Physiology: Retina and Vision Flashcards
describe the passage of light as it enters the eye
passes through ganglion cells and bipolar cells before reaching photoreceptors (rods and cones)
how is the light signal transmitted from photoreceptors
horizontal cells
receive input from photoreceptors cells and project to photoreceptor and bipolar cells
amacrine cells
receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells
photoreceptor function
convert light (electromagnetic radiation) into neural signals (transduction)
photoreceptor proteins in the cell absorb photons triggering a change in the cell’s membrane potential
what does rhodopsin consist of
rhodopsin consists of opsin (a G protein) and retinal (vitamin A derivative)
what do photoreceptors do on light exposure
hyperpolarize
what happens to photoreceptors in the dark
depolarize and release glutamate continuously
in the dark, there is a high concentration of cGMP in cells, which opens ion channels largely Sodium channels)
the positive charges of the ion change the cell’s membrane potential causing depolarization and leading to the release of glutamate (neurotransmitter)
what happens to photoreceptors in the light
- when light hits a photoreceptive pigment, the pigment changes shape
- rhodopsin consists of opsin (a G protein) and retinal (vitamin A derivative)
- it exists in the 11-cis-Retinal form in the dark, and stimulation by light causes it to change to the all-trans-Retinal form
- this structural change causes activation of transducin, which leads to the activation of cGMP phosphodiesterase which breaks down cGMP into 5-GMP
- reduction in cGMP allows the ion channels to close, preventing positive ion influx and hyperpolarizing the cells and stopping the release of neurotransmitters (glutamate)
- = visual phototransduction
dark current
the residual electrical current flowing when there is no incident illumination
unstimulated (dark) cyclic-nucleotide gated channels in the outer segment are open because cGMP is bound to them. Hence, positively charged ions (namely Sodium) enter the photoreceptor depolarizing the cell to about -40mV (higher than most other nerve cells where the resting potential is around -65mV)
the depolarizing current is known as the dark current. steady release of neurotransmitters
visual acuity
the ability to distinguish between 2 nearby points
largely determined by photoreceptor spacing and refractive power
what is daylight vision observed by
cone cells
describe the properties of cone cells
high spatial density in the central fovea
low convergence, low light sensitivity, high visual acuity
chromatic
what is vision in low light observed by
rods
describe the properties of rods
lower spatial resolution, due to spatial summation of rods (eg large number of rods merge into a bipolar cell, connecting to a ganglion cell)
high sensitivity, low acuity
achromatic (one type of pigment)
describe the distribution of cones and rods in the eye
cones are most dense in the central fovea
rods are absent here but dense elsewhere
how do cones view colour
there are 3 different types of cone cell in humans, each absorb photons at a different wavelength of light, this gives us trichromatic vision
- roughly short, medium and long wavelengths
describe the visual fields
each eye sees a part of the visual space - monocular visual field (±45), but their visual fields overlap extensively to form a binocular visual field (±45)
which nerve fibres cross at the optic chiasma
nasal fibres
how does our visual system differentiate two points
- detects local differences in light intensity, not absolute amounts of light
- lateral inhibition exaggerates the differences in stimulus intensity detected by adjacent neurons to aid localisation
on and off centre ganglion cells
- each ganglion cell has a receptive field, arranged into a centre and a surround, each region responds oppositely to light
- there are two types of retinal ganglion cells: on and off centre
- an on centre is only stimulated when the centre is exposed to light etc
- on-centre: stimulation of centre produces depolarization and an increase in firing of the ganglion cell, stimulation of surround produces hyperpolarization and a decrease in firing of the cell
how does an on-centre retinal ganglion cell respond to light that covers both the cntre and the surround
very little/no response as as there is inhibition from the surround
and vice versa
role of horizontal cells
interconnect surround neurons, they release inhibitory GABA to sharpen the edge of a receptive field by inhibiting the surrounding photoreceptor cells
describe the path of the optic tracts
- go to lateral geniculate nucleus in thalamus
- from here, optic radiations are projected to the primary visual cortex
- enter at level 4C
loctaion of V1
in and around calcarine sulcus in the occipital lobe
outline visuotopic organisation
- lower visual field is projected to gyrus superior to calcarine sulcus and conversely
- a disproportionately large area of V1 is dedicated to the small fovea (highest density of cones so transmits highest visual acuity)
Meyer’s loop
- lower optic radiation fibres (corresponding to superior visual quadrants) first loop anteriorly aorund the temporal part of the lateral ventricle and end below the calcarine sulcus
