The Retina and Central Visual Pathways Flashcards
Why does the position of the optic nerve form a ‘blind spot’ of vision?
there are no photoreceptors located in the area of the optic nerve leaving the retina/eye
What is the purpose of the pigmented layer of the reint, and in which population does this pose a potential problem?
prevent excess light reflections within the retina which would negatively affect vision
problem for albino population as they lack melanin, which allows excessive scattering of light on their reitna in bright conditions
Which type of photoreceptor in the eye are responsible for black and white/low level lighting vision?
Rods
(this is the reason we can only detect monochrome colours at night, as the cones are not active)
What is the function of the cone photoreceptors in the eye?
Colour Vision
Detailed Vision in Bright Light
Which cells of the reitna converge to form the optic nerve?
Ganglion Cells
What is the function of horizontal cells in the retina?
lateral inhibition
allows the photoreceptors detecting the brightest light to inhibit neighbouring cells to allow the contract/sharpness of the image to be enhanced
What can be found in the region of the optic disc on the back of the retina?
Optic Nerve
Central Vessels of Optic Nerve
(central retinal artery & vein)
What is the functional importance of the fovea of the retina?
highest density of photoreceptors
higest area of visual acuity
Does the fovea contain a higher density of rod or cone shaped photoreceptors?
Cones
(better for high quality images)
Other than a high density of photoreceptors, how else if the fovea adapted to perform it role of vision?
thinner retinal layer due to absence of retinal ganglion cell axons
What is the underlying pathological process in a detached retina?
detachment of the photoreceptors from the underlying pigmented layer of the retina
Identify the following structures seen on fundoscopy
What symptom do patients commonly present with when suffering from Amaurosis Fugax and what is the underlying pathology?
curtain coming down over vision
transient monocular blindness believed to be due to microemboli or vasospasm
Why can the optic disc appear to be oedematous with raised intracranial pressure?
Optic nerve is connected to the brain and is surrounded by the meningeal layers, which allows any pressure to transmit from the brain along the optic nerve
Identify the structures indicated by the red arrows
Which fibres of the optic nerve decussate at the optic chiasm?
Nasal Fibres
What defect in the visual pathway would lead to a right sided monocular blindness?
lesion of the right optic nerve (1)
e.g. optic sheath meningiomas
What defect in the visual pathway would lead to bitemporal hemianopia?
lesion at optic chiasm (2)
affects both NASAL fibres
e.g. pituitary/anterior communicating artery disease
What defect in the visual pathway would lead to left sided homonomous hemianopia?
lesion of right optic tract (3)
affected right temporal and left nasal fibres
e.g. vascular stroke/neoplasia
Why do patients suffering a stroke in the occipital lobe (caused by occlusion of the posterior cerebral artery), sometimes present with macula sparing?
Occipital lobe has a dual blood supply from:
- posterior cerebral artery
- branch of middle cerebral artery
thus, the blood flow can be preserved to the occipital pole which provides central vision
Outline the pathway of the pupillary light reflex
Retinal Sensory Afferent (via CN II) > pretectal area & lateral geniculate nucleus
pretectal synapses with Edinger-Westphal Nucleus (midbrain) > parasympathetic fibres on CN III leave Edinger-Westphal Nucleus > pass via cillary ganglion > synapse at sphincter pupillae
direct and consensual pupilly constriction
Outline the nervous pathway involved in the accommodation reflex
retina > optic nerve > lateral geniculus >
communicates with Edinger-Westphal nucleus
efferents travel via occulomotor nerve (parasmpathetics) > sphinctor pupillae and medial rectus muscle
