PSC1002/L25 The Eye Flashcards
What is the function of the cornea?
Refraction of light to bring into focus
What is the function of the lens?
Accommodation
Adjusting reflective properties of eye to focus over wide range of distances
What is the function of the iris?
Pigment determines eye colour
Muscles adjust pupil diameter according to light levels and emotional signals
What is the function of the pupil?
Opening in centre of iris allows light entry
What kind of cells exist in the retina? (3)
Photoreceptors
Horizontal cells
Bipolar cells
Amacrine cells
Ganglion cells
Nerve fibres
At which point is light focussed at the back of the eye?
Retina
How is an image projected onto the retina?
Inverted
Left/right reversed
How wide is the visual field of each eye?
150 degrees
How does the lens adjust as an object gets closer? (2)
Becomes more rounded to shorten the focal length
Refracts light more
Which muscles contract to round the lens more?
Ciliary muscles
Which structures are pulled tight to flatten the lens?
Zonulas
Describe myopia (short-sightedness). (3)
Too much focal power for its length
Light rays converge in front of retina
Image at retina is blurred
Describe hyperopia (long-sightedness). (3)
Too little focal power for its length
Light rays converge behind retina
Image at retina is blurred
What is the function of the foveal pit? (2)
Upper layers of retina cells pushed aside to allow light direct access to photoreceptors
Region of highest acuity
Describe the fovea. (3)
Pit in centre of macula
Central part of visual field
Area of highest acuity (highest image resolution) with highest density of cone receptors, colour vision
Describe the blind spot (optic disc). (3)
Region where nerve fibres and blood vessels leave the eye
Blind in this region
Brain ‘fills in’ missing area
What kind of cells make up the retina? (3)
Pigment epithelium
Photoreceptors - rods & cones
Other connecting cells
Describe pigment epithelium of the retina. (3)
Cells at back of retina
Photoreceptors embedded
Contain melanin black pigment to absorb stray light
Briefly describe the 2 types of photoreceptors in the retina.
Rods (scoptic vision - low light level)
Cones (photopic vision - high light levels)
R:C 20:1
Describe rod photoreceptors. (3)
Most sensitive to light
Sensitive to brightness
Only detect contrast
Function at low light levels
Bleached at high light levels
Describe cone photoreceptors. (3)
3 types sensitive to long (red), medium (green) and short (blue) waves
Enable colour vision
Responsible for highest acuity
Work only at high light levels
Explain dark adaptation. (3)
Cones are not sensitive
Rods bleached
Visual sensitivity increases as rods recover
Colour loss as switch from colour vision to contrast only vision
How are cones distributed? (2)
Larger and bigger further away from retina
Allows highest acuity at fovea
What photopigment is present in rods and cones?
Rhodopsin
3 different pigments in cones
Describe rhosdopsin. (4)
Opsin + 11 CIS-retinal
In dark, opsin and retinal bound together
On light exposure, retinal molecule changes shape
Causes split from opsin
‘Bleaching’
Describe how darkness leads to release of neurotransmitter. (5)
cGMP levels high keep Na+ channels open
Na+ current into rod higher than K+ current out
Rod depolarised to -40mV
Transmitter release
Constant release of NTM in dark
Describe how light leads to hyperpolarisation of rods. (6)
Light bleaching of rhodopsin activates G-protein transducin
Decreased cGMP
Na+ channels close
K+ continues to flow out
Rod hyperpolarises to -70mV
Less transmitter release
