Physiology Flashcards
What must occur for us to see?
Light must enter the eye- Not too much or little
Light must fall on photoreceptors
Photoreceptors must generate current
Brain must interpret electrical signals
Where are photoreceptors found?
In the inner layer of the eye- Retina
Where in the laminar structure of the retina are the photoreceptors found?
Deep
What is found on top of the photoreceptors?
Bipolar cells then ganglion cells
What cells link photoreceptor cells with each other and and bipolar cells?
Horizontal cells
What cells link bipolar cells with each other and ganglion cells?
Amacrine cells
What must light pass through to reach the photoreceptors?
Pupil, lens, posterior chamber of the eye, ganglion cells, bipolar cells, amacrine cells and horizontal cells
What kind of photoreceptors do we possess?
Cones
Rods
What do photoreceptors do?
Transduce electromagnetism to neuronal signals
What are the 4 main regions of photoreceptors?
Synaptic terminals
Cell bodies
Inner segment
Outer segment
Where does signal transduction occur in photoreceptors?
Outer segment
In what type of light do rods help us see?
Dim/dark
In what type of light do cones help us to see?
Bright
Describe the basal state of photoreceptors
Marginally depolarised (-20mV)
What happens when photoreceptors are exposed to light?
Becomes hyperpolarized
What is the dark current?
The constantly depolarised state photoreceptors experience in the dark
What causes the dark current?
An open Na channel
Describe the dark current
Continuously open Na channels depolarise photosensitive cell to between Na and K Vm (-20mV)
What does this dark current result in?
Constant neurotransmitter (glutamate) release
How does light switch off the dark current?
Light activates GPCR rhodopsin.
Rhodopsin activates PDE through GMP
PDE hydrolyses cGMP thus reducing its conc
Decrease in cGMP levels close cGMP dependent Na channels leading to hyperpolarization of cell
Is phototransduction high or low gain?
High gain
What is rhodopsin?
A photosensitive GPCR
What is rhodopsin made of?
Retinal (derived from Vit A)
Opsin (GPCR)
Where is rhodopsin found?
Disks in the outer segment
What happens to Rhodesian when light hits it?
Converted to all-trans-retinal
What two pathways are possible after the photosensitive cell in sight?
On pathway
Off pathway
What happens in the On Pathway?
Glutamate from photosensitive cell activates metabotropic R (GPCR) on bipolar cells leading hyperpolarisation.
In turn this then leads to hyperpolarisation of ganglion cell and no AP
When light reduced glutamate levels AP can be generated
What happens in the Off Pathway?
Glutamate from photosensitive cell activates ionotropic R on bipolar cells leading to depolarisation.
In turn this then leads to depolarisation of ganglion cell and generation of AP
When light reduced glutamate levels AP generation is inhibited
What affects visual acuity?
Density of photosensitive cells
What increases visual acuity?
Increasing density of photosensitive cells and convergence
What is convergence?
The number of photosensitive cells that synapse onto a single ganglion cell
Do cones or roads have higher convergence?
Rods
What is high convergence?
Many photosensitive cells synapsing onto one ganglion cell
What is low convergence?
One or two photosensitive cells synapsing onto one ganglion cells
Why are rods high convergence?
Because they’re activated in low light therefore require more input to activate an AP. Only able to fire an AP by pooling signals from many photosensitive cells
Why does acuity decrease as convergence increases?
Because the receptive fields from many photosensitive cells are being combined therefore cannot distinguish between them
Where is the greatest density of cones
In the macula of the eye (0 degrees)
Where are the greatest density of rods found?
Between 10 and 70 degrees (+/-)
Where is degree 0 of the visual field?
Macula
How many types of cone do we have?
3
What colour light do short wave cones tend to see?
Blue
What colour light do medium wave cones tend to see?
Green
What colour light do long wave cones tend to see?
Red
How does the lens change to focus on far away objects?
Tall and flat
How does the lens change to focus on nearby objects?
Short and flat
What is hyperopia?
Farsightedness- trouble seeing nearby objects as focus behind retina
What is myopia?
Nearsightedness- trouble seeing far away objects as focus in posterior chamber
What does hyperopia mean?
‘Oversight’
What does myopia mean?
Shut eyes
Do the eyes detect absolute light or difference in light?
Difference in light/contrast
Describe the optic tracts
Nasal tracts cross
Temporal tracts do not
Where do the optic tracts synapse in the brain?
Lateral geniculate nucleus
How can temporary loss of vision in one eye during development lead to loss of binocular vision?
Axons from both eyes normally compete and provide binocular vision
When one of these is lost the other takes over and provides all input
Where is visual information processed in the brain
Primary visual cortex layer 4
What pathology does a lesion in the optic tract just behind the eye cause?
Monocular blindness
What pathology does a lesion in the optic tract at the optic chiasma cause?
Bitemporal hemianopsia
What pathology does a lesion in the optic tract after the optic chiasm cause?
Contralateral hemianopsia
