Visual Pathways Flashcards
Describe the landmarks of the visual pathway
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
Describe neural pathway from retina
First Order Neurons – Rod and Cone Retinal Photoreceptors
Second order Neurons – Retinal Bipolar Cells
Third Order Neurons –Retinal Ganglion Cells
Why is the optic chiasm important?
- Localising lesions
Lesions anterior to Optic Chiasma affect visual field in one eye only. Lesions posterior to Optic Chiasma affect visual field in both eyes. - Define visual fields
Crossed Fibres – originating from nasal retina, responsible for temporal visual field. Uncrossed Fibres – originating from temporal retina, responsible for nasal visual field.
What are possible visual field defects?
- Bitemporal hemianopia - lesion at optic chiasm so temporal field deficit in both eyes. Damaged crossed ganglion fibres from nasal retina in both eyes.
- Homonymous hemianopia - lesion posterior to optic chiasm. Right sided lesion – Left Homonymous Hemianopia in Both Eyes. Left sided lesion – Right Homonymous Hemianopia in Both Eyes.
Describe how damage along different points of the visual pathway causes visual defects
Lesion of optic nerve - monocular blindness
Lesion on one side of optic chiasm - right/left nasal hemianopia
Lesion at optic chiasm - bitemporal hemianopia
Lesion along optic tract - homonymous hemianopia
Lesion posterior to lateral geniculate nucleus - quadrantopia
Lesion in occipital lobe - macular sparing
What are the different hemianopias usually caused by?
Bitemporal hemianopia - pituitary tumours causing an enlargment
Homonymous - cerebrovascular event like stroke
What causes 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 is the purpose of pupil constriction in light?
Decreases spherical aberrations and glare
Increases depth of field
Reduces bleaching of photo-pigments
Pupillary constriction mediated by parasymapthetic nerve (within CN III)
What is the purpose of pupil dilatation in dark?
Increases light sensitivity in the dark by allowing more light into the eye
Pupillary dilatation mediated by sympathetic nerve
How is pupil size altered?
Parasympathetic stimulation causes circular muscles to contract. Sympathetic stimulation causes radial muscles to contract.
Describe the afferent part of pupillary reflex pathway
From retinal ganglion cell, 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.
Describe the efferent part of pupillary reflex pathway
Edinger-Westphal Nucleus -> Oculomotor Nerve Efferent -> Synapses at Ciliary ganglion -> Short Posterior Ciliary Nerve -> Pupillary Sphincter
It is important to note that afferent pathway from either eye, stimulates the efferent pathway on both eyes.
What is direct and consensual reflex?
Direct Light Reflex refers to the constriction of the pupil from the light-stimulated eye. Consensual Light Reflex refers to the constriction of the pupil from the fellow eye. Occurs because afferent pathway on either side alone will stimulate efferent (outgoing) pathway on both sides.
How are afferent and efferent defects different?
If unilateral afferent pathway defect, no pupillary constriction observed when affected side stimulated. However, when other side stimulated, constriction seen consensually.
If unilateral efferent affected, no matter which eye is stimulated only unaffected eye will constrict.
What is Relative Afferent Pupillary Defect?
Partial pupillary response still present when the damaged eye is stimulated as damage to afferent pathway is usually incomplete or relative. There is some degree of of pupillary constriction, albeit weaker response when the damaged side is stimulated. The best way to demonstrate this weaken response, is to stimulate one eye at a time, alternating between the right and the left eye. Swinging torch test used to do this.
