06-10-23 - The eye and vision (special senses) Flashcards
Learning outcomes
- Understand how the brain creates vision
- Understand visual development
- Appreciate some aspects of cerebral visual impairment
- Know the different forms of visual field loss, the anatomical site of these and possible causes
What structures are at the front and back of the eye?
What connections are there between the eye and the brain?
What 4 lobes of the brain are responsible for vision?
- The eye has an optical front end and retina/optic disc at the back
- Connections between the eye and the brain:
1) Optic nerve
2) Optic chiasm
3) Optic tract
4) LGN (lateral geniculate nucleus) - 4 lobes of the brain are responsible for vision:
1) Occipital
2) Temporal
3) Parietal
4) Frontal lobes
What structures are at the front of the eye?
What makes up the anterior segment of the eye?
What sits behind the anterior segment?
What does the retina form?
- Structures at the front of the eye:
1) Cornea
2) Anterior chamber
3) Iris surrounding pupil - The anterior segment is made up from the anterior and posterior chamber
- The lens sits behind the anterior segment
- The retina forms the optic nerve, which transfers information coming out the back of the eye
What are the anterior and posterior segments of the eye for?
- The anterior part of the eye is for focusing
- The posterior of the eye is for sensing (retina)
What is the cornea?
What is its role?
What is the cornea made up from?
What is the role of endothelial cells in the cornea?
What happens if the endothelial cells don’t function properly?
When can this happen?
- The cornea is the first part of the vision system
- It is a Major light focusing element of the eye – 40D (dioptres) of ‘fixed’ power (the ability to focus on an object or image 1/40 of a meter from the eye)
- The cornea is up from stroma (collagen), which has multiple layers with gaps aligned specifically to maintain transparency
- A thin layer of endothelial cells in the cornea pump water out of the stroma to maintain the gaps in the stroma, and therefore transparency
- If these cells are damaged, this can lead to cornea oedema, which can occur after surgery for cataracts
What is dioptre (D) a unit of measurement for?
- A dioptre is the unit of measurement used for an eye prescription.
- The focusing strength of your glasses or contacts (optical power of a lens) is measured in diopters.
- In other words, diopters measure how powerful your prescription is
- A higher prescription will have larger numbers, whereas a milder prescription will have smaller numbers.
Describe the optical dioptre strength of the:
1) Cornea
2) Lens
3) Whole eye ball
What are the interfaces for the cornea and the lens?
- The optical dioptre strength of the:
1) Cornea
* largest element (40D)
* Interfaces with air (low RI) - big difference
2) Lens
* lesser element (20D)
* Interfaces with aqueous (similar RI) – small difference
* But can vary in power (accommodation)
3) Whole eye ball
* About 60D
What is accommodation in eye?
What occurs during accommodation?
- Accommodation is the process of changing the shape of the lens in order to change the refractive index of the lens to focus on near or distant objects.
- During accommodation, the ciliary muscle contracts and moves the ciliary body anteriorly and deep towards the optic axis
- To focus on a near object – the lens becomes thicker, this allows the light rays to refract (bend) more strongly.
- To focus on a distant object – the lens is pulled thin, this allows the light rays to refract slightly.
What influence do autonomics have on the iris?
How does this affect the ciliary muscles, suspensory ligaments and lenses?
- There is dual innervation of the iris from the autonomics:
- Pupil dilation is mediated by a sympathetic output (fight or flight) acting in opposition to parasympathetically mediated pupil constriction
- Sympathetics relax the ciliary muscle and contract the suspensory ligaments, making the lens thinner
- The parasympathetic constrict the ciliary muscle and relax the suspensory ligaments, making the lens fatter and more curved
What are the 3 parts of the near triad?
How does the near triad affect optical power?
- 3 parts of the near triad:
1) Miosis (shrinking of pupil)
2) Convergence (lens becomes fatter)
3) Accommodation - Optically, we have a higher power in this state, so we can see things closer
What is presbyopia?
Why does it happen?
- Presbyopia is a refractive error that makes it hard for middle-aged and older adults to see things up close.
- It happens because the lens (an inner part of the eye that helps the eye focus) stops focusing light correctly on the retina (a light-sensitive layer of tissue at the back of the eye)
What is myopia and hypertropia?
How are these corrected?
What is emmetropia?
- Hyperopia is a condition in which an image of a distant object becomes focused behind the retina, making objects up close appear out of focus. (long-sighted) – corrected with a convex lens
- Myopia is a condition in which, opposite of hyperopia, an image of a distant object becomes focused in front of the retina, making distant objects appear out of focus. (short-sighted) – corrected with a concave lens
- Emmetropia is the refractive state in a healthy eye in which, any individual achieves the perfect visual function
Myopia, Hypermetropia, Emmetropia diagram (in picture)
What are 2 conditions those with myopia are prone to?
What are 2 conditions those with hypermetropia are prone to?
- 2 conditions those with myopia are prone to:
1) Open angle glaucoma (optic nerve becomes damaged)
2) Retinal detachment - 2 conditions those with hypermetropia are prone to:
1) Angle closure glaucoma
2) Ischaemic optic neuropathy
What is the main cause of visual impairment worldwide?
What is the main type of URE?
Where is myopia especially present
What is hypermetropia associated with?
- Uncorrected refractive error (URE) is the main cause of visual impairment world wide
- Main cause of visual impairment world wide
- Presbyopia (failure to accommodate when you are older) is the main type of URE
- Myopia (short-sightedness) growing epidemic in industrialised regions especially Asia and associated with retinal detachment
- Hypermetropia (long sightedness) is associated with squint and lazy eye (amblyopia) and acute closed angle glaucoma
What are the 2 most common causes of visual impairment in the world?
What are the 2 most common cause of blindness in the world?
- 2 most common causes of visual impairment in the world:
1) AMD - Age-related macular degeneration
2) DR Diabetic retinopathy - 2 most common cause of blindness in the world:
1) CO - Cornea (can become hazy with age)
2) RE - Refractive error
How are visual acuity assessments conducted with a small Snellen chart?
- Visual acuity assessments with a small Snellen chart are conducted at a distance of 3m from the chart and with a matching card
Recording of visual acuity with a large Snellen chart.
What distance is this done from?
When is the Arclight chart used?
How do we document the vision?
- Recording of visual acuity with a large Snellen chart
- 6m is the standard distance for ‘big’ Snellen charts
- The Arclight chart is 50% smaller chart with 50% smaller letters and so you use the chart at 50% distance = 3m
- the top line is still called the 60 line and so you document the vision as 6/60 and 6/36 etc. with the bottom number being the smallest line the patient can read
Describe the 8 steps in testing distance visual acuity. What is this test repeated for?
- 8 steps in testing distance visual acuity:
1) Measure out 3m
2) Cover left eye with palm of hand
3) Ask patient to read from the top of the chart
4) Chart must be held perpendicular to patient in good lighting, smoothed out and flat
5) Record the ‘number’ of the smallest line that can be seen
6) If cannot read even the top letter then go to 1.5m and repeat
7) If cannot read even the top letter at 1.5m then go to 0.5m and repeat
8) If cannot see at 0.5m then try counting fingers (CF) at 1m, hand movements (HM) then perception of light and classify with projection or with no projection of perception of light then finally no perception of light
- Repeat for fellow eye
- Repeat with both eyes together
- Repeat with pinhole and with glasses
Describe the WHO Classification Groups – 0 to 9 for visual impairment/blindness (in picture)
Describe the WHO Classification Groups – 0 to 5 for visual impairment/blindness (in picture)
What is trachoma?
What is it caused by?
What is it a major cause of?
Is it preventable/treatable?
Describe the 5 stages of trachoma (in picture)
- Trachoma is an Infectious disorder of the surface of the eye caused by Chlamydia
- It is one of the major causes of blindness in the world (4th)
- It is preventable & treatable
- 5 stages of trachoma (in picture)
5 stages of trachoma (in picture)
Corneal scarring caused by trachoma (in picture) – eye lashes rubbing on eye
What is the major cause of worldwide blindness?
What is cataracts?
When does it become more common?
- The major cause of worldwide blindness is cataracts
- A cataract is a cloudy area in the lens of the eye (the clear part of the eye that helps to focus light).
- Cataracts are very common as people get older.
Where is the retina located?
What is it covered in?
What is the role of the retina?
- The retina is located at the back of the eye
- It is covered in nervous issue
- The retina collects light information that comes in and transfers it to the brain via the optic nerve
Where are the photoreceptors located on the retina?
Where does information come into the retina and leave the retina?
- Photoreceptors are located at the bottom later of the retina
- Information goes through all of the layers of the retina to the photoreceptors
- It then goes out the axons of the ganglion cells and into the optic nerve towards the brain
Name the 2 types of photoreceptors in the retina (in picture). Describe the following:
1) Number in retina
2) Convergence to ganglion cells
3) Number of types
4) Sensitivity
5) Distribution
6) Spectrum of sensitivity
7) Illumination levels they work best in
8) Best use
Label the structures on the retina (in picture).
What is the macula?
Where is it located?
What is the role of the macula?
What is the fovea?
What is located here?
- Label the structures on the retina (in picture)
- The macula is the round area at the centre of your retina, at the back of your eyeball.
- The macula is the part of your eye that processes what you see directly in front of you (your central vision).
- The fovea is the centre of the macula
- It is where photoreceptor cells (particularly cone cells) are most tightly packed (highest acuity here – sharpness of sight)
Describe the distribution of the photoreceptors in the retina (in picture).
Where is the blind spot of the retina?
Why is it here?
- Distribution of the photoreceptors in the retina (in picture)
- The blind spot of the retina located where the axons of the ganglion cells go out the back of the eye on the optic nerve
- This is because there are no photoreceptor cells here
Why is what we see not actually what we think we see?
- Why is what we see not actually what we think we see?
- There are fewer axons than there are photoreceptor cells
- This means convergence has to occur across these photoreceptor cells
- Convergence of information from rods and cons onto ganglion cells is cell location dependent, so it will vary
Describe the acuity of an image at different parts of the retina
- In the centre of the retina, the image will be in colour and high acuity in bright light (due to cone cells – good at concentrating)
- At the edges, the image will not be in colour, and will be of low acuity (due to rod cells – good at catching motion)
What is the chromophore retinal derived from?
Where is it found?
When is it bound to a protein called opsin?
What opsin is used in rods?
What happens when light hits retinal?
- The chromophore retinal is derived from vitamin A
- It is found in all rods and cones
- Under unstimulated conditions, retinal is bound to opsin
- There are different types of opsins, each specific to a different type of cone (there are 3 different types of cone)
- Rod retinal binding an opsin called rhodopsin
- When light hits retinal, it changes conformation and appears bleached
What is the wavelength of visible light?
What is each opsin sensitive to?
At the most sensitive wavelength, what number of photos can our eyes detect?
When are photoreceptors depolarised and hyperpolarised?
- The wavelength for visible light is 400-700nm
- Each opsin gives sensitivity to a different range of wavelengths of light
- At the most sensitive wavelength, our eyes can detect as little as 5 photos of light
- Contrary to expectation, photoreceptors are depolarised in the dark, and hyperpolarised in the light
What occurs with a vitamin A deficiency?
At what time of the day is the retina most active?
What is this an exploit for?
- Vitamin A deficient results in Night Blindness then total corneal blindness then Death
- The retina is metabolically more active when you are asleep - exploit as therapy for retinal disease
How can cone fatigue be elicited?
What is the after image a result of?
What occurs as cone cells become less tired?
- When you stare at a specific colour for too long, the cells that detect that frequency of light will get fatigued, leading to cone fatigue
- The after image is a result your photoreceptors not being ‘in balance’.
- As the photoreceptors become less tired, which takes between 10 and 30 seconds, the balance is recovered and the after image disappears.
How is the concept of cone fatigue utilised in submarines?
- In submarines, the concept of cone fatigue is utilised by using red light
- This means we are only stimulating red spectrum photoreceptors, so they are adapted to dark environments
- If the red light were to disappear, they would still be able to see, as the photoreceptors are dark adapted (only uses 1 population of cones, the other 2 groups work fine)
What are the 3 other types of cells in the retina?
Why does summation/convergence need to occur?
- 3 other types of cells in the retina:
1) Bipolar cells
* There are two types – ‘ON’ or ‘OFF’
* ‘ON’ = neuro-transmitter released in the light (when photoreceptor cells NOT releasing neuro-transmitter)
* ‘OFF’ = opposite
* These cells allow the retina to pass information to the brain in order to distinguish between boundaries, which is the bases of our vision
2) Horizontal and Amacrine Cells
* Connect bipolar cells and allow for summation of information to allow detection of edges and contrast
- Summation/convergence needs to occur due to their being fewer axons of ganglionic cells than there are photoreceptor cells
Retinal Cell Connectivity diagram (in picture)
What is a receptive field?
Where is there a convergence of information?
Describe the receptive field for bipolar cells (in picture)
- A receptive field is an area in space where the presence of an appropriate stimulus will modify the activity of this neuron
- There is a convergence of information from PRs onto bipolar cells then a ganglion cell
- The receptive field for bipolar cells (in picture)
How is bleeding in the retina caused by diabetes?
How can diabetes cause hard exudate maculopathy?
What causes macular scar toxoplasma?
- Bleeding in the retina can be caused in diabetes because the high blood sugar can damage the walls of the blood vessels in the eye
- Hard exudate maculopathy can be caused by diabetes
- When blood vessels are damaged by high sugar levels, the inside surface of the blood vessels becomes thickened and the blood vessels become smaller
- The tissue becomes ischaemic, meaning new fragile blood vessels form and break, causing bleeding and hard exudate maculopathy
- Macular scar toxoplasma is caused by AMD (age-related muscular degeneration
