Physiology of Vision Flashcards
Vision:
insert diagram
Optics of Vision
- light travels through the layers of
globe - light is refracted
- image formed on the retina by
phototransduction
Phototransduction is
conversion of light/photon energy into neural/electrical potential energy
What structures refract light rays in the globe and how?
- cornea = 2/3 = convex = converge
- lens = 1/3 = biconvex = converge
concave lens = diverge light rays
convex lays = converge light rays
Refractive power of a material:
- higher the power, more
convergent/divergent effect on
rays - Dioptres D
What is the focal length?
- distance from parallel light rays
- entering material t othe point of
convergence of light rays which is
normally on the retina - different dimensions of eye would
cause refractive error, hence not
projecting onto the retina
Which of the following has the largest refractive power?
last lens
steeper angle
Which of the globe structures can change to change refraction of light?
Lens
How is the lens controlled?
What lens shown below? Effect?
- ciliary bodies
- cillary muscle contracts -> zonular
fibers have less tension ->
spherical lens = more focusing
power, near focus = PARASYMP
= closer to eye structures - ciliary muscle relaxes -> zonular
fibers are more tense -> flatter
lens, less focusing power, far focus
= SYMP = long distance vision
Pilocarpine is a muscarinic receptor agonist
Atropine is a muscarinic receptor antagonist
Muscarinic receptors PARASYMPATHETIC activity
Hypermetropia/Hyperopia:
- long sightedness
- eyeball too short or too weak
refraction - blurred near vision, good long
vision
Myopia:
- short sightedness
- eyeball too long or too strong
refraction - blurred far vision, clear near vision
Astigmatism:
imperfect curvature of cornea or lens
blurred near and far vision
Presbyopia:
loss of accomodation of eye so loss of near vision
loss in shape of lens changing for near vision; (parasymp)
Optics of Vision:
insert diagram
Iris Function:
- controls the volume of rays
entering the posterior segment of
the globe - sphincter pupillae -> constricts
pupil, decreases light in response
to bright light = parasymp - dilator pupillae -> dilates pupil,
increases light in response to dim
light or fight/flight responses =
sympathetic
Which layer of the eye contains the retina?
The optic part of the retina has how many layers?
What is/are the layer/s called?
- internal neural layer
- non-visual and optic
- optic further divided into external
pigmented layer and neural
internal layer - external = absorbs excess light
and prevents reflections of light - internal = neurologically
functional layer, photosensitive
How many cell types in the neural internal layer of the retina?
3 neural cell types
1) photoreceptors: rods and cones
2) bipolar neurons
3) ganglion cells
Function of rod cells:
receptive to dim light
light sensitive
Function of cone cells:
- receptive to bright light
- colour sensitive receptors
What pigment in rods?
rhodopsin (light sensitivity)
What pigments in cones?
photopsin (colour sensitivity)
What are the two transverse cell types of the neural internal layer of the retina?
- horizontal cells: lateral inhibition
of bipolar cells - amacrine cells: contrast
enhancement and movement
detection
What cell types are involved in colour vision?
- cones = photoreceptors
- L cones/photopsin I: Red
- M cones/photopsin II: Green
- S cones/photopsin III: Blue
Red-Green Colour Blindness:
X chromosome linked
hence more common in male
only one mutated copy needed for expression
Blue Colour Blindness:
chromosome 7
Visual Fields:
image in the environment/external correct orientation
Retinal Field:
image in inversion of the visual field
image on the retina
light travels in straight lines
Visual and Retinal Fields:
insert slide
Visual Pathway: Components:
optic nerve
optic chiasm
optic tract
lateral genticulate nucleus of thalamus or superior colliculus
optic radiation
primary visual cortex
Ipsilateral Eye Blindness:
- damage to the optic nerve
- loss of sight in one eye
Bitemporal hemianopia:
- loss of vision on temporal field of
both eyes - damage to the optic chiasm
Homonymous hemianopia:
- loss of vision on eg the left visual
field of both eyes hence one
temporal field and one nasal field - damage to the optic radiation
fibers
Each primary visual cortex will have visual information from both eyes.
Draw pathways.
- nasal visual field seen on retinal
temporal field - optic nerve
- fibers pass through genticulate
nucleus on ipsilateral side to the
primary visual cortex
nasal visual field = ipsilateral primary visual cortex
temporal visual field = contralateral primary visual cortex
- temporal visual field seen on
retinal nasal field - optic fiber
- crossed over at optic chiasm
- passes through lateral genticulate
nucleus - to contralateral primary visual cortex
Defects at each number causes?
Pituitary gland tumour may result in what visual defect?
- grows upward
- squashes optic chiasm
- temporal fibers cross and are
hence lost - bitemporal hemianopia
What might cause homonymous hemianopia?
disturbance top optic radiations on one side
stroke, tumour, inflammation, injury
Scotoma:
- blind spot
- spot loss of vision, unilateral,
bilateral, temporary or permanent
Visual Agnosia:
inability to recognise visually present objects despite normal visual fields, acuity and colour
vision
once visual info has gone to primar visual cortex, can project via the temporal ventral route or the parietal dorsal route
*ventral route = recognition of object
dorsal route = visually guided movements
Prosopagnosia:
inability to recognise familiar faces
once visual info has gone to primar visual cortex, can project via the temporal ventral route or the parietal dorsal route
*ventral route = recognition of object
dorsal route = visually guided movements
