Visual Pathway - Part 1 Flashcards
What are the visual pathway landmarks?
Eye Optic nerve Optic chiasm Optic tract Lateral geniculate nucleus Optic radiation Primary visual cortex/striate cortices Extrastriate cortex
What are the first second and third order neurones?
First order- rods and cones
Second order - bipolar neurones
Third order - retinal ganglion cells
What are the fourth order neurones and where do they arise from?
Optic radiation, from lateral geniculate nucleus to primary visual cortex
What is the receptor field of a photoreceptor?
The retinal space surrounding a photoreceptor, where light falling activates or alters the firing rate of the photoreceptor
What is convergence?
The number of lower order neurons (photoreceptors, bipolar neurones) synapsing onto the same higher order neuron (ganglion cell)
Compare cone and rod convergence
Cone system convergence > rod system convergence
Compare central and peripheral retinal convergence
Central retinal convergence > peripheral retinal convergence
Describe the features of low convergence
Small receptor fields
Low light sensitivity
Fine visual acuity (because you can distinguish more finely)
Describe the features of high convergence
Large receptor fields
High light sensitivity
Coarse visual acuity
What are the two classifications of retinal ganglion cells. Describe the difference
On centre and off centre
On centre - activated by light at centre of receptive field, inhibited by light at periphery
Off centre - stimulated by light at periphery of receptive field, inhibited by light at centre
What are the advantages of having on and off centre ganglion cells?
Better contrast sensitivity
Enhanced edge detection
What do lesions anterior to the optic chiasm affect?
One eye only
What do lesions posterior to the optic chiasm affect?
Both eyes, due to crossing over
Which part of the visual field are the crossed fibres of the optic chiasm responsible for?
Temporal visual field
Nerves originate from nasal side
What would result from a lesion just anterior to the optic chiasm?
Bitemporal hemianopia
Where would the lesion be in homonymous hemianopia/
On either the left or right optic tract or occipital lobe
What would a lesion on the right, posterior to the optic chiasm result in?
LEFT homonymous hemianopia
And vice versa.
Give a cause of bitemporal hemianopia
Pituitary adenoma, as pituitary sits underneath optic chiasm
Give a cause of homonymous hemianopia
CVA e.g. stroke
What is another name for the primary visual cortex?
Striate cortex
Where is the primary visual cortex?
Situated along calcarine sulcus within occipital lobe
Describe the representation of the macula in the striate cortex
It’s disproportionately largely represented
Where does the superior visual field project in the cortex?
BELOW calcarine fissure
Where does the inferior visual field project in the cortex?
ABOVE calcarine fissure
What projects to the left primary visual cortex?
RIGHT hemifield from both eyes
What projects to the right primary visual cortex?
LEFT hemifield from both eyes
How is the primary visual cortex organised?
As columns with:
a unique sensitivity to a particular orientation
alternate dominant columns for left and right eye
What is often spared in homonymous hemianopia and why?
Macula is spared because the area which represents the macula has dual blood supply from posterior cerebral arteries from both sides!
What is the extrastriate cortex and what does it do??
The area surrounding the primary visual cortex
Converts basic visual info, orientation and position into complex info
What are the two pathways from the extrastriate cortex?
Dorsal pathway
Ventral pathway
What is the dorsal pathway from the extrastriate cortex?
Primary visual cortex to posterior parietal cortex (WHERE)
What is the ventral pathway from the extrastriate cortex?
Primary visual cortex to inferiotemporal cortex (WHAT)
What is the ventral pathway needed for?
Object recognition
Facial recognition
Detailed central vision
Colour vision
What is the dorsal pathway needed for?
Motion detection
Visually guided actions
What do damages to the ventral and dorsal pathways result in?
Damage to dorsal pathway: motion blindness
Damage to ventral pathway: cerebral achromatopsia
Besides limiting how much light enters the eye, why is pupil constriction important?
Increases depth of field
Reduces bleaching of photo-pigments
Decreases spherical aberrations
Decreases glare
Which nerve does the parasympathetic nerve travel with to control pupil constriction?
CN 3
Describe the afferent pathway of the pupil reflex
Rod and cone photoreceptors
Bipolar neurones
Retinal ganglion cells
Optic nerve
Pretectal nucleus of midbrain (same side)
Edinger-Westphal nucleus of midbrain (both sides)
Describe the efferent pathway of the pupil reflex
Edinger-Westphal nucleus of midbrain (from both sides) Occulomotor nerve Ciliary ganglion Short posterior ciliary nerve Pupillary sphincter
What would you see when you shine light into both eyes if there was damage to the optic nerve leaving the RIGHT eye?
Light shone in right eye: no pupil constriction in both eyes
Light shone in left eye: normal pupil constriction in both eyes
What would you see if there was efferent nerve damage to the RIGHT oculomotor nerve?
No pupil constriction in right eye, regardless of which eye the light is shone into
Normal constriction of left eye despite which eye the light is shone into
If one eye doesn’t constrict regardless of which eye light is shone into, is the damage afferent or efferent?
Efferent
If there is no pupil constriction when you shine the light into one eye, but both eyes constrict normally when the light is shone into the other eye, where is the lesion?
Afferent - side of eye that you shine light into that doesn’t elicit consensual reflex
What is the RAPD swinging torch test? Describe where the damage would be in relation to where the light is shone.
The point of swinging is to see whether, in comparison to the constricted pupil, there is a paradoxical relative dilation in both eyes when the light swings to the DAMAGED side due to afferent pupillary defect
