12 - Neurology of the visual system Flashcards
What are the main landmarks of the visual pathway?
Eye Optic Nerve Optic Chiasm Optic Tract Lateral Geniculate Nucleus Optic Radiation Primary Visual Cortex (Striate Cortex)
What is the visual pathway?
Where does it transmit to in the brain - anatomically, where is this?
the neurological pathway, where by vision is converted to neurological impulses, to be transmitted from the eye to the visual cortex, the posterior part of the brain
Where do retinal ganglion axons coming down the optic nerve synapse?
Lateral Geniculate Nucleus
Where is the lateral geniculate nucleus found?
Thalamus
What are the fibres leaving the lateral geniculate nucleus called?
Optic Radiation
Which order neurones are the optic radiation and where do they terminate?
4th Order Neurones
They terminate in the primary visual cortex
What are the first, second and third order neurones in the visual pathway?
First Order – photo-receptors (rods and cones)
Second Order – bipolar cells
Third Order – retinal ganglion cells
What happens as the retinal ganglion cells enter the optic nerve, which improves the transmission of the signal?
They become myelinated
What percentage of retinal ganglion cell fibres crosses the midline at the optic chiasma?
Around half (53%)
Describe the convergence and receptive field sizes of rods and cones.
Rods have high convergence and large receptive fields
Cones have low convergence and small receptive fields
Describe how the convergence of the rod system differs across different parts of the retina.
The rod system near that macula has lower convergence than in the peripheral retina
What is the benefit of having high convergence and a large receptive field?
Which type of photoreceptor does this relate to?
What is the disadvantage of these types of photoreceptors?
High light sensitivity - reacting to rods
BUT have a coarse visual acuity
What is the benefit of having low convergence and a small receptive field?
Which type of photoreceptor does this relate to?
What is the disadvantage of these types of photoreceptors?
Fine visual acuity - relating to cones
BUT have low light sensitivity
Retinal ganglion cells can be described as on-centre or off-centre. What do these two terms mean?
On-centre - they are stimulated by light falling on the centre of the receptive field and inhibited by light falling on the edge of the receptive field
Off-centre - they are stimulated by light falling on the edge of the receptive field and inhibited by light falling on the centre
This is important in contrast sensitivity and enhanced edge detection
Where do the fibres that decussate at the optic chiasma originate?
Where do fibres that don’t decussate at the optic chiasm originate?
The nasal part of the retina
These fibres are responsible for the temporal half of the visual field
Originate from temporal retina.
They are responsible for nasal visual field.
What effect do lesions anterior to the optic chiasm have on vision?
Affects only ONE eye
What effect do lesions posterior to the optic chiasm have on vision?
Affects BOTH eyes
Right-sided lesion: left homonymous hemianopia
Left-sided lesions: right homonymous hemianopia
What is the effect of a lesion at the optic chiasm?
Bitemporal hemianopia
What is a bitemporal hemianopia typically caused by?
enlargement of the pituitary gland tumour
What is homonymous hemianopia typically caused by?
Strokes and other cerebrovascular accidents
Give another name for the primary visual cortex and state where it is located?
Striate cortex
Located along the Calcarine Fissure in the occipital lobe
Describe which parts of the primary visual cortex are responsible for the different fields of vision.
- The left primary visual cortex is responsible for the right visual field from both eyes
- The right primary visual cortex is responsible for the left visual field from both eyes
- Visual cortex above the calcarine fissure is responsible for the inferior visual field
- Visual cortex below the calcarine fissure is responsible for the superior visual field
How is it possible for the macula to be spared by a stroke in the primary visual cortex leading to homonymous hemianopia?
The area representing the macula in the primary visual cortex has a dual blood supply from both right and left posterior cerebral arteries so, it is less vulnerable to ischaemia
What is the extrastriate cortex?
The Primary Visual Cortex (striate cortex) relays visual information to the Extra-striate Cortex, a region adjacent to the Primary Visual Cortex, for further higher visual processing.
- Area around PVC within the Occipital Lobe
- Converts basic visual information, orientation and position into complex information
What are the two pathways of the extrastriate cortex and what are they responsible for?
Dorsal Pathway – deals with motion detection
Ventral Pathway – handles detailed object recognition and face recognition
Describe the dorsal pathway, what is it responsible for and what damage to to will cause
- Primary Visual Cortex -> Posterior Parietal Cortex
- function in Motion Detection and Visually-Guided Action
- Damage results in Motion Blindness
Describe the ventral pathway, what is it responsible for and what damage to to will cause
- Primary Visual Cortex -> Inferiotemporal Cortex
- function in Object Representation, Face Recognition and detailed fine central and colour vision
- Damage may result in Cerebral Achromatopsia
Describe what happens to the eyes in the light.
- Iris circular muscle contracts
- Constriction of pupillary aperture
- Reduced rate of photopigment bleaching
- Increased depth of field
Describe what happens to the eyes in the dark.
- pupil dilation due to radial muscle contraction
- increased light sensitivity
Describe the Afferent pathway for the pupillary reflex
• Photoreceptors synapse on Bipolar Cells, which synapse on Retinal Ganglion Cells - they send axons via the optic nerve
• Pupil-specific ganglion cells exit at posterior third of optic tract before entering the LGN
(fibres that are responsible for the pupillary reflex will get passed the optic chiasm and then leave the posterior 1/3 of the optic tract before it reaches the LGN)
• Synpases at Brain Stem (Pretectal Nucleus)
• Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
Describe the efferent pathway for the pupillary reflex
A parasympathetic nerve from the Edinger-Westphal nuclei to the ciliary ganglion forms the efferent pathway
Short ciliary nerves travel from the ciliary ganglion to the pupillary sphincter
Summarise the pathway that is responsible for the consensual light reflex (afferent and efferent)
Retinal Ganglion Cell -> Pretectal Nucleus -> Edinger-Westphal Nucleus -> Ciliary Ganglion -> Short Ciliary Nerves -> Sphincter Pupillae
What would the consequences be of a right afferent defect?
Light shone in right eye: no direct or consensual response
Light shone in left eye: direct and consensual response present
What would the consequences be of a right efferent defect?
Light shone in right eye: no direct response, consensual response present
Light shone in left eye: direct response, no consensual response
What does RAPD mean?
Relative Afferent Pupillary Defect
A partial pupillary response is still present despite damage to an eye and its pupillary reflex pathway – there is some degree of constriction
What test would you do to identify RAPD? What would you expect to see in a patient showing a RAPD?
Swinging Torch Test
When the light is shone on the good eye, there will be a direct and consensual response
When the light is then swung and shone at the bad eye, there will be a paradoxical dilation of the iris in the bad eye
This is because the constriction response elicited by the bad eye is weaker than the consensual response elicited by the good eye
Define duction
Movement of one eye
Define version
Simultaneous movement of both eyes
define vergeance
Simultaneous movement of both eyes in opposite directions
define convergeance
Simultaneous adduction of both eyes when viewing a near object
What are the two types of eye movement and how are they different?
Saccade – short fast burst
Smooth Pursuit – sustained slow movement
Name the 6 extra ocular muscles
Where do they originate from?
4 rectus muscles: - Superior rectus - Inferior rectus - Lateral rectus - Medial rectus 2 oblique muscles: - superior oblique - inferior oblique
5 of the 6 muscles come out of a cone from the back of the orbit. The inferior oblique comes in nasally.
What is the term for the elevation of one eye?
Supraduction
What is the term for the depression of both eyes?
Infraversion
Where are the vertical rectus muscle attached? How do they move the eye?
attach anterior to the globe equator, and pull backwards and nasally
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.
Where is the lateral rectus attached? How does it move the eye?
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? How does it move the eye?
It attaches on the nasal side of the eye
It moves the eye toward the middle of the heard (towards the nose)
Describe the attachment and the path of the superior oblique muscle
How does it move the eye?
- Attached high on the temporal side of the eye.
- Passes under the Superior Rectus.
- Moves the eye in a diagonal pattern – down and in.
- Travels through the trochlea
Describe the attachment and the path of the inferior oblique muscle
How does it move the eye?
- Attached low on the nasal side of the eye
- Passes over the Inferior Rectus
- Moves the eye in a diagonal pattern – up and out
State which nerve innervates each of the extrinsic eye muscles.
Lateral Rectus = Abducens (CN VI)
Superior Oblique = Trochlear (CN IV)
Medial Rectus, Superior Rectus, Inferior Oblique, Inferior Rectus and Levator Palpebrae Superioris = Oculomotor (CN III)
Where do all the rectus muscles originate?
Common tendinous ring at the apex of the orbit
Where do the rectus muscles insert?
Into the sclera anterior to the globe equator
The oculomotor nerve has two branches. State what each of these branches innervates.
Superior Oculomotor Nerve • Superior Rectus • Levator Palpebrae Superioris Inferior Oculomotor Nerve • Inferior Rectus • Medial Rectus • Inferior Oblique • Parasympathetic nerve that causes pupil constriction
How would you test the extraocular muscles?
Isolate the muscle to be tested by maximising its action and minimising the action of the other muscles
E.g. to test the superior rectus, make the patient abduct and elevate their eye
Define supraversion
The elevation of both eyes
namely simultaneous right and left eye supraduction.
Define infraversion
Depression of both eyes
namely simultaneous right and left eye infraduction.
What is dextroversion?
right gaze
involves simultaneous right eye abduction, and left eye adduction
What is levoversion?
left gaze
involves simultaneous left eye abduction, and right eye adduction
What is torsion?
rotation of eye around the anterior-posterior axis of the eye
Describe and explain what you would see in a patient with 3rd nerve palsy.
- affected eye pointing down and out
due to the unopposed contraction of lateral rectus and superior oblique - Ptosis – loss of innervation of levator palpebrae superioris
Describe and explain what you would see in a patient with 6th nerve palsy.
- deficit in abduction in the affected eye
This is made apparent when the patient is asked to abduct the affected eye, eye stops around the midline
This is because the lateral rectus isn’t functioning and can’t abduct the eye - affected eye deviates inwards (unable to abduct)
- Double vision worsen on gazing to the side of affected eye
What is Optokinetic Nystagmus?
Form of physiological nystagmus (oscillatory eye movement), triggered by the presentation of a constantly moving grating pattern.
Smooth Pursuit + Fast Phase Reset Saccade
What reflex is used to assess visual acuity in preverbal children?
- may want to reword
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
