Neuro13: Neurology of the visual system

Question 1

Outline the visual pathway anatomy

Answer 1

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

Outline pathway of 1st, 2nd, 3rd and 4th order neurones in the visual pathway

Answer 2

1st order are the rods and cones photoreceptors

2nd order are the retinal bipolar cells

3rd order are the retinal ganglion cells, the axons of which travel in the optic nerve and eventually synapse in the LGN of the thalamus

4th order neurons begin at the LGN adnd go to optic radiation

Question 3

What percentage of 3rd order neurons decussate at the optic chiasm

Answer 3

53% of ganglion fibres cross the midline

Question 4

T/F retinal ganglion fibres are myelinated

Answer 4

T, but only after entering the optic nerve

Question 5

What us the receptive field of a photoreceptor

Answer 5

Retinal space within which incoming light can alter the firing pattern of a neuron- – a small circular space surrounding the photoreceptor

Question 6

What is meant by convergence of receptive field

Answer 6

Number of lower order neurons field synapsing on the same higher order neuron….

retinal ganglion cells can bipolar cells from different photoreceptros synapsing onto them…

the more photoreceptors feeding into 1 retinal ganglion cell, the higher the convergence

Question 7

Which photoreceptors have the highest convergence

Differentiate convergence of rods near macula vs in the periphery

Answer 7

Rods have higher convergence than cones so more rods synapse onto a retinal ganglion cell, than cones onto retinal ganglion cells

Nearer to the macula, there is lower convergence for rods than in the periphery (i.e. in the centre less rods synapse onto each retinal ganglia)

Question 8

Differentiate the size of receptor fields for the ganglion cells in the cone system and rod system (and also rods near the macula vs in the peripher)

Answer 8

Ganglion cells in cone system have smaller receptive field than rod system,

Ganglion cells in rod system near macula have smaller receptive field than those near the peripohery

Question 9

What does a small and large retinal ganglion receptive field give rise to

Answer 9

Small receptive field- fine visual acuity

large receptive field- higher light sensitivity

Question 10

Differentiate on-centre and off centre ganglion cells

Answer 10

On-centre Ganglion:
stimulated by light at the centre of the receptive field
Inhibited by light on the edge of the receptive field

Off-centre Ganglion:
Inhibited by light at the centre of the receptive field
Stimulated by light on the edge of the receptive field

Question 11

What is on-centre and off-centre ganglion cells important for

Answer 11

Contrast Sensitivity

Enhanced Edge Detection

Question 12

Differentiatie the effect of a lesion anterior and posterior to optic chiasma

Answer 12

Lesions anterior to Optic Chiasma affect visual field in one eye only
Lesions posterior to Optic Chiasma affect visual field in both eyes

Question 13

Which parts of the retina do i. crossed fibres (i.e. those than decussate at optic chiasma) and ii. uncrossed fibre come from

Answer 13

crossed fibres: originate from NASAL side of the retina, resposible for temporal visual field

uncrossed fibre: Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field

Question 14

What gives edge detection and sharpness

Answer 14

The positive stimulus of one part of the retina and the negative simulus to the neighbouring part

(i.e. on an off centre retinal ganglion cells, see above)

Question 15

Effect of following lesions:

1. Optic chiasma
2. Lesion posterior to optic chiasma (on the right hand side in this case)

Answer 15

1. You get damage to crossed fibres from nasal retina so loss of temporal vision in both eyes= BILATERAL HEMIANOPIA
2. Right side- damage to uncrossed fibres temporal retina of right eye, and to crossed fibres from the nasal part of the left eye. So you lose temporal vision in left eye and nasal vision in your right eye…. so LEFT HOMONYMOUS HEMIANOPIA (and opposite for the right)

visual pathway lesion posterior to the chiasma produces contralateral Homonymous Hemianopia in both eyes.

Question 16

Explain which lesions would lead to the following visual field disorders:

1. Monocular blindness
2. Right nasal hemianopia
3. Bitemporal hemianopia
4. Homonymous hemianopia
5. Quadrant anopia
6. Macular sparing

Answer 16

1. Optic nerve on one side
2. Half of optic chiasm affected (on the right side at front, as this will have nasal crossed fibres from right eye just before they cross)
3. Middle of optic chiasm
4. Optic tract (post. to optic chiasm)
5. Damage far back in the visual pathway
6. Damage further back (because back in the cortex, you have a really high representation of the macula here, so you need large damage in this area to lose the macula)

The further back you go, the smaller the damage to the receptive field

Question 17

Cause of bitemporal hemianopia

Answer 17

Typically caused by enlargement of Pituitary Gland Tumour

Pituitary Gland sits under Optic Chiasma

Question 18

Cause of homonymous hemianopia

Answer 18

Stroke (Cerebrovascular Accident)

Question 19

Where is primary visual cortex located

Answer 19

Situated along Calcarine Sulcus within Occipital Lobe

Question 20

PVC also known as

Answer 20

Striate Cortex

Question 21

Outline representation on the PVC

Answer 21

Disproportionately large area representing the macula

Superior Visual Field projects to 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 22

Function of PVC

Answer 22

Cortex specializes in processing visual information of static and moving objects.

Question 23

Organisation of the primary visual cortex

Answer 23

In columns which have a unique sensitivity to visual stimulus of a particular orientation

….. right eye and left eye doninant columns interspersed.

Question 24

Outline macular sparing homonymous hemianopia

Answer 24

DUE TO STROKE….

- leads to contralateral homonymous hemianopia but is macular sparing

Question 25

Why is the area of the PVC representing the macula usually spared in Macular Sparing Homonymous Hemianopia

Answer 25

Because this area of PVC receives dual blood supply from posterior cerebral arteries from both sides

Question 26

What is the extrastriate cortex

Answer 26

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

Question 27

Outline the 2 pathways in the extrastriate cortex

Answer 27

DORSAL: Primary Visual Cortex -> Posterior Parietal Cortex for MOTION detection (think about motion in the back p) Visually guided action WHERE VENTRAL: Primary Visual Cortex -> Inferiotemporal Cortex Object Representation, Face Recognition Detailed fine central vision and colour vision WHAT

Question 28

What might damage in dorsal pathway or ventral pathway of the extrastriate cortex result in?

Answer 28

Dorsal: Motion Blindness Ventral:Cerebral Achromatopsia

Question 29

Outline what happens in pupillary constriction in light

Answer 29

decreases spherical aberrations and glare ----- spherical aberration is when light from the outside of the lens bent more than the light coming through the centre of the lens as the outside is more bent. Closing the pupil aperture prevents light from passing through the bent part of the lens, and ensures it only travels through the centre.... allowing less glare due to the reduced spherical anberation 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) iris circular muscle contracts, and constricts the pupillary aperture.

Question 30

What happens in darkness with pupllary dilation

Answer 30

increases light sensitivity in the dark by allowing more light into the eye pupillary dilatation mediated by sympathetic nerve activating the iris radial muscle (but greater spherical aberration so some glare)

Question 31

Outline the afferent pathway in pupillary reflex (was year 1 neuro!)

Answer 31

AFFERENT 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 Synpases at Brain Stem (Pretectal Nucleus) Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem

Question 32

Outline efferent pathway in pupillary reflex

Answer 32

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

Question 33

Differentiate a direct and consensal reflex

Answer 33

Direct Light Reflex –Constriction of Pupil of the light-stimulated eye Consensual Light Reflex – Constriction of Pupil of the fellow (other) eye

Question 34

Explain why you get consensual reflex

Answer 34

Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides

Question 35

Explain the damage to the reflex in the following conditions: 1. Right afferent defect 2. Right efferent defect 3. Unilateral afferent defect 4. Unilateral efferent defect

Answer 35

1. No constriction in either eye when light shines in right. When light shines in left, both eyes constrict 2. Right eye doesn't constrict, whether right or left stimulated w light.... left eye constricts fine with either eye stimulated 3. Difference response pending on which eye stimulated 4. Same unequal response between left and right eye irrespective of the eye stimulated

Question 36

What is the RAPD

Answer 36

Relative afferent pupillary defect..... there is still a small reflex response if there is damage afferent If right is damaged, when left eye stimulated, both constrict normally, when you then swing the torch to shine in the right eye, there will be a dilation (from the constrction state as a result of relatively reduced dive for pupillary constriction IN BOTH EYES

Question 37

``` Define following terms: Duction Version Vergence Convergence ```

Answer 37

Duction – Eye Movement in One Eye Version – Simultaneous movement of both eyes in the same direction (dextroversion to right and levoversion to left) Vergence – Simultaneous movement of both eyes in the opposite direction Convergence – Simultaneous adduction (inward) movement in both eyes when viewing a near object

Question 38

What is a sacade

Answer 38

short fast burst, up to 900deg/sec Reflexive saccade to external stimuli Scanning saccade Predictive saccade to track objects Memory-guided saccade

Question 39

What is a smooth pursuit and differentiate it to saccade

Answer 39

sustain slow movement Slow movement – up to 60°/s Driven by motion of a moving target across the retina. Involuntary A saccade is a quick, simultaneous movement of both eyes between two or more phases of fixation in the same direction.[1] In contrast, in smooth pursuit movements, the eyes move smoothly instead of in jumps. The phenomenon can be associated with a shift in frequency of an emitted signal[clarification needed] or a movement of a body part or device.[2] Controlled cortically by the frontal eye fields (FEF), or subcortically by the superior colliculus, saccades serve as a mechanism for fixation, rapid eye movement, and the fast phase of optokinetic nystagmus

Question 40

Function of extraocular muscles

Answer 40

Attach eyeball to orbit | Straight and rotary movement

Question 41

Name the 4 straight muscles

Answer 41

Superior rectus Inferior rectus Lateral rectus Medial rectus

Question 42

Name the 2 diagonanal muscles

Answer 42

Superior oblique | Inferior oblique

Question 43

Function of the Superior rectus

Answer 43

Attached at 12 o;clock... pulls the eye up

Question 44

Function of inferior rectus

Answer 44

Attached at 6 oclock pulls eye down

Question 45

Function of lateral rectus

Answer 45

Attaches on the temporal side of the eye | Moves the eye toward the outside of the head (toward the temple)

Question 46

Function of medial retus

Answer 46

Attached on the nasal side of the eye | Moves the eye toward the middle of the head (toward the nose)

Question 47

Superior oblique function and attachment

Answer 47

Attached high on the temporal side of the eye. Passes under the Superior Rectus. Moves the eye in a diagonal pattern -- down and in. Travels through the trochlea BE CAREFUL OF THIS.... it's kind of different to the anatomy.... I think the one from steve gentleman with my notes it generally better.

Question 48

Function of the inferior oblique and attahment

Answer 48

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 49

State the innervation of the extraocular muscle

Answer 49

3rd--> superior branch: superior rectus (elevates eye) and lid levator. Inferior branch: medial rectus, inferior rectus, inferior oblique (elevates eye) and parasympathetic nerve to contstrict pupil 4th--> superior oblique 6th--> lateral rectus (abducts eye) there's some more to do with cranial nerve testing. and nerve palsy