6) Retina and Central Visual Pathways Flashcards
What are the layers of the retina from outside to inside?
Pigmented epithelial layer
Photoreceptor cells
Bipolar neurones
Retinal ganglion cells
What is the function of the pigmented epithelial layer?
Anchor for photoreceptor cells
Contain melanin to prevent excessive reflections in the eyeball
In which condition is there a lack of melanin in the retina? What symptoms occur?
Albinism - struggle in bright lights
What are the functions of the photoreceptor cells?
Rods - low level light and black and white
Cones - higher acuity and colour vision
What are the functions of the horizontal cells?
Inhibitory interneurones, help to increase contrast by lateral inhibition
Describe the fovea centralis:
Depression in macula lutea and has highest density of cones to give high resolution, central vision
What clinical features can be seen on fundoscopy?
Retinopathies, vascular occlusions, macula and optic disc (swelling)
What is amaurosis fugax?
TIA blocking ophthalmic artery
Curtain over vision due to hypoxia of retina
Prelude to stroke
What is OCT?
Optical coherence tomography - high resolution images of retina
What is retinal detachment? What symptoms can occur?
When photoreceptors come away from retinal pigment epithelium
Blurring, loss of vision, seeing stars
Describe the general visual pathway:
Optic nerve -> optic tract -> optic radiations -> primary visual cortex
Describe how the path of temporal and nasal fibres differ:
Temporal fibres run ipsilateral
Nasal fibres decussate at optic chiasm
Describe where the optic radiations are located:
Superior in parietal lobe
Inferior in temporal lobe
What fibres are responsible for our lateral field of vision?
Nasal
What fibres are responsible for our medial field of vision?
Temporal
Describe the general features of the signs of a lesion before the optic chiasm:
Signs are unilateral and ipsilateral
Describe the general features of the signs of a lesion at the optic chiasm:
Signs are bilateral
Describe the general features of the signs of a lesion after the optic chiasm:
Signs are bilateral and contralateral
Where would a lesion causing monocular blindness be located?
Optic nerve
What are some examples of causes of monocular blindness?
Optic nerve glioma, retinoblastoma (child), optic sheath meningioma (middle aged)
Where would a lesion causing bitemporal hemianopia be located?
Optic chiasm
What are some examples of causes of bitemporal hemianopia?
Pituitary adenoma
Anterior communicating artery aneurysm
Where would a lesion causing left homonymous hemianopia be located?
Right optic tract - affecting right temporal and left nasal fibres
What are some examples of causes of left homonymous hemianopia?
Vascular (strokes), neoplasia, trauma
What visual field defect would a lesion to the right inferior radiation cause?
Contralateral upper quadrantic anopsia
Why can there be macular sparing in a stroke affecting the posterior cerebral artery?
Occipital lobe has dual blood supply from posterior and middle cerebral arteries (occipital pole). Occipital pole is where macula is so central vision spared
Describe the pathway for the pupillary light reflex:
Light stimulates afferent (CN II) -> synapses in pretectal area -> gives rise to neurones supplying Edinger-Westphal nucleus bilaterally -> both CN III stimulated to cause direct and consensual pupillary constriction via parasympathetic fibres
What are the 3 aspects of the accommodation reflex?
Convergence (medial rectus)
Pupillary constriction (constrictor pupillae)
Convexity of lens (ciliary muscle)
Describe the accommodation reflex pathway:
Follows visual pathway via lateral geniculate nucleus to visual cortex. Fibres from visual cortex to EW nucleus and sphincter pupillae and ciliary muscle. Stimulation of oculomotor nucleus to cause medial rectus contraction
What is the function of the medial longitudinal fasciculus?
Controlling movements of the eyes and making sure eyes compensate for movements of the head
What structures are connected by the medial longitudinal fasciculus?
Oculomotor, trochlear, abducens and vestibular nuclei
