Visual systems pathways Flashcards
What is the main function of the visual pathway?
transmits signal from eye to the visual cortex
What are the visual pathway 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 at Lateral Geniculate Nucleus
Optic Radiation – 4th order neuron
Primary Visual Cortex or Striate Cortes – within the Occipital Lobe
What are the first order neurone?
Rod and Cone Retinal Photoreceptors
What are the second order neurons?
Retinal Bipolar Cells
What are the third order neurons?
Retinal Ganglion Cells
What are the features of the optic nerve?
Optic Nerve (CN II)
Partial Decussation at Optic Chiasma – 53% of ganglion fibres cross the midline
Optic Tract
Destinations
Lateral Geniculate Nucleus (LGN) in Thalamus – to relay visual information to Visual Cortex
What is the optic chiasm?
Important Landmark in Visual Pathway
What do lesion of the optic chiasma result in?
Lesions anterior to Optic Chiasma affect visual field in one eye only
Lesions posterior to Optic Chiasma affect visual field in both eyes
What are the crossed fibres responsible for?
originating from nasal retina, responsible for temporal visual field
What are uncrossed fibres responsible for?
originating from temporal retina, responsible for nasal visual field
Give examples of disorders of the visual pathway
Monocular blindness Bitemporal hemianopia Right nasal hemianopia Homonymous hemianopia Quadrantanopia Macular sparing
What is bitemporal hemianopia caused by?
Typically caused by enlargement of Pituitary Gland Tumour
Pituitary Gland sits under Optic Chiasma
What causes homonymous hemianopia?
Stroke (Cerebrovascular Accident)
What are the features of homonymous hemianopia with macular sparing?
Damage to Primary Visual Cortex
Often due to stroke
Leads to Contralateral Homonymous Hemianopia with Macula Sparing
Area representing the Macula receives dual blood supply from Posterior Cerebral Arteries from both sides
What happens to the pupil in light?
Constriction
decreases spherical aberrations and glare
increases depth of field
reduces bleaching of photo-pigments
What is pupillary constriction mediated by?
parasymapthetic nerve (within CN III)
What happens to the pupil in dark?
Dilatation
increases light sensitivity in the dark by allowing more light into the eye
pupillary dilatation mediated by sympathetic nerve
What are the two pathways involved in pupillary reflex?
Afferent
Efferent
What are the features of the afferent pathway?
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
What are the features of the efferent pathway?
Edinger-Westphal Nucleus -> Oculomotor Nerve
Efferent ->
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 consensusual light reflex?
Constriction of Pupil of the other (fellow) eye
What is the neurological basis of the consensual light reflex?
Afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides
What is the 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 is the 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 is the unilateral afferent defect?
Difference response pending on which eye is stimulated
What is the unilateral efferent defect?
Same unequal response between left and right eye irrespective which eye is stimulated
What is the swinging torch test?
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 undamaged side
Both Pupils paradoxically dilate when light swings to the right damaged side
