Vision Flashcards
Eye anatomy: identify the optical and functional anatomical components of the eye
Eye sits within the orbit
Anterior-‐posterior diameter of the orbit is 24 mm in adults
The coat of the eye has THREE layers:
Sclera -‐ hard and opaque
- Protects the eye and maintains the shape of the eye
- High water content
- Accident may cause a fracture in the orbit
Choroid -‐ pigmented and vascular
- Provides circulation to the eye
- Shields out unwanted scattered light
Retina -‐ neurosensory tissue
Converts light into neurological impulses to be transmitted to the brain via the optic nerve
- The retina gives rise anteriorly to the ciliary body epithelium and the posterior (epithelial) layer of the iris
- Choroid gives rise anteriorly to the ciliary body stroma and the anterior (stromal) layer of the iris
- Uvea = Choroid + Ciliary Body + Iris
Explain the function of the lacrimal system
Lacrimal System
The lacrimal gland is located within the orbit, latero-‐superior to the globe
It produces THREE types of tears:
-
Basal Tears
* These are tears that are produced at a constant level, even in the absence of irritation or stimulation - Reflex Tears
- This refers to increased tear production in response to irritation
- The tear reflex is made up of an afferent pathway, CNS and efferent pathway and the lacrimal glan
- Emotional Tears (crying)
- The cornea is one of the most sensitive tissues in the body
- Irritation is detected within the cornea which is innervated by sensory nerve fibres via the ophthamic branch of the Trigeminal Nerve (CN V)
The efferent pathway is mediated by a parasympathetic nerve which innervates the lacrimal gland
Tear films drains through two puncta from tiny openings on the upper and lower medial lid margins
The puncta form the opening to the superior and inferior canaliculi within the upper and lower eyelids
The two canaliculi converge as one single common canaliculus which drains the tears into the tear sac
The tears are finally drained out of the tear sac via the tear duct (nasolacrimal duct) which opens up in the nasal cavity in the inferior meatus
Explain the function of the tear film
- The tear film is a thin layer of fluid that covers the cornea
- The tear films maintain a smooth cornea-‐air surface
It is important for maintaining clear vision and removing surface debris during blinking
- It is also a source of oxygen and nutrient supply to the anterior segment
- It is a bactericide
The tear film consists of THREE layers:
1. Superficial Oily Layer
- Reduces tear film evaporation
- It is produced by the Meibomian Glands along the lid margin
2. Aqueous Tear Film
- Main bulk of the tear film
- Delivers oxygen and nutrients to the surrounding tissue
- It contains bactericide
3. Mucinous Layer
- Maintains surface wetting
- Ensures that the tear film sticks to the eye surface
- The mucin molecules act by binding water molecules to the hydrophobic corneal epithelial cell surface
- The conjunctiva is a transparent layer on top of the cornea that is very vascular (comes into contact with tear film)-‐ it has goblet cells that produce mucin
- transparent tissue that covers the outer surface of the eye and begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
- nourished by tiny blood vessels that are nearly invisible to the naked eye.
Discuss the structure and function of the cornea
Cornea
- This is the front-most part of the anterior segment
- Continuous with the scleral layer and transparent
- Responsible for 2/3 of the refractive power of the eye
- It has a convex curvature and a higher refractive index than air
- The front surface of the cornea acts as a physical barrier, protecting the eye from opportunistic infection
- Prolonged contact lens wear reduces the oxygen supply to the cornea and compromises the health of the corneal tissue
This is because the cornea gets some oxygen from the atmosphere
Excessive wear considerably increases the risk of serious corneal eye infection
The cornea consists of THREE main layers:
- Epithelium
Bowman’s membrane - Stroma: Contributes to the transparency of the cornea
• Corneal nerve endings provides sensation and nutrients for healthy tissue
• No blood vessels in normal cornea
Descemet’s membrane - Endothelium
- Pumps fluid out of the stroma and prevents stromal edema
- If the endothelium becomes damaged, the stroma will begin to swell and you get blurred vision
Discuss the structure and function of the uvea and its components
Uvea:
Vascular coat of eyeball and lies between the sclera and retina
Uvea is composed of three parts
- Iris
- ciliary body
- choroid
These three portions are intimately connected and disease of one part also affects the other portions though not necessarily to the same degree.
NOTE: uvea means grape
Choroid
The choroid lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.
The Iris
- The coloured part of the eye is called the iris. It controls light levels inside the eye similar to the aperture on a camera.
- The round opening in the centre of the iris is called the pupil.
- The iris is embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
- The Iris is composed of TWO layers:
Anterior Layer -‐ Stromal Layer containing muscle fibres
Posterior Layer -‐ Epithelial
Ciliary Body
- Ciliary body is a ring shaped tissue, surrounding the lens and secretes aqueous fluid in the eye
- Intraocular Aqueous Fluid flows anteriorly into the Anterior Chamber along the green arrow
- Aqueous Fluid supplies nutrient
- Trabecular Meshwork drains the fluid out of the eye
Normal Intraocular Pressure – 12- 21mmHg
It is between the anterior and posterior segments and is located behind the iris
Discuss the structure and function of the lens and lens zonula
Lens
Structure :
- Outer Acellular Capsule
- Regular inner-elongated cell fibers -Transparency
- May lose transparency with age - Cataract
Function
- Transparency - because of its regular structure
- Refractive Power:
- The lens is responsible for 1/3 of the refractive power of the eye
- It has a higher refractive index than the aqueous and vitreous fluid
- The lens can change shape so its refractive power and ability to focus can change
- Accommodation -‐ allows you to focus on near and distant objects by changing their shape
NOTE: cataract is the commonest preventable cause of blindness worldwide
Lens Zonules
The lens is suspended by fibrous bands called lens zonules
It consists of passive connective tissue
They anchor the lens by attaching to the ciliary body
These fibres of the zonules don’t stretch at all -‐ they merely transmit force from the contraction of the ciliary muscles
Discuss the structure and function of the retina, optic nerve and macula
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. Much like the film’s role in photography.
- These light impulses are then sent to the brain for processing, via the optic nerve.
-Optical coherence tomography
Optic nerve
- The optic nerve transmits electrical impulses from the retina to the brain.
- It connects to the back of the eye near the macula.
- The visible portion of the optic nerve is called the optic disc.
Optic Nerve: Blind Spot
Macula
- The macula is located roughly in the centre of the retina, temporal to the optic nerve.
- It is a small and highly sensitive part of the retina responsible for detailed central vision.
- The fovea is the very centre of the macula.
- The macula allows us to appreciate detail and perform tasks that require central vision such reading
Outline and describe anterior and posterior eye chambers
The two segments of the eyeball are separated by the lens
There are also two anatomical spaces within the anterior segment:
- Anterior Chamber (cornea to iris)
- Posterior Chamber (iris to lens)
Anterior Segment = aqueous humour
Posterior Segment = vitreous humour
The part of the optic nerve that is visible at the back of the eye is the optic disc
The zonules are fibrous strands that hold the lens in place in line with the pupil -‐ it is connected to the ciliary muscles
Zonules are also called suspensory ligaments
Anterior Segment
- Aqueous fluid is produced by the ciliary body and it passes into the anterior chamber and then out through the angle via the trabecular meshwork
- This drainage maintains the intraocular pressure
- The role of the aqueous fluid is to supply nutrients to the cornea and other tissue in the anterior chamber
- There are no blood vessels in the middle of the eye because you need a clear window for the light to pass through
- So for the tissue to receive the oxygen and nutrients it needs and to remove waste it needs to be bathed in this fluid
Posterior Segment
- This is located between the lens and the retina
- Vitreous humour is composed of 99% water, trapped inside a jelly matrix
- The jelly substance provides mechanical support to the eye
- There is some collagen and GAGs in the vitreous humour
- The regular structure of the vitreous allows it to be transparent
- As we get older, the vitreous humor loses its jelly consistency, liquefies and detaches from the retina
- The vitreous detachment is experiences as seeing FLOATERS: Normally this is harmless But sometimes it may lead to a small tear in the peripheral retina
Aqueous humour: explain the production, circulation and drainage of the aqueous humour and the importance for maintenance of intraocular pressure
Ciliary Body
- The ciliary body is a ring-shaped tissue, surrounding the lens and secretes aqueous fluid in the eye
- Intraocular Aqueous Fluid flows anteriorly into the Anterior Chamber along the green arrow
- Aqueous Fluid supplies nutrient
- Trabecular Meshwork drains the fluid out of the eye
- Normal Intraocular Pressure – 12- 21mmHg
- It is between the anterior and posterior segments and is located behind the iris
Glaucoma:
Optic neuropathy with characteristic structural damage to the optic nerve, associated with progressive retinal ganglion cell death, loss of nerve fibres and visual field loss
- Condition of sustained raised intraocular pressure
- It results in gradual, accumulative damage to the optic nerve tissue
- There is retinal ganglion cell death and ENLARGED optic disc cupping (seen above)
- Patients with untreated glaucoma lose peripheral vision progressively: visual field loss, blindless
- Untreated glaucoma will eventually lead to blindness
Types of Glaucoma
1. Primary Open Angle Glaucoma
- COMMONEST (the one on the left)
- It is caused by a functional blockage of the Trabecular Meshwork
2. Closed Angle Glaucoma
- Also relatively common
- This can be acute or chronic
- It is caused by the forward displacement of the iris/lens complex -‐ narrowing the trabecular meshwork
- It occurs commonly in patients with small eyes (hypermetropic)
- This can present with sudden painful red eye with acute drop in vision
- Can be treated with peripheral laser iridotomy to create a drainage hole in the iris
Vicious Cycle -‐ the increase in intraocular pressure pushes the iris and lens forward thus narrowing the angle and reducing the drainage -‐ this leads to more of an increase in intraocular pressure leading to more narrowing
Retina: list the main layers, cellular components and synaptic connections of the retina, and explain the basis of phototransduction
Anatomy and Physiology of the Retina
Optic Disc -‐ retinal ganglion cells exit via the optic nerve -‐ this is the physiological blind spot
Macula -‐ has the highest concentration of photoreceptors so is vital for fine vision
Macula Lutea = yellow patch
There are FOUR branches of vessel arcades radiating from the optic disc
- Superior Temporal
- Inferior Temporal
- Superior Nasal
- Inferior Nasal
The retinal arteries and veins provide circulation to the inner 2/3 of the retina
Veins tend to be darker and thicker than the arteries
The outer 1/3 of the retina is supplied by the choroidal vasculature
Retina:
Retina forms the innermost layer of the coat of the eye in the posterior segment (other two layers are sclera and choroid)
It consists of an outer layer of retinal pigment epithelium, immediately in front of the choroid and an inner thicker layer called the neuroretina (made up of photoreceptors and neurones)
The retinal pigment epithelium transports nutrients from the choroid to the photo-‐receptor cells and removes metabolic waste from the retina
Divisions of the Neuroretina:
Outer Layer -‐ photoreceptors (rods and cones)
Middle Layer -‐ bipolar cells (because their axons project in both directions)
Inner Layer -‐ retinal ganglion cells (have their axons running into the optic nerve)
Macula and Fovea
NOTE: macula and macula lutea mean the same thing (macula lutea means ‘yellow patch’ because of the presence of a yellow pigment
The macula is a central region in the retina of about 6 mm in diameter
The fovea is characterised by an anatomical dip known as the foveal pit due to the absence of the overlying ganglion cell layer
The fovea has the highest concentration of photoreceptors for fine vision (cones)
Clinically it can be assessed with an OCT scan
Distinguish between central and peripheral vision
Central and Peripheral Vision
Central Vision
- Detail day vision, colour vision -‐ FOVEA
- The fovea has the highest concentration of cones
- This vision is responsible for reading and facial recognition
- It is assessed by a visual ACUITY assessment
- Loss of foveal vision leads to poor visual acuity
Peripheral Vision
- Shape, movement, night vision
- It is also important for navigation vision
- Assessed by a visual FIELD assessment
- Extensive loss of visual field:
- Unable to navigate the environment
- Patient might still need a white stick even with perfect visual acuity
NOTE: blindness doesn’t necessarily mean that they live in a world of complete darkness. It just means that they can’t see the top of a visual acuity chart
Discuss photoreceptors and photopigments
Photoreceptors
There are TWO main classes of photoreceptors:
o Rods:
- Longer outer segment with photosensitive pigment
- 100 time more sensitive to light than cones
- Slow response to light
- Responsible for night vision (scotopic vision)
- 120 MILLION RODS
Cones
- Less sensitive to light
- FASTER response to light
- Responsible for day light fine vision and colour vision (photopic vision)
- 6 MILLION CONES
- Photopigments are synthesised in the inner photo-‐receptor segment and are then transported to the outer segment discs
Photopigments
Rod Photopigment -‐ Rhodopsin (Opsin is the transmembrane protein)
Cone Photopigments
There are THREE subtypes of photopsin
They react maximally to THREE different light frequencies
Photoreceptor Distribution
- Night vision = SCOTOPIC VISION -‐ rods are responsible for this
- Rod receptors are widely distributed across the retina but the highest density is just outside the macula
- The rod photoreceptors are completely absent within the macula
- The density of rod photoreceptors tails off towards the periphery
- Cone photoreceptors are responsible for day-‐time vision -‐ PHOTOPIC VISION
- Cone photoreceptors are ONLY found within the macula
Colour blindness: explain the most common forms of colour blindness
Rod photopigment has one peak -‐ 498 nm (blue-‐green)
There are THREE cone photopigments:
S-‐cone -‐ short wavelength -‐ 420-‐440 nm (blue)
M-‐cone -‐ medium wavelength -‐ 534-‐545 nm (green)
L-‐cone -‐ long wavelength -‐ 564-‐580 nm (red)
This forms the basis of colour vision
E.g. yellow light has a wavelength between the peak sensitivities of the M and L cones so yellow light stimulates M and L cones equally
Colour Vision Deficiencies
- Congenital colour deficiencies
- The commonest form of colour vision deficiency is Deuteranomaly (red-‐green colour blindness)
This is caused by the shifting of the M-‐cone sensitivity peak towards the L-‐cone peak causing red-‐green confusion
- Anomalous trichromatism - shifted tip
- Dichromatism - 2 cone types
- Monochromatism - no cones
Ishihara test: colour perception test to diagnose deuteranomaly
Light Dark adaptation:
- In dark adaptation, the retina increases its light sensitivity when moving FROM LIGHT TO DARK environment
- Cone photoreceptors adapt much quicker than rod photoreceptors
- Light adaptation is the suppression of light sensitivity when moving FROM DARK TO LIGHT environment
- It is mediated by photopigment bleaching by bright light and neuro-‐adaptation inhibiting rod and cone function
- In light adaptation -‐ rod function is greatly suppressed and cone function takes over within a minute
- The pupil also provides some light and dark adaptation by acting as an adjustable aperture regulating the amount of light that passes into the eye
Optics: explain the basis of physiological optics
The eye functions as a camera:
- Light is refracted by the cornea and lens to focus the incoming light rays onto the retina to form a clear image
- Regulation of Light Entry by the pupil and the pigmented uvea (iris, choroid, ciliary body) -‐ it shields out excess light
- The pigmented uvea absorbs any scattered light once it has entered the eye so that you don’t have any excess light bouncing around inside the eye
Maintenance of the shape of the eye
- Scleral coat
- Maintenance of intraocular pressure by the production in the ciliary body and drainage via the trabecular meshwork
Visual Information Processing
- Optic Nerve
Refraction
Refraction= speed of light in vacuum/ speed of light in a medium (medium V<vacuum></vacuum>
<ul>
<li>All substances have an index of refraction and can be used to identify the material</li>
<li>Some of the light REFLECTS off the boundary and some of the light REFRACTS through the boundary.</li>
<li>Angle of incidence = Angle of Reflection</li>
<li>Angle of Incidence > or < the Angle of refraction depending on the direction of the light</li>
</ul>
<p><strong>Types of lenses </strong></p>
<p>Lenses – An application of refraction There are 2 basic types of lenses</p>
<ol>
<li>A converging lens (Convex) takes light rays and bring them to a point.</li>
<li>A diverging lens (concave) takes light rays and spreads them outward.</li>
</ol>
</vacuum>
Outline the common defects of refraction
1. Emmetropia (healthy)
This is perfect focusing -‐ in emmetropia, parallel rays (from a distance) converge exactly on the fovea forming a clear image on the retina
Refractive (Focusing) Power: Ability to focus light to form an image on the retina
Division of focusing power:
Cornea -‐ 2/3 + Lens -‐ 1/3 People with emmetropia can see clearly at long distances without glasses
2. Hypermetropia -‐ Long Sighted
- In hypermetropia, the parallel rays focus behind the retina
- Causes blurred vision without glasses
- The eye doesn’t have enough focusing power to focus the rays on the retina -‐ it can only focus it behind the retina
- This can be corrected with a convex lens to provide additional converging power
- The blurred vision is exacerbated by near vision
- This is commonly caused by a short eyeball and, more rarely, a flat corneal surface
Lazy eye – if uncorrected hypermetropia in childhood eye ignores the signal
3. Myopia -‐ Short Sighted
Light rays coming from distant objects focus in front of the retinal surface
The cornea and lens have excessive refractive power
This is commonly caused by having a long eyeball or, occasionally, a highly curved cornea
Light rays from near objects require high refractive power to focus an image on the back of the retina so myopic patients can see near objects clearly without glasses
Patients require concave lenses to see clearly at a distance
Symptoms:
- Blurred distance vision
- Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
- Headache (In very high myopia – removal of the lense is preferred treatment)
3. Astigmatism
- In astigmatism, the cornea is OVAL rather than a spherical shape
- The refractive power varies along different planes
- The diagram shows that the light in the vertical plane (green) are focused in front of the retina
- Light in the horizontal plane (purple) are focused just behind the retina
- So this is eye is myopic in the vertical plane and mildly hypertropic in the horizontal plane
- Special astigmatic glasses are required with correction for the different planes
- Parallel rays come to focus in 2 focal lines rather than a single focal point
- Etiology : heredity
- – 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
Outline receptive fields and convergence
Receptive Field of a neurone -‐ retinal space within which incoming light can alter the firing pattern of a neurone
- In general, fewer cone photoreceptors synapse upon the same ganglion cell than in the case of rod photoreceptors
- So the cone system has lower convergence than rods at the peripheral retina
- The rod system near the macula has a lower convergence than the rod system at the peripheral retina
- This means that the ganglion cells in the cone system have a smaller receptive field than ganglion cells in the rod system
- A single retinal ganglion cell will receive input from several photoreceptors (convergence)
- Also, ganglion cells near the macula have smaller receptive fields than ganglion cells originating from the peripheral retina
- Small Receptive Field = Fine Visual Acuity
- LARGE Receptive Field = Higher Light Sensitivity
- Retinal ganglion cells can be described as on-‐centre or off-‐centre
- This is dependent on how surrounding photoreceptors influence the firing of the retinal ganglion cell
- On-‐Centre Ganglion Cell -‐ it is stimulated by light falling on at the centre of its receptive field and inhibited by light falling on the edge of its receptive field
- Off-‐Centre Ganglion Cell -‐ inhibited by light falling on the centre of its receptive field and stimulated by light falling on the edge of its receptive field
- This has an important role in contrast sensitivity and enhanced edge detection in vision
- TAKE HOME MESSAGE: the ganglion cells are NOT all the same -‐ they behave differently depending on whether light is falling on the centre of their receptive fields or on the edge
Explain how specific visual field defects can arise from lesions at different sites of the visual pathway
Optic Chiasma
- Lesions anterior to the optic chiasma affect ONE eye only
- Lesion posterior to the optic chiasma affect visual field in BOTH eyes
- 53% of ganglion fibres cross the midline at the optic chiasma
- The fibres that cross originate from the nasal retina which is responsible for the temporal half of the visual field in each eye
- The uncrossed fibres predominantly originate from the temporal retina and are responsible for the nasal half of the visual field in each eye
Lesion at optic chiasma
Damages fibres that are crossing from the nasal retina of both eyes
Bitemporal Hemianopia
Lesion posterior to the optic chiasma
Right sided lesion -‐ left homonymous hemianopia in both eyes
Left sided lesion -‐ right homonymous hemianopia in both eyes
Visual Defects (pic)
- The optic radiation travels from the lateral geniculate nucleus to the primary visual cortex
- The optic radiation has an upper division (through the parietal lobe) and a lower division (through the temporal lobe)
- Upper Division -‐ INFERIOR visual quadrants
- Lower Division -‐ SUPERIOR visual quadrants
The lower division loops anteriorly forming Meyer’s Loop
When lesion respects the horizontal action -> usually eye issue (glaucoma)
When it respects the vertical line -> usually neurological issue
Meyer’s Loop lesion -‐ vision loss will be in one of the superior quadrants
This is superior homonymous quadrantopia
Lesion in the branch of the optic radiation going through the parietal lobe -‐ vision loss will be in one of the inferior quadrants
This is inferior homonymous quadrantopia
Bitemporal Hemianopia
Typically caused by enlargement of the pituitary gland which sits in the sella turcica beneath the optic chiasma
Homonymous Hemianopia
Typically caused by stroke or other cerebrovascular accidents
Visual reflexes: identify afferent and efferent pathways of the pupillary light reflexes, and the near reflex
Pupillary Function
Regulates light input to the eye like a camera aperture
In LIGHT:
- Iris circular muscle contracts
- Constriction of pupillary aperture
- Reduced light entering the eye
- Reduces the rate of photopigment bleaching
- Increased depth of field
- Action mediated by parasympathetic nerve within CN III
In DARK:
- More light is allowed into the eye so increases light sensitivity
- Pupillary dilation is mediated by a sympathetic nerve activating the iris radial muscle
Afferent pathway (Red & Green)
– Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells
– Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus
– Synapses at Brain Stem (Pretectal Nucleus)
– Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
Efferent pathway (Blue)
– Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent ->
– Synapses at Ciliary ganglion ->
– Short Posterior Ciliary Nerve -> Pupillary Sphincter
IMPORTANT: the afferent pathways from either eye will stimulate the efferent pathway on BOTH eyes
This means that only one eye needs to be stimulated with light to elicit a pupillary response in both eyes
