Visual Pathways and Control of Eye Movements Flashcards
What are the main landmarks of the visual pathway?
- Eye
- Optic Nerve (retinal ganglion nerve fibres)
- Optic Chiasm
- Optic Tract
- Lateral Geniculate Nucleus
- Optic Radiation (4th order neurons from LGN to visual cortex NB retinal ganglion cells= 3rd order neurons)
- Primary Visual Cortex (Striate Cortex)
Where do retinal ganglion axons coming down the optic nerve synapse?
Lateral Geniculate Nucleus, they synapse here with 4th order neurons which form the optic radiation, this goes to the visual cortex
Where is the lateral geniculate nucleus found?
Thalamus
What are the fibres leaving the lateral geniculate nucleus called?
Optic Radiation
Which order neurones are these 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?
53%
Describe the convergence and receptive field sizes of rods and cones.
Rod system has high convergence and hence larger receptive fields for the ganglion cells that they eventually converge onto
Cone system has low convergence (less cones converge to higher order neurons) = smaller receptive fields for the ganglion cells
What is retinal convergence and how does central retinal convergence differ from peripheral retinal convergence
retinal convergence = Basically how many rods converge onto a higher order neuron.
Cones not involved because they are pretty much always 1:1.
central retinal convergence is lower than peripheral ie less rods converge onto one bipolar cell near the macula compared to the periphery where there is more convergence
What is the benefit of having high convergence and a large receptive field?
High light sensitivity (at the cost of visual acuity)
What is the benefit of having low convergence and a small receptive field?
Fine visual acuity (at the cost of 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 ie sensing difference in the colour/brightness of colour of the object which helps u to distinguish it from others in the background
Where do the fibres that decussate at the optic chiasma originate?
The nasal part of the retina
These fibres are responsible for the temporal half of the visual field
What effect do lesions anterior to the optic chiasm have on vision?
Affects only ONE eye (unilateral field loss)
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? What normally causes lesions at optic chiasm which causes this?
Bitemporal hemianopia
Pituitary gland tumour (pituitary is below optic chiasm)
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
Each optic radiation controls the same half of visual field in both eyes. This optic radiation then splits into two branches and so each branch will be in charge of a QUADRANT of the total visual field per eye
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 (upper division goes through parietal lobe because parietal lobe is above temporal lobe and they are all going back towards the occipital lobe where V1 is) 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 Quadrantanopia
Meyer loop= lower branch of optic radiation=controls superior quadrant
What would be the consequence of a lesion of the upper division of the optic radiation?
Inferior homonymous Quadrantanopia
What is homonymous hemianopia typically caused by?
Strokes and other cerebrovascular accidents
Where is the primary visual 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 macula has a disproportionate representation in the primary visual cortex
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
What causes macular sparing homonymous hemianopia? Why is it macular sparing?
Lesion in primary visual cortex (eg stroke)
Macular sparing hemianopia is only present if lesion is in primary visual cortex. If lesion is prior to V1 then it is likely to affect the macula or part of it. This is because the nerve fibres spread more and more as they reach V1 and so the more anterior your lesion is, the more likely that you will damage the fibres supplying the macula as well because all optic fibres are so close together.
Two reasons for macular sparing:
1. 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.
- Also, since such a large proportion of the V1 is for the macula, you need a lot of damage in order to affect the macula/central vision (the nerve fibres from macula spread out and innervate a wide area of V1, so if you damage parts of V1, it is likely that you did not damage enough fibres from macula in order for you to lose central vision)
What are the two pathways of the extrastriate cortex and what are they responsible for?
What are the consequences of damage to these pathways?
Dorsal Pathway – deals with motion detection/peripheral vision (WHERE)
Damage leads to motion blindness
Ventral Pathway – handles detailed object recognition/face recognition, fine central vision/colour recognition (WHAT)
Damage leads to cerebral achromatopsia
Describe the pupil’s response to light and the consequences of this response. What nerve mediates this pupil response?
Iric circular muscle (sphincter pupillae) contracts= pupil constriction
This leads to:
-Reduced rate of photopigment bleaching (light causes bleaching, constriction leads to less light entering and so rate of bleaching decreases)
-Increased depth of field (distance over which an object may be moved without causing a sharpness reduction beyond a certain tolerable amount. )
Oculomotor (III)
Describe the pathway that is responsible for the consensual lightreflex.
Retinal ganglion cells from the retina send they axons back via the optic nerve
The fibres that are responsible for the pupillary reflex will leave at the posterior 1/3 of the optic tract before it reaches the LGN
The axons then go to the pretectal nucleus in the dorsal brainstem
The afferent pathways from each eye then synapse on the Edinger-Westphal nuclei on both sides of the brainstem.
Efferent: Preganglionic parasympathetic fibres (part of CNIII) from the Edinger-Westphal nuclei goes to the ciliary ganglion via Oculomotor nerve. Short ciliary nerves (contain postganglionic parasympathetic fibres of oculomotor) travel from the ciliary ganglion to the pupillary sphincter
Summary:
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 (right eye does not constrict), consensual response present (left does)
Light shone in left eye: direct response (left eye does constricts), no consensual response (right doesnt)
What does RAPD mean?
Relative Afferent Pupillary Defect
This is seen in the swinging torch test. A PARTIAL pupillary response is still present despite damage to an eye and its pupillary reflex pathway – there is some degree of constriction but not as much as normal healthy constriction, and therefore when you move the torch onto the defected eye it looks like it dilates a bit because it does not constrict as much as the healthy eye before
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:
a. Duction
b. Version
c. Vergeance
d. Convergeance
a. Duction Movement of one eye b. Version Simultaneous movement of both eyes c. Vergence Simultaneous movement of both eyes in opposite directions d. Convergence Simultaneous adduction of both eyes when viewing a near object
What is the term for the elevation of one eye?
Supraduction
What is the term for the depression of both eyes?
Infraversion
What is the technical term for right gaze?
Dextroversion dextro=on the right side
What is the technical term for left gaze?
Levoversion levo=left
What are the two types of eye movement and how are they different?
Saccade – short fast burst
Smooth Pursuit – sustained slow movement
What reflex is used to assess visual acuity in preverbal children?
Optokinetic Nystagmus Reflex
It is a form of physiological nystagmus triggered by the presentation of a constantly moving grating pattern
Optokinetic- visual movement
ie reflex nystagmus as a result of a visual movement stimulus
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 (annulus of zinn, annulus=ring)
Where do the rectus muscles insert?
Into the sclera anterior to the globe equator
In what position would the eye have to be to get maximum elevation/depression due to:
a. Superior and Inferior Recti
b. Superior and Inferior Obliques
a. Superior and Inferior Recti
Abducted
b. Superior and Inferior Obliques
Adducted
Explain why this is with respect to the anterior-posterior axis of the eye.
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
Where do oblique muscles attach to the eye?
Into the sclera posterior to the globe equator
They pull the eye forwards and nasally
This is because of the pulley system established by the trochlea and the oblique muscles
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
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.
Eye deviates medially
When asked to 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
Double vision worsens when asked to gaze to the side of the affected eye. Eg if left eye affected and you asked patient to look left with both eyes, they will not be able to abduct on their left eye due to 6th nerve palsy but their right eye will adduct, this will lead to two eyes looking at different directions and hence double vision
What is Hering’s Law of Equal Innervation?
Muscles from both eyes involved in conjugate movement receive equalinnervation
What structure in the brainstem acts as a synchronising link between the eyes, allowing paired eye movements?
Medial Longitudinal Fasciculus
What can damage to the MLF cause?
Internuclear Opthalmoplegia
E.g. right abduction wont be accompanied by left adduction
Could be accompanied by nystagmus on right gaze
What is Sherrington’s Law of Reciprocal Innervation?
Agonist muscles contract while antagonist muscles relax
State a condition in which Sherrington’s Law is violated.
Duane’s Syndrome – congenital absence of abducens (CN VI)
Both lateral and medial recti are innervated by CN III (oculomotor)
