visual systems 3 Flashcards
what is the anatomy of the visual pathway?
transmits the signal from eye to visual cortex
landmarks:
eye
optic nerve - ganglion nerve fibres
optic chiasm - half of the nerve fibres cross here
optic tract - ganglion nerve fibres exit as optic tract
lateral geniculate nucleus - ganglion nerve fibres synapse here
optic radiation - 4th order neurone
primary visual cortex or striate cortes - within the occipital lobe
what is the visual pathway in the retina?
first order neurones - rod and cone retinal photoreceptors
second order neurones - retinal bipolar cells (improve contrast sensitivity)
third order neurones - retinal ganglion cells:
optic nerve
partial decussation at the optic chiasm (53%)
optic tract
destinations (lateral geniculate nucleus in thalamus, to relay visual info to visual cortex (4th order)
what is the optic chiasm?
lesions anterior to this affect vision in one eye only
posterior, they effect both eyes
53% of ganglion fibres cross here
crossed fibres - originating from nasal retina, responsible for temporal visual field (peripheral)
uncrossed fibres - originating from temporal retina, responsible for nasal visual field
(so effectively the lateral parts of visual field cross over)
what happens in a lesion to the optic chiasm?
damages crossed ganglion fibres from nasal retina in both eyes
temporal field deficit in both eyes - bitemporal hemianopia
what happens in a lesion posterior to the optic chiasm?
right sided lesion: left homonymous (both same sides eg. one nasal, one temporal) hemianopia in both eyes
left sided lesion: right homonymous hemianopia in both eyes
what are the hemianopias?
bitemporal
typically caused by an enlargement of pituitary gland tumour
pituitary gland sits inder optic chiasm
homonymous:
eg. left sides of both eyes
lesion in one side post optic chiasm
due to stroke (cerebrovascular accident)
what is a homonymous hemianopia with macular sparing?
damage to primary visual cortex
often due to stroke
leads to contralateral homonymous hemianopia with macular sparing
area representing the macula receives dual blood supply from both the posterior and middle cerebral arteries
what is the function of the pupil?
regulates the amount of light entering the eye
what happens to the pupil in bright light?
pupil constricts
decreases spherical aberrations and glare
increases depth of visual field (near response triad)
reduces bleaching of photo pigments
pupillary constriction mediated by parasympathetic nerve
what happens to the pupil in the dark?
pupil dilates
increases light sensitivity in the dark by allowing more light into the eye
pupillary dilation is mediated by sympathetic nerves
how does the pupillary reflex work?
afferent pathways:
Rod and Cone Photoreceptors synapsing on Bipolar Cells synapsing on Retinal Ganglion Cells
Pupil-specific ganglion cells exits at posterior third of optic tract before entering the Lateral Geniculate Nucleus
Afferent (incoming) pathway from each eye synapses on Edinger-Westphal Nuclei on both sides in the brainstem
efferent pathway:
Edinger-Westphal Nucleus -> OCCULOMOTOR NERVE Efferent ->
Synapses at Ciliary ganglion ->
Short Posterior Ciliary Nerve -> Pupillary Sphincter
what is the direct vs consensual pupillary reflex?
direct: constriction of pupil in light stimulated eye
consensual: constriction of pupil in other eye
neurological basis:
afferent pathways on either side alone will stimulate efferents on BOTH sides
what are afferent vs efferent defects?
Right AFFERENT Defect:
E.g. damage to optic nerve
No pupil constriction in both eyes when right eye is stimulated with light
Normal pupil constriction in both eyes when left eye is stimulated with light
Right EFFERENT Defect:
E.g. 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
UNILATERAL AFFERENT Defect:
Different response pending on which eye is stimulated
UNILATERAL EFFERENT DEFECT:
Same unequal response between left and right eye irrespective which eye is stimulated
what is the swinging torch test?
tests for relative afferent pupillary defect in one eye
Partial pupillary response still present when the damaged eye is stimulated. (Damage to afferent pathway is usually incomplete, or relative.)
Elicited by the swinging torch test – alternating stimulation of right and left eye with light
Both Pupils constrict when light swings to left undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
