Neurology of the visual system Flashcards
What is the complete visual pathway?
- Eye
- Optic nerve – ganglion nerve fibres
- Optic chiasm – half of fibres decussate here
- Optic tract – ganglion fibres exit as optic tract
- Lateral Geniculate Nucleus – ganglion fibres synapse in nucleus
- Optic radiation – 4th order neuron
- Primary visual cortex (lower visual processing) or striate cortes -> extrastriate cortex (higher processing)
What is the visual pathway from the retina?
- First order neurones (R+C)
- Second order neurones (retinal bipolar)
- Third order neurones (retinal ganglion cells)
- Optic nerve
- Partial decussation at chiasmd (53% of fibre)
- Optic tract
- Synpase at lateral geniculate nucleus in thalamus
What happens to retinal ganglion fibres to improve signal transmission?
They become myelinated after entering the optic nerve
What is the receptive field of a neurone?
Retinal space within which incoming light can alter the firing pattern of a neuron.
Compare the receptive fields of a single photoreceptor compared to a ganglion cell
In the case of a single photoreceptor, any light that falls within its immediate neighbourhood on the retina, will excite the photo-receptor.
The receptive field of ganglion cells covers a much larger area than that of a single photoreceptor.
It includes the receptive fields of all the photoreceptors, that synapse upon the ganglion cells indirectly via bipolar cells.
What is convergence?
Number of lower order neurons synapsing on the same higher order neuron.
Compare cone and rod system convergence
Compare central retinal and peripheral retinal convergence
Cone system convergence < Rod system convergence
Central retina convergence < peripheral retinal convergence
What does a low convergence mean?
- effect on receptive field, visual acuity and light sensitivity
- Small Receptive Field
- Fine Visual Acuity
- Low Light Sensitivity
What does a high convergence mean?
- effect on receptive field, visual acuity and light sensitivity
- Large Receptive Field
- Coarse Visual Acuity
- High Light Sensitivity
What are on and off centre ganglion cells?
On-centre Ganglion: Stimulated by light at the centre of the receptive field. Inhibited by light on the edge of the receptive field
Off-centre Ganglion: Inhibited by light at the centre of the receptive field. Stimulated by light on the edge of the receptive field.
What are on and off centre ganglion cells important for?
Contrast sensitivity and enhanced edge detection
How will lesions before and after the optic chiasm differ in their effects on vision?
- Lesions anterior to optic chiasm affect visual field in one eye only
- Lesions posterior to optic chiasm affect visual field in both eyes
Which are the crossed and uncrossed fibres at the optic chiasm?
Crossed Fibres – originating from nasal retina, responsible for temporal visual field
Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field
What will lesions at the optic chiasm cause?
- Damages crossed ganglion fibres from nasal retina in both eyes
Temporal field deficit in both eyes – bitemporal hemianopia
What will lesions posterior to the optic chiasm cause?
Right sided lesion – left homonymous hemianopia in both eyes
Left sided lesion – right homonymous hemianopia (half eye blindness) in both eyes
What are some visual defects?
- monocular blindness (one eye)
- bitemporal heminanopia
- right nasal hemianopia
- homonymous hemianopia
- quadrantanopia
- macular sparing (macula - central - vision spared)
What causes bitemporal hemianopia and homonymous hemianopia?
Bitemporal Hemianopia
- Typically caused by enlargement of pituitary gland tumour pressing on optic chiasm from below
(pituitary gland sits under optic chiasm)
Homonymous Hemianopia
- Stroke
- Cerebrovascular accidents
How is vision loss different in glaucoma and neurological conditions?
Glaucoma often affects the horizontal plane of loss of vision
Neurological problems often affect the vertical plane of loss of vision
Where is the primary visual cortex?
- Situated along calcarine sulcus within occipital lobe
- Also known as striate cortex
- Characterized by a distinct stripe derived from the myelinated fibre of the optic radiation projecting into the visual cortex
What is represented within the visual cortex?
- Disproportionately large area representing the macula
- Superior visual field projects to below the calcarine fissure
- Inferior visual field projects to above the calcarine fissure
- The right hemifield from both eyes projects onto left PVC
- The left hemifield from both eyes projects onto right PVC
What is the role of the PVC?
Processing visual information of static and moving objects
How is the PVC organised?
- Organized as columns
- Each column is sensitive to a visual stimulus of a particular orientation
- Right eye and left dominant columns intersperse each other
What is macular sparing homonymous hemianopia?
- Caused by damage to primary visual cortex
- Often due to stroke
- Leads to contralateral MSHH
- Macula is spared as it has dual blood supply from the L/R posterior cerebral arteries
Where and what is the extrastriate cortex?
- Area around PVC within the occipital lobe
- Converts basic visual information, orientation and position into complex information
What is the dorsal pathway (how pathway) in the PVC?
What does it detect?
What does damage lead to?
- PVC -> posterior parietal cortex
- Motion detection, visually-guided action
- Damage results in motion blindness
What is the ventral pathway (the what pathway) in the PVC?
What does it detect?
What does damage lead to?
- PVC -> inferiotemporal cortex
- Object representation, face recognition
- Detailed fine central vision and colour vision
- Damage may result in cerebral achromatopsia (partial/total loss of colour vision)
What happens to the eye in the light?
- constricts due to the circular iris muscle contracting:
- decreases spherical aberrations and glare
- increases depth of field and reduces amount of light entering the eye
- reduces bleaching of photopigments
PUPIL CONSTRICTION BY CN 3 PNS
What happens to the eye in the dark?
- pupil dilates: radial muscle contracts
- more light into eye
- sympathetic nerve
What is the afferent pathway in the pupillary reflex?
Rod and cone photoreceptor -> bipolar cells -> RGCs -> nerves exit at posterior 1/3rd of optic tract and enter LGN -> synapse at brainstem (Pretectal nucleus) -> synapse at Edinger Westphal nucleus
What is the efferent pathway in the pupillary reflex?
Edinger Westphal -> oculomotor nerve efferent -> synapse at ciliary ganglion -> short posterior ciliary nerve -> pupillary sphincter (BOTH EYES)
What is the direct light reflex?
constriction of pupil of the light stimulated eye
What is the consensual light reflex?
constriction of pupil of the other eye
What happens if there is a right afferent defect e.g. optic nerve damage?
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 if there is a right efferent defect e.g. right oculomotor nerve damage?
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 is the swinging torch test?
- Done to demonstrate weakness of the afferent pathway
- The damage is usually incomplete or relative
What will happen in the swinging torch test if there is a relative afferent pupillary defect in the right eye?
- 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
What is duction, version, vergence and convergence?
Duction – eye movement in one eye
Version – eye movement in both eyes in the same direction, dextroversion is to the right, levo- is to the left
Vergence – eye movement in both eyes in opposite directions
Convergence – simultaneous adduction (inwards) movement in both eyes when viewing a near object
What is saccade and smooth pursuit?
Saccade – short, fast burst of movement (900deg/second) – reflexive saccade, scanning saccade, predictive saccade and memory-guided saccade
Smooth pursuit – sustained slow movement (60deg/second) – driven by motion of a moving target across the retina
What are the 6 extraocular muscles?
- Attach eyeball to orbit
- straight and rotatory movements
- 4 straight: superior, inferior, medial and lateral rectus
- superior and inferior oblique muscles
Where are the superior and inferior rectus attached and what do they do?
Superior rectus Attached to the eye at 12 o’clock, moves the eye up and adduct
Inferior rectus Attached to the eye at 6 o’clock, moves the eye down and adduct
Where is the lateral rectus attached and what does it do?
- It attaches on the temporal side of the eye
- It moves the eye toward the outside of the head (toward the temple
Where is the medial rectus attached and what does it do?
- It attaches on the nasal side of the eye
- It moves the eye toward the middle of the heard (towards the nose)
Where is the superior oblique muscle attached and what does it do?
What does it pass under and what is it innervated by?
- Attached high on the temporal side of the eye
- Passes under the superior rectus
- Moves the eye down and in
- Trochlea
Where is the inferior oblique attached, what does it do, what does it pass and what is it innervated by?
- Attached low on the nasal side of the eye
- Passes over the inferior rectus
- Moves the eye up and out
- Occulomotor innervation
What innervates the superior and inferior rectus muscles?
superior - superior occulomotor nerve
inferior - inferior occulomotor
What innervates the medial rectus?
inferior occulomotor
What innervates lid levator?
superior occulomotor
What innervates the inferior oblique?
inferior occulomotor
What innervates the superior oblique?
fourth cranial nerve
What innervates the lateral rectus?
sixth cranial nerve
How can the eye movements be tested?
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
What is supraversion?
The elevation of both eyes, namely simultaneous right and left eye supraduction.
What is infraversion?
Depression of both eyes, namely simultaneous right and left eye infraduction.
What is abduction and adduction?
Abduction refers to duction movement of one eye, moving away from the nose. Adduction refers to duction movement of one eye, moving towards the nose.
What is dextroversion and levoversion?
Dextroversion involves simultaneous right eye abduction, and left eye adduction.
Levoversion involves simultaneous left eye abduction, and right eye adduction.
What is supraduction and supraversion?
Up (Elevation): Supraduction – one eye
Supraversion – both eyes
What is torsion?
rotation of eye around the anterior-posterior axis of the eye
What is third nerve palsy?
Only muscles not innervated by CN III in the affected eye are working
- Lateral rectus muscle and superior oblique muscle
- Affected eye down and out
- Droopy eyelid (ptosis)
- Unopposed superior oblique innervated by fourth nerve (down)
- Unopposed lateral rectus action innervated by sixth nerve (out)
What is sixth nerve palsy?
- Sixth nerve palsy presents with deficit in abduction in the affected eye
- This is made apparent when the patient is asked to abduct the affected eye
- The affected eye deviates inwards (unable to abduct)
- Double vision worsen on gazing to the side of affected eye
What is optokinietic nystagmus reflex?
Oscillatory eye movement
- smooth and fast paced movement between two points
What is optokinietic nystagmus useful for?
To test visual acuity in pre-verbal children by observing presence of nystagmus movement
- reflex is physiological and shows they can percieve motion
