Physiology of photoreceptors Flashcards
What is the direct pathway for signal transmission?
photoreceptors deduce light into electrical current -> bipolar cells -> ganglion cells (axons comprise the optic nerve)
What is the role of horizontal cells?
recieve input from photoreceptors and project to other photoreceptors and bipolar cells
What is the role of amacrine cells?
recieve input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
What is the role of photoreceptors?
convert electromagnetic radiation to neural signals
What happens to vertebrate photoreceptor cells membrane potentials when they are exposed to light?
membrane potential hyper polarises
Why is a vertebrate photoreceptor cells membrane potential positive?
the dark current
What is the dark current?
a cGMP gated Na channel that is open in the dark due to the release of neurotransmittors (glutamate) and closed during light
How does the dark current allow the brain to percieve objects?
the change of Na due to light signals to the brain
What cells can cause action potentials?
ganglion cells
What neurotransmittor is released to open the Na channels?
glutamate
What is the membrane potential equation in the dark?
Pna = Pk Vm = between End and Ek
What is rhodopsin (rods) made up from?
retinal - vit A derivative
opsin - G protein coupled receptor
Where are rods present?
membrane folds - called discs in outer segment
What does light convert 11-cis-retinal to?
all-trans-retinal
What is the role of all-trans-retinal?
activates transducin -> molecular cascade -> decrease cGMP which leads to the closure of cGMP gated Na channels
What does decreased Na entry lead to for cGMP gated Na channels?
hyperpolarisation
What is transducin?
a G protein that activates phosphodiesterase (PDE) which hydrolyses cGMP leading to decreased cGMP levels
What are the benefits of rods?
good for seeing in dim light - high convergence, low visual acuity
What are the benefits of cones?
good for seeing in normal daylight with high visual acuity - high density of cones, high visual acuity
How do we see shades?
cones of different wavelengths overlap
What type of cone sees blue?
short wave cone
What type of cone sees red?
long wave cone
What type of cone sees green?
middle wave cone
Where are rods found in the retina?
peripheral retina
Where are cones found in the retina?
center of the retina - fovea
What is the monocular visual field?
what each eye sees individually
What is the binocular visual field?
what both eyes overlap to see
Where would the right visual field go to?
left cortex
Where would the superior visual field go to?
lower cortex
What cells recieve mononoclar vision?
4c layer
What cells recieve binocular vision?
all cells outside the 4c layer
What is ambylopia/cortical blindness?
no problem with the eye, but one eye has better vision than the other
What is ambylopia caused by?
strabismus - wandering eye in infancy - must be corrected by eye patch over good eye as signals from eyes need to compete equally
What is Hebbs postulate?
basis for the loss of binocularity
basically: if axon A is near enough to touch axon B and takes part in exciting it and firing it, a growth process takes place in one or both cells allowing As efficiency to fire cell B to increase - long term potination
What can Hebbs postulate lead to?
one eye being deprived of axons
Where is a colloid cyst found?
intravertebral foramen
What does an ependymanos arise from?
ependymal cells
Where is a epidural haematoma found?
between skull and dura - arterial bleed
Where is a subdural haematoma found?
between dura and arachnoid - venous bleed
What is hydrocephalus?
accumulation of CSF in the ventricular system or around the brain, enlargement of the ventricles increase CSF pressure
What is hydrocephalus due to?
too much CSF production
obstruction of flow
