Physiology 2 - Retina and Vision Flashcards
What are the vision receptors and where are these located?
Rods and cones
In the retina
What cells do photoreceptors pass their signal to?
Bipolar cells
what cells do bipolar cells pass their signals to?
Ganglion cells
What is the order of photoreceptors, ganglion cells and bipolar cells from the front to the back of the eye?
Ganglion cells
Bipolar cells
Photoreceptors
What do ganglion cell axons project to the forebrain in?
The optic nerve
What cells receive input from photoreceptors and project to other photoreceptors and bipolar cells?
Horizontal cells
What cells receive input from bipolar cells and project to ganglion cells, bipolar cells and other cells the same type as it?
Amacrine cells
What is the purpose of photoreceptors?
To convert electromagnetic radiation to neural signals (transduction)
What are the 4 main regions of photoreceptors?
Outer segment
Inner segment
Cell body
Synaptic terminal
What are the 4 different types of photoreceptors?
Short-wave cone
Middle-wave cone
Long-wave cone
Rod
What type of photoreceptors have the longer outer segment?
Rod photoreceptors
What type of membrane potential do vertebrate photoreceptors have?
Depolarised rmp (Vm)
what is the approximate resting membrane potential of photoreceptors?
About -20mV
How does the resting membrane potential of photoreceptors compare to other neurones?
More positive
What happens to the membrane potential of photoreceptors on light exposure?
Membrane potential hyperpolarises
What causes the change in the membrane potential of photoreceptors due to light?
The “Dark Current”
What is the dark current?
A cGMP-gated Na+ channel that is open in the dark and closes in the light
What is the name of the visual pigment molecules?
What are these found in?
Rhodopsin Rods (present in membrane folds)
What is the purpose of Rhodopsin?
It is extremely sensitive light and therefore enables vision in low-light conditions
What are the 2 things that make up Rhodopsin?
Retinal (Vitamin A derivative)
Opsin (GPCR)
What does light do to Rhodopsin?
Converts 11-cis-retinal to all-trans-retinal (activated form)
What does all-trans-retinal cause?
Activates a molecular cascade which leads to decreased cGMP leading to closure of cGMP-gated Na+ channel -> hyperpolarisation
What molecule opens that dark current channel?
cGMP (nucleotide-gated channel)
What molecule is the dark current permeable to?
Na+
What neurotransmitter do photoreceptors release?
Glutamate
What 2 factors largely determine visual acuity?
Photoreceptor spacing
Refractive power
Do more photoreceptors lead to better or worse visual acuity?
Better
What type of photoreceptors allow you to see in dim light?
Rods
What type of photoreceptors allow you to see in normal daylight?
Cones
What does a high convergence mean?
many rods feed into one ganglion with large spacing = over density (opposite for high)
What does the higher convergence in rod system mean?
increased sensitivity whilst decreasing acuity
What type of photoreceptors perceive colour?
Cones
What colours do short-wave cones perceive?
Blue
What colours do middle-wave cones perceive?
Green
What colours do long-wave cones perceive?
Red
Where are the rods mainly located on the retina?
peripheral retina
Where on the retina are the cones mainly located?
Central retina (foevea)
Do rods have a high or low convergence?
high
Do cones have a high or low convergence?
Low
Do rods or cones have a higher light sensitivity?
Rods
Do rods or cones have a higher visual acuity?
Cones
Does our visual system detect the absolute amount of light?
No, it detects the local differences in light intensity
What is the name of the part of the visual space perceived by each eye?
Monocular visual field
What is the name for the part of the visual space overlapped by both the eyes?
Binocular visual field
What 2 halves can the retina be divided into?
A nasal and temporal hemiretina
Where do the nerve fibres from the nasal half of each retina cross over?
At the optic chiasm
How much of the visual field is perceived by both the eyes?
60% (the 2 retina)
How does a discrete point of light activate many cells in the target structure?
Due to ovrelapping receptive fields
What is perception of vision based on?
The brain’s interpretation of distributed patterns of activity
What happens to the visual field in the brain? (what changes are made)
Left and right bifurcation and up and down invertion
What part of the cortex does the right visual field do to?
Left cortex
What part of the cortex does the superior visual field go to?
Lower cortex
What is another name for the visual cortex?
Striate cortex
What layer of the primary visual cortex are eye specific inputs segregated in?
Layer 4 (cells outside of layer 4 receive input from both eyes)
What is amblyopia?
Cortical blindness = variety of visual disorders where the optics and retina of the eye are okay but one eye has better vision than the other
