17. Retinal Function and Visual Perception Flashcards
What range of electromagnetic wave length is visible to the eye?
400-750 nm
What does length of an electromagnetic wave correspond to?
Colour
What does amplitude of an electromagnetic wave correspond to?
Brightness
What effect does a closer light source have on the focal distance?
Longer focal distance
What is accommodation?
Lens alters focal length
What are the functions of the pupil?
Control light entering eye
Depth of focus
How does the pupil control depth of focus?
Constriction cuts out peripheral light
What is the effect of sympathetics on the pupil?
Contract radial muscle of iris
Cause dilation
What is the effect of parasympathetics on the pupil?
Contract circular muscle of iris
Cause constriction
What is used to test for visual acuity?
Snellen chart
What is visual acuity?
Ability to distinguish between 2 nearby points
What does each part of 20/20 stand for?
First 20 is distance in feet from Snellen chart
Second is size of the smallest letter read
What are the 3 cell types in the retina?
Photoreceptors
Bipolar
Ganglion
What do rods detect?
Light intensity
Why is visual acuity low in rods?
Many rods converge to the same ganglion
What do cones detect?
Colour
What is the equation for photon energy?
hv (planck constant x frequency)
hc/ λ (planck constant x speed of light/ wavelength)
Which cells are non-image forming photoreceptors?
Retinal Ganglion Cells
What do Retinal Ganglion cells control?
Circadian rhythm
What light-sensitive pigement is used by retinal ganglion cells?
Melanopsin
What is the name for loss of photoreceptor cells?
Macular degeneration
What is found in the outer segment of cones?
Discs: specialised receptor region
What is found in the inner segment of cones?
Mitochondria and other organelles
What is phototransduction?
Converting light to an electrical signal
What pigment is used in rods?
Rhodopsin
What is rhodopsin composed of?
Retinal and opsin
What neurotransmitter is released by rods?
Glutamate
What is meant by the dark current?
Na+ enters causing depolarisation
K+ leaves causing repolarisation
What happens once light hits rhodopsin?
11-cis retinal converted to 11-trans retinal
What does 11-trans retinal activate?
Transducin
What does transducin activate?
Phosphodiesterase
What is the function of phosphodiesterase?
Cleave cGMP to GMP
Low cGMP closes Na+ channels: hyperpolarisation
What are the 3 types of cones?
Blue (short)
Green (medium)
Red (long)
What are the percentage population of each type of cone?
Short: 16%
Medium: 10&
Long: 74%
What is colour blindness?
Inherited absence of one or more class of colour pigment L and M cones are x-linked
What is it called when someone lacks red cones?
Protanope
What is it called when someone lacks green cones?
Deuteranope
What is it called when someone lacks blue cones?
Tritanope
How do photoreceptors adapt to the dark?
Light is insufficient to activate cones
Wait for regeneration of rod rhodopsin (20-30 mins)
What is night vision called?
Scotopic vision
How do photoreceptors adapt to strong light?
Pigment in rods bleaches
Brain switches to cone-only info processing
Which fibres of the optic nerve decussate?
Nasal
Temporal don’t
Where in the eyes is the left visual field taken in from?
Right side of each eye
Where is visually detected movement processed?
Parietal lobe
Where is perception of shape and colour processed?
Temporal lobe
