Flashcards in Structure and function of the eye Deck (58):
What is the difference between basal and reflex tears?
Basal tears are produced in absence of irritation or stimulation.
Reflex tears are a result of irritation.
Describe the production of reflex tears
Afferent detection of irritation from the cornea innervated by V1 ophthalmic division of trigeminal nerve.
Efferent to parasympathetic nerve to lacrimal gland
ACh is the neurotransmitter
Describe the production and drainage of tears during crying
Tears are produced by the lacrimal gland
They drain via two pact and the upper and lower medial lid margins
Pass through superior and inferior canniculi which converge to a single canniculi
Drains the tear sac via the tear duct/nasolacrimal duct -> inferior meatus
What is the purpose of the tear film?
Maintains a smooth cornea-air surface
Supplies oxygen and nutrients as there is no blood supply
Clears surface debris
What are the layers of the tear film?
Superficial oily layer: reduces tear film evaporation, produced by Meibomian glands
Aqueous tear film; contains oxygen, nutrients and bactericide. Lubricates the eye.
Mucinous layer; Makes sure tear film adheres to the eye. Mucin molecules bind water molecules to hydrophobic corneal epithelial surface.
What is the conjunctiva?
Thin layer above the cornea; highly vascular, has conjunctival goblet cells which produce mucin molecules
What are the three layers of the coating of the eye and their function?
Sclera: maintains shape of the eye, has a high water content
Choroid: provides circulation
Retina: (neurosensory) converts light into nerve impulses to be sent to the brain via optic nerve
Describe the cornea and features of it
Dome-shaped convex curvature
Higher refractive index than air
provides 2/3rd of eye focusing power
Continuous with the sclera
Low water content
What are the layers of the cornea?
Stroma; provides sensation and nutrients
What is the purpose of the endothelium layer of the cornea?
Pumps fluid out of the stroma to prevent corneal oedema and blurred vision
What is the uvea?
Consists of the iris, ciliary body and choroid
Describe the lens structure and function
Outer acellular capsule
Regular inner elongated cell fibres; contributes to transparency
Higher refractive index than aqueous/vitreous humour
Provides 1/3rd of eye refractive power
Able to change shape and focus
What is meant by accommodation?
When the lens changes its shape in order to focus on near and distant objects e.g. near objects need thicker lens
What are lens zonules?
Also known as the suspensory ligaments.
They suspend the lens by connecting them to ciliary muscles. (passive connective tissue)
They do not contract, just pass force from contraction of ciliary muscles.
What is the macula?
Central part of retina, temporal to optic nerve
Responsible for detailed vision.
The fovea is the centra of the macula.
What are the two anatomical segments of the eye?
Anterior: cornea to lens contains aqueous humour. Supplies nutrients.
Posterior: posterior to the lens contains vitreous humour
Where is the ciliary body located?
Between the anterior and posterior segments, behind the iris.
What are the layers of the iris?
Anterior; stromal layer which is provided by the choroid, contains muscle fibres
Posterior; epithelial layer which is provided by the retina
Where is aqueous fluid produced and drained?
Produced by the ciliary body
Does not require a concentration gradient, it is energy dependent.
Aqueous fluid travels to the anterior chamber to circulate,
It is reabsorbed in the scleral-corneal angle where the trabecular meshwork is.
What are the two pathways of drainage for the aqueous fluid?
Uvea-scleral pathway: 20% drainage occurs here, resistance dependent pathway, fluid leaks between the sclera and choroid.
Schlemm's canal/trabecular meshwork: 80% of drainage, goes to the aqueous veins and bloodstream
What is the normal eye intraocular pressure?
How do you treat increased eye pressure?
What is a 'definition' of glaucoma?
Optic neuropathy with retinal ganglion death, optic nerve fibre loss, enlarged optic disc cupping, visual field loss
What is the pressure standard for ocular hypertension?
21+mmHg with no development of glaucoma
What is primary open angle glaucoma?
Functional blockage of the trabecular meshwork
What is closed angle glaucoma, risk factors, presentation?
Chronic or acute
Forward displacement of iris/lens causes narrowing of trabecular meshwork pathway
Risk factors; small eye (hypermetropia)
Presents with painful red eye and acute drop in vision
What is the treatment for closed angle glaucoma?
Peripheral laser iridotomy; drainage hole in the iris is created
What is meant by central vision?
Necessary for focussed vision
Day and coloured vision
Fovea has the highest conc. of cones
Needed for facial recognition and reading
Assessment of visual acuity
What is meant by peripheral vision?
Necessary to detect motion, shape, night vision
Assessed by visual field assessment
Intact peripheral vision with loss of visual acuity can be classified as blindness
What is the connection between the choroid and retina?
The choroid is the blood supply to the outer 1/3rd of the retina (photoreceptors)
Describe the structure of the retina
Retinal pigment epithelium; provides nutrients and removes metabolic debris from the photoreceptors
Describe the layers of the neuroretina
Outer layer: photoreceptors capture light
Horizontal cells modulate the signal from the photoreceptors
Middle layer: bipolar cells regulate the sensitivity
Inner layer: retinal ganglion cells pass the signal from the eye to the brain
How can you visualised the macula?
Use optical coherence tomography
Describe rod photoreceptors and their location
Long outer segment with photo-sensitive pigment
Needed in night vision, detecting motion, peripheral vision (scotopic vision)
Slow response to light
100 times more sensitive than cones
Highly concentrated outside the macula, 20-40 degrees away from the fovea
120 million rods
Describe cone photoreceptors and their location
Less sensitive to light
Detailed daylight and colour vision (photopic vision)
Only concentrated within the macula, 0 degrees from the fovea
60 million cones
What is the frequency of the human visual spectrum?
What are the rod and cone photopigments?
Rod: 498nm, blue-green
Cone: S (blue), M (green), L (red)
What is the most common colour vision deficiency called and caused by?
Deuteranomly (red-green colour blindness); M cone sensitivity shifts towards L-cone so causes red-green confusion
What is meant by dichromatism and monochromatism?
Dichromatism; absence of one of the cone photopigments
Monochromatism; complete absence of colour vision
What is meant by anomalous trichromatism?
When colour blindness is caused by a shifted peak
How is red-green colour blindness identified?
Ishihara colour perception test
What is dark adaptation?
When photoreceptors become more sensitive to light in the dark
Cones; adapt within 7 minutes
Rods; more sensitive so adapt in 30mins, must generate rhodopsin
What is light adaptation?
Adaptation from dark to light
Takes 5 minutes
Bleaching of photopigments
Rod/cone function inhibition
What is pupil adaptation?
Constriction of pupil in response to light
What is the index of refraction formula?
Index of refraction= speed of light in a vacuum/speed of light in medium
Difference between convex and concave lens?
Convex: light rays converge onto focal point
Concave: light rays diverge
Which structures in the eye help concentrate light?
Cornea and lens focus light rays onto the retina
Which structures in the eye regulate light entry?
Pupil and pigmented uvea (absorbs excess scattered light within the eye)
What is meant by emmetropia?
Eyes are relaxed and lens is focused on object far away. Light rays are almost parallel and rays are focused on the retina without effort ( no accomodation).
What is meant by ametropia?
When there is axial length and refractive power mismatch. Parallel light rays do not fall on the retina.
Near-sightedness (myopia), Far-sightedness (hyperopia)
What is the pathology of myopia?
Light rays converge at a point anterior to the retina
Eyeball is longer (axial/globe length)
Excessive refractive index
Squint to improve visual acuity
Correct using concave lenses
What is the pathology of hyperopia?
Light rays converge at a point posterior to the retina
Short globe/axial length
Inadequate refractive power (flat corneal surface)
Asthenopic/eye strain symptoms; eye pain, headache in frontal region, burning
Correct using convex lenses or intraocular lenses
What is Amblyopia?
Lazy eye, uncorrected hyperopia of more than 5 diopters
What is astigmatism?
Parallel rays converge in two lines rather than one.
Cornea is of oval not spherical shape.
Refractive power varies along different planes.
Asthenotopic symptoms; blurred vision, spinning
Treat using cylinder lenses or rigid contact lenses or surgery.
What is accommodation and how does it occur?
Contraction of ciliary muscles, relaxation of zonule fibres.
Absence of zonular tension causes lens to relax into convex shape due to elasticity.
Increases refractive power of the lens
Mediated by CNIII
Describe the near response triad
1. Pupillary mitosis; increases depth of field
2. Convergence; medial recti of both eyes
3. Accomodation; makes lens thicker to increase refractive power
What is presbyopia?
Naturally occurring (40+ yrs) loss of accomodation
Due to hardening of crystalline lens
Loss of visual acuity in near objects
May still be emmetropic for far objects
Treat using positive convex/converging lens to increase optical power