Neurology 13 - Neurology of the Visual System Flashcards
Describe the visual pathway anatomy
- Eye
- Optic nerve - ganglion nerve fibres
- Optic chiasm - half the nerve fibres cross here
- Optic tract - glanglion nerve fibres exit
- Lateral geniculate nucleus (synapse)
- Optic radiation (4th order neuron)
- Primary visual cortex or striate cortex within the occipital libe
- Extrastriate cortex
Describe the visual pathway in the retina
- First order neurons are rod and cone retinal photoreceptors
- Second order neurons are the retinal bipolar cells
- Third order neurons are the retinal ganglion cells (optic nerve, decussation at chiasm, optic tract)
Describe the receptive field
- Retinal space within which incoming light can alter the firing pattern of a neuron
- Photoreceptors are a small circular space surrounding the photoreceptor
- Retinal ganglion cells have input from neighbouring photoreceptors (convergence)
Describe the convergence of receptive field in the eyes
- Number of lower order neurons field synapsing on the same higher order neuron
- Cone system convergence is larger than in rod system
- Central retina convergence is larger than peripheral retina convergence
- Low convergence results in small receptive field, fine visual acuity and low light sensitivity
- High convergence results in large receptive field, with high light sensitivity and coarse visual acuity
What are the on-centre and off-centre ganglion cells?
- On centre are stimulated by light at the centre of the receptive field
- Off centre ganglion are stimulated by lack of light in the centre of the receptive field
- Important for contrast sensitivity and enhanced edge detection
What is the optic chiasma?
- 53% of ganglion fibres cross at the optic chiasma
- Uncrossed fibres are responsible for temporal visual field
- Crossed fibres are responsible for nasal visual field
What do lesions anterior and posterior to the optic chiasma cause?
- Lesions anterior to the optic chiasm affect one eye only
- Lesions at the optic chiasm affect visual field in both eyes
- Posterior cause left homonymous hemianopia in both eyes (right sided) or right homonymous hemianopia in both eyes (left sided)
List the disorders of visual pathway
- Monocular blindness (one eye)
- Bitemporal hemianopia
- Nasal hemianopia
- Homonymous hemianopia
- Quadrantopia
- Macular sparing (central sparing)
List the causes of bitemporal hemianopia
- Enlargement of pituitary gland tumour
- Pituitary sits under optic chiasma
What causes homonymous hemianopia?
Stroke (cerebrovascular accident)
Describe the structue of the primary visual cortex
- Situated alone the calcarine sulcus within the occipital lobe
- Known as striate cortex
- Characterised by a sidtinct shape derived from the myelinated fibre of the optic radiation projecting into the visual cortex
Describe the representation in the primary visual cortex
- Disproportionately large area represents 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 the right primary visual cortex
What is the function of the primary visual cortex?
- Organised in columns with unique sensitivity to visual stimulus of a particular ortientation
- Right eye and left dominant columns intersperse each other
List the causes of macular sparing homonymous hemianopia
- Damage to primary visual cortex, often due to stroke
- Area representing the macula receives dual blood supply from the posterior cerebral arteries
What is the extrastriate cortex?
- Area around the primary visual cortex within the occipital lobe
- Converts basic visual information, orientation and oposition into complex information
What is the ventral pathway of the extrastriate cortex?
- Primary visual cortex to inferiotemporal cortex
- Object representation, face regognition, detailed fine central vision and colour vision
- Damage may result in cerebral achromatopsia
What is the dorsal pathway of the extrastriate cortex?
- Primary visual cortex to posterior parietal cortex
- Involved in motion detection, visually guided action
- Damage results in motion blindness
Describe pupillary funtion
- Regulates light input to the eye
- In light, there is pupil constriction to decrease glare and spherical aberrations, increases depth of field, mediated by the oculomotor nerve
- In the dark, there is dilation to increase light sensitivity, mediated by sympathetic nerve
Describe the afferent pathway of the pupillary reflex
- Rode and cone photoreceptors synapse on bipolar cells which synapse on retinal ganglion cells
- Pupil specific ganglion cells exit at posterior third of optic tract before entering the lateral genticulate nucleus
- Synapses at brainstem
- Synapses on the edinger westphal nuscli on either side
Describe the efferent pathway of the pupillary reflex
- Edinger westphal nuclus to ocular motor nerve efferent
- Synapses at the ciliary ganglion
- Short posterior ciliary nerve to pupillary sphincter
What is the direct light reflex?
- Constriction of pupil of stimulated eye
What is the consensual light reflex?
- Constricion of the other eye
- Afferent pathway on either side stimulates the efferent pathway on both sides
What is the result of a right afferent defect?
- Eg. damage to optic nerve
- No constriction in both eyes when right eye stimulates
- Normal constriction when left eye is stimulated with light
What is the result of a right efferent defect?
- Damage to right oculomotor nerve
- No right pupil constriction whether right or left eye is stimulated with light
- Left pupil constricts whether the right or left eye is stimulated
Compare unilateral afferent defects to unilateral efferent defects
- Difference in response depending on which eye is stimulated (afferent)
- Same unequal response with either eye stimulated (efferent)
What is the swinging torch test?
- Used to identify pipullary response
- Alternating stimulation of right and left eye with light
Describe eye movement
- Voluntary or involuntary
- Necessary for aquiring and tracking visual stimuli
- Facilitated by the 6 extraocular muscles innervated by 3 cranial nerves
What is duction of the eye?
Movement in one eye
What is version of the eye?
Simultaneous movement of both eyes in the same direction
What is vergence of the eyes?
Simultaneous movement of both eyes in the opposite direction
What is convergence of the eyes?
Simultaneous adduction (inward) movement in both eyes when viewing a near object
Describe the possible speeds of eye movement
- Saccade - short fast burst (reflexive, scanning, predictive or memory guided)
- Smooth pursuit - sustain slow movement, driven by motion of a moving target
List the extraocular muscles of the eye
- 6 muscles
- Attach eyeball to orbit
- Straight and rotary movement
- Four straight muscles (superior, inferior, lateral and medial rectus)
Where are the superior and inferior muscles attached? What is their function?
Superior rectus
- Attached to the eye at 12 o’clock
- Moves the eye up
Inferior rectus
- Attached to the eye at 6 o’clock
- Moves the eye down
Describe the location and function of the lateral rectus muscle
- Also called the external rectus
- Attaches on the temporal side of the eye
- Moves the eye towards the outside of the head (toward the temple)
Describe the location and function of the medial rectus
- Also called the internal rectus
- Attached on the nasal side of the eye
- Moves the eye towards the middle of the head
Describe the location and function of the superior oblique muscle
- 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
Describe the location and function of the 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)
Describe the innervation of the extraocular muscles
Third cranial nerve
- Superior branch supplies the superior rectus and lid levator
- Inferior branch supplies inferior and medial recus, inferior oblique and constricts pupil (parasympathetic nerve)
Fourth cranial nerve (superior oblique - depresses the eye)
Sixth cranial nerve
- Lateral rectus - abducts eye
How are eye movements tested?
- Isolate the muscle to be tested by maximising its action and minimising the action of other muscles
- Abduction - lateral retus
- Adduction - medial rectus
- Elevated and abducted - superior rectus
- Depressed and abducted - inferior rectus
- Elevated and adducted - inferior oblique
- Depressed and adducted - superior oblique
How is elevation of the eye described?
- Supraduction (one eye)
- Supraversion (both eyes)
How is depression of the eye described?
- Infraduction (one eye)
- Infraversion (both eyes)
How is dextroversion of the eye described?
- Movement right
- Right eye abduction
- Left eye adduction
How is levoversion of the eye described?
- Movement of the eyes to the left
- Left eye adduction
- Right eye abduction
Define torsion of the eye
- Rotation of the eye around the anterior-posterior axis of the eye
What happens in third nerve palsy?
- Affected eye down and outwards
- Droopy eyelid
- Unopposed superior oblique innervated by the fourth nerve (down)
- Unopposed lateral rectus action innervated by the sixth nerve (out)
What happens in sixth nerve palsy?
- Affected eye is unable to abduct and therefore deviates inwards
- Double vision which worsens on gazing to the side of the affected eye
When is reviewing the optokinetic nystagmus reflex useful?
- When testing visual acuity in pre-verbal children
- The presence of nystagmis movement (in response to moving grating patters of various spatial frequencies) signifies the subject can percieve the grating pattern
- Smooth pursuit and fast saccade
