The Neurology of the Visual System Flashcards
Describe the route of the visual pathway
- Eye
- Optic Nerve – Ganglion Nerve Fibres
- Optic Chiasm – Half of the nerve fibres cross here
- Optic Tract – Ganglion nerve fibres exit as optic tract
- Lateral Geniculate Nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus
- Optic Radiation – 4th order neuron
- Primary Visual Cortex or Striate
Cortes – within the Occipital Lobe - Extrastriate Cortex
What are the first, second and third order neurons form the retina
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second Order Neurons – Retinal Bipolar Cells
Third Order Neurons – Retinal Ganglion Cells
Describe the retinal ganglion cells
Optic Nerve (CN II) Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline – Optic Tract Travels to. Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex
What is the receptive field
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
– Inputfrom neighbouring photoreceptors (convergence)
What is convergence
Number of lower order neurons field synapsing on the same higher order neuron
Compare the convergence of cone and rod systems and central vs peripheral retina
Cone System Convergence > Rod System Convergence
Central Retina Convergence > Peripheral Retina Convergence
Describe low convergence
Small receptive field
Fine visual acuity
Low light sensitivity
Describe high convergence
Large receptive field
Coarse visual acuity
High light sensitivity
Describe the 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 the off-centre ganglion cells
Inhibited by light at the centre of the receptive field
Stimulated by light on the edge of the receptive field
What is the importance of retinal ganglion cells
Contrast sensitivity
Enhanced edge detection
Describe the optic chiasm
53% of ganglion fibres cross here
Crossed fibres originate from the nasal retina, responsible for the temporal visual field.
Uncrossed fibres originate from the temporal retina, responsible for nasal visual field
Describe the affects of lesions o the optic chiasm
Anterior to chiasm - one eye only
At the chiasm - Causes bitemporal hemianopia
Posterior to chiasm - visual field in both eyes affected
Right side lesion - left homonymous hemianopia in both eyes
Describe bitemporal hemianopia
BitemporalHemianopia
Typically caused by enlargement of Pituitary Gland Tumour
Pituitary Gland sits under Optic Chiasma
Homonymous Hemianopia
Stroke (Cerebrovascular Accident)
Describe the 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
What projects to the different parts of the primary visual cortex
Below calcarine fissure - superior visual field
Above calcarine fissure - inferior visual field
Left primary visual cortex - right hemifield
Right primary visual cortex - left hemifield
What are the functions of the primary visual cortex
Organised as columns with unique sensitivity to visual stimulus or a particular orientation
Right eye and left dominant columns intersperse each other
Describe the common cause of primary visual cortex damage and what it leads to
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 sides
What is the function of the pupil and what is contraction and dilation mediated by
Regulates light input to the eye
contraction - parasympathetic (CN III)
dilation - sympathetic
What are the effects of pupil constriction
Decreases spherical aberrations and glare
Increases depth of field
Reduces bleaching of photo-pigments
What are the effects of pupil dilations
Increases light sensitivity in the dark by allowing more light in
Explain the pupillary reflex
- Rod and cone photoreceptors synapse on bipolar cells synapsing on retinal ganglion cells
- Pupil-specific ganglion cells exist at the posterior 1/3 of the optic tract before entering the lateral geniculate nucleus
- Synapses at the brain stem (pretectal nucleus)
- Afferent pathway from each eye synapses on the Edinger-Westphal nuclei on both sides in the brainstem
- Path to oculomotor nerve efferent
- Synapse at ciliary ganglion
- Short posterior ciliary nerve -> pupillary sphincter
Compare direct to consensual light reflex and explain why this occurs
Direct - constriction of the pupil that is light-stimulated
Consensual - constriction of the other eye
Afferent pathway on either side will stimulate efferent pathway on both sides
Describe what occurs in a right afferent defect
Normal pupil constriction in both eyes when left eye is stimulated with light
No pupil constriction in both eyes when the right eye is stimulated with light
e.g. damage to optic nerve
Describe what occurs in a right efferent defect
Left eye constricts whether right or left eye is stimulated with light
No right pupil constriction whether right or left eye is stimulated with light
e.g. damage to right 3rd nerve
Compare unilateral afferent and efferent defects
afferent - difference response pending on which eye is stimulated
efferent - same unequal response between left and right eye irrespective of the eye stimulated
Define duction, version, mergence and convergence
Duction = Eye Movement in One Eye
Version = Simultaneous movement of both eyes in the same direction
Vergence = Simultaneous movement of both eyes in the opposite direction
Convergence = Simultaneous adduction (inward) movement in both eyes when viewing a near object
What is saccade movement
Short, fast burst up to 900deg/sec Reflexive saccade to external stimuli Scanning saccade Predictive saccade to track objects Memory-guided saccade
What is smooth pursuit movement
Sustained slow movement
Up to 60deg/sec
Driven by motion of a moving target across the retina
What are the straight muscles of the eye
Superior rectus
Inferior rectus
Medial rectus
Lateral rectus
What movements of the eye do the rectus muscles assist in
SR - elevation
IR - depression
LR - abduction
MR - adduction
Describe the superior oblique muscle
Attached on the temporal side of eye
Passes under the superior rectus
Moves the eye down and in
Travels through the trochlea
Describe the inferior oblique muscle
Passes over the inferior rectus
Moves the eye up and out
Which extra ocular muscles does the superior branch of the oculomotor nerve supply
Superior rectus - elevates eye
Lid levator - raises eyelid
Which extra ocular muscles does the inferior branch of the oculomotor nerve supply
Inferior rectus - depression
Medial rectus - adduction
Inferior oblique - elevates eye
Parasympathetic - constricts pupil
Which extra ocular muscles does the trochlear nerve supply
Superior oblique - depression
Which extra ocular muscles does the abducens nerve supply
Lateral rectus - abduction
What are the directions of eye movement
Elevation - supraduction, supraversion
Depression - infraduction, infraversion
Dextroversion (right) - Right adduction, left abduction
Levoversion - Right adduction, left abduction
Torsion - rotation of eye around the anterior-posterior axis of the eye
Describe third nerve palsy
Down and out movement affected
Droopy eyelid
Unopposed superior oblique innervated by fourth nerve (down)
Unopposed lateral rectus action innervated by sixth nerve (out)
Describe sixth nerve palsy
Affected eye unable to abduct and deviates inwards
Double vision worsen on gazing to the side of the affected eye
What is nystagmus and optokinetic nystagmus
Oscillatory eye movement
Smooth pursuit + fast phase rest saccade
What is the optokinetic nystagmus reflex useful for
testing visual acuity in pre-verbal children
Observe presence of nystagmus movement in response to moving grating patterns of various spatial frequencies
Signifies the patient has sufficient visual acuity to perceive grating pattern
