L33 - visual defects Flashcards
vitreous humour
viscous jellylike substance that lies between the lens and the retina
conversion of light into neural activity
- light is focused by the conea, then the lens, then passes through the vitreous humour to the retina
- retina lies in front of the pigment epithelium which absorbs any light not absorbed by the retina
pigment epithelium
filled with black pigment melanin which absorb any light not absorbed by the retina
- stops light bouncing inside the eye, improving resolution
what do bipolar neurones release
glutamate
phototransduction
conversion of light energy into electrical energy
where in cones does phototransduction occur
invaginated membrane
where in rods does phototransduction occur
membranous discs
photoreceptors in the dark
depolarised
continuously release glutamate
photoreceptors in the light
- depolarised ion channels close
- photons are absorbed
- membrane is hyperpolarised
- glutamate release reduced
photopigments
individual transmembrane proteins
G-protein coupled receptors
retinal
- embedded within photopigments
- synthesised from vitamin A
what happens when retinal is hit with a photon if light
it undergoes a conformational change, activating Rhodopsin molecule
- G-protein coupled amplification
- activates transuding and phosphodiesterase
- breakdown of cyclic GMP
- when levels fall, ion channels are closed
- hyperpolarisation
resolution when denser photoreceptors
better
on bipolar cells in light
depolarise
off bipolar cells in light
hyperpolarise
mechanism of off bipolar cells
- glutamate receptors
- Na can permeate through and depolarise the cell
- when light comes along, glutamate decreases and unbinds
- ion channels close
- hyperpolarisation occurs
mechanism of on bipolar cells
- glutamate activates G-protein coupled receptor, inhibiting an ion channel
- when light comes along, glutamate decreases, preventing inhibiting of the ion channel
- depolarisation
- releasing inhibition
off bipolar cells as light gets dimmer
depolarises
on bipolar cells as light gets dimmer
hyperpolarises
receptive field
the area of the retina that causes any change in response of a neurone
outer retina receptive field
horizontal cells shape receptive fields of bipolar cells
inner retina receptive field
amacrine cells shape receptive fields of ganglion cells
lateral inhibition
regions of retina which are outside the lateral extent of the dendrites of the ganglion cells
- form inhibitory synapses with ganglion cells
In dim light, the presence of which molecules causes ion channels in rod cells to open?
cGMP molecules
The 3 types of cone cells can detect which 3 colours (each cone has 1 of 3 opsins)?
blue
green
red
What happens to the size of photoreceptors further away from the fovea?
Photoreceptors further from the fovea are larger in size
What is the name of the pigment in all rod cells called?
Rhodopsin
What are the 2 colour opponent pathways?
red/green
blue ON/yellow OFF
opsin
receptor protein of rhodopsin
what colour do we see when all cone cells are equally active
white
what type of firing if just red hits the centre of the receptive field
maximum firing
firing if green surrounds and red is in the centre
green inhibits the excitation of red in the centre
What are ipRGCs (intrinsically photosensitive retinal ganglion cells) required for?
pupillary light reflex
they connect the Edinger-Westphal nuclei
striate cortex
primary visual cortex
melanospin
5th photopigment
expressed by retinal ganglion cells
where is melanopsin expressed
intrinsically photosensitive retinal ganglion cells
groups of layers of the LGN
parvocellular
magnocellular
koniocellular
layers 3-6 of the LGN
parvocellular (small cell bodies)
layers 1-2 of LGN
magnocellular layers (large cell bodies)
What is the name of the small layers between layers 1-6 in the LGN?
Koniocellular layers
What do magnocellular cells receive input from?
M-type ganglion cells
Magnocellular cells
- large receptive field
- important for detection of stimulus movement
- 5% population
parvocellular cells
- smaller cell type
- 90% populations
What do koniocellular cells receive input from?
K-type
What do parvocellular cells receive input from?
P-type
koniocellular cells
- medium cell type
- 5% population
ganglion cells
M ganglion cells
P ganglion cell
K ganglion cell
M ganglion cell
Large fields allow them to be responsive to motion as they can compare the visual fields the whole time
P ganglion cell
- Provides more high resolution information
- Only receiving input from bipolar cells connected to photoreceptors
- Concerned with working out what the stimulus is
K ganglion cells
works out which direction someone is coming towards you / object motion sensitive
What can be said about all neurons in the vertical column?
They display the same orientation specificity
what can be said about neighbouring columns
similar orientation but slightly shifted in angle
What is an orientation column?
A column of the cortex of neurones that are all connected to each other and are selective to the same orientation of a stimulus
Neurones in oblique rows display what kind of orientation specificity?
heterogenous
what are blobs
- pillars of neurones in the primary visual cortex
- rich with cytochrome oxidase
- where colour-processing occurs
Information from the primary visual cortex is transmitted to which lobe for processing of visual motion?
parietal lobe
Information from the primary visual cortex is transmitted to which lobe for recognition of objects?
temporal
What is the part of the brain that is involved in working out where objects are relative to ourselves?
Superior colliculus
cytochrome oxidase
mitochondrial enzyme - cell metabolism
