27 Monitoring visual function Flashcards
what are the surfaces off which light refracts?
cornea
lens 1
lens 2
how does the lens focus on a distant object?
ciliary muscles relaxed
sclera pulls on ciliary muscles
suspensory ligaments pulled
lens pulled thin ∴ bends light less
how does the lens focus on a nearby object?
ciliary muscles contract
no tension on sclera
suspensory ligaments ‘slacken’
lens is not stretched ∴ bends light more
what are the components of rhodopsin in the dark?
opsin (protein)
11-cis retinal (vitamin A derivative)
why are rod cells considered to have ‘low’ visual acuity?
many rod cells form a synapse with a single bipolar neurone
what is the photosynthetic pigment found in cone cells?
iodopsin
how do cone cells produce trichromatic vision?
3 types of cone cell, each with either red/green/blue iodopsin which differ in sensitivity to photons of different wavelengths
where is the highest concentration of cone cells?
in the fovea centralis of the macula
what is macular degeneration?
a loss of cone cells from the fovea
why are cone cells considered to have ‘high’ visual acuity?
each cone cell has its own bipolar neurone
what happens in the retina when no light is present?
Na+/K+ pump actively transports Na+ out and K+ in
open Na+ channels allow Na+ to diffuse in
open K+ channels allow K+ to diffuse out
rod cells release small amounts of neurotransmitter (glutamate)
NT inhibits depolarisation of bipolar cells
ganglion cell not depolarised ∴ no impulse
what happens in the retina in the presence of light?
retinal goes through allosteric change from 22 cis-retinal to all trans retinal –> active site no longer complementary to opsin ∴ rhodopsin becomes unstable
chemically gated Na+ channels close
Na+/K+ pump continues –> cell becomes depolarised
voltage gates Ca2+ channels close at -40 mV
less NT released to bipolar cells
bipolar cell no longer inhibited ∴ becomes depolarised
creates generator potential –> release of neurotransmitter to ganglion neurone
generator potentials summate –> action potential along optic nerve
how frequently should you have an eye test?
every 2 years
what conditions can eye tests detect?
blindness
signs of tumours
diabetes
outline the visual acuity test
uses a Snellen chart
reading letters < 1 cm in height
20/20 - being able to read the letters at 20ft
variables to be controlled:
- distance to check
- light intensity
- cover one eye
- random letters (to prevent learning)
outline the colour vision test
Ishihara test - series of pictures of coloured spots
diagnosis of red/green colour deficiencies
can alter colour combinations
BUT might not know numbers
outline the Farnsworth-Munselll 1000 hue test
arranging coloured caps in hue order
can be used as an industry test
what is an OCT scan?
optical coherence tomography
optical beam directed at tissue (normally retina) –> light can be reflected and is collected
what conditions can an OCT scan diagnose?
ARMD
diabetic retinopathy
macular holes
macular oedema
what is the consequence of retinal detachment?
retina comes away from blood supply in choroid
–> lack of oxygen and glucose to retinal cells for aerobic respiration
outline some effects of ageing on the eye
lens loses elasticity
change in composition of lens
lens cannot refract light sufficiently
loss of efficiency of ciliary muscles
how does the eye recover after exposure to light?
rod cells detect low intensity light
rhodopsin present in rods –> broken down by light into retinal and opsin
takes time to resynthesise during dark period
outline the role of the autonomic NS in controlling pupil size
autonomic NS controls smooth muscle in the iris
iris has two sets of antagonistic muscles: circular and radial
sympathetic: radial –> open pupil
parasympathetic: circular –> closed pupil
light intensity detected by retina –> optic nerve –> brain
too much light –> reflex action –> impulses sent to iris
circular muscles contract/radial muscles relax –> pupil constricts
too little light –> reflex action –> impulses sent to iris
radial muscles contract/circular relaxes –> pupil dilates
