neurobiology of vision Flashcards
action of the brain lobes in relation to vision
- occipital
- parietal
- temporal
- frontal
what is and role of retina
membrane on the inner aspect of the eye that contains photoreceptors
made up of multiple layers
pupil
lens
transparent section behind the pupil that is enclosed in a thin transparent capsule
can be changed shape to help focus/refract light onto the retina by having its shape changed by the ciliary muscle
choroid
middle layer of the eye between the retina and the sclera
contains a pigment that absorbs excess light, preventing blurring of vision
ciliary muscle
a ring of smooth muscle in the eye’s middle layer (vascular layer) that controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humor into Schlemm’s canal.
links the choroid layer to the iris
macula
small yellow spot on the retina at the back of the eye.
surrounds the fovea
fovea
small indentation at the very back of the eye
thin so light can more directly reach photoreceptors
composed of closely packed cones
low ratio between cones and ganglion cells, high resolution abd gives us our best acuity
optic nerve
CN II carrying sensory info from thre retinal ganglion cells to the brain, syanpsing in the LGN
where it joins the retina there are no photoreceptors, giving us an anatomical blind spot
sclera
tough protective layer that covers the eye- the cornea is an anterior projection
iris
regulates the amount of light that can enter the eye
coloured part in fornt of the lens
night enters through it via the pupil
allows for dilation and constrictiuon of the pupil
which 2 structures focus light via refraction onto the retina
cornea and lens
anterior cavity
space between cornea and lens
filled with watery substance known as aqeous humor
posterior cavity
space behind the lens
filled with vitrous humor
the retina is made up of which 2 kinds of photoreceptor
what is the name of the pigments that absorb light
cone cells
- Fewer in number
- conical shaped
- low sensitivity to light
- responsible for colour vision
- localised at fovea
rod cells
- More abundant
- cylindrical shaped
- high sensitivity to ligh
- function in night vision
- low visual acuity
- absent at the fovea.
contain an inner segment within which there are normal organelles
outer segment is specialised for photoreception containing rhodopsins
layers and their cells of the retina (3)
1) outer plexiform layer- includes photreceptors
2) inner plexiform- contains bipolar cells, horizontal cells and amacrine cells
3) ganglion cell layer- contains ganglia cells
horizontal cells
- forms synapses with bipolar cells and photoreceptor cells
- laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina
- help integrate and regulate the input from multiple photoreceptor cells
bipolar cells
what is their role in phototransduction
exist between photoreceptors and ganglion cells
They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.
In the dark, a photoreceptor (rod/cone) cell will release glutamate, which inhibits (hyperpolarizes) the ON bipolar cells and excites (depolarizes) the OFF bipolar cells. In light, however, light strikes the photoreceptor which causes the photoreceptor to be inhibited (hyperpolarized) due to the activation of opsins which activate All trans-Retinal, giving energy to stimulate G-Protein coupled receptors to activate phosphodiesterase (PDE) which cleaves cGMP into 5’-GMP.
how do photoreceptors link to ON and OFF bipolar cells
In the dark, a photoreceptor (rod/cone) cell will release glutamate, which inhibits (hyperpolarizes) the ON bipolar cells and excites (depolarizes) the OFF bipolar cells.
In light, however, light strikes the photoreceptor which causes the photoreceptor to be inhibited (hyperpolarized) due to the activation of opsins which activate All trans-Retinal, giving energy to stimulate G-Protein coupled receptors to activate phosphodiesterase (PDE) which cleaves cGMP into 5’-GMP.
amacrine cell
interneurone in the retina that allows ganglia to send temporarily correlated signals to the brain
what kind of molecules capture photons
there are different kinds of these in rods and cones, name them (1 rod and 3 cones)
in cones, one type of ospin usually predominates
in colour blindness the red/ green opsins in cones can be swapped around
pigment molecules
receptor potentials in photoreceptors
phototransduction and visual pathway
Light waves enter the pupil after being refracted from the tear layer and cornea.
- The lens further refracts the light onto the retina which is where the photoreceptors are populated (specifically the fovea)
- Photons from the light are absorbed by the photopigment (rhodopsin/iodopsin), specifically by opsin. This tunes the light and detects the particular wavelength on the spectrum.
- The absorption of light triggers the activation of transducin which activates a phosphodiesterase that hydrolyses cGMP.
- In darkness, high levels of cGMP in the outer segment keep sodium channels open, however in the light cGMP levels drop and some of the channels close leading to hyperpolarization of the outer segment. This ultimately reduces the opening of Calcium channels at the synaptic membrane and reduces glutamate being released into the synapse
- In the dark, the photoreceptors are in a depolarized state (membrane potential of roughly -40mV). As there is a progressive increase in the intensity of light, it causes the potential across the receptor membrane to become more negative (reaching -65mV)
- The drop in the glutamate neurotransmitter signals that light is present
- Photoreceptors in the outer plexiform layer stimulate horizontal cells which helps to identify the information that is passing through.
- Alongside this, the bipolar cells create direct or indirect connections to the ganglion cells in the inner plexiform layer
- The ganglion cells process the electrical information (alongside the amacrine cells). Their axons collectively form the initial part of optic nerve.
- The optic nerve (cranial nerve II) exits via the optic disc on the retina
a. Anything from the nasal visual field is projected onto the temporal retina
b. Anything from the temporal visual field is projected onto the nasal retina
c. (for example, light from the right visual field will hit the left eye’s temporal retina while hitting the right eye’s nasal retina) - After the formation of the optic nerve, it leaves the bony orbit via the optic canal, a passageway through the sphenoid bone. It then enters the cranial cavity running along the surface of the middle cranial fossa.
- The optic nerve meets at the chiasm in which;
- Axons from the nasal retina cross over to the opposite sides, while the temporal retina information remain on the same side (this means that all information from the left visual field stay together and all information from the right visual field stay together)
- The optic tract then synapses with the cells in the lateral geniculate nucleus (part of the thalamus)
- Information from the contralateral side goes to layers 1, 4 and 6 of the LGN while information from the ipsilateral side goes to layers 2,3 and 5
- It then travels to the primary visual cortex in the occipital lobe via two main pathways;
a. Upper optic radiation – (baum loop) this pathway carries fibres from the superior retinal quadrants (corresponding to the inferior visual field quadrants)
b. Lower optic radiation – (Meyers loop) carries fibres from the inferior retinal quadrants (corresponding to the superior visual field quadrants) - In the higher visual centre, it takes either the;
a. Dorsal pathway (to parietal cortex) which subserves spatial vision providing an image of where the object is
b. Ventral pathway (to inferotemporal cortex) providing an image of what the object is
describe the reltionship between the retinal sides and the visual field
the temporal retina receive light from the inner visual fields. their ganglia do not decussate
nasal
how do the retinal zones correlate to the visual fields
the temporal retinal zones receive light from the inner visual field and their ganglia do not decussate at the optic chaism
the nasal aspects do the opposite
nerve fibres from the LGN go where to be processed?
in what layer for they mainly synapse?
where is the primary visual cortex found
primary visual cortex
layer 5
in the occipital lobe enar the calcari sulcus
the LGN is arranged in a what style of fashion
layered modular
what are the 2 routes to higher visual centres from the primary visual cortex?
dorsal pathway- where- sptial vision- goes to parietal lobe
ventral pathway- what- subserves colour+ object vision- goes to temporal lobe
Glaucoma
risk factors
causes thinning of the optic nerve fibre layer in the retina and graduallly results in vision loss and ifnleft untreated—– blindness
risk factors:
family history of glaucoma
increasing age
prolonged steroid use
age related macula degeneration (ARMD)
diabetic neuropathy
deterioration of the tight blood vessels causing blockage, exudates (plasma leakage), haemorrahges and micro aneurysms
retinal vein occlusion
blockage of retinal vein
predisposing factors are:
increasing age
systemic hypertension
increased IOP
long sighted eye corrections
presbyopia
cataracts
any opcaity of the lens
gradual blurring of vision, seeing double in one eye or increasing sensitivity to glare
risk factors: increasing age, excessive unprotected eye exposure to UV light, smoking, poor nutrition, prolonged steroid use
may be surgically removed by replacing the lens w a clear plastic lens implant
Balint syndrome
- neurological visual condition
- bilateral damage to the parito occipital region (region between the parietal and occipital lobes)
- spatial perception deficits
- dorsal stream is only affected
includes:
- optic ataxia- difficulaty reaching for objects under visual guidance
- occular apraxia- difficulaty in visual scanning
- simultanagnosia-
damage to the visual system of the brain
neurological visual condition
damage to V1- no conscious awareness of being able to see– blindsight impaired
info goes straight to processing
damage to V4- no colour perception or memory of colour, achromatopsia
damage to V5- cannot process motion so objects moving become invisible- akinetopsia
- visual agnosia
- apperceptive visual agnoisa
- prosopagnosia
cannot perceive visual stimuli correctly
cannot perceive objects
failure to recognise faces- damage to fusiform face area
associative visual agnosia
inability to identify objects perceived visually
form can be matched w similar objects or drawn from memory
just can’t link photos w words
disruption to connections in the ventral stream o f the visual cortex but no damage to the dorsal stream
achromatopsia
inability to discriminate among different hues, cannot see colour at all
damage to visual association cortex either bilateral or unilateral
also affects memory of colour
