Neuro13: Neurology of the visual system Flashcards
Outline the visual pathway anatomy
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
Outline pathway of 1st, 2nd, 3rd and 4th order neurones in the visual pathway
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
What percentage of 3rd order neurons decussate at the optic chiasm
53% of ganglion fibres cross the midline
T/F retinal ganglion fibres are myelinated
T, but only after entering the optic nerve
What us the receptive field of a photoreceptor
Retinal space within which incoming light can alter the firing pattern of a neuron- – a small circular space surrounding the photoreceptor
What is meant by convergence of receptive field
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
Which photoreceptors have the highest convergence
Differentiate convergence of rods near macula vs in the periphery
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)
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)
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
What does a small and large retinal ganglion receptive field give rise to
Small receptive field- fine visual acuity
large receptive field- higher light sensitivity
Differentiate on-centre and off centre ganglion cells
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
What is on-centre and off-centre ganglion cells important for
Contrast Sensitivity
Enhanced Edge Detection
Differentiatie the effect of a lesion anterior and posterior to optic chiasma
Lesions anterior to Optic Chiasma affect visual field in one eye only
Lesions posterior to Optic Chiasma affect visual field in both eyes
Which parts of the retina do i. crossed fibres (i.e. those than decussate at optic chiasma) and ii. uncrossed fibre come from
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
What gives edge detection and sharpness
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)
Effect of following lesions:
- Optic chiasma
- Lesion posterior to optic chiasma (on the right hand side in this case)
- You get damage to crossed fibres from nasal retina so loss of temporal vision in both eyes= BILATERAL HEMIANOPIA
- 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.
Explain which lesions would lead to the following visual field disorders:
- Monocular blindness
- Right nasal hemianopia
- Bitemporal hemianopia
- Homonymous hemianopia
- Quadrant anopia
- Macular sparing
- Optic nerve on one side
- 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)
- Middle of optic chiasm
- Optic tract (post. to optic chiasm)
- Damage far back in the visual pathway
- 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
Cause of bitemporal hemianopia
Typically caused by enlargement of Pituitary Gland Tumour
Pituitary Gland sits under Optic Chiasma
Cause of homonymous hemianopia
Stroke (Cerebrovascular Accident)
Where is primary visual cortex located
Situated along Calcarine Sulcus within Occipital Lobe
PVC also known as
Striate Cortex
Outline representation on the PVC
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
Function of PVC
Cortex specializes in processing visual information of static and moving objects.
Organisation of the primary visual cortex
In columns which have a unique sensitivity to visual stimulus of a particular orientation
….. right eye and left eye doninant columns interspersed.
Outline macular sparing homonymous hemianopia
DUE TO STROKE….
- leads to contralateral homonymous hemianopia but is macular sparing
Why is the area of the PVC representing the macula usually spared in Macular Sparing Homonymous Hemianopia
Because this area of PVC receives dual blood supply from posterior cerebral arteries from both sides
What is the extrastriate cortex
Area around Primary Visual Cortex within the Occipital Lobe
Converts basic visual information, orientation and position into complex information
Outline the 2 pathways in the extrastriate cortex
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
What might damage in dorsal pathway or ventral pathway of the extrastriate cortex result in?
Dorsal: Motion Blindness
Ventral:Cerebral Achromatopsia
Outline what happens in pupillary constriction in light
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.
What happens in darkness with pupllary dilation
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)
Outline the afferent pathway in pupillary reflex (was year 1 neuro!)
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
Outline efferent pathway in pupillary reflex
Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent ->
Synapses at Ciliary ganglion ->
Short Posterior Ciliary Nerve -> Pupillary Sphincter
Differentiate a direct and consensal reflex
Direct Light Reflex –Constriction of Pupil of the light-stimulated eye
Consensual Light Reflex – Constriction of Pupil of the fellow (other) eye
Explain why you get consensual reflex
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
Explain the damage to the reflex in the following conditions:
- Right afferent defect
- Right efferent defect
- Unilateral afferent defect
- Unilateral efferent defect
- No constriction in either eye when light shines in right. When light shines in left, both eyes constrict
- Right eye doesn’t constrict, whether right or left stimulated w light….
left eye constricts fine with either eye stimulated - Difference response pending on which eye stimulated
- Same unequal response between left and right eye irrespective of the eye stimulated
What is the RAPD
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
Define following terms: Duction Version Vergence Convergence
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
What is a sacade
short fast burst, up to 900deg/sec
Reflexive saccade to external stimuli
Scanning saccade
Predictive saccade to track objects
Memory-guided saccade
What is a smooth pursuit and differentiate it to saccade
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
Function of extraocular muscles
Attach eyeball to orbit
Straight and rotary movement
Name the 4 straight muscles
Superior rectus
Inferior rectus
Lateral rectus
Medial rectus
Name the 2 diagonanal muscles
Superior oblique
Inferior oblique
Function of the Superior rectus
Attached at 12 o;clock… pulls the eye up
Function of inferior rectus
Attached at 6 oclock pulls eye down
Function of lateral rectus
Attaches on the temporal side of the eye
Moves the eye toward the outside of the head (toward the temple)
Function of medial retus
Attached on the nasal side of the eye
Moves the eye toward the middle of the head (toward the nose)
Superior oblique function and attachment
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.
Function of the inferior oblique and attahment
Attached low on the nasal side of the eye.
Passes over the Inferior Rectus.
Moves the eye in a diagonal pattern – up and out.
State the innervation of the extraocular muscle
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
