Neurology of the visual system Flashcards
Summarise the visual pathway
Visual pathway: transmit signal from eye to visual cortex
Optic nerve: contains ganglion nerve fibres
Optic chiasm: half of fibres cross here
Optic tract: ganglion nerve fibres exit as tract
Lateral geniculate nucleus: fibres synapse at LGN
Optic radiation: 4th order neurones that connect to primary visual/striate cortexes (occipital lobe)
Also the extra-striate cortex
Describe the visual pathway in the 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
What is important to remember about the optic tract
Once past the optic chiasm, the ganglion cell axons on each side form the optic tract. Thus, the optic tract, unlike the optic nerve, contains fibres from both eyes.
What is the destination of the optic tract
• Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex
Describe the purpose of the decussation at the optic chiasm
The partial decussation of the ganglion cell axons at the optic chiasm allows information from corresponding points on two retinas to be processed by approximately the same cortical site in each hemisphere.
Where is the optic nerve formed
Ganglion cell axons exit the retina through a circular region in its nasal part called the optic disk (or optic papilla), where they bundle together to form the optic nerve.
Axons in the optic nerve run a straight course to the optic chiasm at the base of the diencephalon.
Why do we need a receptive field
Photoreceptors firing all the time
So we need an on/off system
On- this photoreceptor stimulated- surrounding ganglion cells inhibited- but the retinal ganglion in light path stimulated
Off- some ganglion cells off- surroundings on
Allows us to perceive the contrast and shape of object- according to which part of hit.
Summarise what is meant by a receptive field
- Receptive Field of a Neuron –Retinal space within which incoming light can alter the firing pattern of a neuron
- Photoreceptor –a small circular space surrounding the photoreceptor
- Retinal Ganglion Cell – Input from neighbouring photoreceptors (convergence
Describe the convergence of the receptive field
• Convergence –Number of lower order neurons field synapsing on the same higher order neuron
• Cone System Convergence > Rod System Convergence
• Central Retina Convergence > Peripheral Retina Convergence
• Low Convergence – Small Receptive Field – Fine Visual Acuity – Low Light Sensitivity
• High Convergence – Large Receptive Field – Coarse Visual Acuity – High Light Sensitivity
This is why visual acuity is greatest at the fovea (one retinal ganglion cell synapses with one bipolar cell)
Further away from fovea- more photoreceptors feed into retinal ganglion cells
Also why you it’s easier to spot movement in the periphery- as the light will be detected by many photoreceptors which then synapse with many more ganglion cells- whereas in central vison- the object has to move from one photoreceptos to the next to be recognised as movement.
What can retinal ganglion cells be divided into and why is this division important
Retinal Ganglion Cells –can be divided into On-Centre and Off Centre
Important for – Contrast Sensitivity – Enhanced Edge Detection
We know something is happening at that point- because nothing is happening around it (relative difference between photoreceptors that are firing ant those that aren’t).
Describe on centre ganglion cells
– stimulated by light at the centre of the receptive field – Inhibited by light on the edge of the receptive field
Describe off centre ganglion cells
– Inhibited by light at the centreof the receptive field
– Stimulated by light on the edge of the receptive field
Why is the visual system complicated
Need x,y,z perception
Colour perception
Depth perception
Movement perception (peripheral and central)
Summarise the 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 Fibrescross at Optic Chiasma
Where do the crossed and uncrossed fibres of the optic chiasma originate from
- Crossed Fibres–originating from nasal retina, responsible for temporal visual field
- Uncrossed Fibres–originating from temporal retina, responsible for nasal visual field
Where is the right visual hemisphere processed
For example, the right visual hemisphere is processed in the left visual cortex, but is constructed from the temporal (outside) portion of the left retina and the nasal (inside) portion of the right eye (Fig. 8.12). Clearly the nasal fibres will need to cross over so they can project onto the contralateral thalamus; this process occurs in the optic chiasm, which lies in front of the pituitary stalk
Summarise our central visual field
The central portion of the visual world can be viewed with both eyes separately. This corresponds to the binocular region and explains how we can see, for instance, a small nasal portion of the left hemifield when we close our left eye
Where will inferior and superior meridians hit the retina
Inferior meridians will hit the top of the retina
Superior meridians will hit the bottom of the retina
Like a camera
So all images of the retina will be inverted and upside down.
So if image hits nasal part of retina- person will see it on temporal side- why decussation is important.
Describe our visual field
Each retina and its corresponding visual field are divided into quadrants. In this scheme, vertical and horizontal lines that intersect at the centre of the fovea, subdivide the surface of the retina.
The vertical line divides the retina into temporal and nasal divisions, and the horizontal line divides the retina into superior and inferior divisions.
Corresponding vertical and horizontal meridians intersect at the point of fixation (the point of visual space that falls onto the fovea) and define the quadrants of the visual field.
Describe the crossing of light rays diverging from different points on an object
The crossing of light rays diverging from different points on an object at the pupil cause the images of objects in the visual field to be inverted and left-right reversed on the retinal surface. As a result, objects in the temporal part of the visual field are seen by the nasal part of the retina, and objects in the superior part of the visual field are seen by the inferior surface of the retina.
Compare the binocular and monocular visual fields
With both eyes open, the two foveas normally align on a single target in visual space, causing the visual fields of both eyes to overlap extensively. This binocular field consists of two symmetrical visual hemidfields (left and right). The left binocular hemifield includes the nasal visual field of the right eye and the temporal visual field of the left eye.
The temporal visual fields are more extensive than the nasal visual fields, reflecting the sizes of the nasal and temporal visual fields respectively.
As a result, vision in the periphery of the field of view is strictly monocular, mediated by the most medial portion of the nasal retina (remember temporal retina is responsible for nasal vision)
Describe how the shape of the face and nose affect the extent of the region of binocular vision
In particular, the inferior nasal visual fields, and consequently the binocular field of view is smaller in the lower visual field than the upper.
Describe visual field defects at the level of the optic chiasma
• Lesion at Optic Chiasma
– Damages crossed ganglion fibres from nasal retina in both eyes
– Temporal Field Deficit in Both Eyes –Bitemporal Hemianopia
Describe visual field defects posterior to the level of the optic chiama
• Lesion Posterior to Optic Chiasma
– Right sided lesion –Left Homonymous Hemianopia in Both Eyes (can’t see nasal vision of right eye or temporal vision of left eye)
– Left sided lesion –Right Homonymous Hemianopia in Both Eyes (can’t see nasal vision of left eye or temporal vision of right)
Summarise bitemporal hemianopia
– Typically caused by enlargement of Pituitary Gland Tumour
– Pituitary Gland sits under Optic Chiasma
Nasal fibres can’t cross- so lose temporal vision on both eyes
Summarise homonymous hemianopia
– Stroke (Cerebrovascular Accident)
Macular sparing homonymous hemianopia: damage to primary visual cortex due to stroke, leading to contralateral homonymous hemianopia - macular area supplied by PCAs so spared
All fibres coming from the right (and so represented on the left cortex) travel in one tract now
Describe visual lesions anterior the optic chiasm
monocular blindness
Summarise visual field defects
Visual field defects: defects that don’t cross horizontal lines tend to be within the eye (ganglion cell or optic neuritis, glaucoma, or eyelid low- peripheral), defects that don’t cross vertical lines tend to be neurological (bitemporal hemianopia)
Describe right nasal hemianopia
External part of chiasm affected
Can’t see nasal vision of right eye
Binasal visual defect uncommon- unlikely to have compressing mass on both hemispheres.
Describe quadrantanopia
Fibres start spreading out- to reach the right portion of occipital cortex
Further back lesion is- smaller defect is
Tract widens- so bigger insult needed to effect all of the tract- so smaller homonymous quadrantanopia.
Describe injuries at the level of the occipital cortex
Macula sparing homonymous hemianopia- central vision spared- biggest representation on occipital cortex.
Unless the injury is massive.
Where is the primary visual cortex found, describe its key properties
• 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
Describe the representation of images in 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
– Right Hemifield from both eyes projects to Left Primary Visual Cortex
– Left Hemifield from both eyes projects to Right Primary Visual Cortex
Summarise the organisation of the visual cortex
– Organized as columns with unique sensitivity to visual stimulus of a particular orientation
– Right eye and left dominant columns intersperse each other
Different coordinates in each eye- will arrive at coordinates close to each other- allowing us to judge depth perception.
Left, right, left, right (left hemisphere)
Right, left, right, left (right hemisphere)
Area activated depending on which eye the light comes from- different areas in each hemisphere stimulated when seeing image
Describe the causes of macular sparing homonymous hemianopia
• 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 side
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) meaning that it is less vulnerable to ischaemia
How can occlusion of the central retinal artery lead to macular sparing vision
Occlusion of the central retinal artery is a medical emergency- need to thrombolyse the patient and give blood thinners to dissolve the clot- or can remove clot with massage- if not treated-patient will be blind within hours.
However- macular vision may be spared- some patients have pseudo-retinal artery which enters globe of eye just behind central retinal artery to supply the macula
Describe the extrastriate cortex
– Area around Primary Visual Cortex within the Occipital Lobe
– Converts basic visual information, orientation and position into complex information
Describe the dorsal pathway of the extrastriate cortex
– Primary Visual Cortex -> Posterior Parietal Cortex
– Motion Detection – Visually-Guided Action – Damage results in Motion Blindness
Allows for muscle memory
Describe the ventral pathway of the extra-striate cortex
– Primary Visual Cortex -> InferiotemporalCortex – Object Representation, Face Recognition
– Detailed fine central vision and colourvision
– Damage may result in Cerebral Achromatopsia (loss of the ability to see the world in colour)
What can the pupillary reflex tell you a lot about
the extra-cortical part of the brain
What is the key function of the pupil
Regulates light input to the eye (but less than 2 log unit change) like a Camera Aperture
Describe the pupillary response to 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 parasymaptheticnerve (within CN III)
Autonomic nerves surround the eye
Describe the pupillary response to dark
pupil dilatation – increases light sensitivity in the dark by allowing more light into the eye – pupillary dilatation mediated by sympathetic nerve
Describe the afferents in the pupillary reflex pathway
– 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
– Synpases at Brain Stem (Pretectal Nucleus)
– Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
Neurones from nasal retina of each eye cross
Pretectum lies between thalamus and midbrain
Neurones from temporal retina remain ipsilateral.
Describe the efferents in the pupillary reflex
– Edinger-Westphal Nucleus -> OculomotorNerve Efferent -> – Synapses at Ciliaryganglion -> – Short Posterior CiliaryNerve -> Pupillary Sphincter
Edinger-Westphal nucleus is a small group of nerve cells that lies close to the nucleus of the oculomotor nerve in the midbrain. The Edinger-Westphal nucleus contains the preganglionic parasympathetic neurones that send their axons via the oculomotor nerve to terminate on neurones in the ciliary ganglion.
Compare the direct response to the consensual response
- Direct Light Reflex – Constriction of Pupil of the lightstimulated eye
- Consensual Light Reflex –Constriction of Pupil of the fellow (other) eye- neurones from pretectal nucleus project bilaterally to each Edinger-Westphal nucleus.
Describe the neurological basis of the consensual response
• Neurological Basis – Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
Bilateral projection from the retina to the pretectum and bilateral projections from the pretectum to the Edinger-Westphal nucleus.
Describe a right afferent 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 (efferent pathways intact)
Could also be damage to the right retina.
Describe a right efferent defect
• 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
Afferent pathway intact and left efferent pathway intact
Could also be due to damage to right Edinger-Westphal nucleus
Compare unilateral afferent defects to unilateral efferent defects
- 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
Describe the 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
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
o Both pupils will paradoxically dilate when light swings to the right eye as a result of relatively reduced drive for pupillary constriction in both eyes.
Summarise the roles of the extra-ocular muscles
• Voluntary or involuntary of movement of eyes • Necessary for acquiring and tracking visual stimuli
• Facilitated by the six extraocular rmuscles innervated by the three cranial nerves
4 rectus muscles
2 oblique muscles
What is meant by duction
Eye Movement in One Eye
What is meant by version
Simultaneous movement of both eyes in the same direction (conversion, diversion)
What is meant by vergence
Simultaneous movement of both eyes in the opposite direction
What is meant by convergence
Simultaneous adduction (inward) movement in both eyes when viewing a near object
Describe what is meant by a saccade
• Saccade –short fast burst, up to 900deg/sec –Reflexive saccade to external stimuli –Scanning saccade –Predictive saccade to track objects –Memory-guided saccade
Input onto fovea small- need rapid movement to tract object
Describe what is meant by smooth pursuit
• Smooth Pursuit –sustain slow movement –Slow movement –up to 60°/s –Driven by motion of a moving target across the retina.
The
What are the extraocular muscles
6 muscles – Attach eyeball to orbit – Straight and rotary movement – Four straight muscles 1. Superior rectus 2. Inferior rectus 3. Lateral rectus 4. Medial rectus
Describe the superior and inferior rectus muscles
• Superior rectus – Attached to the eye at 12 oclock – Moves the eye up
. • Inferior rectus – Attached to the eye at 6 oclock – Moves the eye down.
Describe the actions of the lateral rectus
• 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)
Describe the medial rectus
• 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)
Describe the superior oblique muscle
• 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 inferior oblique muscle
• Attached low on the nasal side of the eye. • Passes over the Inferior Rectus. • Moves the eye in a diagonal pattern –up and out.
Describe the role of the oculomotor nerve in the innervation of the extraocular muscles
– Superior Branch • Superior Rectus –elevates eye • Lid Levator–raises eyelid – Inferior Branch • Inferior Rectus –depresses eye • Medial Rectus –adducts eye • Inferior Oblique –elevates eye • Parasympathetic Nerve – constricts pupil
Describe the role of the trochlear nerve and abducens nerve in the innervation of the extraocular muscle
- Fourth Cranial Nerve – Superior Oblique –depresses eye
* Sixth Cranial Nerve – Lateral Rectus –abducts eye
In what position would the eye have to be to get maximum elevation/depression due to:
Explain why this is with respect to the anterior-posterior axis of the eye
a. Superior and Inferior Recti
Abducted
b. Superior and Inferior Obliques
Adducted
The anterior-posterior axis of the eye is aligned with the axis of the vertical recti when the eye is abducted
If the eye is adducted, the axes are not aligned and contraction of the vertical recti would cause torsion
How can we isolate each extraocular eye muscle clinically
• ExtraocularMuscle Testing
–Isolate muscle to be tested by maximizing its action and minimizing the action of other muscles
– 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
Describe the up and down movements of the eye
• Up (Elevation) – Supraduction – one eye – Supraversion – both eyes • Down (Depression) – Infraduction – one eye – Infraversion – both eyes
Describe the right and left movements of the eyes
• Right – Dextroversion – Right Abduction – Left Adduction • Left – Levoversion – Right Adduction – Left Abduction
Describe what is meant by torsion
• Torsion – rotation of eye around the anteriorposterior axis of the eye
Describe and explain what you would see in a patient with 3rd nerve palsy.
Their affected eye would point down and out
This is because of the unopposed contraction of lateral rectus and superior oblique
Ptosis – because of the loss of innervation of levator palpebrae superioris
Pupil dilation – loss of parasympathetic innervation to the eye via CN III
Describe and explain what you would see in a patient with 6th nerve palsy.
When asked the abduct the affected eye, they eye will stop around midline
This is because the lateral rectus isn’t functioning and can’t abduct the eye
This can lead to blurred vision
– Affected eye unable to abduct and deviates inwards – Double vision worsen on gazing to the side of the affected eye
What is meant by a nystagmus
- Nystagmus –Oscillatory eye movement
* Optokinetic Nystagmus = Smooth Pursuit + Fast Phase Reset Saccade
Describe the clinical use of the optokinetic nystagmus
- 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
Presence in babies shows that their visual acuity is good but still in development.
Describe some other important targets of the retinal ganglion cell axons
Retinal ganglion cell axons also have other important targets. One is the suprachiasmatic nucleus of the hypothalamus, a small group of neurones at the base of the diencephalon. The retinohypothalamic pathway is the route by which variation in light levels influences a spectrum of visceral functions that are entrained by the light-day cycle.
Another target is the superior colliculus, a prominent structure visible on the dorsal surface of the midbrain. The superior colliculus coordinates head and eye movements to visual (as well as other targets).
What structure in the brainstem acts as a synchronising link between the eyes, allowing paired eye movements?
Medial Longitiduinal Fasiculus
Damage to MLF:
Internuclear Opthalmoplegia
E.g. right abduction wont be accompanied by left adduction
Could be accompanied by nystagmus on right gaze
Which part of the brain does the upper division of the optic radiation travel through and which parts of the visual field is it responsible for?
Parietal Lobe
Responsible for the inferior visual quadrants
Which part of the brain does the lower division of the optic radiation travel through and what part of the visual field is it responsible for?
Temporal Lobe
Responsible for the superior visual quadrants
The lower division loops inferiorly and anteriorly before going posteriorly towards the primary visual cortex. What is this loop called
Meyer’s Loop
. What would be the consequence of a lesion in Meyer’s loop?
Superior homonymous quadrantopia
What would be the consequence of a lesion of the upper division of the optic radiation?
Inferior homonymous quadrantopia
