The Structure of the Eye Flashcards
Label the eye
Cornea Anterior chamber Pupil Iris Lens Conjunctiva Sclera Ciliary body Vitreous body Optic disc Optic nerve Choroid Retina Macula
What is the anterior-posterior diameter of the orbit in adults
24mm
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
What are the two anatomical spaces within the anterior segment
Anterior chamber (cornea → iris) Posterior chamber (iris → lens)
Describe the optic disc
Part of the optic nerve visible at the back of the eye
Describe the zones/suspensory ligaments
Fibrous strands that hold the lens in line with the pupil
Connected to pupil by ciliary muscles.
Describe the cornea
Transparent window
2/3 refraction
Physical and infection barrier
Describe the uvea
Vascular coat
= iris +ciliary bodies + choroid
Describe the iris
Coloured part of eye
Controls the diameter and size of the pupil and therefore the amount of light reaching the retina
Describe the retina
Captures light rays that fall on the eye
What are the layers of the eye
Sclera
Choroid
Retina
Describe the sclera layer of the eye
Hard
Opaque
Protects and maintains the shape of the eye
Describe the choroid layer of the eye
Pigmented
Vascular
Shields out unwanted scattered light
Provides circulation to the eye
Describe the retina layer of the eye
Neurosensory tissue
Converts lights to neurological impulses that are transmitted via optic nerve
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
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 thecorneaandlensto 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
- Pupillary Miosis – mediated by the sphincter pupillae
- Convergence of the eyes – mediated by the medial recti muscles
- Lens Accommodation (becomes thicker) – mediated by the circular ciliary muscle
