SPECIAL 130 Sudden Visual Disturbance Flashcards
What focuses the light before it reaches the retina?
Cornea and Lens
What are the two types of photoreceptors?
Rods and cones
What are the cells in the eye that transmit impulses to the CNS?
Ganglion Cells
What cells in the eye send excitatory signals to the ganglion cells from the photoreceptors?
Bipolar cells
What cells provide lateral inhibition?
Horizontal and amacrine cells
Where is there a 1:1:1 ratio of photoreceptors, bipolar and ganglion cells in the eye? What does this result in?
Fovea centralis - increased visual acuity
What is the receptive field?
A limited area corresponding with a group of photoreceptors that synapse with a particular ganglion cell
Explain ON centred bipolar cells
they increase firing when the centre of the receptive field is stimulated by light
they decrease firing when the peripheral receptive field is stimulated
Explain OFF centred bipolar cells
they decrease firing when centre of receptive field is stimulated by light
they increase firing when the peripheral receptive field is stimulated by light
What is the state of photoreceptor cells in the dark and why?
They are relatively depolarised (~40mV) as cGMP is raised keeping Na+ channels open
What happens to the photoreceptor cells in the light and why?
becomes hyperpolarised due to closing of Na+ channels
What happens to 11-cis retinal in response to light?
It becomes all-trans retinal
Why does the molecule retinal detach after light exposure?
Rhodopsin cannot bind to retinal in the all-trans state
What occurs after retinal detachment in the process of phototransduction in the eye?
Transducin is activated which subsequently activates phosphodiesterate
What does phosphodiesterase activation do in the eye?
Converts cGMP to 5’GMP
Where do the two optic nerves meet?
Optic chiasm at the base of the skull
What optic fibres remain ipsilateral after the optic chiasm?
Fibres from the temporal region of the corresponding eye
What optic fibres cross to the contralateral side after the optic chiasm
Fibres from the nasal region of the corresponding eye
Where do the optic fibres connect to after the optic chiasm?
Lateral geniculate nucleus, superior colliculus or the pretectal nucleus of the brainstem
What and where is the optic radiation?
It is the optic fibres from the lateral geniculate nucleus of the thalamus projecting upward and outwards through the parietal and temporal lobes to the primary visual cortex
What is the pupilliary light reflex?
Light stimulus in one eye produces a response in both
In the pupilliary light reflex what structures/areas in the brain do the fibres pass to after the optic chiasm?
Pretectal nuclei –> edinger-westphal nucleus (each receives information from both pretectal nuclei) –> oculomotor nerve –>ciliary ganglia –> constrictor muscles of iris
What 3 movements can the pupil make in the pupilliary near response?
Convergence, Accommodation and Constriction
In the pupilliary near response what pathway does it follow to the visual cortex but what happens after?
Normal sensory pathway. From visual cortex there are projections to the frontal eye field –> EW nuclei –> effector muscles
What is the most common cause of blindness/partial sightedness?
Age-related macular degeneration
Describe non-exudative, dry AMD
Painless and progressive loss of central vision
What degenerative disorder has abnormal neovascularisation in the macula?
Exudate, wet AMD - severe central vision loss
What is glaucoma?
Group of diseases where intraocular pressures rises to cause optic nerve damage
What is a relative afferent pupilliary defect?
A medical sign observed in the swinging-flashlight test where the pupil has a consensual response but not a direct light response
What are the support cells of the eye?
Muller cells
What are the 8 histological layers of the eye from surface to deep (humour to choiroid)
Inner limiting membrane Nerve fibre layer (ganglia) Inner nuclear layer (synapses) Inner plexiform layer (processor cells e.g. bipolar) Outerplexiform layer (synapses) Photoreceptors Retinal pigment epithelium Choroid
What is the choroid in the eye?
the vascular supply of the retina
What does the retinal pigment epithelium do?
prevents reflection of light and phagocytoses debris
