Visual Pathways Flashcards
What are the retinal neural layers?
Outer - Photoreceptors
Middle - Bipolar cells (Improve contrast, regulate sensitivity)
Inner - Retinal ganglion cells
Where do visual stimuli travel?
Eye
Optic nerve
Optic chiasm (53% fibres cross)
Optic tract - ganglion nerve fibres exit as this
Lateral Genticulate nucleus - ganglion nerve fibres synapse here (in thalamus)
Optic Radiation - 4th order neuron
Primary visual cortex (Striate Cortes) - Within occidental lobe
Extrastiate cortex
What is a visual receptive field?
retinal space within which incoming light alters firing pattern of a neurone
What is receptive field convergence?
Convergence of receptive field - number of lower order neurone fields synapsing on a single higher order neurone
Cone convergence > Rod divergence
Central convergence > Peripheral divergence
What are on and off centre ganglion cells?
On centre ganglion cells - stimulated by light at centre of receptive field, inhibited by light at periphery
Off centre ganglion cells - Inhibited by light at centre of receptive field, stimulated by light at periphery
Helps contrast sensitivity and edge detection
What is the optic chiasma
Point where the optic nerves cross
Lesions anterior to chiasma - ipselateral defect
Lesions posterior to chiasma - bilateral defect
(Right-sided lesion: left homonymous hemianopia
Left-sided lesions: right homonymous hemianopia)
Lesion ON chiasma - Bitemporal hemianopia
Crossed fibres from nasal retina responsible to temporal visual field
Uncrosses fibres from temporal retina responsible for nasal visual field
Describe the primary visual cortex
Along Calcarine sulcus - occipital lobe
Distinct stripe - myelinated optic radiation fibre
Representation
Macula disproportionate large
Superior visual field projects to below calcarine fissure
Inferior visual field projects to above calcarine fissure
Ipsilateral hemifield respresentation for both eyes
(left from both eyes goes to right)
Organised with columns (right and left dominant interspersed) sensitive to particular orientation. Allows depth perception
Explain Macular sparing homonymous hemionopia
Macula representing area has dual blood supply - Post cerebral arteries. In stroke one blood suply usually unaffected - Macula function spared
Describe the extrastriate cortex
Around primary cortex in occipital lobe
Converts visual info into position info
Dorsal pathway - Motion detection, visual guided action, projects to post parietal cortex
Ventral pathway - Object and face recognition, detail and colour vision, projects to inferiotemporal cortex
damage causes cerebral achromatopsia
What is the function of the Pupillary reflex?
Regulate light input (less than 2 log unit change)
In light - decreases glare and spherical aberrations, reduces photo-bleaching CN III mediated
In dark - Increases light sensitivity, sympathetic mediated
What is the afferent pathway of the pupillary reflex?
Rod and cone cells -> Bipolar cells -> Retinal ganglion cells
Pupil specific ganglion cells -> exit post 1/3 of optic tract -> Lateral geniculate nucleus
Lateral geniculate nucleus -> pretectal nucleus in brainstem
Pretectal nucleus-> interneurones -> edinger-westphal
What is the efferent pathway of the pupillary reflex?
Edinger-westphal -> Occulomotor nerve -> ciliary ganglion -> Short post ciliary nerve -> pupillary sphincter
Direct and consensual reflex in physiology
What do defects in the afferent and efferent pupillary reflex pathways cause?
Afferent defect - No response in either eye if light in eye with defect Direct and consensual reflex if light in eye without defect
Efferent defect - Defect eye will not respond directly or consensually. Non defect eye responds directly and consensually
How is Relative Afferent Pupillary Defect tested?
Swinging torch - Pardox pupil dilation in defective eye in non-complete damage. This is because the constriction response elicited by the bad eye is weaker than the consensual response elicited by the good eye
What happens as the retinal ganglion cells enter the optic nerve, which improves the transmission of the signal?
They become myelinated
Describe the convergence and receptive field sizes of rods and cones.
Rods have high convergence and large receptive fields Cones have low convergence and small receptive fields
Describe how the convergence of the rod system differs across different parts of the retina.
The rod system near that macula has lower convergence than in the peripheral retina
What is the benefit of having high or low convergence and a large or small receptive field?
High, large - High light sensitivity
Low, small - Fine visual acuity
Which part of the brain does the upper division of the optic radiation travel through and which parts of the visual field is it responsible for?
Parietal Lobe Responsible for the inferior visual quadrants
Which part of the brain does the lower division of the optic radiation travel through and what part of the visual field is it responsible for?
Temporal Lobe Responsible for the superior visual quadrants
The lower division loops inferiorly and anteriorly before going posteriorly towards the primary visual cortex. What is this loop called?
Meyer’s Loop
What would be the consequence of a lesion in Meyer’s loop?
Superior homonymous quadrantopia
What would be the consequence of a lesion of the upper division of the optic radiation?
Inferior homonymous quadrantopia
What is homonymous hemianopia typically caused by?
Strokes and other cerebrovascular accidents
What are the Types of eye movement?
Duction - unilateral eye movement
Version - simultaneous same direction movement of eyes
Vergence - simultaneous opposite direction movement of eyes
Convergence - Simultaneous adduction of both eyes in near vision
What are the speeds of eye movement?
Saccade - short fast 900 deg/sec
- Reflexive
- Scanning
- Predictive to track object
- Memory guided
Smooth persuit - sustained slow (up to 60 deg/sec)
- Accurate tracking
What is the term for the elevation of one eye?
Supraduction
What is the term for the depression of both eyes?
Infraversion
What is the technical term for right gaze?
Dextroversion
What is the technical term for left gaze?
Levoversion
What reflex is used to assess visual acuity in preverbal children?
Optokinetic Nystagmus Reflex It is a form of physiological nystagmus triggered by the presentation of a constantly moving grating pattern
What are the Six extraocular muscles?
Superior oblique - moves down and in (CN IV)
Inferior oblique - moves up and out (CN III inf branch)
Superior rectus - moves up (CN III sup branch)
Inferior rectus - move down (CN III Inf branch)
Lateral rectus - moves out (CN VI)
Medial rectus - move in (CN III inf branch)
How would you test the extraocular muscles?
Isolate the muscle to be tested by maximising its action and minimising the action of the other muscles E.g. to test the superior rectus, make the patient abduct and elevate their eye
Describe the CN III, IV and VI nerve palsys
Third nerve palsy - down (unopposed CN IV) and out (unnoposed CN VI), ptosis
Sixth nerve palsy - affected eye deviate inwards, double vision when looking to side
Fourth nerve palsy - upwards (Unopposed CN III) double vision looking down
What is Hering’s Law of Equal Innervation?
Muscles from both eyes involved in conjugate movement receive equal innervation
What structure in the brainstem acts as a synchronising link between the eyes, allowing paired eye movements?
Medial Longitudinal Fasciculus
What can damage to the MLF cause?
Internuclear Opthalmoplegia
E.g. right abduction wont be accompanied by left adduction Could be accompanied by nystagmus on right gaze
What is Sherrington’s Law of Reciprocal Innervation?
Agonist muscles contract while antagonist muscles relax
State a condition in which Sherrington’s Law is violated.
Duane’s Syndrome – congenital absence of abducens (CN VI) Both lateral and medial recti are innervated by CN III (oculomotor)
