Physiology Vision Flashcards
what is the order of cells light travels through in the retina
ganglion cells FIRST
bipolar cells
photoreceptors
what is the order of cells the signal is transmitted through in the retina
photoreceptors
bipolar cells
ganglion cells
what do horizontal cells in the retina do
receive input from photoreceptors and project to other photoreceptors and bipolar cells
inhibits bipolar cells by releasing GABA- lateral inhibition
what do amacrine cells in the retina do
receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
what is the dark current
darkness causes a greater release of glutamate from photoreceptors which is passes on to bipolar cells
this causes a stream of sodium which depolarises the photoreceptor releasinf glutamate
in response to light PNa is reduced and the photoreceptor hyperpolarises
what does light convert 11-cis-retinal to
all-trans-retinal (activated form)
what is rhodopsin
opsin + 11-cis-retinal
what is present in the dark that allows the dark current
cGMP - allows sodium channel to open= depolarisation
in light cGMP decreases so Na channel closes
what is the role of rods and cones
rods- dim light
cones- colours
where is there highest visual acuity- how
in fovea of eye - highest conc of cones
is there more convergence in rods or cones
more convergence in rods- increases sensitivity but decreases acuity
what is convergence
number of photoreceptors communicating with a ganglion cell
what allows you to see different colours
different opsins for discrete wavelengthd
are rods or cones chromatic
cones
rods are achromatic
where in retina are the cones and rods
rods- peripheral retina
cones- central (fovea)
do rods or cones have higher light sensitivity
rods have high sensitivity
cones low
what are the ‘off’ and ‘on’ pathways
off pathway bipolar cells release ionotropic glutamate, are hyperpolarised by light
on pathway bipolar cells release metabotropic glutamate
are depolarised by light
can be on- centre/ off- surround or vice versa
what is the role of lateral inhibition in the retina
exaggerates the difference in stimulus intensity detected by adjacent neurones (parallel neural pathways), aids in locaisation
explain the receptive field of the retina
two concentric circles create a centre e.g.an on centre/ off surround field:
surround field:
in centre are ‘on’ photoreceptors
surround are ‘off’ photoreceptors
when light hits centre of field firing rate (photoreceptors are hyperpolarised) is maximal
when the light extends out towards the surround there is lateral inhibition and the firing returns to a baseline level
when light only on surround not centre then firing is suppressed
what is retinotopy
the mapping of visual information from the retina to neurons in the brain
fibres from what visual field cross at the chiasm
nasal half of each retina so temporal visual field
how is the visual field organised
visuotopic- scaling not consistent- fovea has a large area
what is the path of the optic radiations
from lateral geniculate nucleus to layer 4 of the visual cortex
what is V1
brodmans area 17- visual cortex
what is layer four of the visual cortex
main input area
what is the alpha layer of the visual cortex
movement ganglion layer
what is the beta layer of the visual cortex
parasol ganglion cells
what is the striate cortex
the part of the occipital cortex that receives the fibers of the optic radiation from the lateral geniculate body and is the primary receptive area for vision
what is the competition hypothesis
Connections from the two eyes compete with each other in cortex
In the cortex, monocular deprivation during a ‘critical period’ in development results in active afferents from one eye and lower activity from the other eye (leads to alteration in the structure of the cerebral cortex)
