Visual defects and anatomy of the eye Flashcards
Is the retina considered part of the CNS?
Yes
What is refraction and what 2 structures in the eye enables refraction to occur?
Refraction is the bending of light rays
Happens as light passes through the cornea and the lens
Why is refraction important?
So that light rays that strike the curved surface of the cornea bend so they converge on the back of the eye, the lens also refracts light rays passing through it to produce a sharp image
What is the cornea?
The glassy, transparent external surface of the eye
How is the cornea nourished?
Lacks blood vessels and is nourished by the aqueous humour
What is the function of lens accommodation?
Closer images require greater refractive power by the lens to bring them into focus - this is brought about by the lens changing shape
What is lens accommodation?
Rounding of the lens to increase the curvature of the lens surface and increase the refractive power - the ability to accommodate the lens changes with age
Lens accommodation involves which muscle?
The ciliary muscle
How does contraction of the ciliary muscle allow rounding of the lens?
Contraction of the ciliary muscle relieves the tension on the zonule fibres, this allows the lens to become rounder due to its natural elasticity
What is an emmetropic eye?
A normal eye which doesn’t need glasses to aid focus
What is hyperopia?
Long sightedness
Why does hyperopia occur?
The eyeball is too short from front to back and light rays are focused at some point behind the retina, as a result the retina sees a blurry circle
Accommodation of the lens is needed for distant objects and near objects cannot be brought into focus
How is hyperopia corrected?
A convex lens is placed in front of the eye (glasses) which provides the necessary refraction to allow near objects to be brought into focus
What is myopia?
Nearsightedness
Why does myopia occur?
The eye ball is too long from front to back, parallel light rays converge at some point infront of the retina, as a result the retina sees an unfocused blurry circle.
How is myopia corrected?
A concave lens is placed in front of the eye (glasses) which provides the necessary refraction to allow distant objects to be brought into focus on the retinal surface
How does corrective laser surgery work?
(Also called photorefractive keractectomy) uses a lase to reshape the cornea and increase or decrease the amount of refraction possible
What fills cells in the retinal pigment epithelium and what is its function?
Melanin - absorbs any light not absorbed by the retina
What are the 5 differences between rods and cones?
Rods:
1) High sensitivity to light - specialised for night vision
2) More photopigment - capture more light
3) High amplification
4) Low temporal resolution - slower response
5) More sensitive to scattered light
Cones:
1) Lower sensitivity - specialised for day vision
2) Less photopigment
3) Lower amplification
4) High temporal resolution
5) More sensitive to direct light rays
Where does the process of absorption occur in the photoreceptors?
In the outer segments of the photoreceptors (bits closest to the RPE) which contain a stack of membranous discs which contain light sensitive photopigments
What is the main difference in morphology between cone cells and rod cells?
Rod cells - long cylindrical outer segment containing many discs
Cone cells - short, tapering out segments with few membranous discs
What is the function of photoreceptors?
Transduce light energy into changes in membrane potential
What are the 4 differences between the rod system and the cone system of vision?
Rod system: 1) Low acuity 2) Not present in fovea 3) Highly convergent pathway 4) Achromatic (one type of rod pigment) Cone system: 1) High acuity 2) Concentrated in fovea 3) Dispersed pathway 4) Trichromatic (3 types of cone pigment, each with a different wavelength preference)
What is scotopic lighting, which cells function in it?
Night time lighting - rods only
What is mesopic lighting, which cells function in it?
Twilight - both rods and cones
What is photopic lighting, which cells function in it?
Daytime lighting - cones only
The pigment in all rod cells is called what?
Rhodopsin
How do cone cells allow colour vision?
1) In each cone cell there is one of 3 types of opsin
2) Each cone opsin has different spectral sensitivity (responds to light of different wavelengths ie. different colours)
3) Have blue cones, green cones and red cones
4) Colour perception is determined by the relative contribution of the blue, green and red cones to the retinal signal
What is the Young-Helmholtz trichromatic theory?
1) Every colour in the rainbow can be obtained by mixing the proper ratio of red, green and blue light
2) At each point in the retina there exists a cluster of 3 receptor types, each receptor type being maximally sensitive to either blue, green or red
3) The brain assigns colours based on a comparison of the 3 cone types
4) Eg. when all cone types are equally active we perceive white
What genetic finding supports the Young-Helmholtz trichromatic theory?
Genes for the cone pigments have been identified, and are missing in some colour blind patients
A person with normal colour vision is referred to as trichromat, what is an anomalous trichromat?
A person with an alternate perception of colour
Someone with colour blindness eg. red-green colour blindness is referred to as what, what is the genetic defect?
A dichromat - lack the correct gene for red or green pigment
Why is red-green colourblindness more common in males?
The genes for the red and green pigments are carried on the X chromosome
What is the genetic defect in a person lacking all colour vision?
Lack the gene for red and green pigments - world is seen in black and white
What is the function of bipolar cells in the retina?
Create the direct pathway from photoreceptors to ganglion cells
What is the function of horizontal/amacrine cells?
Indirect pathways - ie act as modulators
How does signal transduction occur in photoreceptors?
Unusual pathway
1) If light of the correct wavelength hits a photoreceptor then this STOPS the photoreceptor from releasing neurotransmitter
2) This is due to cGMP
3) When light of the correct wavelength hits the photoreceptor then cGMP is converted to GMP and can no longer open sodium channels thus neurotransmitter release decreases
What is the arrangement of connections of photoreceptors to bipolar cells?
- Have a receptive field for each bipolar cell, as each bipolar cell is connected to more than one photoreceptor
- Photoreceptors in the receptive field center make direct contact with the bipolar cell
- Photoreceptors in the receptive field surround make indirect contact with the bipolar cells via horizontal cells
This is referred to as Centre-Surround receptive field
What is the receptive field of a bipolar cells?
The area of retina that alters the polarisation (Vm) of a bipolar cell in response to light
When light is in the center of a receptive field how does this affect the membrane potential of the bipolar cell?
1) Photoreceptor becomes hyperpolarised
2) Less glutamate is released from the photoreceptor
3) mGluR6 (Glutamate receptor) on the bipolar cell surface is less active allowing Na channels to open, causing depolarisation
How does glutamate and thus the activity of its receptor mGluR6 affect the membrane potential of a bipolar cell?
Decreased activity of glutamate on the bipolar cell allows Na+ channels to open causing depolarisation
When light hits the photoreceptors in the receptive field surround and its dark in the center, how does this affect bipolar cells?
1) Photoreceptor in the center is depolarised so glutamate released from the center photoreceptor
2) This causes the glutamate receptor (mGluR6) on the bipolar cell surface to be MORE active which closes the Na channel causing hyperpolarisation
3) At the same time, light is on the photoreceptors in the receptive field surround so they become hyperpolarised and stop releasing glutamate
4) Less glutamate causes the horizontal cells to become hyperpolarised
5) The horizontal cells thus release less GABA and this depolarises the photoreceptors in the receptive field center producing more bipolar cell hyperpolarisation
How does glutamate release from photoreceptors affect the membrane potential of horizontal cells?
Less glutamate causes horizontal cells to become hyperpolarised and reduces the release of GABA (glutamate has the opposite effect on horizontal and bipolar cells)
How does GABA released from horizontal cells affect photoreceptors in the central receptive field?
Causes them to become depolarised and release more glutamate (Which hyperpolarises the bipolar cell)
Do photoreceptors in the central receptive field cause depolarisation of the bipolar cells when light is on them or in the dark?
In the light
Do photoreceptors in the receptive field surround cause depolarisation of the bipolar cells when light is on them or in the dark?
In the dark
What are the 3 types of retinal ganglion cells, what is there size, and what percentage of the ganglion population do they make up?
1) Magnocellular type (M type) - Larger cell type, 5% of the population, large receptive field and important for detection of stimulus movement
2) Parvocellular type (P-type) - Smaller cell type, 90% of the population, sensitive to stimulus and fine detail
3) Non-M non-P type (K-type) - Medium cell type, 5% of population
When will an on center ganglion cell be depolarized?
When a small spot of light is projected onto the middle of its receptive field
When will an off center ganglion cell be depolarized?
Responds to a small dark spot presented to the middle of its receptive field
Ganglion cells are mainly responsive to differences in illumination which occur where?
Within their receptive field
Which 2 types of ganglion cells are sensitive to differences in wavelength of light and what are they therefore called?
P type K type (non M non P type) Colour sensitive neurons are called colour-opponent cells
Which 2 types of opponent ganglion cells provide our perception of colour?
1) Red-green opponent cells
2) Blue-yellow opponent ganglion cells
The retinofugal projection describes the course of optic neurons, what are the 4 targets of optic neurons?
1) Main target is lateral geniculate nucleus
2) Some go to hypothalamus - to do with circadian rhythm
3) Some go to the superior colliculus and is involved in reflex responses
4) Some go to the pretectum
How many layers can the lateral geniculate nucleus be separated into?
6
Where do the koniocellular layers lie in the lateral geniculate nucleus?
Ventral to each of the 6 principal layers - in between the 6 principle layers
Which type of ganglion cell projects to the koniocellular layer of the lateral geniculate nucleus?
nonM-nonP type
Which type of ganglion cell projects to the magnocellular layers (layers 1 and 2) of the lateral geniculate nucleus?
M type
Which type of ganglion cell projects to the parvocellular layer of the lateral geniculate nucleus (layers 3-6)?
P type
Neurons of the lateral geniculate nucleus project to the visual cortex via what?
Optic radiation
In the primary visual cortex, what is the role of M channels?
Analysis of object motion
In the primary visual cortex, what is the role of P-IB channels?
Analysis of object shape
What is the function of blobs in the primary visual cortex?
Analysis of object colour
What is the function of orientation columns in the primary visual cortex?
Tells us the angles that light is coming in at
What is the function of ocular dominance columns in the primary visual cortex?
Gives us information about which eye is receiving which eye - as they only receive info from one eye
There are areas of the cortex involved in visual processing called the extrastriate cortex - what 2 cortical streams of visual processing are thought to exist?
1) Striate cortex towards parietal lobe - involved in visual motion
2) Striate cortex towards temporal lobe - involved in recognition of objects
