2.16 - Visual system Flashcards
What are the components of the eye? (10 - Label slide)
- upper eyelid
- palpebral fissure
- lateral canthus
- lower eyelid
- pupil
- iris
- sclera
- medial canthus
- caruncle
- limbus (border between cornea and sclera)
What is the normal antero-posterior diameter of the eye in adults?
24mm
What are the three layers of the coat of the eye?
- sclera - hard and opaque
- choroid - pigmented and vascular
- retina - neurosensory tissue
What is the sclera?
- commonly known as the ‘white of the eye’
- tough opaque tissue that serves as the eye’s protective outer coat
- high water content
What is the uvea?
Vascular coat of eyeball and lies between the sclera and retina
Consists of iris, ciliary body and choroid
What are the three parts of the uvea and how are they connected?
- iris
- ciliary body
- choroid
- intimately connected and a disease of one part also affects the other portions (though not necessarily to the same degree)
What is the retina?
- very thin layer of tissue that lines the inner part of the eye
- responsible for capturing the light rays that enter the eye (like film in photography)
- these light impulses are sent to brain via optic nerve for processing
What is the optic nerve and where is it?
- 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 (blind spot)
What is the blind spot?
Where the optic nerve meets the retina there are no light sensitive cells (anatomical landmark is optic disc - visible portion of optic nerve)
What is the macula and where is it?
- 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 (e.g. reading or facial recognition)
- (fovea is the centre of the macula and has high concentration of cone photoreceptors)
What is the fovea?
The very centre of the macula
What is central vision responsible for? (4)
- detail day vision
- colour vision (fovea has highest concentration of cone photoreceptors)
- reading
- facial recognition
What is central vision assessed with?
Visual acuity assessment
What does a loss of foveal vision lead to?
Poor visual acuity
What is peripheral vision responsible for? (4)
- shape
- movement
- night vision
- navigation vision
What is peripheral vision assessed with?
Visual field assessment
What would a loss of visual field cause?
Unable to navigate in environment, patient may need white stick even with perfect visual acuity
What are the three layers of the retina?
- outer layer - photoreceptors (1st order neuron)
- middle layer - bipolar cells (2nd order neuron)
- inner layer - retinal ganglion cells (3rd order neuron)
What is the function of the outer layer of the retina?
- detection of light
- photoreceptors (1st order neuron)
What is the function of the middle layer of the retina?
- local signal processing to improve contrast sensitivity
- bipolar cells (2nd order neuron)
What is the function of the inner layer of the retina?
- transmission of signal from eye to the brain
- retinal ganglion cells (3rd order neuron)
What are the two types of photoreceptors?
- rods (periphery)
- cones (centre)
What do rod cells do?
- 100x more sensitive to light that cones
- slow response to light
- responsible for night vision (scotopic vision)
- 120 million rods per eye
What do cone cells do?
- less sensitive to light, but faster response
- responsible for daylight fine vision and colour (phototopic vision)
- 6 million cones per eye
- (think C=colour)
What is refraction?
As light goes from one medium to another, the velocity changes (causing the light to bend)
What are the two basic types of lenses?
- converging lens (convex) - takes light rays and brings them to a point (focal point)
- diverging lens (concave) - takes light rays and spreads them outward
What is emmetropia?
- adequate correlation between axial length and refractive power
- parallel light rays fall on the retina
What is ametropia (refractive error)?
- mismatch between axial length and refractive power
- parallel light rays do not fall on the retina
- e.g. near-sightedness (myopia), far-sightedness (hyperopia), presbyopia
What is myopia?
- parallel rays converge at a focal point anterior to the retina
- aetiology unclear - genetic factor
What are the causes of myopia?
- excessive long globe (axial myopia) - more common
- excessive refractive power (refractive myopia) - lens too strong
What are the symptoms of myopia?
- blurred distance vision
- squint in an attempt to improve uncorrected visual acuity when gazing into distance
- headache
What is hyperopia?
- parallel rays converge at a focal point posterior to the retina
- aetiology unclear - inherited?
What are the causes of hyperopia?
- excessive short globe (axial hyperopia) - more common
- insufficient refractive power (refractive hyperopia) - weak lens
What are the symptoms of hyperopia?
- visual acuity at near tends to blur relatively early - nature of blur varies from inability to read fine print to clear near vision but sudden and intermittently blur
- blurred vision more noticeable if person tired, weak printing or inadequate light
- asthenopic symptoms - eyepain, headache in frontal region, burning sensation in eyes
What is the near response triad used for?
Eye’s adaptation for near vision
What does the near response triad consist of?
- 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 refractive power of lens (thicken lens) for near vision, circular ciliary contracts and zonules relax
What is presbyopia?
- naturally occurring loss of accommodation (focus for near objects)
- onset from 40 years
- distant vision intact
How is presbyopia corrected?
Corrected by reading glasses (convex lenses) to increase refractive power of eye
What kind of lens is used to treat myopia?
Concave lens
Where does the visual pathway transmit signals from and to?
Transmits signal from eye to visual cortex
Describe the order of the visual pathway from eye to visual cortex?
- eye (neurons in retina)
- optic nerve - ganglion nerve fibres
- optic chiasm (optic nerves from both eyes converge at the optic chiasm, 53% decussate to form contralateral optic tract)
- optic tract (ganglion nerve fibres continuation)
- lateral geniculate nucleus (relay centre within thalamus, where ganglion nerve fibres synapse)
- optic radiation (4th order neuron, relay signal from lateral geniculate ganglion to primary visual cortex)
- primary visual cortex / striate cortex (within occipital lobe, relays to extra-striate cortex for higher visual processing)
What is the order of neurons in the retina?
- 1st order neurons - rod and cone retinal photoreceptors
- 2nd order neurons - retinal bipolar cells
- 3rd order neurons - retinal ganglion cells
- fibres from these go to form optic nerve
Which eyes control which visual fields?
For each eye, respective nasal retinas responsible for temporal fields, and temporal retinas responsible for nasal fields
L temporal retina = L nasal field
L nasal retina = L temporal field
R temporal retina = R nasal field
R nasal retina = R temporal field
What happens at the optic chiasm?
- partial decussation - 53% of ganglion fibres cross at optic chiasm
- crossed fibres originate from nasal retina and are responsible for temporal visual field
- uncrossed fibres originate from temporal retina and are responsible for nasal visual field
What do lesions anterior to the optic chiasm affect?
Affect visual field in one eye only (as fibres have not crossed) - affect temporal and nasal field
What do lesions at the optic chiasm affect?
- damages crossed ganglion fibres from nasal retina (controlling temporal field) in both eyes
- temporal field deficit in both eyes - bitemporal hemianopia
What do lesions posterior to the optic chiasm affect?
- affect visual field in both eyes
- right-sided lesion - left homonymous hemianopia in both eyes (cannot see left visual field both eyes)
- left-sided lesion - right homonymous hemianopia (cannot see right visual field both eyes)
Affects ipsilateral temporal retina (ipsilateral nasal field) and contralateral nasal retina (contralateral temporal field)
What usually causes bitemporal hemianopia?
Typically caused by enlargement of pituitary gland tumour (sits under optic chiasm)
What usually causes homonymous hemianopia?
Stroke (cerebrovascular accident) causing primary visual cortex damage –> contralateral homonymous hemianopia with macula sparing
Why does macular sparing occur in homonymous hemianopia from stroke?
Area representing the macula in primary motor cortex receives dual blood supply from posterior cerebral arteries from both sides
What is pupillary function?
Regulates light input to the eye (like a camera aperture)
What happens to pupillary function in light?
- pupil constriction - parasympathetic stimulation causes circular muscles to contract
- decreases glare
- increases depth of field (Near Response Triad)
- pupillary constriction mediated by parasympathetic nerve (within CN III)
What happens to pupillary function in the dark?
- pupil dilatation - sympathetic stimulation causes radial muscles to contract
- increases light sensitivity in the dark by allowing more light into the eye
- pupillary dilatation mediated by sympathetic nerve
Describe the afferent pathway of the pupillary reflex.
- a small sub-section of retinal ganglion cells participate in the pupillary reflex pathway
- pupil-specific ganglion cells exit at posterior third of optic tract before entering lateral geniculate nucleus
- afferent pathway from each eye synapses on Edinger-Westphal nuclei on both sides in the brainstem
Describe the efferent pathway of the pupillary reflex.
- oculomotor nerve efferent emerges from the Edinger-Westphal nucleus (where afferents from both sides synapse)
- this synapses at the ciliary ganglion
- a short posterior ciliary nerve emerges and goes to the pupillary sphincter
What is the direct vs consensual pupillary reflex?
- direct light reflex - constriction of pupil of the light-stimulated eye
- consensual light reflex - constriction of pupil of the other (fellow) eye
What is the neurological basis of the consensual light reflex?
Afferent pathway on either side alone will stimulate efferent pathway on both sides (as synapses on Edinger-Westphal nucleus on both sides)
What happens in a right afferent defect?
And what is an example of an 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
What happens in a left afferent defect?
- no pupil constriction in both eyes when left eye is stimulated with light
- normal pupil constriction in both eyes when right eye is stimulated with light
What happens in a right efferent defect?
And what is an example of an efferent defect?
- e.g. damage to right CN III
- no right pupil constriction whether right or left eye is stimulated with light
- left pupil constriction whether right or left eye is stimulated with light
What happens in a left efferent defect?
- no left pupil constriction whether left or right eye is stimulated with light
- right pupil constriction whether left or right eye is stimulated with light
What is the swinging torch test?
Light is shone rapidly between two eyes to test for relative afferent pupillary defect
What happens in a relative afferent pupillary defect?
(Damage to afferent pathway is usually incomplete/relative)
- partial pupillary response still present when damaged eye is stimulated
- elicited by swinging torch test - alternating stimulation of right and left eye with light
- both pupils constrict when light swings to undamaged eye
- both pupils paradoxically dilate when light swings to damaged side (not neurologically dilate, just in comparison to constriction)
What is eye movement necessary for?
Acquiring and tracking visual stimuli
How many extraocular muscles are there and what three cranial nerves are involved?
Eye movement facilitated by six extraocular muscles innervated by three cranial nerves (III, IV, VI)
What do the extraocular muscles attach?
Attach eyeball to orbit
What do the extraocular muscles allow?
Straight and rotary movement
What are the four straight extraocular muscles?
- superior rectus
- inferior rectus
- lateral rectus
- medial rectus
Where is the superior rectus attached to and what is the function?
- attached to the eye at 12 o’clock
- moves the eye up
Where is the inferior rectus attached to and what is the function?
- attached to the eye at 6 o’clock
- moves the eye down
Where is the lateral rectus (AKA external rectus) attached to and what is the function?
- attaches on the temporal side of the eye
- moves eye towards outside of head (abducts eye)
Where is the medial rectus (AKA internal rectus) attached to and what is the function?
- attaches on the nasal side of the eye
- moves eye towards middle of head (nose) - adducts eye
Where is the superior oblique and what is the function?
- attached high on the temporal side of the eye
- passes under superior rectus
- travels through the trochlea
- moves the eye in a diagonal pattern down and out
Where is the inferior oblique and what is the function?
- attached low on the nasal side of the eye
- passes over the inferior rectus
- moves the eye in a diagonal pattern up and out
What does the superior branch of the oculomotor nerve CN III innervate?
- superior rectus - elevates eye
- levator palpebrae superioris - raises eyelid
What does the inferior branch of the oculomotor nerve CN III innervate?
- inferior rectus - depresses eye
- medial rectus - adducts eye
- inferior oblique - elevates eye
- parasympathetic nerve - constricts pupil
What does the trochlear nerve CN IV innervate?
Superior oblique - depresses eye
What does the abducens nerve CN VI innervate?
Lateral rectus - abducts eye
What muscle do we isolate when testing abduction?
Lateral rectus
What muscle do we isolate when testing adduction?
Medial rectus
What muscle do we isolate when testing elevated and abducted?
Superior rectus
What muscle do we isolate when testing depressed and abducted?
Inferior rectus
What muscle do we isolate when testing elevated and adducted?
Inferior oblique
What muscle do we isolate when testing depressed and adducted?
Superior oblique
What happens in CN III palsy?
- muscles innervated by oculomotor nerve do not work leading to overaction of muscles innervated by CN IV and VI
- lateral rectus (CN VI) –> outward motion of eye
- superior oblique (CN IV) –> downward motion of eye
- ptosis (droopy eyelid) due to loss of levator palpebrae superioris action
- dilated pupil due to lack of parasympathetic stimulation
What happens in CN IV palsy?
Muscles innervated by trochlear nerve (superior oblique) do not work –> eye cannot look down/out, unopposed action of eye looking up and in
What happens in CN VI palsy?
Muscles innervated by abducens nerve (lateral rectus) do not work –> eye cannot look outwards (abduct), unopposed action of eye looking in (adducted)
