Visual System - Pathways Flashcards
What is the visual pathway? What does it do?
What is the anatomy of the visual pathway?
Visual pathway transmits signal from the eye to the visual cortex
Light travels: into the eye, via the retina, through the optic nerve and optic chiasm, down the optic tracts to the lateral geniculate nucleus, which synpases onto the optic radiation (4th order neuron) and finally to the Primary Vidual Cortex or Striate Cortes
What type of nerve fibres are the optic nerve and optic tracts?
What occurs at the optic chiasm?
Ganglion nerve fibres
Half of the ganglion nerve fibres from the optic nerve cross over, from then they are called the optic tract
What is the retina?
What type of neurons does it contain and what is its function?
First step of the visual pathway
Contains: 1st order neurons = red and cone retinal photoreceptors
2nd order neurons = retinal bipolar cells
3rd order neurons = retinal ganglion cells. These transmit the signal down the optic nerve
What is the optic chiasm?
53% of the ganglion nerve fibres cross:
Crossed Fibres - originating from nasal retina responsible for temporal visual field
Uncrossed fibres - originating from temporal retina, responsible for nasal visual field
What happens if there is a lesion anterior or posterior to the optic chiasm?
What happens if there is a lesion on the optic chiasm?
Therefore, lesions anterior to the optic chiasm affects visual field in only one eye, lesions posterior to the optic chiasm affects visual fields in both eyes (right sided lesion = left homonymous hemianopia in both eyes, left sided lesion right homonymous hemianopia in both eyes)
Lesion on the optic chiasm = damaged crossed fibres from nasal retina in both eyes and temporal fibres from both sides (bitemporal hemianopia)
What would be the disorders of the visual pathways if there is damage to the following areas? 1 2 3 4 5 6 7
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What causes a bitemporal hemianopia?
What causes a homonymous hemianopia?
Englargement of the pituitary gland (as the optic chiasm sits on top) - e.g. pituitary tumour
Stroke
What is homonymous hemianopia with Macular Sparing?
What may cause this?
Damage to the Primary Visual Cortex
Often due to stroke - macula is spared because it receives dual blood supply from the poesterior cerebral arteries
What is the function of the pupils?
How do the pupils react in lots of light?
How do the pupils react in the dark?
Regulates light input into the eye
Pupil constriction - parasympathetic innervation from CN III. This decreases glare, increases dept of field and reduced bleadhing of photopigments
Pupil dilation - sympathetic innervation from CN III. This increases light sensitivity in the dark by allowing more light into the eye
How does the pupillary reflex take place?
What are the 2 pathways and how do they work?
Afferent pathway - pupil-specific ganglion cells exit the optic tract and enter the lateral geniculate nucleus, where they synapse on the Edinger-Westphal nuclei on both sides of the brainstem
Efferent pathway - Edinger-Westphal nucleus synapses onto CN III efferent, which synapses on the ciliary gangion, that then travels up the short posterior ciliary nerve to act on the pupillary sphincter muscles
What is the difference between direct VS consensual reflex?
Why does the consensual reflex happen?
Direct = pupil constriction in the eye being stimulated by light
Consensual = pupil constriction of the other eye when only one eye is being stimulated by light
Afferent pathway on either side alone will stimulate efferent pathway on both sides
What occurs if there is a right afferent defect?
e.g. if there is damage to the 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 occurs if there is a right efferent defect?
e.g. if there is damage to the 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 occurs if there is unilateral afferent defect?
Difference in response depending on which eye is stimulated
What occurs if there is a unilateral efferent defect?
Same unequal response between left and right eye irrespective which eye is stimulated
