Visual System Pathways Flashcards
What happens are the optic chiasm?
Half of the nerve fibres cross here
What happens at the Optic tract?
Ganglion nerve fibres exit as optic tract
What happens at the lateral geniculate nucleus?
Ganglion nerve fibres synapse at this place
What is the optic radiation?
4th Order neuron
What is the primary visual cortex found?
Within occipital lobe
What is the primary visual cortex also called?
Striate cortes
What are the 2nd order neurons called?
Retinal bipolar cells, come after the first order photoreceptors
What are the 3rd order neurons?
Retinal ganglion cells (includes optic nerve, decussation, tract, destination)
If a lesion affects the area anterior to the optic chiasma what does this to the visual field?
Affects one eye only
If a lesion is posterior to the optic chiasma what does this do to the visual field?
affects both eyes
What are the crossed fibres at the optic chiasma responsible for?
The temporal visual field
these fibres originate from the nasal retina
What are the uncrossed fibres responsible for?
Responsible for the nasal visual field
These fibres originate from the temporal retina
How does a lesion at the optic chiasma present as?
Temporal field deficit in both eyes = bitemporal hemianopia
- damages crossed fibres from the nasal retina in both eyes
How does a lesion posterior optic chiasma present?
Right sided lesion :left homonymous hemianopia in both eyes
Life sided lesion : right homonymous hemianopia in both eyes
What are the causes of bitemporal hemianopia?
- Typically caused by enlargement of Pituitary Gland Tumour
- Pituitary Gland sits under Optic Chiasma
What are the causes of homonymous hemianopia?
Stroke (Cerebrovascular Accident)
What causes Homonymous hemianopia with macula sparing?
Damage to primary visual cortex itself
Often due to stroke
Why is the macula spared?
Area representing macular recieves dual blood supply from posterior cerebral from both right and left arteries
What does pupil constriction do?
in light :
decreases spherical aberrations and glare
Increases depth of field
reduced bleaching of photo pigments
pupillary constriction mediated by parasympathetic nerve
What does pupil dilation do?
increased light sensitivity in the dark by allowing in more light
mediated by sympathetic nerve
Describe the afferent pathway of pupillary reflex? (3)
Rod and cone receptors synapse in bipolar cells which synapse on retinal ganglion cells
Pupil specific ganglion cells exit at posterior 1/3rd of optic tract before entering lateral geniculate nucleus
Afferent pathway from both eye synapse on edinger-westphal nuceli on both sides in brainstem
How does the efferent pathway in the pupillary reflex work? (3)
Edinger-westphal nucleus –> occulomotor nerve efferent
Synapses at ciliary ganglion
Short posterior ciliary nerve –> pupillary sphincter
What is direct light reflex?
Refers to constriction of pupil from light stimulated eye
WHat is consensual light reflex?
Constriction of pupil from fellow eye
- due to afferent pathway on either side alone stimualting efferent pathways on both sides
What may cause right afferent defect?
How will this present?
Damage to optic nerve
reflex test : no pupil constriction in both eyes when right is stimulated
normal pupil constriction in both eyes when left eye is stimulated
What may cayse right efferent defect?
Damage to Right 3rd Nerve
No right pupil constriction whether right or left eye is stimulated with light
Left pupil constricts whether right or left eye is stimulated with light
What is unilateral afferent defect?
Difference response pending on which eye is stimulated
What is Unilateral efferent defect?
Same unequal response between left and right eye irrespective which eye is stimulated
What is the swinging torch test?
Elicited by the swinging torch test – alternating stimulation of right and left eye with light
How does swinging torch test show relative afferent pupillary defect?
Partial pupillary response still present when the damaged eye is stimulated
Both Pupils constrict when light swings to left undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
