Visual System - Pathways Flashcards
The visual pathway transmits signals from where to where?
eye
visual cortex
List visual pathway landmarks.
eye, optic nerve, optic chiasm, optic tract,
lateral geniculate nucleus – Ganglion nerve fibres synapse at Lateral Geniculate Nucleus in thalamus, optic radiation – 4th order neuron, primary visual cortex or Striate Cortes – within the Occipital Lobe
First order neurons made up of?
rod and cone retinal photoreceptors
Second order neurons made up of?
retinal bipolar cells
Third order neurons made up of?
retinal ganglion cells
53% of ganglion fibres decussate at what location?
optic chiasm
How do lesions anterior to the optic chiasm affect the visual field?
only one eye affected
How do lesions posterior to the optic chiasm affect the visual field?
both eyes affected
Crossed fibres originate from what part of the retina?
nasal retina
Crossed fibres are responsible for what part of the visual field?
temporal visual fibres
Uncrossed fibres originate from what part of the retina?
temporal retina
Uncrossed fibres are responsible for what part of the visual field?
nasal visual field
When there is a lesion at the optic chiasm, what visual deficit occurs?
bitemporal hemianopia
A right sided lesion posterior to the optic chiasm leads to what visual defect?
left homonymous hemianopia in both eyes
A left sided lesion posterior to the optic chiasm leads to what visual defect?
right homonymous hemianopia in both eyes
Bitemporal hemianopia is typically caused by?
enlargement of pituitary gland tumour
Homonymous hemianopia is typically caused by?
stroke (cerebrovascular accident)
What is macular sparing?
visual field loss that preserves vision in the center of the visual field
Damage to the primary visual cortex leads to?
contralateral homonyous hemianopia with macula sparing
Homonymous hemianopia with macular sparing is typically due to?
stroke
The area representing the macula receives blood supply from?
posterior cerebral arteries from both sides
In light, the pupillary reflex causes the pupil to?
constrict
In darkness, the pupillary reflex causes the pupil to?
dilate
Pupil constriction causes?
decreases spherical aberrations and glare
increases depth of field
reduces bleaching of photo-pigments
Pupillary constriction is mediated by what nerve?
parasympathetic nerve within CN III
Pupil dilation causes?
increases light sensitivity in the dark by allowing more light into the eye
Pupillary dilatation is mediated by what nerve?
sympathetic
Describe the afferent pathway of the pupillary reflex.
rod/cone photoreceptors > bipolar cells > retinal ganglion cells > pupil specific ganglion cells exit posterior 1/3 of optic tract > lateral geniculate nucleus
Afferent reflex pathways from each eye synapse on what nuclei on both sides of the brainstem?
Edinger-Westphal
Describe the efferent pathway of the pupillary reflex.
Edinger-Westphal nucleus > oculmotor nerve > synapses at ciliary ganglion > short posterior ciliary nerve > pupillary sphincter
What is the direct light reflex?
constriction of pupil of the light stimulated eye
What is the consensual light reflex?
constriction of pupil of the non-light stimulated eye
Explain the neurological basis of the consensual light reflex.
afferent pathway on either side alone will stimulate efferent pathway on both sides
What happens to pupillary reflex with 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
What happens to pupillary reflex with 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
What happens to pupillary reflex with unliateral afferent defect?
Difference response pending on which eye is stimulated
What happens to pupillary reflex with unliateral efferent defect?
Same unequal response between left and right eye irrespective which eye is stimulated
What is the relative afferent pupillary defect?
Partial pupillary response still present when the damaged eye is stimulated
What test elicits the response from the relative afferent pupillary defect?
swinging torch test
What happens in the swinging torch test with the relative afferent pupillary defect?
Both Pupils constrict when light swings to left undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
What is the swinging torch test?
alternating stimulation of right and left eye with light
