The Structure of the Eye Flashcards by Alexandra Chan

Label the eye

Cornea
Anterior chamber
Pupil
Iris
Lens
Conjunctiva 
Sclera
Ciliary body 
Vitreous body 
Optic disc
Optic nerve
Choroid
Retina
Macula

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What is the anterior-posterior diameter of the orbit in adults

24mm

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What type of humour is in the two segments and what are they separated by

Anterior segment - aqueous humour
Posterior segment - vitreous humour

separated by the lens

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What are the two anatomical spaces within the anterior segment

Anterior chamber (cornea → iris)
Posterior chamber (iris → lens)

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Describe the optic disc

Part of the optic nerve visible at the back of the eye

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Describe the zones/suspensory ligaments

Fibrous strands that hold the lens in line with the pupil

Connected to pupil by ciliary muscles.

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Describe the cornea

Transparent window
2/3 refraction
Physical and infection barrier

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Describe the uvea

Vascular coat

= iris +ciliary bodies + choroid

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Describe the iris

Coloured part of eye

Controls the diameter and size of the pupil and therefore the amount of light reaching the retina

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Describe the retina

Captures light rays that fall on the eye

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What are the layers of the eye

Sclera
Choroid
Retina

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Describe the sclera layer of the eye

Hard
Opaque
Protects and maintains the shape of the eye

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Describe the choroid layer of the eye

Pigmented
Vascular
Shields out unwanted scattered light
Provides circulation to the eye

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Describe the retina layer of the eye

Neurosensory tissue

Converts lights to neurological impulses that are transmitted via optic nerve

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What is the ciliary body

Ring shaped tissue surrounding the lens between the anterior and posterior segments and is located behind the iris
Contains the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces aqueous humor

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What are the layers of the iris

Anterior Layer Stromal Layer containing muscle fibres

Posterior Layer -Epithelial

What is the normal intra-ocular pressure and how is fluid drained

12-20mmHg

Drained via the tribecular network

What are the 2 portions of intraocular fluid

Aqueous humor - in front of lens, freely flowing

Vitreous humor - between posterior surface of the lens and the retina, gelatinous mass held by proteoglycan molecules, little change

Describe aqueous humour

Nourishes the cornea and lens by supplying nutrition such as amino acids and glucose.
Transports vit C in the front segment to act as an antioxidant agent.
Each eye forms AH at 2-3 microlitres/minute

What is uveoscleral outflow

drainage of ocular aqueous humour from the anterior chamber to anterior chamber angle that is NOT through the trabecular network

Describe the pathway of intraocular fluid drainage

Ciliary body
Anterior chamber of the eye
Trabecular meshwork
Canal of Schlemm (thinly walled vein that extends circumferentially all around the eye)
Extraocular veins

Describe glaucoma

Condition caused by sustained, raised intraocular pressure
Axons of the optic nerve are compressed at the optic disk. causing a lack of appropriate nutrition to fibres
This results in retinal ganglion cell death and cupping of the optic disc

What is the optic cup

The central portion of the optic disc, of which there is an increase in glaucoma

Describe primary open angle glaucoma

Commonest type

Due to a blockage in the trabecular meshwork

Describe closed angle glaucoma

Increased pressure in the eye pushes the lens/iris forward, making it difficult to drain aqueous humor Patients present with painful red eyes and an acute drop in vision

What are the risk factors for closed angle glaucoma

Small eyes | Narrow trabecular meshwork

What are the 2 main photoreceptors in the retina

Rods – 120 million present, 100X more sensitive to light Cones – 6 million present, Less sensitive to light but faster response

What occurs with photoreceptors during dark and light adaptation

Dark adaption - retina increases sensitivity so that cones are suppressed and rods take over Light adaptation - rod function is suppressed and cone function takes over

Describe refraction of light in the eye

Light is refracted by the cornea and lens to focus the incoming light rays onto the retina to form a clear image The refractive power of the lens = 1/3 The refractive power of the cornea = 2/3

What is emmetropia

Perfect correlation between the axial length of the eye and it’s refractive index Therefore, parallel light waves fall perfectly on the retina and there is 20:20 vision

What is ametropia

Axial length of the eye does not match its refractive power As a result, parallel light rays don't fall on to the retina People with ametropia require corrective lenses

Give 4 examples of ametropia

Nearsightedness (Myopia) Farsightedness (Hyperopia) Astigmatism Presbyopia

Describe myopia

Near-sightedness Parallel rays converge at a focal point anterior to the retina Due to a long globe or excessive refractive power Requires a concave lens

Describe hyperopia

Far-sightedness Parallel rays converge at a focus point posterior to the retina Due to a short globe (axial hyperopia - more common) or inadequate refractive power (refractive hyperopia) Requires a convex lens

Describe astigmatism

Parallel rays do not come to focus on a single point, instead we end up with two focal lines This occurs because the eye is an irregular shape: oval/not circular It can be hereditary

Describe presbyopia

Naturally occurring loss of accommodation with old age as the lens hardens and is unable to change shape Onset from age 40 years Distant vision intact Corrected by reading glasses (convex lenses)

What is anisometropia

Condition where 2 eyes have unequal refractive power

What is the near response triad

1. Pupillary Miosis – mediated by the sphincter pupillae 2. Convergence of the eyes – mediated by the medial recti muscles 3. Lens Accommodation (becomes thicker) – mediated by the circular ciliary muscle