Physiology II Flashcards
Describe the passage of light inside the retinal cells
- Photoreceptors
- Bipolar cells
- ganglion cells
What is the role of horizontal cells?
Receive input from photoreceptors and project to other photoreceptors and bipolar cells
What is the role of amacrine cells?
Receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
What are the four main regions of the photoreceptor cell?
Outer segment
Inner segment
Cell body
Synaptic terminals
What are the types of photoreceptor?
Rods and cones
Cones- short, medium, long wave
What causes cells to release glutamate?
Light causes conversion of chemicals in photopigment regions causing the cell to release glutamate
What is the membrane potential of vertebrate photoreceptors?
Depolarised -20mV
What happens to membrane potential in photoreceptors with light exposure?
Hyperpolarisation
What is the dark current?
A cGMP gated Na+ channel that is open in the dark and closes in light
Describe the dark current in the dark
PNa = PK (Na channels in the outer segment)
Vm therefore between ENa and EK
Describe the dark current in the light
PNa is reduced (outer segment channels close), PK > PNa
therefore, Vm —> EK, hyperpolarizes
Why are photoreceptor signals amplified?
so we can interpret a small change in light
What is the visual pigment in rods?
Rhodopsin- vitamin A derivative + opsin (GPCR)
Where is rhodopsin present?
membrane folds (called discs in outer segment)
What does light convert 11-cis-retinal to?
all-trans-retinal (activated form)
In the dark there is … glutamate
more
What is visual acuity?
ability to distinguish two nearby points.
determined largely by photoreceptor spacing and refractive power
When do rods/cones see?
rods= see in dim light
cones= see in normal daylight
Where is the highest density of cones?
Fovea
Why do rods have such a high convergence?
Many rods can access one ganglion giving poor acuity but allowing you to see in the dark; the signals assimilate to one larger signals
What allows us to see in colour?
differnet rods and cones for specific spectra
What does our visual system detect?
local differences in light intensity- not the absolute amounts of light
What is our visual system good at?
Identifying differences
What causes amblyopia?
multitude of issue
No problem with optics and retina
What happens to the visual cortex in amblyopia?
The dominant eye will take over
Describe hebb’s postulate
Cells that fire together, wire together
What are the 5 characteristics of rods and cones
Rods : Cones
Achromatic : Chromatic
Peripheral retina : Central retina
High convergence : low convergence
High light sensitivity : low light sensitivity
Low visual acuity : high visual acuity
