surround concentric receptive fields
explain centre surround receptive fields for o+ O-
eg. light centre dark surround.
centre:
light hits photoreceptor-> cone depolarises-> reduced glu release->on bipolar cells depolarise and release glu-> excites RGC
surround
no light hits-> horizontal cells not activated no inhibition
if light hits surround:
horizontal cells hyperpolarise->reduced GABA release-> central photoreceptor depolarisation->increased glu release->bipolar cells hyperpolarise-> RGC inhibited
what are the types of RGC and their relative abundence
Midget ganglion cells- 80%
Parasol ganglion cells- 10%
Koniocellular ganglion cells- 10%
what is the function of Midget GCs
static stimuli
detail and colour
luminance sensitive and colour sensitive
what is the function of parasol cells
motion and change
luminance sensitive
what is responsible for focusing light on retina
refractive indices of different media- cornea, lens, vitreous
what is responsible for most of the refraction seen
cornea 75%
lens 25%
