Sensory Systems - Ophthalmology - How We See Flashcards
what is refraction?
Bending of light when it passes from one optical medium to another
what is accomodation?
We can focus on far off or near objects by changing how much we bend the light rays
what leads to refractory errors
Sometimes there is a mismatch between how much we bend light rays leading to
what is a bioconvex lens?
Biconvex lens is a simple lens which comprises two convex surfaces in spherical form, generally having the same kind of radius of curvature.
what is a bioconcave lens?
Bi-concave lenses have two inward curved surfaces. These lenses have a negative focal length.
what is found in the fibrous coat of the eyeball?
cornea, sclaera, aqueous humor
what is found in the vascular coat?
iris, ciliary body, choroid (aqueous humour)
what is found in the sensory coat of the eye?
retina, vitreous humor
How does a sharp image form on the retina?
Light waves from an object bend at the cornea, bend some more at the lens to form a clear image on the retina. This bending of light waves is called Refraction
what are the two media of the eye responsible for the bending of light?
cornea and the lens
which of the cornea and lens is more powerful at bending light?
Cornea is the most powerful “bender” of light (45D) but lens (15D) has the capacity to change its “bending power”
when objects are distant >6 metres which rays reach the eye?
When objects are distant (>6m) then only parallel rays from the object reach the eye
when objects are up close which rays reach the eye?
But when objects are up close divergent rays from the object reach the eye. They need to be “bent” more to bring them to a focus on the retina - in order to do this, the lens becomes thicker, which is something that forms part of accommodation
How does a sharp image form on the retina?
When an object comes closer, the eye needs more bending power to focus on an object🡪 The lens becomes thicker & hence more powerful, and a clear image is formed on the retina again.
what is accomodation?
Our eyes have the capacity to change focus from distant objects (infinity) to close objects (20cm).
The changes occurring in both eyes as it changes focus from a distant to a close object is called as accommodation.
3 things happen simultaneously & comprise accommodation, what are they?
Lens changes shape (becomes thicker & more spherical)
Pupil constricts
Eyes converge
what is the process of making the lense thicker (4)?
Ciliary muscle contracts making the ciliary body bulge
Space in the middle decreases
Suspensory ligaments become lax
Lens is no longer under stretch
LENS BECOMES THICKER
what causes the ciliary muscle to contract?
Ciliary body contraction (parasympathetic) causes lens to become thicker & more spherical
what causes the pupil to constrict?
Pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation from CN3
when does the pupil need to constrict?
When we are looking at an object up close, we need a sharp focus.
To sharpen focus the pupil constricts to allow only a few rays (those from the object) to pass through.
when are your eyes required to converge?
When focussing on an object up close, our eyes have to turn in to look at the object🡪 convergence.
which muscles are used to converge the eye?
We use our medial rectus muscles of both eyes to converge.
what are the medial rectus muscles both innervated by?
Cranial nerve 3
which muscles are thicker medial recti muscles or lateral recti muscles?
Humans spend a lot of time doing ‘close’ work, so our Medial recti muscles are thicker than our lateral recti muscles.
what is myopia?
short-sightedness
rays are focused infront of the retina
what is hyperopia?
long-sightedness
what is astigmatism?
non-spherical curvature of cornea (or lens)
what is presbyopia?
long-sightedness of old age
what is emmetrope?
perfect vision
how do objects appear in myopia?
Close objects look clear, distant objects appear hazy
what is the most common cause of myopia?
eyeball too long
what is the pathophysiology of myopia?
eyeball too long
So when the cornea + lens bend rays of light, they make the image form IN FRONT OF the retina. So far off objects not seen clearly.
(Basically the “bending power” of cornea + lens is too much for that eyeball)
what happens in myopia when an object is brought closer?
When the object is brought closer, divergent rays from the object reach the cornea; and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens (i.e. without using the accommodative power.
Can see near things clearly i.e. near-sighted
what are symptoms of myopia?
Headaches, Complain of not being able to see blackboard/ distant objects.
what is often seen in infants and preverbal children with myopia?
divergent squint
what is often seen in toddlers with myopia?
loss of interest in sports/people. More interest in books, pictures.
how urgently must a divergent squint be dealt with?
deal with that aggressively if you’re not to make one of the eyes lose their vision or make it a lazy eye
what is the treatment for myopia?
Bending power needs to be decreased.
Biconcave lenses
- spectacles
- contact lenses
- laser eye surgery
when is laser eye surgery of greatest benefit?
this procedure is of greatest benefit when you have a steady power,
which means that your power doesn’t change any more as you grow or you have a higher power
something like minus four, minus five and beyond, where you cannot manage without glasses.
how do objects appear in hyperopia?
Close objects look hazy, distant objects appear clear
how does an emmetrope use accomodative power?
If you’re an emmetrope and you’re looking at somebody far away,
you should be able to see them without using your accommodative power.
But this person here is sitting within that six metre distance. So in order to see him, you are going to start using up your accommodative power.
how would a hypermetrope use accomodative power?
a hypermetrope now has to use their accommodative power in order to be able to see all these people far away.
And so people far away are quite clear. You can see them because you’re using your accommodative power.But by the time you’ve come to six metres, while an emmetrope would have started using their accommodative power to start seeing them,
a hypermetrope has no more accommodative power at their disposal and therefore near objects appear quite hazy.
what is the cause of hyperopia?
Eyeball too short or cornea + lens too flat.
explain the pathophysiology of hyperopia?
Eyeball too short or cornea + lens too flat.
So the image of a distant object is formed BEHIND THE RETINA.
The person then automatically starts to use his accommodative power and makes the lens thicker. This causes the image to form on the retina.
So he is using his lens power to see far off things (that he should normally be seeing without using any power).
When seeing closer objects, he uses more and more power until ultimately, his power is all used up!
So he can’t see nearby objects ie. “longsighted
what are symptoms of hyperopia in a young individual?
eyestrain after reading/ working on the computer in a young individual.
what are symptoms of hyperopia in a child/toddler?
Convergent squint in children/ toddlers – needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent a “lazy eye”.
what is the treatment for hyperopia?
Biconvex glasses alleviates use of glasses for focussing distant objects and ‘rests’ the accomodative power
Contact lenses
Laser Eye surgery
how do objects appear in astigmatism?
Close and distant objects appear hazy
what is the pathophysiology of astigmatism?
cornea is shaped more like a rugby ball than a football
So the bending of light rays along one axis will never be the same as that of the other axis.
So image formed is always hazy, whatever the distance of the object.
what is the treatment for astigmatism?
So to correct it we need special glasses – called cylindrical glasses (which are curved in only one axis).
Laser eye surgery can also be used to correct the defect.
Need special contact lenses called toric lenses.
what is the pathophysiology of presbyopia?
With age the lens gets less mobile/elastic.
So when the ciliary muscle contracts, it is not as capable as before to change shape.
So seeing near objects/ reading the newspaper starts to become difficult🡪 needs glasses to read.
Usually starts in 5th decade of life.
how is presbyopia corrected?
Correction is by using biconvex “reading glasses”
in a prescription note what does BC stand for?
base curvature and it is telling you how curved your cornea is so that the contact lens can be shaped to your cornea
in a prescription note what does DIA stand for
DIA shows you the diameter of the cornea so you can make your contact lenses as big as that to cover the entire cornea
In a prescription what does CYL stand for?
power required to correct astigmatism
In a prescription what does the axis represent?
number between 1-180 showing the degree at which the astigmatism lies
what is visible light a form of?
electromagnetic wave
what do the energy in light waves need to stimulate?
photoreceptor cells on the retina (phototransduction)
what is phototransduction defined as?
Phototransduction is defined as the conversion of light energy to an electrochemical response by the photoreceptors (rods and cones)
what do the phototransduced roads and cones need to do?
need to activate optic nerve cells (generate an action potential)
label the different features of road & cone?
what does this image show?
what is each lamellae made up of?
Each lamellae is made up of cell membrane. Integrated into this cell membrane is the visual pigment rhodopsin in rods and cone opsins S, M and L in cones. These molecules differ in their spectral sensitivity
label this diagram?
chromophore nests in the opsin what is this molecule formed from?
dietary vitamin A
what happens when light falls of 11-cis retinal?
it isomerises to all-trans retinal
what is an issue with all-trans retinal?
cannot fit into the opsin. So rhodopsin splits. This results in BLEACHING of the visual purple
describe what is shown in the image?
So if you think of this as the retina and it normally looks this reddish colour.
When light falls on it and the 11-cis retinal has become all-trans retinal and therefore moves away from the opsin,
you find this bleaching until you have a bleached compound.
describe what is shown in the image?
So if you think of this as the retina and it normally looks this reddish colour.
When light falls on it and the 11-cis retinal has become all-trans retinal and therefore moves away from the opsin,
you find this bleaching until you have a bleached compound.
How does bleaching of the visual pigment result in phototransduction?
Phototransduction cascade.
What role does Vitamin A play in the visual pigment?
visual pigment regeneration
study the phototransduction cascade?
study visual pigment regeneration?
what symptoms will someone suffereing with vitamin A deficiency present with?
supplied through the diet, any condition that affects vitamin A absorption will affect vision – (night)blindness. Vitamin A is also essential for healthy epithelium. So conjunctiva and corneal epithelium are also abnormal.
what conditions might vitamin A deficiency present in?
conditions such as malnutrition, malabsorption syndromes such as coeliac disease, sprue.
what are characteristic features of vitamin A deficiency that may be seen in the eyes?
Bitot’s spots in conjunctiva are sometimes the first indication of Vitamin A deficiency
Corneal ulceration ( green colour is dye to show up extent of ulcer)
corneal melting
Which leads to future opacification of the cornea
what is the role of intrinsic occular muscles?
control pupil diameter & helps alter lens curvature to enable us to see near objects
what is the role of extrinsic occular muscles?
move the eye.
what are the six extrinsic occular muscles of the eye?
4 Straight muscles called recti
Medial rectus (MR)
Lateral rectus (LR)
Inferior rectus (IR)
Superior rectus (SR)
2 oblique muscles
Superior Oblique (SO)
Inferior Oblique (IO)
what are the four straight extrinsic occular muscles of the eye?
medial rectus (MR)
Lateral rectus (LR)
Inferior rectus (IR)
Superior rectus (SR)
what are the 2 oblique extrinsic muscles of the eye?
2 oblique muscles
Superior Oblique (SO)
Inferior Oblique (IO)
where do the recti muscles of the eye arise from?
The recti muscles arise from the apex of the orbit from an annular fibrous ring
where do the superior oblique muscles arise from?
The superior oblique muscle arises from the roof of the orbit posteriorly
where do the inferior oblique muscles arise from?
The inferior oblique muscle arises from the floor of the orbit anteriorly
what lies just above the superior rectis?
the LPS (levator palpebrae superioris) is a muscle running to the upper eyelid to elevate it
how do the recti muscles insert into the sclera versus the obliques?
The recti muscles insert onto sclera anteriorly
Obliques insert posteriorly
what is the origin and insertion of the L.P.S?
origin - roof of orbit
insertion - upper eyelid
what is the origin and insertion of all recti?
origin - tendinous ring
insertion - sclera anteriorly
what is the origin and insertion of the superior oblique?
origin - lesser wing of sphenoid
insertion - sclera posteriorly
what is the origin and insertion of inferior oblique?
orgin - medial part of orbit floor
insertion - sclera posteriorly
what does the trochlear (IV) nerve supply?
supplies muscle with trochlea (SO)
what does the abducent nerve (VI) supply?
supplies muscle which abducts (LR)
what nerve is the rest of the eye supplied by?
Everything else – Oculomotor (III) nerve
uniocular?
located/confined to one eye
binocular?
both eyes
what is intorsion?
when the top of the eyeball rotates towards the nose
what is extorsion?
when the top of the eyeball rotates away from the nose
what is the primary action of the medial rectus?
adduction
what is the primary action of the lateral rectus?
abduction
what is the primary action of the superior rectus?
Elevation
Adduction
Intorsion
what is the primary action of the inferior rectus?
depression
adduction
extorsion
what is the primary action of the superior oblique?
Intorsion
depression
abduction
what is the primary action of the inferior oblique?
extorsion
elevation
abduction
what are the action of EOM influences by?
The muscles are attached along the orbital axis and not the optical axis, so they pull on the eyeball at an angle. This is a muscle has more than one action
The Oblique muscles are attached to the posterior part of the sclera, so they pull the posterior part of the eyeball up/down & the anterior part moves in the opposite direction
label the nerve and muscle associated with each movement?
what do obliques do when eye is adducted?
elevate/ depress when the eye is adducted
what do recti do when eye is abducted?
elevate/depress when the eye is abducted.
what is strabismus?
Misalignment of the eyes
Esotropia (manifest convergent squint)
Exotropia (manifest divergent squint)
what are functional consequences of a squint?
amblyopia
diplopia
what is amblyopia?
(lazy eye) where brain supresses the image of one eye leading to poor vision in that eye without any pathology (correctable in early years using eye patches to stimulate the “lazy” eye to work)
what is diplopia?
(double vision) – usually occurs in squints occuring as a result of nerve palsies.
what are the instrinsic eye muscles?
Ciliaris muscle in ciliary body
Constrictor pupillae in iris at pupillary border
Dilator pupillae radially running muscle in iris
what innervates both cillaris muscle and constrictor pupillae?
1 and 2 innervated by parasympathetic (IIIn)
what innervated dilator pupillae?
3 by sympathetic (from plexus around blood vessels)
what does pathology of innervation of intrinsic muscles leads to?
pupillary abnormalities
what is the pupillary responce to increased illumination?
increased illumination🡪 parasympathetic🡪 both pupils constrict
what is the puppilary response to decreased illumination?
Decreased illumination🡪 sympathetic🡪 pupils dilate
how do you elicit the pupillary reflex?
location - dimly lit room
Pen torch in front of one eye 🡪 check for both pupils constricting (Direct and consensual reflex)
Swing light to other side 🡪 should remain both pupils constricted
describe the pathway of light reflex in the afferent limb?
describe the pathway of light reflex in the efferent limb?
what are some pupil abnormalities?
Pupils maybe of different sizes – anisocoria eg: Horner’s syndrome
Pupils may look normal but react abnormally to light (abnormal light reflex)
what are some common causes of absent/abnormal pupillary reflex?
Diseases of the retina – detachment/ degenerations or dystrophies
Diseases of the optic nerve – such as in optic neuritis (frequently seen in MS)
Diseases of the III cranial nerve (efferent limb)
3rd nerve cranial palsy?
In III n palsy due to a medical cause such as diabetes, there is usually no damage to parasympathetic fibres. So if you see a patient with a IIIn palsy, check pupillary
reflex🡪 if absent🡪 suspect a cerebral artery aneurysm 🡪emergency
what is horners syndrome?
Anisocoria due to damage to the sympathetic innervation to the pupil.
You might also see ptosis (drooping of the eyelid) on the affected side
Other signs – anhidrosis (loss of sweating on the affected side)
sympathetic innervation of the eye?
remember the thoracolumbar outflow of the sympathetic
Remember the sympathetic chain and cervical ganglia
Remember that in the head and neck – postganglionic sympathetic fibres travel along with blood vessels
