Structure and function of the eye Flashcards
What does the eye sit within?
The orbit
What are the three layers of the coat of the eye?
Sclera, choroid and retina
What is the sclera like and what does this help?
Hard and opaque- protects the eye and maintains its shape
What is the choroid like and what does it do?
It is pigmented and vascular and provides circulation to the eye and retina
What is the retina like and what does it do?
Neurosensory tissue- converts light into nerve impulses which are transmitted to the brain via the optic nerve
What are the anterior and posterior segments of the eye separated by?
Lens
What sort of humour is in the anterior and posterior segments?
Anterior- aqueous
Posterior- vitreous
What are zonules?
Fibrous strands that holds the lens in place in line with the pupil- they are connected to ciliary muscles
What does the ciliary body produce?
Aqueous humour
Where does aqueous humour go after production?
Passes into the anterior chamber and then drains out through the angle via trabecular meshwork
What is this drainage important for?
Maintaining intraocular pressure
What is the aqueous fluid important for
Supply nutrients to the cornea and other tissues in the anterior chamber
What is unique about the middle part of the eye?
No blood vessels as you need a clear window for light to pass through
What is the posterior segment located between?
Lens and retina
The vitreous humour is made up of 99% water trapped inside a jelly matrix, what is the jelly matrix for?
Mechanical support to the eye
What is happening when you see floaters?
The vitreous humour is detaching, as we get older, the vitreous humour loses its jelly constituency, liquefies and detaches from the retina
What can go wrong with vitreous humour detachment?
It can lead to a small tear in the peripheral retina, the liquid vitreous could pass into the sub retinal space and lead to detachment which requires prompt treatment or can lead to blindness
What is the name of the ring shaped tissue surrounding the lens
Ciliary body
What are the two layers of the iris and what are they composed of?
Anterior- Stromal layer containing muscle fibres
Posterior- Epithelia
What is the uvea made up of?
Choroid, ciliary body and iris
What is the normal intraocular pressure
11-12 mmHg
What is glaucoma?
Condition of sustained intraocular pressure
What does glaucoma result in?
Accumulative damage to the optic tissue
Retinal ganglion cell death and enlarged optic disc cupping
What is the commonest form of glaucoma?
Primary open angle glaucoma
What is primary open angle glaucoma caused by?
Functional blockage of trabecular meshwork
What is closed angle glaucoma caused by?
Forward displacement of the iris/lens complex narrowing the trabecular meshwork
What sort of patients does closed angle glaucoma occur commonly in?
Small eyed patients (hypermetropic)
What is the lens made up of and what function does this have?
Regular inner elongated cell fibres- transparency
What fraction of the eye’s refractive power is the lens responsible for?
1/3
What is accommodation?
The lens can change shape so it can alter its refractive power and ability to focus can change so it allows you to focus on near or distant objects
What is a cataracts?
When the lens loses its transparency
Where is the cornea in the eye?
Front most part of the anterior segment
What is there cornea continuous with?
Sclera
What fraction of the eyes refractive power is the cornea responsible for?
2/3
What is the function of the front surface of the cornea
Physical barrier
What are the three main layers of the cornea?
Epithelium, stroma and endothelium
What is the stroma responsible for?
Transparency
What does the endothelium do?
Pumps fluid out of the stroma and prevents stromal oedema
What is the tear film?
Thin layer of fluid that covers the cornea
What is the tear film important for?
Maintaining clear vision and removing surface debris during blinking
Source of oxygen and nutrients to anterior segment
Bactericide
What are the three layers of the tear film?
Superficial oily layer
Aqueous tear layer
Mucinous layer
What does the superficial oily layer do?
Reduces tear film evaporation
What is the superficial oily layer secreted by?
Melbomian glands along the lid margin
What does the aqueous tear film do?
Delivers oxygen and nutrients to surrounding tissues and contains bactericide
What does the mucinous layer do?
Maintains surface wetting
Ensures that the tear film sticks to the eye
How do mucin molecules act?
Binding water molecules to the hydrophobic corneal epithelial cell surface
What is the conjunctiva?
Transparent layer on top of the surface of the cornea that is very vascular- it has goblet cells that produce mucin
Where is the lacrimal gland situated?
Within the orbit, latero-superior to the globe
What are the three types of tear that the lacrimal gland produces?
Basal tears- produced at constant level
Reflex- In response to irritation
Emotional- crying
What is the tear reflex pathway?
Irritation is detected by the incredibly sensitive cornea which is innervated by the sensory fibres from the ophthalmic branch of the trigeminal nerve and the efferent pathway is mediated by a parasympathetic nerve that innervates the lacrimal gland
What regulates light entry into the eye?
Pupil and pigmented uvea
What maintains the shape of the eye?
Scleral coat
What is emmetropia
Perfect focusing, parallel rays converge directly on the fovea forming a clear image
What is hypermetropia more commonly known as?
Long sighted
Where do the light rays focus in hypermetropia?
Behind the retina
What is myopia?
Short sighted
Where do the light rays focus in myopia and why?
In front of the retina- cornea and lens have excess refractive power
What sort of lens do people with myopia require to see objects far away?
Concave
What is the cornea like in astigmatism?
The cornea is oval shaped instead of spherical
Describe the mechanism of accommodation
The circular ciliary muscles within the ciliary body contract which leads to relaxation of the zonules so the lens returns to its natural convex shape and increases the refractive power of the lens
Which nerve is accommodation mediated by?
Efferent oculomotor nerve
What are the three pathways of the near response triad for near vision?
Pupillary miosis (constriction)- increases depth of field
Convergence
Accommodation
What is presbyopia?
Naturally occurring loss of accommodation with age
What happens at the optic disc?
Retinal ganglion cells leave the eye via the optic nerve
What is important about the macula?
It has the highest concentration of photoreceptors so is vital for fine vision
What are the four branches of vessel arcades that radiate from the optic disc?
Superior temporal
Inferior temporal
Superior nasal
Inferior nasal
When looking at an eye, how can you tell the difference between an artery and vein?
Veins are darker and thicker
Where do the retinal arteries and veins provide circulation to?
Inner 2/3 of retina
What provides circulation to outer 1/3?
Choroidal vasculature
What sort of vision is the fovea responsible for and why?
Central- Detailed day/colour vision - it is the area with the highest concentration of cones
How do you assess fovea function?
Visual acuity test
What is peripheral vision used for?
Shapes, movement and night vision
How is peripheral vision assessed?
Visual field assessment
What are the two layers of the retina?
Outer layer- Retinal pigment epithelium
Inner layer- Neuroretina (made up of photoreceptors and neurones)
What is the function of the retinal pigment epithelium?
Transports nutrients from the choroid to the photoreceptor cells and removes metabolic waste from the retina
What are the three divisions of the neuroretina?
Outer layer- photoreceptors
Middle layer- bipolar cells (axons project in both directions)
Inner layer- retinal ganglion cells
What is the fovea anatomically characterised by?
Foveal pit- it is there due to the lack of overlying ganglion cell layer
What is the main structural difference between cones and rods?
Rods have longer outer segments
How many cones and how many rods are there roughly?
120 million rods
6 million cones
What is the rod photopigment?
Rhodopsin
What is another term for night vision?
Scotopic vision
Where is the highest density of rod receptors?
Just outside the macula
Where is there no rod receptors?
Macula
On the frequency spectrum, how many peaks does the rod photopigment(s) have?
One peak- 498nm (blue/green)
How many peaks does the cone photopigments(s) have
Three peaks
S-cone - 420-440nm (blue)
M-cone - 534-545nm (green)
L-cone - 564-580nm (red)
What is the commonest form of colour blindness?
Deuteranomaly (red-green colour blindness)
What is deuteranomaly caused by?
Shifting of M cone peak towards L cone peak
What is an anomalous trichromatism?
When a vision deficit is caused by a shifted frequency peak
What is monochromatism?
Complete absence of colour vision
What is a common test used to test for colour blindness?
Ishihara
What happens to rod receptors in daylight?
They are suppressed to reduce sensitivity to light
