Visual System Flashcards
Eye anatomy
Palpebral fissure Lateral canthus Medial canthus Iris Pupil Caruncle Limbus (border between cornea and sclera)
Lacrimal System
Tears – basal (constant), reflex and emotional (crying)
Afferent – cornea, cranial nerve V1 – ophthalmic trigeminal
Efferent – parasympathetic
Neurotransmitter - acetylcholine
Tears produced by lacrimal gland
The lacrimal gland is located within the orbit, Latero-superior to the globe.
Drain through the two puncta, openings on medial lid margin
Flow through superior and inferior canaliculi
Gather in tear sac
Exit tear sac through tear duct into nasal cavity
Tear Film
Maintains smooth cornea-air surface
Oxygen supply to Cornea – normal cornea has no blood vessels
Removal of debris (tear film and blinking)
Bactericide
Composed of three layers
Superficial lipid layer to reduce tear film evaporation - produced by a row of Meibomian Glands along the lid margins
Aqueous (water) tear film (tear gland)
Mucinous Layer corneal surface - maintains surface wetting
Conjunctiva
Thin, transparent tissue that covers the outer surface of the eye
It begins at the outer edge of the cornea, covers the visible part of the eye, and lines the inside of the eyelids
It is nourished by tiny blood vessels that are nearly invisible to the naked eye
Coat of the Eye
3 layers
Sclera - hard, fibrous and opaque - maintain shape - High water content
Choroid - pigmented and vascular - provide circulation
Retina - neurosensory tissue - convert light into electrical impulses
Cornea
5 layers
1 – Epithelium
2 – Bowman’s membrane
3 – Stroma – its regularity contributes towards transparency
4- Descemet’s membrane
5- Endothelium – pumps fluid out of corneal and prevents corneal oedema
The transparent, dome-shaped window covering the front of the eye.
Low water content
Powerful refracting surface, providing 2/3 of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through
Uvea
Vascular coat of eyeball and lies between the sclera and retina.
Composed of three parts – iris, ciliary body and choroid.
Intimately connected and a disease of one part also affects the other portions though not necessarily to the same degree.
Choroid
Lies between the retina and sclera. It is composed of layers of blood vessels that nourish the back of the eye.
Iris
Controls light levels inside the eye similar to the aperture on a camera.
Round opening in the centre is the pupil.
Embedded with tiny muscles that dilate (widen) and constrict (narrow) the pupil size.
Lens
Outer acellular capsule
Regular inner elongated cell fibres – transparency
May lose transparency with age – cataract
Transparency Regular structure Refractive Power 1/3 of the eye focusing power - higher refractive index than aqueous fluid and vitreous Accommodation Elasticity
Retina
Very thin layer of tissue that lines the inner part of the eye.
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.
Optic Nerve
transmits electrical impulses from the retina to the brain
connects to the back of the eye near the macula
visible portion is called the optic disc
Optic Nerve: Blind Spot
Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot
Macula
Located roughly in the centre of the retina, temporal to the optic nerve
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.
Fovea
Fovea is the most sensitive part of the retina
It has the highest concentration of cones, but a low concentration of rods
This is why stars out of the corner of your eye are brighter than when you look at them directly.
But only your fovea has the concentration of cones to perceive in detail.
Central vision
Detail day vision, colour vision – fovea has the highest concentration of cone photoreceptors
Reading, facial recognition
Assessed by visual acuity assessment
Loss of foveal vision – Poor visual acuity
Peripheral Vision
Shape, movement, night Vision
Navigation vision
Assessed by visual field assessment
Extensive loss of visual field – unable to navigate in environment, patient may need white stick even with perfect visual acuity
Retinal Structure
Outer layer
Photoreceptors (1st order neuron)
Detection of Light
Middle layer
Bipolar Cells (2nd order neuron)
Local signal processing to improve contrast sensitivity, regulate sensitivity
Inner layer
Retinal ganglion cells (3rd order neuron)
Transmission of signal from the eye to the brain
Visual processing - photoreceptors
Rods
Longer outer segment with photo-sensitive pigment
100 times more sensitive to light than cones
Slow response to light
Responsible for night vision (Scotopic Vision)
120 million rods
Cones
Less sensitive to light, but faster response
Responsible for day light fine vision and colour vision (Photopic Vision)
6 million cones
Photoreceptor distribution
Rod (scotopic) vision
Peripheral and night vision More photoreceptors, more pigment, higher spatial and temporal (time) summation
Recognizes motion
Cone (photopic) vision
Central and day vision
Recognizes colour and detail
Frequency Spectrum
Like a CCD camera the eye captures different colours through different photoreceptors:
S-Cones: Blue
M-Cones: Green
L- Cones: Red
Rods are used for night vision and spatial recognition and are not really sensitive to any particular colour
Colour Vision Deficiencies
Deuteranomaly also known as Daltonism is the most frequent form of colour blindness.
People with deuteranomaly are not completely colour blind but they don’t perceive the colour red.
Full colour blindness which occur only in a very small percentage of the population is called achromatopsia
Index of refraction
Speed of light in vacuum/ speed of light in medium
Adequate correlation between axial length and refractive power
Parallel light rays fall on the retina (no accommodation)
Emmetropia
Mismatch between axial length and refractive power
Parallel light rays don’t fall on the retina (no accommodation)
Ametropia (refractive error)
Near-sightedness (Myopia)
Farsightedness (Hyperopia)
Astigmatism
Presbyopia
Myopia
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
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
Hyperopia
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)
visual acuity at near tends to blur relatively early
nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur
asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
Amblyopia – uncorrected hyperopia > 5D
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 cylinder lenses, surgery
Near Response Triad
Adaptation for Near Vision
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
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
Treatment convex lenses in near vision Reading glasses Bifocal glasses Trifocal glasses Progressive power glasses
Types of optical correction
Contact lenses
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 Pre operative eye Initial cutting of corneal flap Cutting of corneal flap Flipping of corneal flap Photorefractive treatment (laser) Corneal stroma reshaped post laser Corneal flap back in position
The Staar intra-collamer lens (ICL) is inserted into the eye for the correction of myopia and astigmatism
Clear lens extraction + IOL: IOL: Intra ocular lens. Same as cataract extraction. Implantation of artificial lens. Lose accommodation (patient will need reading glasses).
Visual Pathway Anatomy
Visual Pathway transmits signal from eye to the visual cortex
Visual Pathway Landmarks
Eye
Optic Nerve – Ganglion Nerve Fibres
Optic Chiasm – Half of the nerve fibres cross here
Optic Tract – Ganglion nerve fibres exit as optic tract
Lateral Geniculate Nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus
Optic Radiation – 4th order neuron
Primary Visual Cortex or Striate Cortes – within the Occipital Lobe
Visual Pathway Retina
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second order Neurons – Retinal Bipolar Cells
Third Order Neurons –Retinal Ganglion Cells
Optic Nerve (CN II)
Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline
Optic Tract
Destinations
Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex
Optic Chiasma
Optic Chiasma – Important Landmark in Visual Pathway
Lesions anterior to Optic Chiasma affect visual field in one eye only
Lesions posterior to Optic Chiasma affect visual field in both eyes
53% Ganglion Fibres cross at Optic Chiasma
Crossed Fibres – originating from nasal retina, responsible for temporal visual field
Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field
Visual Field Defects
Lesion at Optic Chiasma
Damages crossed ganglion fibres from nasal retina in both eyes
Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia
Lesion Posterior to Optic Chiasma
Right sided lesion – Left Homonymous Hemianopia in Both Eyes
Left sided lesion – Right Homonymous Hemianopia in Both Eyes
Bitemporal Hemianopia
Bitemporal Hemianopia
Typically caused by enlargement of Pituitary Gland Tumour
Pituitary Gland sits under Optic Chiasma
Homonymous Hemianopia
Stroke (Cerebrovascular Accident)
Homonymous Hemianopia with Macular Sparing
Damage to Primary Visual Cortex
Often due to stroke
Leads to Contralateral Homonymous Hemianopia with Macula Sparing
Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides
Pupillary Function
Regulates light input to the eye like a Camera Aperture
In light: pupil constriction
decreases spherical aberrations and glare
increases depth of field – see Near Response Triad from Previous Lecture
reduces bleaching of photo-pigments
Pupillary constriction mediated by parasymapthetic nerve (within CN III)
In dark: pupil dilatation
increases light sensitivity in the dark by allowing more light into the eye
pupillary dilatation mediated by sympathetic nerve
Pupillary pathways
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
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
Direct vs. Consensual Reflex
Direct Light Reflex –Constriction of Pupil of the light-stimulated eye
Consensual Light Reflex – Constriction of Pupil of the other (fellow) eye
Neurological Basis
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
Afferent vs. Efferent Defect
Right Afferent Defect
E.g. damage to optic nerve
No pupil constriction in both eyes when right eye is stimulated with light
Normal pupil constriction in both eyes when left eye is stimulated with light
Right Efferent Defect (Pupil Constriction)
E.g. Damage to Right 3rd Nerve
No right pupil constriction whether right or left eye is stimulated with light
Left pupil constricts whether right or left eye is stimulated with light
Unilateral Afferent Defect
Difference response pending on which eye is stimulated
Unilateral Efferent Defect
Same unequal response between left and right eye irrespective which eye is stimulated
Swinging Torch Test
Relative Afferent Pupillary Defect
Partial pupillary response still present when the damaged eye is stimulated
Elicited by the swinging torch test – alternating stimulation of right and left eye with light
Both Pupils constrict when light swings to left undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
Eye Movement - Terminology
Duction – Eye Movement in One Eye
Version – Simultaneous movement of both eyes in the same direction
Vergence – Simultaneous movement of both eyes in the opposite direction
Convergence – Simultaneous adduction (inward) movement in both eyes when viewing a near object
Speed of Eye Movement
Saccade – short fast burst, up to 900°/sec Reflexive saccade to external stimuli Scanning saccade Predictive saccade to track objects Memory-guided saccade
Smooth Pursuit – sustain slow movement
Slow movement – up to 60°/s
Driven by motion of a moving target across the retina.
The Muscles of the Eye (Extraocular muscles)
Six muscles Attach eyeball to orbit Straight and rotary movement Four straight muscles Superior rectus Inferior rectus Lateral rectus Medial rectus
Superior oblique
Inferior oblique
Superior and Inferior rectus
Superior rectus
Attached to the eye at 12 o’clock
Moves the eye up.
Inferior rectus
Attached to the eye at 6 o’clock
Moves the eye down.
Lateral Rectus
Also called the external rectus
Attaches on the temporal side of the eye
Moves the eye toward the outside of the head (toward the temple)
Medial Rectus
Also called the internal rectus
Attached on the nasal side of the eye
Moves the eye toward the middle of the head (toward the nose)
Superior Oblique
Attached high on the temporal side of the eye.
Passes under the Superior Rectus.
Moves the eye in a diagonal pattern down and out
Travels through the trochlea
Inferior Oblique
Attached low on the nasal side of the eye.
Passes over the Inferior Rectus.
Moves the eye in a diagonal pattern - up and out.
Innervation of Extraocular Muscles
Third Cranial Nerve (oculomotor) Superior Branch Superior Rectus – elevates eye levator palpebrae superioris - raises eyelid (not shown) Inferior Branch Inferior Rectus – depresses eye Medial Rectus – adducts eye Inferior Oblique – elevates eye Parasympathetic Nerve – constricts pupil
Fourth Cranial Nerve (trochlear)
Superior Oblique – depresses eye
Sixth Cranial Nerve (abducens)
Lateral Rectus – abducts eye
Eye Movement Testing
Abduction – Lateral Rectus Adduction – Medial Rectus Elevated and Abducted – Superior Rectus Depressed and Abducted – Inferior Rectus Elevated and Adducted – Inferior Oblique Depressed and Adducted – Superior Oblique
Directions of Eye Movement
Up (Elevation)
Supraduction – one eye
Supraversion – both eyes
Down (Depression)
Infraduction – one eye
Infraversion – both eyes
Right – Dextroversion
Right Abduction
Left Adduction
Left – Levoversion
Right Adduction
Left Abduction
Torsion – rotation of eye around the anterior-posterior axis of the eye
Third Nerve Palsy
Affected eye down and out Droopy eyelid (loss of elevator palpebrae superioris) Unopposed superior oblique innervated by fourth nerve (down) Unopposed lateral rectus action innervated by sixth nerve (out)
Sixth Nerve Palsy
Affected eye unable to abduct and deviates inwards
Double vision worsen on gazing to the side of the affected eye
Optokinetic Nystagmus Reflex
Nystagmus – Oscillatory eye movement
Optokinetic Nystagmus = Smooth Pursuit + Fast Phase Reset Saccade
Optokinetic Nystagmus Reflex is useful in testing visual acuity in pre-verbal children by observing the presence of nystagmus movement in response to moving grating patterns of various spatial frequencies
Presence of Optokinetic Nystagmus in response to moving grating signifies that the subject has sufficient visual acuity to perceive the grating pattern
