The Neurology go the Visual System Flashcards
Visual pathway
• transmits signal from eye to the visual cortex
• Visual Pathway Landmarks
– 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
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
– Destinations: • Lateral Geniculate Nucleus
(LGN) in Thalamus – to relay visual information to Visual Cortex
What is a receptive field of a neurone?
- “retinal space within which incoming light can alter the firing pattern of a neurone”
- similar to skin
What is the receptive field of a photoreceptor?
- ” a small circular space surrounding the photoreceptor”
What is the receptive field of a retinal ganglion cell?
Input from neighbouring photoreceptors (convergence)
What is convergence of receptive fields?
- Number of lower order neurones field synapsing on the same higher order neurone
- Cone system convergence > rod system convergence
- central retina convergence > peripheral retina convergence
What are the consequences of low vs. high convergence of receptive fields?
Low Convergence
– Small Receptive Field
– Fine Visual Acuity
– Low Light Sensitivity
High Convergence
– Large Receptive Field
– Coarse Visual Acuity
– High Light Sensitivity
What categories can retinal ganglion cells be subdivided into?
On-centre and off-centre
On-centre ganglion cells
– stimulated by light at the centre of the
– Inhibited by light on the edge of the receptive field
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 are on- and off-ganglions important for?
- contrast sensitivity
- enhanced edge detection
- relative: balance between parts of the retina that are firing and parts that are not firing.
How do lesions anterior to and posterior to the optic chasm present?
- anterior: affect visual field in one eye
- posterior: affect visual field in both eyes
What fraction of ganglion fibres cross at the optic chiasm?
53%
Optic chiasm - crossed vs. uncrossed fibres?
- Crossed Fibres – originating from nasal retina, responsible for temporal visual field
- Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field
What are the consequences of a lesion at the optic chiasm?
– Damages crossed ganglion fibres from nasal retina in both eyes
– Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia
What are the consequences of lesions posterior to the optic chiasm?
– Right sided lesion – Left Homonymous Hemianopia in Both Eyes
– Left sided lesion – Right Homonymous Hemianopia in Both Eyes
Homonymous heimanopia
- the same side of visual field is affected on both eyes (e.g. you cannot see the right so side meaning medial in left eye and lateral in right eye)
- caused by lesions posterior to the optic chiasm
Monocular blindness
- due to lesion anterior to the optic chiasm on the affected side
Bitemporal hemianopia
- caused by lesions at the optic chiasm
- e.g. (suprasellar) pituitary tumor >10mm
- causes loss of lateral / temporal vision fields on both sides
Right nasal hemianopia
- caused by lesion affecting uncrossed fibres on the affected side
What visual field does the right/left side of the brain process?
- Right side of the brain processes the left field of view
- Left side of the brain processes the right field of view
=> contralateral
Homonymous hemianopia
- loss of vision fiel on one side (e.g. right or left)
- lesion posterior to the optic chiasm affecting the crossed and uncrossed fibres
- the further back the lesion is the smaller the defect is.
- “postchiasmatic lesion”
Quadrant anopia
- lesion of optic radiation
- can be caused by lesions in temporal or parietal lobes but is most commonly caused by lesions in the occipital lobe
- the further back the lesion the smaller the visual defect is
Macular sparing
- lesion at the back, almost at the occipital cortex
- unless the injury is very massive, a part of the central vision will always be spared (there is dual blood supply to the area of the cortex that represents the macula from posterior cerebral arteries on both sides.)
Visual defects affecting horizontal and vertical line
- most visual field defects that affect something across a horizontal line (superior or inferior) is usually an eye problem e.g. glaucoma
- most visual fields affectecting sides of the vertical line are caused due to neurological defects.
What are the causes of homonymous Hemianopia?
- most commonly stroke (cerebrovascular accident)
Where is the primary visual cortex located?
- situated along the Calcarine sulcus within occipital lobe
- also known as the striate complex
- Characterized by a distinct stripe derived from the myelinated fibre of the Optic Radiation projecting into the Visual Cortex
Representation in the primary visual cortex?
– Disproportionately large area representing the macula – Superior Visual Field projects 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
What is the function of the primary visual cortex?
– Organized as columns (right and left interspersed) with unique sensitivity to visual stimulus of a particular orientation
– Right eye and left dominant columns intersperse each other
– If signals in the brain are elicited if both sides by both eyes will elicit the image in cortex
– This makes depth perception possible because the signal will have slightly different coordinates in both eyes and you can recognise the distance of an object.
Macular sparing in 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 sides
Extrastriate complex
Area around Primary Visual Cortex within the Occipital Lobe
– Converts basic visual information,
orientation and position into complex information
Dorsal Pathwyas
– Primary Visual Cortex -> Posterior Parietal Cortex – Motion Detection – Visually-Guided Action – Damage results in Motion Blindness
Ventral Pathway
– Primary Visual Cortex -> Inferiotemporal Cortex
– Object Representation, Face Recognition
– Detailed fine central vision and colour vision
– Damage may result in Cerebral Achromatopsia
What is the function of the pupils?
- Regulates light input to the eye (but less than 2 log unit change)
- in light: constriction
- in darkness: dilation
What is the purpose of 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 parasymapthetic nerve (within CN III)
What is the role of pupil dilation?
– increases light sensitivity in the dark by allowing more light into the eye
– pupillary dilatation mediated by sympathetic nerve
What are the afferent pathways of the pupillary reflex?
– 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 (take a different path than the R/L visual field nerve fibres)
– Synpases at Brain Stem (Pretectal Nucleus)
– Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides of the brainstem.
What are the efferent pathways of the pupillary reflex?
– Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent ->
– Synapses at Ciliary ganglion ->
– Short Posterior Ciliary Nerve -> Pupillary Sphincter
What is the difference between a direct and a consensual pupillary reflex?
- Direct Light Reflex – Constriction of Pupil of the light- stimulated eye
- Consensual Light Reflex – Constriction of Pupil of the fellow (other) eye
=> Neurological Basis: Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
How can yo differentiate between an afferent and efferent defect in the pupillary reflex?
• 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
• 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
• 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
What happens when there is an afferent defect on the right side?
– 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
What happens when there is an efferent defect on the right side?
– 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
What is the winging torch test?
Relative Afferent Pupillary Defect
– Partial pupillary response still present when the damaged eye is stimulated
– Elicitedbytheswinging torch test – alternating stimulation of right and left eye with light
– BothPupilsconstrict when light swings to left undamaged side
– BothPupilsparadoxically dilate when light swings to the right damaged side
Eye Movement
- Voluntary or involuntary of movement of eyes
- Necessary for acquiring and tracking visual stimuli
- Facilitated by the six extra-ocular muscles innervated by the three cranial nerves
What nerves are responsible for innervation of eye muscles?
CN 3, 4, 6
Eye movement terminology - what is duction?
Eye Movement in One Eye
Eye movement terminology - what is version?
Simultaneous movement of both eyes in the same direction
Eye movement terminology - what is vergence?
Simultaneous movement of both eyes in the opposite direction
Eye movement terminology - what is convergence?
Simultaneous adduction (inward) movement in both eyes when viewing a near object
Speed of eye movement - terms?
• Saccade – short fast burst, up to 900deg/sec – Reflexive saccade to external stimuli
– Scanning saccade
– Predictive saccade to track objects
– Memory-guided saccade
• Smooth Pursuit – sustain slow movement
– Slow movement – up to 60°/s
– Driven by motion of a moving target across the retina.
The muscles of the eye - function and names?
= extraocular muscles
- attach eyeball to orbit
- provide straight and rotary movement
- 4 straight muscles: superior inferior, lateral and medial rectus muscle
- superior and inferior oblique muscles
Superior rectus muscle
– Attached to the eye at
12 o’clock
– Moves the eye up.
Inferior rectus muscle
– Attached to the eye at 6 o’clock
– Moves the eye down.
Lateral rectus muscle
– 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)
Medial Rectus muscle
= internal rectus
– Attached on the nasal side of the eye
– Moves the eye toward the middle of the head (toward the nose)
Superior oblique muscle
- attached high on the temporal side of the eye
- passes under the superior rectus
- moves the eye in a diagonal pattern
- travels through the trochees
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.
How are the extra-ocular muscles innervated?
CN3 – 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
CN4
– Superior Oblique – depresses
eye
CN6
– Lateral Rectus–abductseye
How do yo test eye movement?
Extraocular Muscle 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
What is the movement of eyes up (=elevation) called?
– Supraduction – one eye – Supraversion – both eyes
What is the downwards movement (depression) of eyes called?
– Infraduction – one eye
– Infraversion – both eyes
What is the rightwards movement (dextroversion) of eyes called?
– Right Abduction
– Left Adduction
What is the leftwards movement (levoversion) of eyes called?
– Right Adduction
– Left Abduction
Eye movement - what is torsion?
Rotation of eye around the anterior- posterior axis of the eye
What are the effects of thrid nerve palsy on the eyes?
– Affected eye down and out
– Droopy eyelid
– Unopposed superior oblique innervated by fourth nerve (down)
– Unopposed lateral rectus action innervated by sixth nerve (out)
What are the effects of sixth nerve palsy on the eye?
– Affected eye unable to abduct and deviates inwards
– Double vision worsen on gazing to the side of the affected eye
QUIZ
3,4,6 Palsy
What is a nystagmus?
Oscillatory eye movement
-> tracking motion in one direction and then the eye snaps back.
Optokinetic nystagmus reflex
- Optokinetic Nystagmus = Smooth Pursuit + Fast Phase Reset Saccade
- 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
