Visual Systems Flashcards
Label the anatomy of the eye*
Describe the lacrimal system
The lacrimal gland is located within the orbit,
latero-superior to the globe.
It produces tear at a constant level,
even in the absence of irritation or stimulation.
This is known as Basal Tear.
Reflex tear refers to the increased tear production,
in response to ocular irritation.
The tear reflex pathway is made up of the afferent pathway,
the central nervous system, the efferent pathway,
and the lacrimal gland.
The cornea is one of the most sensitive tissues in the body.
It is innervated by the sensory nerve fibres via the Ophthalmic Branch of the Trigeminal Nerve.
As afferent pathway, the trigeminal nerve relays signal to the central nervous system.
The efferent pathway is mediated by the parasympathetic nerve.
Neurotransmitter - acetylcholine
It innervates the lacrimal gland.
Tear film drains through the two puncta,
tiny openings on the upper and lower medial lid margins.
The puncta form the opening of the superior and inferior canaliculi within the upper and lower eyelids.
Both canaliculi converge as one single common canaliculus,
and drain tear into the tear sac.
Tear is finally drained out of the tear sac,
into the nasal cavity through the tear duct.
Where are tears produced?
Lacrimal gland
Describe the tear film
The healthy Cornea is constantly covered by the tear film,
a thin layer of fluid.
The tear film maintains a smooth cornea-to-air surface.
This is important for maintaining clear vision,
and removing surface debris during blinking.
The tear film is also a source of oxygen and nutrient supply to the anterior segment.
The tear film consists of three layers:
The lipid layer on the top responsible for protecting the tear film from rapid evaporation.
The lipid layer is secreted by the Meibomian Glands,
situated along the eyelid margins.
The Aqueous Tear Film Layer in the middle,
forms the main bulk of the tear film.
It delivers oxygen and nutrient to the surrounding tissue.
It contains factors against potentially harmful bacteria.
The bottom Mucinous Layer ensures that the tear film sticks to the eye surface.
This renders the surface of the eye “wettable”.
The mucin molecules act by binding water molecules,
to the hydrophobic corneal epithelial cell surface.
Describe the tear film simple
Maintains smooth cornea-air surface
Oxygen supply to Cornea – normal cornea has no blood vessels
Removal of debris (tear film and blinking)
Bactericide
Composed of three layers
What are the three layers of the tear film
Superficial lipid layer to reduce tear film evaporation - produced by a row of Meibomian Glands along the lid margins
Aqueous (water) tear film (tear gland)
Mucinous Layer corneal surface - maintains surface wetting
- contains epithelial cells and microvillus
Describe the conjunctiva
Thin, transparent tissue that covers the outer surface of the eye
It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
It is nourished by tiny blood vessels that are nearly invisible to the naked eye
Describe the coat of the eye
Antero-posterior diameter of the eye - 24mm in adults
Sclera - hard and opaque
Choroid - pigmented and vascular
Retina - neurosensory tissue
The eye sits within the anatomical space known as the orbit.
It is enclosed by bony walls, laterally, medially, superiorly and inferiorly.
The topic of Orbit Anatomy is outside the scope of this lecture,
and will be covered in a separate session.
The human eye is roughly spherical,
measuring 24 mm in diameter from back to front.
The coat of the eye is composed of three layers.
The outer fibrous opaque layer called the Sclera,
responsible for protecting the eye,
and maintaining the shape of the eye.
The middle pigmented vascular layer called the Choroid,
responsible for providing circulation to the eye,
and shielding out unwanted scattered light.
The innermost Neurosensory Layer called the Retina,
responsible for converting light into neurological impulses,
to be transmitted to the brain via the Optic Nerve.
Describe the sclera
commonly known as “the white of the eye,”
the tough, opaque tissue that serves as the eye’s protective outer coat.
High water content
Describe the cornea
The transparent, dome-shaped window covering the front of the eye.
Low water content
Powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through
What are the 5 layers of the cornea
1 – Epithelium
2 – Bowman’s membrane
3 – Stroma – its regularity contributes towards transparency
4- Descemet’s membrane
5- Endothelium – pumps fluid out of corneal and prevents corneal oedema
Describe the uvea and the three parts
Vascular coat of eyeball and lies between the sclera and retina.
Composed of three parts – iris, ciliary body and choroid.
Intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree.
Choroid?
Choroid - lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye
Iris?
Controls light levels inside the eye similar to the aperture on a camera.
Round opening in the centre is the pupil.
Embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
Describe the structure and the function of the lens
Outer acellular capsule
Regular inner elongated cell fibres – transparency
May loose transparency with age – cataract
Transparency
Regular structure
Refractive Power
1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous
Accommodation
Elasticity
Describe the retina
Very thin layer of tissue that lines the inner part of the eye.
Responsible for capturing the light rays that enter the eye. Much like the film’s role in photography.
These light impulses are then sent to the brain for processing, via the optic nerve.
Optic nerve?
transmits electrical impulses from the retina to the brain
connects to the back of the eye near the macula
visible portion is called the optic disc
What is the blind spot in the optic nerve
Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot
Macula?
Fovea?
Located roughly in the centre of the retina, temporal to the optic nerve
A small and highly sensitive part of the retina responsible for detailed central vision
The fovea is the very centre of the macula. The macula allows us to appreciate detail and perform tasks that require central vision such reading.
Describe the fovea and hence the test
Fovea is the most sensitive part of the retina
It has the highest concentration of cones, but a low concentration of rods
This is why stars out of the corner of your eye are brighter than when you look at them directly.
But only your fovea has the concentration of cones to perceive in detail.
Central vision?
Detail day vision, colour vision – fovea has the highest concentration of cone photoreceptors
Reading, facial recognition
Assessed by visual acuity assessment
Loss of foveal vision – Poor visual acuity
Peripheral vision?
Shape, movement, night Vision
Navigation vision
Assessed by visual field assessment
Extensive loss of visual field – unable to navigate in environment, patient may need 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, regulate sensitivity
Inner layer:
Bipolar Cells (2nd order neuron)
Local signal processing to improve contrast sensitivity, regulate sensitivity
What are the 2 main classes of photoreceptors
Rods:
Longer outer segment with photo-sensitive pigment
100 times more sensitive to light than cones
Slow response to light
Responsible for night vision (Scotopic Vision)
120 million rods
Cones:
Less sensitive to light, but faster response
Responsible for day light fine vision and colour vision (Photopic Vision)
6 million cones
Explain the photoreceptor distribution
Rod (scotopic) vision;
Peripheral and night vision More photoreceptors, more pigment, higher spatial and temporal (time) summation
Recognizes motion
Cone (photopic) vision:
Central and day vision
Recognizes colour and detail
Frequency spectrum?
Like a CCD camera the eye captures different colours through different photoreceptors:
S-Cones: Blue
M-Cones: Green
L- Cones: Red
Rods are used for night vision and spatial recognition and are not really sensitive to any particular colour
Deuteranomaly?
Deuteranomaly also known as Daltonism is the most frequent form of colour blindness
People with deuteranomaly are not completely colour blind but they don’t perceive the colour red.
Achromatopsia?
Full colour blindness which occur only in a very small percentage of the population
Ishihara test?
Colour blindness test
