04-05. Neurons (in the eye) Flashcards
Sequence for neuronal communication
- membrane permeability changes
- Sodium flows in
- Inside more positive (depolarization)
- Critical Value (~ –45 mv) reached and action potential occurs.
- Potassium flows out (repolarization)
- Action potential travels down the axon and stimulates synaptic vesicles
- Synaptic vesicles release NTs into the synapse that influence the permeability of the next neuron.
–> makes it more/less likely to have
action potential
3 important facts about action potentials??
- ALL-OR-NONE
- REFRACTORY PERIOD
- BACKGROUND firing rate
All or nothing means?
- action potential or there isn’t
- one size only, no bigger or smaller
- to communicate bigger stimulus, they change the RATE at which they have action potentials
Refractory period?
- period where another Act. P can’t occur
- limits the rate at which they can have Act. Ps
Background firing?
- Act. Ps all on their own, without any input or stimulus
- you know if it receives an inhibitory input, because it will drop below its normal rate
3 membranes in the eye?
outer: sclera
–> tough protective cover
mid: choroid*
–> blood vessels
inner: retina
Function of ciliary muscles/zonule fibers?
- they ADJUST (change the shape of) the LENS so you can focus on things at diff. distances
- they attach to the choroid
Optic axis? Fovea?
Imaginary line of sight that is your direct focus, which runs through the lens and directly to the fovea (most sensitive part of the retina)
5 types of cells in retina??
- photoreceptors
- horizontal cells
- bipolar cells
- amacrine cells
- retinal ganglion cell
Why are the cells in the order they are in the retina? (even tho it seems a lil backwards)
- photoreceptors (PR) –> only cells sensitive to light
- PRs need to be attached to the pigment epithelium to keep functioning (not see-thru)
- so, PR are at the back, next to PE, and the rest of the cells are in front of them
What happens to the photopigment molecules?
- process of photoisomerization –> molecule changes shape
- releases NT
- Act. P in bipolar/horizontal cell
- photopigment regeneration –> puts the molecule back in its original shape
–> this is why PRs need pig. epith.
What is the blind spot?
its the bundle of ganglion cells that form the optic nerve, where it exits the eye back towards the brain
Why don’t we notice blind spot? If we isolate it, what do we see?
- we will see the background of whatever’s near it continued
- the blind spot is background, its far from the fovea
- we fill in the gaps
- our eyes are frequently moving
7 differences between rods and cones?
- shape
- number
- spatial distribution
- adaptation to darkness
- spectral sensitivity
- light sensitivity
- spatial acuity
- shape ?
rods - straight
cones - cone / triangle ish
- number ?
rods - 100M /eye –> more
cones - 5M /eye –> less
- spatial distribution ?
(density in specific places on retina)
cones - highly dense in the fovea, not anywhere else
rods - none in fovea, highly dense a little ways away from the fovea, overall more dense than cones
- adaptation to darkness ?
process of adjusting retinal sensitivity (changing operating range)
adapt at diff. rates
cones:
- adapt really fast (≈4 - 8 min max)
–> ≈8 min is the rod-cone break
- much LESS sensitive in low light
- abs. thresh. is much higher (need more stim)
rods:
- adapt slower ( ≈25 - 30 min)
- much MORE sensitive in low light
- abs. thresh. and max visual sensitivity are much lower (need less stim)
Spectral sensitivity def?
sensitivity to diff. wavelengths (colors)
(degree to which a photopigment molecule absorbs light of diff. wavelengths)
Photopigment def? types?
molecule (within photoreceptors) with the ability to absorb light and start transduction of light into neural signals
1 type for rods, 3 types in cones (each cone has diff. photopigment)
- spectral sensitivity ?
rods:
- low (only 1 type)
- overall shorter than cones
cones:
- high (3 types – R/B/G)
- short, medium, and long
Purkinje shift?
There’s an overall difference in spect. sensitivity of rods and cones
(rods generally shorter)
Rods are much much more sensitive in the dark
So, when you’re dark adapted, something that’s blue will look brighter than something red
Red colors appear to fade faster than blue
Convergence def?
Ratio in retinal circuits where…
many photoreceptors :: 1 ganglion cell
Spatial summation def?
- Involves convergence
- all PRs in a circuit are from a specific AREA on the retina, and they “ADD UP” to affect the ganglion cell’s response
- works like a funnel
- Sensitivity to light ?
- rods are more sensitive to light because their convergence ratio is higher
- dim light stimulates each rod, which all “adds up” to make the ganglion cell respond
- cones don’t add anything up, so they need more stimulus (brighter light) to produce a response
- spatial acuity ?
rods:
- high convergence = low acuity
cones:
- low convergence = high acuity
Why not cones everywhere?
- blind in the dark
- too much sensory info
–> (rods periphery) - cortical magnification around fovea
–> it would take up too much space
