Physiology of vision Flashcards
As light enters the eye the pupil size can increase or decrease depending on the amount of light it is exposed to. What happens if there is brighter or dimmer light?
- brighter light = pupil constricts and reduces in size
- dimmer light = pupil dilates and gets bigger
There are a few layers light must pass before it reaches the retina, including the cornea and lens. Does this light pass straight through in a straight line to the retina?
- no
- light rays bend when they enter a different medium from air at an angulated surface
There are a few layers light must pass before it reaches the retina, including the cornea and lens. The light rays are bent when they enter a different medium from air at an angulated surface, which is performed by the cornea and the lens. How much does each bend the light?
- cornea does 2/3 of the ray bending
- lens does the other 1/3, but also allows the focus to vary
When we see an image light passes through the cornea, iris and lens and onto the macula of retina, which is the site for the best visual acuity. Does this image appear as we see it or in another way?
- it would appear upside down
- the brain doesn’t interprets the image like this
What is refraction?
- the change in direction of a wave as it passes from one medium to another or from a gradual change in the medium
- same thing happens when light hits the cornea and lens
Refraction is the change in direction of a wave as it passes from one medium to another or from a gradual change in the medium. Do concave (shape curves inwards) and convex (shape curves outwards) shapes reflect light in the same way?
- no
- concave = focuses light rays
- convex = diverges light rays
Refraction is the change in direction of a wave as it passes from one medium to another or from a gradual change in the medium. What is refraction power measured in?
- Diopters (D)
- a 3D lens has a higher refractive power (D) than a 2D lens
What is focal length?
- distance beyond where the light converges on a convex lens to the focal point
- can be shortened using convex and lengthened using concave lens
Emmotropia is derived from the greek ‘well proportioned’ and is a term used to describe perfect vision. When this occurs the lens of the eye is said to be in what state?
- neutral or relaxed state
- ciliary are relaxed and the lens is flat
What is hypermetropia more commonly known as in relation to the eyes?
- hyper = long sighted
- eyes are generally shorter
- light focuses on a point behind the eye
What is myopia more commonly known as in relation to the eyes?
- myo = shorter
- eyes are generally longer
- light focuses on a point in front of the retina
Hypermetropia more commonly known as long sightedness is where light focuses on a point behind the eye. How is this corrected with glasses?
- convex lens glasses can be worn
- convex lenses shortens the focal point of the light
- the light will be brought forward and onto the retina
Myopia more commonly known as short sightedness is where light focuses on a point that does not reach the retina. How is this corrected with glasses?
- concave lens is used
- increases the focal point
- focal point now reaches the retina
What is astigmatism which is greek for without point?
- a type of refractive error
- one or more surface of the cornea or lens are not spherical
- this causes multiple points of focus, resulting in blurred vision
What is Presbyopia, which is greek for old man loss?
- loss of your eyes ability to focus on nearby objects
- lens becomes thinner and/or the ciliary muscles become too weak (both cause a shortening of the lens)
- focus becomes difficult
The following are types of disorders of the refraction ability of the eye.
- Hypermetropia (long sighted) common in children
- Myopia (short sighted) common in Asian population
- Astigmatism (cornea and/or lens are not spherical, increased focal points and blurred vision)
- Presbyopia (eyes ability to focus is impaired due to weak lens or ciliary muscles) common in older age
Which of these refractory disorders is the most common?
- Astigmatism
A patient presents with myopia. Where does image form in the retina and what sort of lens do you need as a corrective lens?
- front of the retina display (short sightedness)
- concave lens as it increases focal length diverges the light rays further back in the retina
The retina contains 2 types of photoreceptors, what are they and what colours are they able to detect?
- rods = black and white
- cones = detect red, blue and green
What does phototransduction mean?
- conversion of light to electrical impulse
What is the optic disc?
- where the optic nerve leaves the eye
- same place where blood vessels enter and leave
What are the 4 layers of the retina?
- pigmented epithelial cells
- photoreceptors (rods and cones)
- bipolar neurons
- ganglion cells (form the the optic nerve)
The photoreceptors are present in the retina, are there more rods or cones?
- rods = 120 million (dim light)
- cones = 5 million cones (red, green, blue)
When light enters the retina and hits the rods and the cones and come into contact with discs within the rods and cones. When it does this it hits photosensitive pigments which are also GPCRs, which are called what?
- opsins
- rods = rhodopsin
- cones = photopsin
When light enters the retina and hits the rods and the cones and come into contact with discs within the rods and cones. When it does this it hits photosensitive pigments, called opsins: rods = rhodopsin and cones = photopsin (both GPCRs). This then causes an intracellular cascade. What is this cascade?
- guanylate cyclase converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP) and pyrophosphate
- cGMP can then open Na+ and Ca2+ channels
- THIS ONLY OCCURS WHEN THERE IS NO LIGHT, ITS CONSTITUTIVE ACTIVITY
- EYES HAVE TO WORK HARDER TO GENERATE ACTION POTENTIALS AS THERE IS NOT MUCH LIGHT
We are able to see lots and lots of colours, but only posses rods which gives us black and white and cones that give us red, green and blue. How do we generate all the different colours?
- cones and rods are stimulated in varying ratios
- in the image below, if we see orange, the red and green cones have been stimulated
If the red, green and blue cones are stimulated equally, what colour does this generate in our eyes?
- white lights
Some patients can be colour blind, or colour blind to specific colours, why is this?
- loss or modification of one or more of the three cones
- visual pigments (opsins) are lost
Genes for red and green pigments are present on the X chromosome. Women have 2 X chromosomes and men have 1 X and 1 Y chromosome. If the X chromosome is damaged or dysfunctional, this may cause red and/or green colour blindness. Is this likely to be more common in men or women?
- men
- women have another chromosome that contains the genes
The optic nerves leave the orbit before some of the nerves from the eyes decussate. Where do they decussate?
- optic chiasm
In each eye we have 2 visual fields, a nasal and a temporal fields of view. When light enters the eye from the right hand side (temporal vision), does it enter the left or the right side of the hemiretina (HR) (one half of the retina)?
LE = left eye
RE = right eye
- LE = nasal hits temporal HR
temporal hits nasal HR - RE = nasal hits temporal HR
temporal hits nasal HR - essentially both go to the opposite HR of the eye
In each eye we have 2 visual fields, a nasal and a temporal fields of view. Light that enters the left eye (LE) then does the following: nasal light hits temporal HR and temporal hits nasal HR. The right eye (RE) then does the following: nasal hits temporal HR and temporal hits nasal HR. Essentially the light from the nasal and temporal sides of visual fields go to the opposite sides of the eye. Once the light has been transmitted to the optic nerve, which signals are sent to the left and right side of the chiasma?
- LE nasal remains on the left side of the chiasma (left side of the brain)
- LE temporal switches to the right side of the chiasma (right side of the brain)
- RE nasal switches to the left side of the chiasma (left side of the brain)
- RE temporal remains on the right side of the chiasma (right side of the brain)
Why does the temporal view from the left eye and nasal view from the right eye visual fields switch sides?
- ensure vision from the left and right sides are processed together
- left visual fields of both eyes end up on right side of brain
- right visual fields of both eyes end up on left side of brain
Once the light leaves the optic chiasma it travels to where in the thalamus?
- lateral geniculate nucleus
- relay centre in thalamus for visual pathways
Once the light leaves the optic chiasma it travels to the lateral geniculate nucleus in the thalamus. Where does it then travel to?
- primary visual cortex in the occipital lobe
Which bordmann area is the primary visual cortex in the occipital lobe located?
- bordmann area 17
What is homonymous hemianopia?
- homonymous - same side
- hemianopsia is greek for half without
- loss of either the two right or the two left halves of the visual fields of both eyes
What is bitemporal hemianopsia?
- visual loss in both the left and right visual fields
- this is because this part of the optic nerve decussates
If we have a lesion anywhere in the visual pathway this can affect different fields of vision. In the image below demonstrating a lesion on the right between the chiasma and the lateral geniculate nucleus, what vision would we lose?
- left visual field in left eye (temporal) and in right eye (nasal)
- this is called right homonymous hemianopia
If we have a lesion anywhere in the visual pathway this can affect different fields of vision. In the image below demonstrating a lesion on the left sided optic nerve, prior to the chiasma, what vision would we lose?
- left eye blindness
If we have a lesion anywhere in the visual pathway this can affect different fields of vision. In the image below demonstrating a lesion on the chiasma, which could be caused by pituitary tumour, what vision would we lose?
- nerve fibres that decussate would be lost
- loss of temporal view due to nasal retina optic nerve loss
If we have a lesion anywhere in the visual pathway this can affect different fields of vision. In the image below demonstrating a lesion on the primary visual cortex, what could we except to see in regards to loss of vision?
- areas partial vision loss
- some parts can be fine but others are affected
What does the image below illustrate an example of?
- a scotoma
- greek for dim vision
The occipital lobes has two information processing streams originating from it. What are they called and where do they go?
1 - ventral (bottom of brain) = occipital to temporal
2 - dorsal (top of brain) = occipital to parietal
The occipital lobes has two information processing streams originating from it. They are the ventral stream (occipital to temporal) and the dorsal stream (occipital to parietal). What information does the ventral (bottom of brain) stream process?
- dorsal think dorsal fin so head top of the brain
- involved in location, motion and action
- motor cortex is located dorsally on the brain
The occipital lobes has two information processing streams originating from it. They are the ventral stream (occipital to temporal) and the dorsal stream (occipital to parietal). What information does the ventral stream process?
- object (and face) identity, and conscious perception
- linked with memory and hippocampus
What is visual agnosia?
- normal visual field remains
- BUT impairment in recognising visually presented objects
- BUT patients can recognise objects using other sensory modalities
Prosopagnosia is a special case of agnosia, inability to recognise and identify objects, persons, normally due to damage of the temporal lobe. What is prosopagnosia?
- prosopagnosia = greek for face and lack of knowledge
- patients cannot recognise people’s faces
- may mistake people for objects
