TSM46: VISUAL PATHWAY
23/10/08
LEARNING OUTCOMES
Describe the anatomy of the visual pathway
FEATURES OF THE RETINA
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Light entering the eye is focussed and refracted onto the retina by the cornea and lens
o Due to refraction the image reaching the retina is inverted
The optic disc is said to be in the centre of the retina but contains no photoreceptors
o The disc is in fact slightly medial to the central point of the visual axis (see below)
o This results in a physiological scotoma roughly 2mm in diameter for each eye
o These blind spots are not normally perceived since each one is in the peripheral vision of the
other eye (however they can be perceived if one eye is closed)
Light travelling centrally along the visual axis reaches the macula in the posterior pole
o 5mm circular area densely populated with photoreceptors
o Contains the fovea at its centre roughly 5mm temporal from the optic disc
The fovea is a 1mm circular area which has only retinal cones and no rods
o The site of highest visual acuity in the eye
VISUAL FIELDS
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The human eye receives and processes information from a number of different visual fields:
o Central – reaching the fovea and surrounding macula; areas of highest visual acuity
o Peripheral – can be divided into four quadrants
 Upper and lower temporal quadrants
 Upper and lower nasal quadrants
Due to image inversion onto the retina:
o The temporal field is represented on the nasal retina and vice versa
o The upper quadrant fields are represented on the lower retinal quadrants and vice versa
VISUAL PATHWAYS
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Afferent fibres from the temporal and nasal areas of the retina utilise different visual pathways:
o Temporal fibres proceed unilaterally
o Nasal fibres decussate at the optic chiasm and proceed contralaterally
Mixed fibres continue along the optic tracts and synapse on the lateral geniculate nuclei
o Laminated retinotopic structure
o Macular fibres synapse in dorsal regions
o Peripheral fibres synapse in ventral regions
 Upper retinal quadrant fibres synapse medially
 Lower retinal quadrant fibres synapse laterally
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Second order neurones arise as optic radiations and travel along the geniculocalcarine tract
o Upper retinal quadrant fibres project directly posteriorly
o Lower retinal quadrant fibres initially project temporally in Meyer’s loop
All fibres terminate on the primary visual cortices where vision is first realised
Each side of the brain therefore receives contralateral temporal fields and ipsilateral nasal fields
o Macular fibres terminate posteriorly
o Peripheral fibres terminate more anteriorly
Third order neurones make complex connections to other areas of the visual cortex via two ‘streams’:
o Ventral stream – what; recognition, identification and categorisation
o Dorsal stream – where; localisation, spatial attention
Describe the relationship between visual deficits and the underlying anatomy
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Lesions of specific regions of the visual pathway result in characteristic deficits:
o Optic nerve lesions result in ipsilateral anopia
o Optic chiasm lesions result in bitemporal hemianopia
o Optic tract lesions result in contralateral homonymous hemianopia
o Meyer’s loop lesions result in contralateral homonymous upper quadrantanopia
Lesions of the optic radiations or primary visual cortex will result in hemianopia but may feature
macular sparing
Some lesions of the optic nerve or demyelination may result in a relative afferent pupil defect (RAPD)
o Shining light in one eye triggers the pupillary light reflex of pupillary constriction in both eyes
 Afferent retinal fibres synapse on the pretectal area
 Neurones project to the Edinger-Westphal nucleus
 Parasympathetic fibres via the ciliary ganglion cause pupillary constriction
o Damage to afferent fibres involved in this reflex will result in RAPD
o Can be tested routinely by eliciting direct and consensual pupillary responses
Describe the basic mechanism of phototransduction
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Photons strike the photosensitive cells of the retina and hyperpolarise them
o Retina, a light-sensitive vitamin A derivative, is isomerised triggering a signal cascade
o Phosphodiesterase lowers cGMP concentrations causing closure of sodium channels
o The cell releases less glutamate as a result of hyperpolarisation
o Bipolar cells are depolarised generating an electric signal
o Perhaps counter-intuitively, photoreceptors cells are depolarised in the dark
Describe the basis of retinal colour blindness
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The photosensitive retinal cones are responsible for colour vision
o There are three different cone-associated proteins that facilitate primary colour recognition
Defective genes coding for retinal cones can result in an inability to detect parts of the visible spectrum
o Two or more defective cone proteins result in complete monochromatic colour blindness
Be aware of the dual functions of vision
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Perception – being able to recognise and distinguish between objects
Action – being able to interact with objects