Neuro 13 - Neurology of the Visual System Flashcards
Describe the visual pathway anatomy
Landmarks:
Eye –> optic nerve –> optic chasm –> optic tract —> lateral geniculate nucleus (ganglion fibres synapse here) —> optic radiation (4th order neuron) —> primary visual cortex (also extra striate cortex)
Where is the primary visual cortex located?
Occipital lobe
Describe the visual pathway in the retina
- First order neurones = rod and cone retinal photoreceptors
- 2nd order neurones = retinal bipolar cells
- 3rd order neurones = retinal ganglion cells (i.e. Optic nerve - which partially decussates at optic chiasm (53% crossing midline), optic tract)
Describe the retinal field of a neuron
Retinal space within which incoming light can alter firing pattern of a neurones
Photoreceptors feed into bipolar cells which feed into ganglion cells (+/- signals). Retinal ganglion cells get input from neighbouring photoreceptors (convergence)
What is convergence
Number of lower order neurones fields synapsing on same higher order neuron
Cones have lower convergence than rods. Central convergence is …. than peripheral convergence
Central convergence < peripheral convergence
Contrast high and low convergence
Low convergence (cones):
- Small receptive field
- Fine visual acuity
- Low light sensitivity
High convergence (rods):
- Large receptive field
- Coarse visual acuity
- High light sensitivity
Describe the 2 types of retinal ganglion cells
On centre:
- Stimulated by light at centre of receptive field
- Inhibited by light on edge of receptive field
Off centre:
- inhibited by light in centre of receptive field
- stimulated by light on edge of receptive field
On/off centre ganglion cells are important for contrast Sensitivity and enhanced edge detection
Describe crossing and uncrossing of fibres in the optic chiasm
Crossing fibres = originate from nasal retina, responsible for temporal visual field
Uncrossing fibres = originate from temporal retina, responsible for nasal visual field
Describe how lesions may affect the visual field
Lesions anterior to optic chiasm = affects visual field in one eye only
Lesions posterior to optic chiasm = affects vision in both eyes
When might one get left anopia (i.e. blindness in left side)
If left optic nerve damaged/lesioned
When might one get bitemporal hemianopia (i.e. outside temporal visual fields not present)
If lesion at optic chiasm
When might one get right homonymous hemianopia (i.e. can’t see in Lt nasal and Rt temporal fields)
Left sided lesion posterior to optic chiasm
Homonymous hemianopia is typically due to?
Stroke
Bitemporal hemianopia is typically due to?
Pit gland tumour - most common
When might right nasal hemianopia occur
If only half of the optic chiasm is affected
Most visual field defects affect the vertical line and not the horizontal line (except in quadrant anopia). What eye condition can affect the horizontal field line?
Glaucoma
Where is the primary visual cortex located
Along the cal calcarine sulcus in the occipital lobe (has distinct stripe from myelinated fibre of optic radiation projecting into visual cortex)
How is the primary visual cortex represented
- Disproportionately large area for macula
- Superior visual field –> projects BELOW calcarine fissure
- Inferior visual field –> projects ABOVE calcarine fissure
- Rt hemifield (stuff seen from R side of both eyes) —> L primary visual cortex
- Lt hemifield (stuff seen from L side of both eyes) —> R primary visual cortex
Describe the arrangement of the primary visual cortex in relation to its function
Arranged in columns —> R eye and L eye dominant columns intersperse each other
Each ganglion goes to a specific nucleus. This is what makes rewiring difficult.
Each column has unique sensitivity to visual stimulus of a particular orientation
How does macular sparing homonymous hemianopia arise
- Damage to primary VC due to stroke
- This leads to contralateral homonymous hemianopia (with macula sparing)
- The macula is spared because the area representing the macula receives dual blood supply from posterior cerebral arteries of both sides
Describe the extra striate cortex pathways
(Extrastriate cortex = area around primary VC in occipital lobe - it converts basic visual info, orientation fn position into complex information)
2 pathways-
- Dorsal pathway - Primary VC to posterior parietal cortex. Needed for motion detection, visually guided action. Damage results in motion blindness
- Ventral pathway - Primary VC to inferotemporal cortex. Needed for object representation, face recognition, detailed fine central vision and colour vision. Damage may lead to cerebral achromatopsia
Pupillary function is to regulate light input into the eye (less than 2 log unit change). How does this differ in light and dark
In light: constriction (circular)
- Decreases spherical aberrations and glare, increases depth of field, reduces bleaching of photo-pigments (less light enters eye)
- Constriction mediated by PNS (CN3)
In dark: dilatation (radial)
- increases light sensitivity by allowing more light into eye
- mediated by SNS
Describe the pathways of the pupillary reflex
Afferent:
- Rod and cone photoreceptors synapse on bipolar cells, which synapse on retinal ganglion cells
- Retinal ganglion cells exit at posterior third of optic tract–> then enter lateral geniculate nucleus
- Synapses at brainstem (in pretectal nucleus)
- Synapses on Edinger-Westphal nuclei on both sides in brainstem
Efferent pathway:
1. Edinger-westphal —> oculomotor nerve —> synapses at ciliary ganglion —-> short posterior ciliary nerve —> pupillary sphincter
What is the neurological basis for the consensual light reflex
Afferent pathway on either side stimulates efferent on both sides
Contrast defects in (right) afferent and efferents
Right afferent defect (e.g. damage to optic nerve) –> No constriction in either eye when R stimulated, but normal constriction in both when L stimulated
Right efferent defect (e.g. damage to oculomotor nerve) –> No R constriction when R or L stimulated, but L constricts when either eye stimulated
What is the swinging torch test used for
Testing relative afferent pupillary defects
Alternating stimulation of R and L eye
Both constrict when light moves to undamaged side
Both pupils (paradoxically) dilate when light swings to damaged side (as relatively reduced drive for pupillary constriction)
Define duction, version, vergence, convergence
Duction - eye movement in one eye
Version - simultaneous dual eye movement in same direction (dextroversion - gazing to R, levoversion - gazing to L)
Vergence - simultaneous dual eye movement in opposite direction
Convergence - simultaneous adduction in both eyes (when viewing near object)
What are the terms used for different eye movement speeds
- Saccade - short, fast burst (900 degrees /s) - voluntary or involuntary
- can be reflexive saccade to external stimuli
- scanning saccade (e.g. reading)
- Predictive (to track objects)
- memory guided saccade - Smooth pursuit - sustained slow movement (unto 60 degrees/s)
- driven by motion of moving target across retina
There are 6 extra ocular eye muscles. 5 out of 6 come from cone from back of orbit. Which is the exception and where does it come from
Inferior oblique - comes in nasally
6 extra ocular muscles used for straight and rotary movement
Describe the 4 straight muscles of the eye
- Superior rectus (attached to eye at 12 o’clock, moves eye up)
- Inferior rectus (6 o’clock, moves eye down)
- Lateral rectus - attaches high on temporal side of eye - moves eye towards temple
- Medial rectus - attached on nasal side of eye, moves eye towards nose
Describe the 2 rotary muscles of the eye
- Superior oblique - attached high on temporal side of eye. Passes under superior rectus. Anatomically moves eye down and in. Travels through trochlea
- Inferior oblique - attached low on nasal side of eye. Passes over inferior rectus. Anatomically, moves eye up and outwards
The oculomotor nerve supplies 4/6 extra ocular muscles. Describe the extraocular muscles it innervates and other things it innervates
CN3 divides into superior and inferior branches when it enters orbit
Superior branch - 1. Superior rectus- elevate eye
2. Lid levator - elevates eyelid
Inferior branch -
- Inferior rectus - depresses eye
- Medial rectus - adducts eye
- Inferior oblique - elevates eye
- PNS nerve - constricts pupil (circular muscle)
CN4 innervates which extra ocular muscle?
Superior oblique = trochlear
Superior oblique depresses eye
CN6 innervates which extra ocular eye muscle?
Lateral rectus - abducts eye
What can a CN3 palsy cause
Affected eye goes down and out (as superior oblique and lateral rectus only muscles not working)
Also causes droopy eyelid (ptosis) –> lid elevator muscle not functioning
What may a CN6 palsy cause?
Affected eye unable to abduct (LR not working)
Double vision also made more apparent on gazing to side of affected eye
What is optokinetic nystagmus reflex
Optokinetic nystagmus = smooth pursuit + fast-phase reset saccade
