Neurology of the Visual System Flashcards
What is the visual pathway?
- Eye
- Optic nerve – ganglion nerve fibres
- Optic chiasm – half of fibres decussate here
- Optic tract – ganglion fibres exit as optic tract
- Lateral Geniculate Nucleus – ganglion fibres synapse in nucleus
- Optic radiation – 4th order neuron
- Primary visual cortex OR Striate Cortes (occipital lobe)
What are the different types of neurons in the retina?
First order neurons – Rod and Cone retinal photoreceptors
Second order neurons – Retinal bipolar cells
Third order neurons – Retinal ganglionic cells
- Travel down the length of the optic nerve and have a PARTIAL decussation (53% cross) at the optic chiasm
- Fibres synapse in the LGN (Lateral Geniculate Nucleus) located within the thalamus
What is the receptive field?
the retinal space within which incoming light can alter the firing pattern of a neuron
What is convergence?
number of lower order neurons field synapsing on the same higher order neuron
The cone system has a lower order of convergence than the rod system (as the cones tend to be one-to-one whereas many photoreceptors link to a single RGC in the rod system)
Low Convergence
- Small Receptive Field
- Fine Visual Acuity
- Low Light Sensitivity
High Convergence
- Large Receptive Field
- Coarse Visual Acuity
- High Light Sensitivity
What are on centre and off centre ganglion cells?
On-centre ganglionic cells:
- Stimulated by light at the CENTRE of the receptive field
- Inhibited by light on the EDGE of the receptive field
- Off-centre ganglionic cells:
- Inhibited by light at the CENTRE of the receptive field
- Stimulated by light at the EDGE of the receptive field
This is important for – contrast sensitivity and edge detection
What are crossed fibres?
originate from nasal retina, responsible for temporal visual field
What are uncrossed fibres?
originate from temporal retina, responsible for nasal visual field
What does a lesion at the optic chiasm cause?
damages crossed RGC fibres from nasal retina in both eyes -> temporal field deficit in both eyes – Bitemporal hemianopia.
What would a lesion posterior to the optic chiasma cause?
Right = left homonymous hemianopia in both eyes
Left = right homonymous hemianopia in both eyes
What are causes of bitemporal and homonymous heminopia?
Bitemporal hemianopia
- usually enlargement of pituitary gland tumour.
Homonymous hemianopia
- stroke
What happens in macula sparing disorder?
loss of peripheral vision but you retain central activity
What’s the difference between the consequence of glaucoma and neurological problems on vision?
Glaucoma often affects the HORIZONTAL plane of loss of vision
Neurological problems often affect the VERTICAL plane of loss of vision
What is the primary visual cortex?
*striate cortex
Situated along the Calcarine fissure
Characterised by a distinct stripe derived from the myelinated fibre of the optic radiation projection
Representation:
- Disproportionately large area representing the macula – due to a higher density of RGCs (lower convergence).
- Visual fields:
> Superior visual field projects BELOW the Calcarine fissure
> Inferior visual field projects ABOVE the Calcarine fissure
> Left hemi-field projects to the right
> Right hemi-field projects to the left
How is the primary visual cortex organised?
Organised as columns with unique sensitivity to visual stimulus of a particular orientation
Right and left eye columns intersperse each other
What are common causes of macula sparing homonymous heminopia?
- Commonly due to damage of the primary visual cortex
- Often due to stroke
- Contralateral homonymous hemianopia with central macula sparing
- Macula is spared due to dual blood supply.
What is the extrastiate cortex?
the area surrounding the visual cortex
Converts basic visual information, orientation and position into complex information
What are the pathways in the extrastriate cortex?
Dorsal pathway:
- Primary visual cortex -> posterior parietal cortex
- Motion detection, visually-guided action.
- Damage results in motion blindness
Ventral pathway:
- Primary visual cortex -> Inferiotemporal cortex
- Object representation, face recognition, detailed fine central vision and colour vision
- Damage results in cerebral Achromatopsia
What is the function of the pupillary?
In light – decreases spherical aberrations and glare, increases depth of field, reduces bleaching of photopigments
- Constriction mediated by PNS nerve within CN III
In dark – allows more light into eye
- Dilation mediated by SNS nerve.
What happens in the pupillary reflex?
- look at diagram
Afferent pathway:
Rod and cone photoreceptor -> bipolar cells -> RGCs -> nerves exit at posterior 1/3rd of optic tract and enter LGN -> synapse at brainstem (Pretectal nucleus) -> synapse at Edinger Westphal nucleus
Efferent pathway :
Edinger Westphal -> oculomotor nerve efferent -> synapse at ciliary ganglion -> short posterior ciliary nerve -> pupillary sphincter
- the afferent pathway on either eye stimulates the efferent pathway in BOTH eyes
** Direct light reflex – constriction of pupil of the light stimulated eye
*** Consensual light reflex – constriction of pupil of the other eye
What happens in right afferent defect?
damage to CN II
no pupil constriction in both eyes when right is stimulated, pupil constriction in both eyes when left eye is stimulated
What happens in right efferent defect?
damage to right CN III
no right pupil constriction at all, left eye constricts whether left or right is stimulated
What can you do to demonstrate weakness of the afferent pathway?
swinging torch test
The damage to the afferent pathway is usually incomplete or relative e.g Relative afferent pupillary defect in right eye
- Partial pupillary response still present when the damaged eye is stimulated - Elicited by the swinging torch test – alternating stimulation of right and left eye with light - Both pupils constrict when light swings to left eye with intact pathway - Both pupils will paradoxically dilate when light swings to the right eye as a result of relatively reduced drive for pupillary constriction in both eyes
What happens in unilateral afferent and efferent defects?
Unilateral Afferent Defect
- Difference response pending on which
eye is stimulated
Unilateral Efferent Defect
- Same unequal response between left and
right eye irre
What are the movements of the eye?
Duction – eye movement in one eye
Version – eye movement in both eyes in the same direction, dextro- is to the right, levo- is to the left
Vergence – eye movement in both eyes in opposite directions
Convergence – simultaneous adduction (inwards) movement in both eyes when viewing a near object
