Session 4 - Central Visual Pathways Flashcards
What are the two layers of the retina?
Pigmented layer (retinal pigmented epithelium) Neural layer
What layer covers the retinal pigmented epithelium (RPE)?
Choroid
What pigment is found in the RPE?
Melanin
Why do albinos struggle to see in bright light?
They have no melanin in their retinal pigmented epithelium. This means less light is absorbed in the retina and more is reflected, making it difficult for albinos to see in bright light.
What are the two types of photoreceptor cells?
Rods and cones
What is the function of rod cells?
Receptor cells for black and white vision, low level lighting.
What is the function of cone cells?
Receptor cells for colour vision, high definition
Other than rod and cone cells, what other cell types are present in the retina?
Bipolar cells
Horizontal cells
Amacrine cells
Ganglion cells
Which area of the retina has the highest concentration of cone cells, and therefore the greatest ability for definition?
Fovea
Raised intracranial pressure can result in what condition seen on fundoscopy?
Papilloedema
What structures should be visible in a normal fundoscopy?
Optic disc Fovea Macula Central retinal artery and vein Retinal arterioles and venules
What conditions can be visible on fundoscopy?
Retinopathies Vascular occlusions - branch of central retinal artery - amaurosis fugax Papilloedema
Which fibres decussate and which run ipsilateral?
Nasal fibres decussate
Temporal fibres run ipsilateral
Which fibres are responsible for relaying the nasal fields of vision and which relay the temporal fields?
Nasal fibres relay the temporal fields
Temporal fibres relay the nasal fields
Where do the nasal fibres decussate?
Optic chiasm
Where do the optic tracts run from and to?
From the optic chiasm to the lateral geniculate nucleus.
Superior optic radiations are responsible for which field of vision?
Inferior field of vision
Inferior optic radiations are responsible for which field of vision?
Superior filed of vision
Which lobes of the brain do the optic radiations project into?
Superior radiations project into the parietal lobe
Inferior radiations project into the temporal lobe
Where is the primary visual cortex located?
Occipital lobe
How are visual field defects named?
Named based on the area of visual loss rather than the site of the lesion.
Lesions before the optic chiasm produce what signs relative to the lesion?
Signs are unilateral and ipsilateral
What is monocular blindness and what lesion causes it?
Complete loss of either the left or right visual field. Lesion of optic nerve on ipsilateral side.
E.g. lesion of right optic nerve causes right monocular blindness
Give examples of conditions that can cause monocular blindness.
Optic nerve glioma or retinoblastoma (children)
Optic sheath meningiomas (middle aged)
What is bitemporal hemianopia, what lesion causes it?
Tunnel vision.
Lesion at the optic chiasm affecting both nasal fibres resulting in the loss of both temporal fields.
What is left hemianopia, what lesion causes it?
Lesion of the right optic tract affecting the right temporal and left nasal fibres, resulting in loss of the right nasal and left temporal fields.
What is right hemianopia, what lesion causes it?
Lesion of the left optic tract affecting the left temporal and right nasal fibres, resulting in the loss of the left nasal and right temporal fields.
What are the possible causes of hemianopia?
Vascular causes are the most common (e.g. stroke)
Also neoplasia and trauma
What lesions cause quadrantanopias?
Lesions of the superior or inferior optic radiations
The occipital lobe is supplied by which two arteries?
Posterior cerebral artery
Middle cerebral artery (occipital pole)
Why does macular sparing occur in a stroke affecting the posterior cerebral artery?
In a stroke affecting the posterior cerebral artery, most of the occipital lobe will be affected. However, the middle cerebral artery’s supply to the occipital lobe will still be intact. The middle cerebral artery supplies the macula therefore macular function (central vision) will be spared.
Describe the light reflex.
Light simulates the optic nerve (CN II) - afferent
Synapses in the pretectal area - gives rise to neurones supplying Edinger-Westphal nuclei bilaterally
Both oculomotor nerves (CN III) are stimulated to cause direct and consensual pupillary constriction - parasympathetic fibres
What are the three aspects of the accommodation reflex?
Convergence (medial rectus)
Pupillary constriction (constrictor pupillae)
Convexity of the lens to increase refractive power (ciliary muscle)
What is the function of the medial longitudinal fasciculus?
Links the three main nerves controlling eye movement (oculomotor, trochlear, abducens) to allow coordination of eye movements.
