Visual System Flashcards by Elizabeth Akeju

Label this image of the eye

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Describe the production and drainage of tears

Produced by lacrimal gland
Drain through two puncta (opening on medial lid margin)
Flow through superior and inferior canaliculi
Gather in tear sac
Exit tear sac through tear duct into nasal cavity

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Describe innervation to the lacrimal system

Afferent - cornea, cranial nerve V1 - ophthalmic branch of trigeminal
Efferent - parasympathetic
Neurotransmitter - acetylcholine

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Describe the different triggers of tears

Basal tears - cover eye
Reflex
Emotional (crying)

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Describe the functions of the tear film

Maintains smooth cornea-air surface
Oxygen supply to cornea
Removal of debris (tear film and blinking)
Bactericide

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Does a normal cornea have blood vessels?

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What three layers compose the tear film and what are their functions?

Superficial lipid layer - reduces tear film evaporation
Aqueous (water) tear film (from tear gland)
Mucinous layer on corneal surface - maintains surface wetting

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What is the superficial lipid layer of the tear film produced by?

Produced by a row of Meibomian Glands along the lid margins

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What is the conjunctiva?

Thin, transparent tissue that covers the outer surface of the eye

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What is the function of the conjunctiva?

Begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids

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What supplies the conjunctiva?

Nourished by tiny blood vessels that are nearly invisible to the naked eye

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What is then antero-posterior diameter of the eye in adults?

24mm

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What are the three layers of the eye?

Sclera - hard and opaque
Choroid - pigmented and vascular
Retina - neurosensory tissue

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Label this diagram of the eye

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Describe the features and function of the sclera

“The white of the eye”
Tough, opaque tissue that serves as the eye’s protective outer coat
High water content

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Describe the cornea

Transparent, dome-shaped window covering the front of the eye
Low water content
Clear window to look through
Powerful refracting surface

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What are the 5 layers of the cornea?

Epithelium
Bowman’s membrane
Stroma - its regularity contributes towards transparency
Descemet’s membrane
Endothelium - pumps fluid out of cornea and prevents corneal oedema

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Where does the cornea receive its oxygen and glucose?

Oxygen from air - exposed to the air most of the time
Glucose - from fluid that sits between iris and cornea (aqueous humour) absorbed by the endothelium

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What structure provides 2/3 of the eye’s focusing power?

Cornea

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What happens if you hydrate the cornea?

It becomes white

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What is the uvea?

Vascular coat of eyeball and lies between the sclera and retina

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What parts compose the uvea?

Composed of three parts - iris, ciliary body and choroid
Intimately connected

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What are the implications of having the parts of the uvea initimately connected?

Disease of one part affects the other portions, though not necessarily to the same degree

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What is the choroid?

Lies between retina and sclera
Composed of layers of blood vessels that nourish the back of the eye

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What is the iris and what is it’s function?

Foremost part of uvea Controls light levels in eye similar to aperture of camera Round opening in centre is the pupil Embedded with tiny muscles that dilate and constrict the pupil

What is the lens?

Outer acellular capsule Regular inner elongated cell fibres - transparency

What is it called when the lens loses transparency with age?

Cataract

What is the function of the lens?

Transparency Refractive power Accommodation - long vs short distance Elasticity

What structure has 1/3 of the eyes refracting power?

Lens

What is the retina and what is its function?

Very thin layer of tissue that lines the inner part of the eye Responsible for capturing light rays that enter the eye (like the film in photography) Light impulses are then sent to the brain for processing via the optic nerve

What is the role of the optic nerve?

Transmits electrical impulses from the retina to the brain Connects to the back of the eye near the macula

What is the visible portion of the optic nerve called?

Optic disc/papilla

Where is the ‘blind spot’ formed?

Where the optic nerve meets the retina No light sensitive cells

What is the macula and where is it located?

Small, highly sensitive part of the retina responsible for detailed central vision Located roughly in the centre of the retina, temporal to the optic nerve

What structure is at the very centre of the macula called?

Fovea Most sensitive part of retina

What is the function of the macula?

Allows us to appreciate detail and perform tasks that require central vision such as reading

Fovea has the highest concentration of rod cells, true or false?

False, fovea has the highest concentration of cone cells, low concentration of rods Only fovea has the concentration of cones that allows us to see fine detail

Why are starts brighter out of the corner of your eye?

Rods are more sensitive to light (periphery of retina is packed with rods) Cone cells of fovea (central vision) are for finer detail

Describe central vision and give examples of tasks requiring it

Detailed day vision and colour vision Important for reading and facial recognition Fovea - highest conc. of cone photoreceptors

How is central vision assessed and what does loss of foveal vision result in?

Assessed by visual acuity assessment Loss of foveal vision - poor visual acuity

Describe peripheral vision and its uses

Identify shape, movement, night vision Important for navigation

How is peripheral vision assessed?

Visual field assessment

What does extensive loss of visual field result in?

Inability to navigate an environment Patient may need a white stick even with perfect visual acuity

Describe the retinal structure

Outer layer: - Photoreceptors (1st order neuron) - Detection of light Middle layer: - Bipolar cells (2nd order neuron) - Local signal processing to improve contrast sensitivity (regulates sensitivity) Inner layer: - Retinal ganglion cells (3rd order neuron) - Transmission of signal from eye to the brain

Describe rod cells and their function

Longer outer segment with photo-sensitive pigment Slow response to light Responsible for night vision

Describe cone cells and their function

Shorter outer segment than rod cells Faster response to light Responsible for daylight fine vision and colour vision

Rod cells are more sensitive to light than cone cells, true or false?

True, rod cells are 100x more sensitive to light than cones

There are more cone than rod cells, true or false?

False, there are 120 million rods and only 6 million cones

What are other terms for day/colour vision and night vision?

Daylight and colour vision - photopic Night vision - scotopic

Describe scotopic vision with regards to photoreceptor distribution

Rod cells Peripheral and night vision More photoreceptors, more pigment, higher spatial and temporal summation Recognises motion

Describe photoreceptor distribution in the eye

Where can you find the highest concentration of rod photoreceptors in the retina?

20-40 degrees away from the fovea

Are all colours captured through the same type of cone cell?

No different colours are captured by different photoreceptors S-Cones: Blue M-Cones: Green L-Cones: Red

Are rod cells sensitive to any specific colours?

No, used for night vision and spatial recognition so not really sensitive to particular colours

Why is the most common form of colour blindness?

Deuteranomaly (also known as Daltonism)

Describe Deuteranomaly

Not completely colour blind Cannot perceive the colour red

What is the term for full colour blindness?

Achromatopsia Very rare (<0.0001%)

Describe the prevalence of the different colour vision deficiencies

How is colour blindness tested?

Colour blindness test - Ishihara Test

What happens to light as it goes from one medium to another?

Velocity changes (refraction) Path changes

How is the index of refraction calculated?

n = speed of light in a vacuum (air)/speed of light in a medium (c/vm)

When calculating a mediums index of refraction, why will the value always be greater than or equal to 1?

The denominator will always be smaller (when compared to the speed in a vacuum)

What exactly is light doing as it reaches a new medium?

Some light reflects off the boundary and some light refracts through the boundary Angle of incidence >/< the angle of refraction, depending on the direction of the light

How do the angle of incidence and angle of reflection compare?

They are equal

What are the two basic types of lenses and what do they do?

Convex - converging lens Takes light rays and brings them to a point Concave - diverging lens Takes light rays and spreads them outward

Describe emmetropia

'Perfect eye' Adequate correlation between axial length and refractive power Parallel light rays fall on the retina (without accommodation)

What is ametropia and what are the different types?

Mismatch between axial length and refractive power Parallel lines don't fall on retina without accommodation - Myopia (near-sightedness) - Hyperopia (far-sightedness) - Astigmatism - Presbyopia

What happens in myopia?

Parallel rays converge at a focal point anterior to the retina Etiology is unclear - genetic factor?

What are the causes of myopia?

Excessive long globe (axial myopia) - most common Excessive refractive power (refractive myopia)

What are the symptoms of myopia?

Blurred distance vision Squint in attempt to improve uncorrected visual acuity when gazing into the distance Headache

How is myopia treated?

Concave lens correction Contact lens correction Correction by removing lens to reduce refractive power of the eye