Structure and Function of the Eye Flashcards
What are the different parts of the eye?
- upper and lower eyelid
- palpebral fissure
- lateral canthus
- pupil
- iris
- sclera
- medial canthus
- caruncle
- limbus
What are the types of tears?
Basal tears
Reflex tears – in response to irritation
- Afferent CN V1, Efferent PNS
Crying tears
How are tears produced?
Lacrimal system
tears produced in lacrimal glands -> drain through two puncta -> through sup. and inf. Canaliculi -> collect in tear sac and drain through tear duct into nose
What are the properties of the tear film?
- Maintains smooth corneal-air surface
- Facilitates oxygen supply to cornea
- Removes debris
- Bactericide
What are the layers of the tear film?
Superficial oily layer – reduce tear film evaporation
- Produced by Meibomian glands
Aqueous layer – produced by tear gland
Mucin layer – maintains wet corneal surface
What is the conjunctiva?
a thin transparent tissue that covers the outside surface of the eye
Begins at the outer edge of the cornea and covers the visible surface of the eye and lines the eyelids
Nourished by near-invisible blood vessels
What are the layers of the coat of the eye?
Retina – a thin layer of photo-sensitive tissue that captures light rays
Choroid – is a component of the uvea (iris, ciliary body and choroid) and is composed of layers of blood vessels
Sclera – a tough opaque white tissue that covers the outside of the eye and is CONTINUOUS with the cornea
- High water content.
What are the layers of the cornea?
- Epithelium
- Bowman’s membrane
- Stroma – thickest layer, contains nerve endings
- Descemet’s membrane
- Endothelium – pumps fluid OUT of cornea and prevents corneal oedema.
- Only 1 cell thick and has NO capacity to regenerate
- Cell density decreases with age and can result in corneal oedema and corneal cloudiness
- LOW water content
- Provides 2/3rds of the eye’s focussing power
- ** Higher refractive index than air, convex
What is the uvea?
the vascular coat of the eye ball that lies between the sclera and the retina
Composed of 3 parts;
- iris - contorls light levels inside eye
- ciliary body
- choroid - nourishes back of eye
Close connections of the structures mean disease of one often affect the other two
Describe the structure and the function of the eye?
Structure - Outer Acellular Capsule - Regular inner elongated cell fibres – transparency - May loose transparency with age – Cataract
Function - Transparency > Regular structure - Refractive Power > 1/3 power > Higher refractive index than aqueous fluid and vitreous - Accommodation > Elasticity
What suspends the lens?
fibrous ring called the lens zonules and consists of passive connective tissue
What is the retina?
The retina is a very thin
layer of tissue that lines the inner part of the eye.
It is responsible for
capturing the light rays that enter the eye.
These light impulses are
then sent to the brain for processing, via the optic
nerve.
What is the macula?
located temporal to the optic nerve and is a small and highly sensitive part of the retina involved in detailed central vision
fovea is the very centre of the macula - allows us to have very focused vision
What are the segments of the eye?
The segments are divided by the lens
Anterior:
- Between the cornea and the lens
- Filled with clear fluid (aqueous humour) and provides nutrients
Posterior:
Filled with vitreous humour
How is aqueous produced?
produced by ciliary body
Aqueous flows into the posterior chamber -> flows into the posterior chamber -> into the scleral angle (with trabecular meshwork)
The aqueous is then absorbed via two methods:
- Uveal-scleral Outflow – aqueous leaks between the sclerous and the choroid
- 20% of drainage
- Prostaglandin analogues target this. - Schlemm’s canal & TM – aqueous goes to blood stream
- 80% of drainage
What is glaucoma?
a disease of sustained high intraocular pressure (a risk factor)
Characteristics are retinal ganglionic cell death and enlarged optic disc cupping (the optic disc enlarges due to absence of retinal ganglionic cells that have died) - results in visual field loss and blindness
Types of glaucoma:
- Primary open angle glaucoma (most common):
- Trabecular meshwork dysfunction. - Closed angle glaucoma (acute or chronic):
- The increased IOP causes the lens/iris to bulge out and restrict access to the TM and thus limit outflow. Risk factors include having a small eye and having a naturally small angle
- Treated with peripheral laser iridotomy to create drainage holes
What are the layers of the retina?
Outer layer – 1st order neurones (photoreceptors), detection of light
Middle layer – 2nd order neurones (bipolar cells), regulate/improve sensitivity and process light
Inner layer – 3rd order neurones (Retinal ganglionic cells), transmission of signal to brain
What are rod photoreceptors?
Scotopic = rod vision
- Longer outer segment that is 100x more sensitive to light
- It has a SLOW response to light
- Responsible for peripheral, night vision and recognises motion
- 120m rods
What are cone photoreceptors?
Photopic = cone vision
- Less sensitive to light but a FASTER response
- Responsible for central colour, day vision and detail
- 6m cones
- no rods found in fovea
What id the difference between central and peripheral vision?
Central vision
- macular degeneration affects ACUITY:
- Detailed day vision
- For reading and facial recognition
- Assessed by the visual ACUITY assessment
Peripheral vision:
- For shape, movement and night vision
- Assessed by the visual FIELD assessment
- Extensive loss of Visual Field – unable
to navigate in environment, patient may need white stick even with perfect
visual acuity
What is the blind spot?
where optic nerve meets retina - no ligth sensitive cells
What is the most commonest form of colour vision deficiency?
how do you test it?
red-green confusion or Deuteranomaly (M-cone peak shifted to L-cone peak)
M- and L-cone peaks are VERY close to each other
500nm is the wavelength of the rods peak sensitivity
Ishihara test is a colour perception test and tests for red-green deficiencies
Patients that suffer from deuteranomaly will not be able to distinguish the numbers from the backgrounds
What is light-dark adaptation?
Dark adaptation:
- There is an increase in light sensitivity in the dark
- Biphasic process:
> Cone adaptation – 7 minutes
> Rod adaptation – 30 minutes (to regenerate the rhodopsin)
Light adaptation:
- Occurs over 5 minutes
- Occurs via; neuro-adaptation, bleaching of photo-pigments and inhibition of rod/cone function
- Pupil adaptation – a minor effect of pupil constriction
What is emmetropia?
Adequate correlation between axial length
and refractive power
Parallel light rays fall on the retina (no
accommodation)
What is ametropia?
refractive error
Mismatch between axial length and refractive
power
Parallel light rays don’t fall on the retina (no accommodation) – Nearsightedness (Myopia) – Farsightedness (Hyperopia) – Astigmatism – Presbyopia
What is myopia?
nearsightedness
Parallel rays converge at a focal point anterior to the retina
Etiology : not clear , genetic factor
Causes:
– excessive long globe (axial myopia) : more common
– excessive refractive power (refractive myopia)
Symptoms:
– Blurred distance vision
– Squint in an attempt to improve uncorrected
visual acuity when gazing into the distance
– Headache
What is hyperopia?
farsightedness
Parallel rays converge at a focal point posterior to the retina
Etiology : not clear , inherited
Causes:
– excessive short globe (axial hyperopia) : more common
– insufficient refractive power (refractive hyperopia)
Symptoms:
– visual acuity at near tendsto blur relatively early
- nature of blur is vary from inability to read fine print to near
vision is clear butsuddenly and intermittently blur
- blurred vision is more noticeable if person istired , printing is
weak or light inadequate
– asthenopic symptoms: eyepain, headache in frontal
region, burning sensation in the eyes,
blepharoconjunctivitis
Amblyopia – uncorrected hyperopia > 5D
What is astigmatism?
Parallel rays come to focus in 2 focal lines rather than a single
focal point
Etiology : heredity
Cause :
refractive media is not spherical–>refract differently
along one meridian than along meridian perpendicular to it–
>2 focal points ( punctiform object is represent as 2 sharply
defined lines)
Symptoms – asthenopic symptoms ( headache , eyepain) – blurred vision – distortion of vision – head tilting and turning
Treatment
– Regular astigmatism :cylinder lenses with or
without spherical lenses(convex or concave), Sx
– Irregular astigmatism : rigid CL , surgery
What is near response triad?
Adaptation for Near
Vision
Near Response Triad
– Pupillary Miosis (Sphincter Pupillae) to increase depth of field
– Convergence (medial recti from both eyes) to align both eyes towards a near object
– Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision
What is presbyopia?
Naturally occurring loss
of accommodation
(focus for near objects)
Onset from age 40 years
Distant vision intact
Corrected by reading
glasses (convex lenses)
to increase refractive
power of the eye
What are types of optical correction?
Spectacle lenses
– Monofocal lenses : spherical lenses , cylindrical
lenses
– Multifocal lenses
Contact lenses
– higher quality of optical image and less influence
on the size of retinal image than spectacle lenses
– indication : cosmetic , athletic activities ,
occupational , irregular corneal astigmatism , high anisometropia , corneal disease
– disadvantages : careful daily cleaning and
disinfection , expense
– complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe
chronic conjunctivitis
Intraocular lenses
– replacement of cataract crystalline lens
– give best optical correction for aphakia , avoid significant magnification and distortion caused by
spectacle lenses
Surgical correction
– Keratorefractive surgery :RK, AK, PRK, LASIK,
ICR, thermokeratoplasty
– Intraocular surgery : clear lens extraction (with
or without IOL), phakic IOL
What is the mechanism of accomodation?
Contraction of the Circular Ciliary Muscle inside the Ciliary Body
This relaxes the zonules that are normally stretched
between the ciliary body attachment and the lens capsule attachment
- zonules are passive
elastic bands with no active contractile muscle
In the absence of zonular tension, the lens returns to
its natural convex shape due to its innate elasticity
This increases the refractive power of the lens
Mediated by the efferent Third Cranial Nerve
