1B visual system Flashcards
What are the three layers of the coat of the eye?
- Sclera
- Choroid
- Retina
Describe the sclera and its function
- hard, tough and opaque
- High water content
- responsible for protecting the eye and maintaining its shape
Describe the retina and its function
- 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.
What is the uvea?
Vascular coat of eyeball and lies between sclera and retina
What are the three parts of the uvea?
- Iris
- Ciliary body
- Choroid
These 3 are intimately connected and a disease of one part also affects the other parts though not necessarily to the same degree
What is the retina?
- Very thin layer of tissue that lines inner part of 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 does the optic nerve do 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
What is the blind spot?
Where the optic nerve meets the retina there are no light sensitive cells- that’s the blind spot
Where is the macula and what does it do?
- 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
What is central vision?
- Detailed day vision and colour vision- fovea has highest conc of cone photoreceptors
- Used for reading and facial recognition
What is central power assessed with?
Visual acuity assessment
What does loss of foveal vision lead to?
Poor visual acuity
What is peripheral vision responsible for?
- shape
- movement
- navigation
- night vision
What is peripheral vision assessed with?
Visual field assessment
What would extensive loss of visual field cause?
Unable to navigate in environment, patient may need white stick even with perfect visual acuity
Describe the retinal structure
- Outer thin layer of retinal pigment epithelium situated in front of choroid
- Inner thicker layer called neuroretina
What is the neuroretina made up of?
- Outer layer of photoreceptors (1st order neurones): detection of light
- Middle layer of bipolar cells (2nd order neurones): Local signal processing to improve contrast sensitivity and regulate sensitivity
- Inner layer of retinal ganglion cells (3rd order neurones): transmission of signal from eye to brain
What are the 2 types of photo receptors?
- Rods
- Cones
What is the purpose and properties of rods?
- 100x more sensitive to light than cones
- Slow response to light
- Responsible for night vision (scotopic vision)
- We have 120 million rods per eye
- There is the highest concentration of rods in the periphery of the eye
What is the purpose and properties of cones?
- Less sensitive to light but faster response
- Responsible for daylight fine vision and colour vision (photopic vision)
- We have 6 million cones per eye
What is the distribution of cone cells?
There is the highest conc in the centre of the eye
What about rod cells and colour?
Rods are used for night vision and spatial recognition and aren’t really sensitive to any particular colour
Define refraction
As light goes from one medium to another, the velocity changes
This causes the light to bend- if an object is seen through water, it is not directly in a straight line path but its image appears that way- the actual object is on either side of the image being viewed
What is a medium’s index of refraction?
The ratio of the speed of light in a vacuum to its speed in the medium
The denominator will always be smaller and produce a value ≥1
What happens to light when it reaches a new medium?
- Some is reflected off the boundary
- Some is refracted through the boundary
- Angle of incidence = angle of reflection
What are the 2 types of lens?
- Converging lens (convex)- takes light rays and brings them to a point
- Diverging lens (concave)- takes light rays and spreads them outwards
What is emmetropia?
- When there’s 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
- Where parallel light rays don’t fall on the retina (no accommodation)
What is the near response triad and what is it used for?
It’s the eye’s adaptation for near vision. Consists of:
- Pupillary miosis (sphincter pupillae) to increase depth of field
- Convergence (medial recti from both eye) 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 40 years of age
- Distant vision is intact
How is presbyopia corrected?
By reading glasses (convex lenses) to increase refractive power of the eye
Describe the visual pathway from eye to visual cortex
- Eye
- Optic nerve- ganglion nerve fibres
- Optic chiasm (half the nerve fibres cross here)
- Optic tract (ganglion nerve fibres exit as optic tract)
- Lateral geniculate nucleus (in thalamus) where ganglion nerve fibres synapse
- Optic radiation (4th order neuron)
- Primary visual cortex or striates cortes- within occipital lobe
Describe the optic chiasm and where fibres will go
- 53% of ganglion fibres cross at the optic chiasm
- Crossed fibres originate in nasal retina and are responsible for temporal visual field
- Uncrossed fibres originate from the temporal retina and are responsible for nasal visual field
What do lesions at the chiasm do?
Lesions at the optic chiasm damages ganglion fibres from nasal retina in both eyes causing temporal field deficit in both eyes- bitemporal hemianopia
What do lesions anterior to the chiasm do?
Lesions anterior to optic chiasm affect visual field in 1 eye only
Lesions posterior to optic chiasm affect visual field where?
Both eyes
What does this diagram show?
The blue and grey on the left just shows the pathways from eye to each side of brain
Blue and grey on right shows what works (blue) and what doesn’t (grey)
What usually causes bitemporal hemianopia?
Typically caused by enlargement of pituitary gland tumour which sits under the optic chiasm
What can cause homonymous hemianopia?
Damage to Primary Visual Cortex- usually a stroke (cerebrovascular accident) causing primary visual cortex damage
Leads to contralateral homonymous hemianopia with macula sparing
Why does macular sparing occur?
Area representing the macula 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 the pupillary function in light?
- Pupil constricts due to parasympathetic nerve causing circular muscles to contract
- Increases depth of field
- Decreases glare
What happens in the dark for the pupillary function?
- Pupil dilates due to sympathetic stimulation of radial muscles causing them to contract
- Increases light sensitivity in the dark by allowing more light into eye
Describe the afferent pathway of the pupillary reflex
- Rod and cone photoreceptors synapsing on bipolar cells synapsing on retinal ganglion cells
- Pupil specific ganglion cells exit at posterior third of optic tract before entering the 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
- This synapses at the ciliary ganglion
- A short posterior ciliary nerve emerges and goes to the pupillary sphincter
What is the direct vs consensual reflex?
- Direct light reflex- constriction of the pupil of the eye that’s been stimulated by light
- Consensual light reflex- constriction of pupil of the other eye
Why does the direct vs consensual reflex happen?
Afferent pathway on either side along will stimulate the efferent pathway on both sides
What happens in a 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
What happens in a right efferent defect?
- 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
What happens in a unilateral afferent defect?
Different response depending on which eye is stimulated
What happens in a unilateral efferent defect?
Same unequal response between left and right eye irrespective which eye is stimulated
What is the swinging torch test?
- When light is shone rapidly between 2 eyes
- If there’s a relative afferent pupillary defect:
- There’s partial pupillary response when damaged eye is stimulated
- Both pupils constrict when light swings to undamaged side (left here)
- Both pupils paradoxically dilate when light swings to the damaged side (right here)
How many extraocular muscles are there and innervated by which CN?
6 extraocular muscles innervated by 3 CN
What are the 6 extraocular muscles that control eye movement?
- Attach eyeball to orbit
- Straight and rotary movement
Four straight muscles:
- Superior rectus
- Inferior rectus
- Lateral rectus
- Medial rectus
Which nerves innervate eye movement?
- Oculomotor (III)
- Trochlear (IV)
- Abducens (VI)
What does the oculomotor nerve innervate and describe the purpose of the muscles?
- Superior branch
- Superior rectus- elevates eye
- Levator palpebrae superioris- raises eyelid
- Inferior branch
- Inferior rectus- depresses eye
- Medial rectus- adducts eye
- Inferior oblique- elevates eye
- Parasympathetic nerve- constricts pupil
What does CN IV innervate and what’s the role of those muscles?
Superior oblique- depresses eye
What does CN VI innervate and what is the role of the muscles?
Lateral rectus: abducts eye
Where is the superior rectus and what does it do?
- Attached to the eye at 12 o’clock
- Moves the eye up.
Where is the inferior rectus and what does it do?
- Attached to the eye at 6 o’clock
- Moves the eye down.
Where is the lateral rectus and what does it do?
- 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)
Where is the medial rectus and what does it do?
- 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)
Where is the superior oblique and what does it do?
- 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
Where is the inferior oblique and what does it do?
- Attached low on the nasal side of the eye.
- Passes over the Inferior Rectus.
- Moves the eye in a diagonal pattern - up and out.
