The Neurology go the Visual System

Question 1

Visual pathway

Answer 1

• 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

Question 2

Visual pathway in the Retina

Answer 2

• 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

Question 3

What is a receptive field of a neurone?

Answer 3

• “retinal space within which incoming light can alter the firing pattern of a neurone”
• similar to skin

Question 4

What is the receptive field of a photoreceptor?

Answer 4

• ” a small circular space surrounding the photoreceptor”

Question 5

What is the receptive field of a retinal ganglion cell?

Answer 5

Input from neighbouring photoreceptors (convergence)

Question 6

What is convergence of receptive fields?

Answer 6

• 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

Question 7

What are the consequences of low vs. high convergence of receptive fields?

Answer 7

Low Convergence
– Small Receptive Field
– Fine Visual Acuity
– Low Light Sensitivity

High Convergence
– Large Receptive Field
– Coarse Visual Acuity
– High Light Sensitivity

Question 8

What categories can retinal ganglion cells be subdivided into?

Answer 8

On-centre and off-centre

Question 9

On-centre ganglion cells

Answer 9

– stimulated by light at the centre of the

– Inhibited by light on the edge of the receptive field

Question 10

Off-centre ganglion cells

Answer 10

– Inhibited by light at the centre of the receptive field

– Stimulated by light on the edge of the receptive field

Question 11

What are on- and off-ganglions important for?

Answer 11

• contrast sensitivity
• enhanced edge detection
• relative: balance between parts of the retina that are firing and parts that are not firing.

Question 12

How do lesions anterior to and posterior to the optic chasm present?

Answer 12

• anterior: affect visual field in one eye

- posterior: affect visual field in both eyes

Question 13

What fraction of ganglion fibres cross at the optic chiasm?

Answer 13

53%

Question 14

Optic chiasm - crossed vs. uncrossed fibres?

Answer 14

• Crossed Fibres – originating from nasal retina, responsible for temporal visual field
• Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field

Question 15

What are the consequences of a lesion at the optic chiasm?

Answer 15

– Damages crossed ganglion fibres from nasal retina in both eyes
– Temporal Field Deficit in Both Eyes – Bitemporal Hemianopia

Question 16

What are the consequences of lesions posterior to the optic chiasm?

Answer 16

– Right sided lesion – Left Homonymous Hemianopia in Both Eyes
– Left sided lesion – Right Homonymous Hemianopia in Both Eyes

Question 17

Homonymous heimanopia

Answer 17

• 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

Question 18

Monocular blindness

Answer 18

• due to lesion anterior to the optic chiasm on the affected side

Question 19

Bitemporal hemianopia

Answer 19

• caused by lesions at the optic chiasm
• e.g. (suprasellar) pituitary tumor >10mm
• causes loss of lateral / temporal vision fields on both sides

Question 20

Right nasal hemianopia

Answer 20

• caused by lesion affecting uncrossed fibres on the affected side

Question 21

What visual field does the right/left side of the brain process?

Answer 21

• Right side of the brain processes the left field of view
• Left side of the brain processes the right field of view

=> contralateral

Question 22

Homonymous hemianopia

Answer 22

• 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”

Question 23

Quadrant anopia

Answer 23

• 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

Question 24

Macular sparing

Answer 24

• 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.)

Question 25

Visual defects affecting horizontal and vertical line

Answer 25

- 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.

Question 26

What are the causes of homonymous Hemianopia?

Answer 26

- most commonly stroke (cerebrovascular accident)

Question 27

Where is the primary visual cortex located?

Answer 27

- 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

Question 28

Representation in the primary visual cortex?

Answer 28

``` – 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 ```

Question 29

What is the function of the primary visual cortex?

Answer 29

– 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.

Question 30

Macular sparing in homonymous hemianopia

Answer 30

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

Question 31

Extrastriate complex

Answer 31

Area around Primary Visual Cortex within the Occipital Lobe – Converts basic visual information, orientation and position into complex information

Question 32

Dorsal Pathwyas

Answer 32

``` – Primary Visual Cortex -> Posterior Parietal Cortex – Motion Detection – Visually-Guided Action – Damage results in Motion Blindness ```

Question 33

Ventral Pathway

Answer 33

– Primary Visual Cortex -> Inferiotemporal Cortex – Object Representation, Face Recognition – Detailed fine central vision and colour vision – Damage may result in Cerebral Achromatopsia

Question 34

What is the function of the pupils?

Answer 34

- Regulates light input to the eye (but less than 2 log unit change) - in light: constriction - in darkness: dilation

Question 35

What is the purpose of pupil constriction?

Answer 35

– 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)

Question 36

What is the role of pupil dilation?

Answer 36

– increases light sensitivity in the dark by allowing more light into the eye – pupillary dilatation mediated by sympathetic nerve

Question 37

What are the afferent pathways of the pupillary reflex?

Answer 37

– 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.

Question 38

What are the efferent pathways of the pupillary reflex?

Answer 38

– Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent -> – Synapses at Ciliary ganglion -> – Short Posterior Ciliary Nerve -> Pupillary Sphincter

Question 39

What is the difference between a direct and a consensual pupillary reflex?

Answer 39

* 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

Question 40

How can yo differentiate between an afferent and efferent defect in the pupillary reflex?

Answer 40

• 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

Question 41

What happens when there is an afferent defect on the right side?

Answer 41

– 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

Question 42

What happens when there is an efferent defect on the right side?

Answer 42

– 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

Question 43

What is the winging torch test?

Answer 43

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

Question 44

Eye Movement

Answer 44

* 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

Question 45

What nerves are responsible for innervation of eye muscles?

Answer 45

CN 3, 4, 6

Question 46

Eye movement terminology - what is duction?

Answer 46

Eye Movement in One Eye

Question 47

Eye movement terminology - what is version?

Answer 47

Simultaneous movement of both eyes in the same direction

Question 48

Eye movement terminology - what is vergence?

Answer 48

Simultaneous movement of both eyes in the opposite direction

Question 49

Eye movement terminology - what is convergence?

Answer 49

Simultaneous adduction (inward) movement in both eyes when viewing a near object

Question 50

Speed of eye movement - terms?

Answer 50

• 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.

Question 51

The muscles of the eye - function and names?

Answer 51

= 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

Question 52

Superior rectus muscle

Answer 52

– Attached to the eye at 12 o'clock – Moves the eye up.

Question 53

Inferior rectus muscle

Answer 53

– Attached to the eye at 6 o'clock | – Moves the eye down.

Question 54

Lateral rectus muscle

Answer 54

– 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)

Question 55

Medial Rectus muscle

Answer 55

= internal rectus – Attached on the nasal side of the eye – Moves the eye toward the middle of the head (toward the nose)

Question 56

Superior oblique muscle

Answer 56

- 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

Question 57

Inferior oblique muscle

Answer 57

* Attached low on the nasal side of the eye. * Passes over the Inferior Rectus. * Moves the eye in a diagonal pattern -- up and out.

Question 58

How are the extra-ocular muscles innervated?

Answer 58

``` 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

Question 59

How do yo test eye movement?

Answer 59

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

Question 60

What is the movement of eyes up (=elevation) called?

Answer 60

– Supraduction – one eye – Supraversion – both eyes

Question 61

What is the downwards movement (depression) of eyes called?

Answer 61

– Infraduction – one eye | – Infraversion – both eyes

Question 62

What is the rightwards movement (dextroversion) of eyes called?

Answer 62

– Right Abduction | – Left Adduction

Question 63

What is the leftwards movement (levoversion) of eyes called?

Answer 63

– Right Adduction | – Left Abduction

Question 64

Eye movement - what is torsion?

Answer 64

Rotation of eye around the anterior- posterior axis of the eye

Question 65

What are the effects of thrid nerve palsy on the eyes?

Answer 65

– Affected eye down and out – Droopy eyelid – Unopposed superior oblique innervated by fourth nerve (down) – Unopposed lateral rectus action innervated by sixth nerve (out)

Question 66

What are the effects of sixth nerve palsy on the eye?

Answer 66

– Affected eye unable to abduct and deviates inwards | – Double vision worsen on gazing to the side of the affected eye

Question 67

QUIZ

Answer 67

3,4,6 Palsy

Question 68

What is a nystagmus?

Answer 68

Oscillatory eye movement -> tracking motion in one direction and then the eye snaps back.

Question 69

Optokinetic nystagmus reflex

Answer 69

* 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