The Retina Flashcards
What are the factors that determine visual acuity?
Neural factors and optical factors
What is visual acuity?
Ability to resolve fine detail
What does the denominator of visual acuity mean eg 6/12?
What a normal person sees at 12m, this person sees at 6m (this is the legal driving limit)
What are the optical factors that affect visual acuity?
pupil size; clarity of optical media: cataracts, corneal opacities; refractive errors (cause blurriness): usually caused by length of eyeball eg myopia, hypermetropia, astigmatism, presbyopia
Visual acuity is defined by
density of cones; tf fovea has highest visual acuity
Phototopic light levels
daylight
Scotopic light levels
dark light levels
At phototopic light levels, the best VA is
6/6 or better
At scotopic light levels, the best VA is
6/60
Visual acuity decreases as you move away from the fovea proportional to
decrease in cones
T/F The best visual acuity under scotopic conditions is at the fovea
False; it’s about 8 degrees from the fovea where the concentration of rods is increasing
What are the 6 types of neurons in the retina?
rods, cones, horizontal cells (HCs), bipolar cells (BCs), amacrine cells (ACs), and ganglion cells (GCs)
The output neurons of the retina are
ganglion cells (GCs)
Where in the retina do the neurons synapse?
Outer and inner plexiform layers
T/F Photoreceptors are the first cells light hits
False; they are the LAST cells, at the back of the retinal layers
95% of photoreceptors are
rods
T/F Rods are more sensitive to light than cones
True
What are the 3 types of cones?
Red, green, and blue
What is the order of layers in the retina from where light contacts it?
nerve fibre layer
ganglion cell layer
inner plexiform layer (bipolar-amacrine-ganglion synapses)
inner nuclear layer (nuclei of horizontal, bipolar, and amacrine cells)
outer plexiform layer (bipolar-horizontal-photoreceptor synapses)
outer nuclear layer (nuclei of photoreceptors)
photoreceptor layer
pigment epithelium
choroid
Bipolar cells are in which layer of the retina?
Inner nuclear layer
What are the 10 different types of bipolar cells?
1x for rods; 9x for cones
Bipolar cells are important for
spatial vision and colour vision
OFF BCs
always HYPERpolarize when light shines on the retina
ON BCs
always DEpolarize when light falls on the retina
Bipolar cells (BCs) receive inout from
photoreceptors
What is the function of horizontal cells?
modulation in outer retina: receive input from photoreceptors and provide output to other photoreceptors - mainly INHIBITORY
Horizontal cells are mainly __________ to photoreceptors
Inhibitory, via GABA
Horizontal cells use which NT?
GABA tf inhibitory
Horizontal cells respond to light by
HYPERpolarizing
ON ganglion cells
DEpolarize when light falls on the retina
OFF ganglion cells
HYPERpolarize when light falls on the retina
What is unique about ganglion cells in the retina?
They are the only cells that fire APs; the rest of the retina fires only graded potentials
Ganglion cells release
Glutamate (excitatory)
What is the role of ganglion cells?
Main output neuron of the retina
T/F The visual system is a detection system
False; it is a comparator - how we interpret what we see (greyness, colour, motion) relates to the context in which it is presented
What is meant by receptive field of a ganglion cell or bipolar cell?
area of retina that when stimulated with light changes the cell’s membrane potential
T/F Ganglion cells respond to light by firing action potentials
False; GCs respond to light by increasing or decreasing their AP firing rate - depending on where in the receptive field the light is shone
What are photopigments?
Proteins in photoreceptors: rods contain rhodopsin, cones contain cone-opsins (1 of 3) - all bind to vitamin A (retinal); they convert light into neural signals
How do photoreceptors turn light into neural signals?
Photopigments (rhodopsin, coneopsins) are conformationally changed when light interacts with their bound retinal
Photoreceptors release NT relative to
graded membrane potential; lower (hyperpolarized/more negatie) means less firing and less release of NT
Photoreceptors
HYPERpolarize in response to light
Photoreceptors use ________ as their NT
glutamate (excitatory)
In the dark, photoreceptors are ___________ due to _________
depolarized; Na+ cycling in and out of the photoreceptors via cGMP-gated channels
In the light, photoreceptors are _________ due to _________
hyperpolarized; phototransduction leads to formation of PDE which breaks down cGMP, closing the Na+ channel thus the cell hyperpolarizes
What is phototransduction?
Light activates rhodopsin via vitamin A; this initiates the production of transducin, then PDE; PDE breaks down cGMP that, in the dark, keeps cGMP Na+ channels open (depolarizes cell); the cGMP Na+ channels close, leading to hyperpolarization of the cell
The photoreceptors that form the central part of a ganglion cell receptive field are
those that directly contact ganglion cells via bipolar cells ie Ph-BC-GCs
The central response is generated by
light contacting photoreceptors in the central part of the ganglion cell receptive field
The central response of ganglion cells to light is
depolarization
The receptive field of a retinal ganglion cell is
the area of retina that the ganglion cell will respond to
Light shone on the periphery of a receptive field generates the _______ response
surround response
The surround response is generated by
shining light on the periphery of a ganglion cell receptive field
Photoreceptors in the surround send information to
horizontal cells
Which cell type creates the ‘centre-surround’?
Horizontal cells
