Visual Pathways and Control of Eye Movements Flashcards
What are the main landmarks of the visual pathway?
Eye
Optic Nerve
Optic Chiasm (1/2 ganglion nerve fibres cross)
Optic Tract
Lateral Geniculate Nucleus
Optic Radiation
Primary Visual Cortex (AKA. Striate Cortex)
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 optic radiations and where do they terminate?
4th Order Neurones
Terminate in the primary visual cortex
What are the first, second and third order neurones in the visual pathway?
First Order: photo-receptors (rods + 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.
Rods have high convergence + large receptive fields
Cones have low convergence + 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?
High light sensitivity
What is the benefit of having low convergence and a small receptive field?
Fine visual acuity
Retinal ganglion cells can be described as on-centre or off-centre. What do these two terms mean?
On-centre: stimulated by light at centre of receptive field + inhibited by light at edge of receptive field
Off-centre: stimulated by light at edge of the receptive field + inhibited by light at centre of receptive field
Important in contrast sensitivity + enhanced edge detection
Where do the fibres that decussate at the optic chiasma originate? Where do uncrossed fibres originate?
Crossed: Nasal part of the retina
Responsible for the temporal half of the visual field
Uncrossed: Temporal retina
Responsible for nasal visual field
What effect do lesions anterior to the optic chiasm have on vision?
Affects only 1 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? What typically causes this defect?
Bitemporal hemianopia
Typically caused by pituitary tumour
What would a visual defect with respect to the horizontal line (superior/ inferior) most likely be related to?
Eye e.g. ganglion cells
What would a visual defect with respect to the vertical line (not crossing vertical line) most likely be related to?
Neurological problem
What is homonymous hemianopia typically caused by?
Strokes + other cerebrovascular accidents
Where is the primary visual cortex located?
Along the Calcarine sulcus in the occipital lobe
Describe which parts of the primary visual cortex are responsible for the different fields of vision.
Macula has a disproportionate representation in the PVC
Left PVC: responsible for right visual field from both eyes
Right PVC: responsible for left visual field from both eyes
Visual cortex above calcarine fissure is responsible for inferior visual field
Visual cortex below calcarine fissure is responsible for 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?
Area representing the macula has a dual blood supply (from posterior cerebral arteries from both sides)
Thus is less vulnerable to ischaemia
What are the 2 pathways of the extrastriate cortex and what are they responsible for? What does damage result in for each?
Dorsal Pathway: motion detection. Damage results in motion blindness
Ventral Pathway: detailed object + face recognition. Damage results in Cerebral Achromatopsia
Describe what happens to the eyes in the light.
Circular muscle contracts
Constriction of pupillary aperture
Reduced rate of photopigment bleaching
Increased depth of field
Describe the pathway that is responsible for the consensual light reflex.
Retinal ganglion cells send axons back via the optic nerve
Fibres responsible for the pupillary reflex exit at posterior 1/3 of the optic tract before entering LGN
Axons then go to the pretectal nucleus in the brainstem
Afferent pathways from each eye synapses on the Edinger-Westphal nuclei on both sides of the brainstem.
PNS nerve from the E-W nuclei to the ciliary ganglion forms the efferent pathway
Short ciliary nerves travel from the ciliary ganglion to the pupillary sphincter
What would the consequences be of a right afferent defect e.g. damage to optic nerve?
Light shone in right eye: no direct or consensual response
Light shone in left eye: direct + consensual response present
What would the consequences be of a right efferent defect e.g. damage to right 3rd nerve?
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
Partial pupillary response still present when damaged eye is stimulated- 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 light is shone on the good eye, there will be a direct + consensual response
When the light is then swung + 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
Duction: Movement of 1 eye
Version: Simultaneous movement of both eyes
Vergeance: Simultaneous movement of both eyes in opposite directions
Convergeance: 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
What is the technical term for left gaze?
Levoversion
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
A form of physiological nystagmus triggered by the presentation of a constantly moving grating pattern
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 + 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
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
Where do oblique muscles attach to the eye?
Into the sclera posterior to the globe equator
Pull the eye forwards and nasally
Because of the pulley system established by the trochlea + the oblique muscles
The oculomotor nerve has 2 branches. State what each of these branches innervates.
Superior: Superior Rectus + Levator Palpebrae Superioris
Inferior: Inferior + Medial Rectus, Inferior Oblique, Parasympathetic nerve that causes pupil constriction
How would you test the extraocular muscles?
Isolate muscle to be tested by maximising its action + minimising the action of the other muscles
E.g. to test superior rectus, make the patient abduct + elevate their eye
Describe and explain what you would see in a patient with 3rd nerve palsy.
Affected eye would point down + out
Because of unopposed contraction of lateral rectus + superior oblique
Ptosis: because of the loss of innervation of levator palpebrae superioris
Pupil dilation: loss of PNS 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
Because the lateral rectus isn’t functioning + can’t abduct the eye
Can lead to blurred vision
Describe the organisation of the primary visual cortex
Columns with unique sensitivity to visual stimulus of a particular orientation
Right + left eye dominant columns intersperse each other
What is pupillary constriction mediated by?
Parasympathetic nerve (within CNIII)
What happens to the pupils in the dark? Why? What mediates this?
Pupil dilation
Increases light sensitivity by allowing more light into the eye
Mediated by sympathetic nerve
Describe the attachment of the superior + inferior rectus
Superior: attached to eye at 12 o’clock. Moves eye up
Inferior: attached to eye at 6 o’clock. Moves eye down
Define Torsion
Rotation of eye around the anterior-posterior axis of the eye
