Visual Pathways and Control of Eye Movements Flashcards
Signals received from eye sent to which part of brain?
What are the main landmarks of the visual pathway?
Visual cortex
-Eye
-Optic Nerve (retinal ganglion nerve fibres)
-Optic Chiasm
-Optic Tract– Ganglion nerve fibres
exit as 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)
Primary visual cortex found where?
Where do retinal ganglion axons coming down the optic nerve synapse?
Within occipital lobe
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
– stimulated by light falling on the centre of the receptive field
-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 both eyes
Left-sided lesions: right homonymous hemianopia both eyes
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
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. )
Decreases spherical aberrations and glare
Oculomotor (III)
Describe the pathway that is responsible for the consensual light reflex.
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?
What is torsion
Levoversion levo=left
Rotation of eye around the anterior-posterior axis of the eye
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
triggered by the presentation of a constantly moving grating pattern of various spatial freq
Optokinetic- visual movement
Smooth pursuit, fast phase reset saccade.
State which nerve innervates each of the extraocular muscles that facilitate eye movement by attaching eye to orbit
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)
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 (innerv by 6th–out) and superior oblique (innerv by 4th-down)
Describe and explain what you would see in a patient with 6th nerve palsy.
Affected eye unable to abduct and deviates inwards
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 structure in the brainstem acts as a synchronising link between the eyes, allowing paired eye movements?
Medial Longitudinal Fasciculus
Consensual reflex
Afferent pathways on either side will stimulate effect
Unilateral afferentdefect
Different response pending on which eye is stimulated
Unilateral efferent defect
Same unequal response between left and right
irrespective which eye is stimulated
How many extraocular muscles
Functions of extraocular muscles
6
Attach eyeball to orbit
Straight and rotary mvmt
4 straight muscles - rectus
Saccade
Fast
- Reflexive
- Scanning
- Predictive
- Memory guided
Smooth
Slow
-Driven by motion of a moving target across retina
Superior rectus muscle eye mvmt
Moves eye up
Lateral rectus
Attaches on termporal side of eye
Medial rectus
Attached on nasal side of eye
Superior oblique
-Attached where
- Mvmt of eye
- In relation to superior rectus
- travels through
Attached high on temporal side of eye
Diagonal- down and in
Passes under superior rectus
Travels through trochlear
Inferioir oblique
in relation to inferior rectua
mvmt of eye
- Passes over inferior rectus
- Up and out
Eye mvmt testing
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
3rd order neurons path
3rd order neurons-retinal ganglion
- Optic nerve
- Partial decuss at optic chiasma 53% of gang
- Optic tract
- Destinations
- -> Lateral Geniculate Nucleus - in thalamus- to relay visual information to visual cortex
Homogenous hemianopia is due to
stroke, cerebrovascular accidents
where do crossed fibres originate from
responsible for
Nasal retina
Temporal visual field
Where do uncrossed fibres originate from
responsible for
Temporal retina
Nasal visual field
