vision Flashcards
function of anterior compartmetn lens system of the eye
focus lgiht on retina
function of posterior compartment of the eye
transduce light into electricl signal
project visual signal to cortex
center of the retina, color vision, only cones present, highest visual acuity
fovea
what do clinicans use to look at increased optic pressure?
optic disc
what is responsible for the blood supply to the retina?
optic disc
optic disc with an inverted, diffuse margin
glaucoma
optic disc with an everted, diffuse margin
papilledema
What is the light sensitive region of the eye?
retina
How many cones in the retina?
3 million
what are cones and rods responsible for?
cones- color
rods- night
abundant photopigment, high sensitivity, saturated in daylight, slow response, 1 visual pigment
rods
low sensitivity, less photopigment, fast response, 3 visual pigments
cones
What is the visual pigment cycle?
- rods (and cones) contain visual pigment- light causes photoactivation (11-cis to all-trans retinal)
- metarhodopsin II (all trans config) triggers phototransduction
- Rhodopsin contains scotopsin and visual pigment
- all-trans retinal recycled by isomerase to 11-cis retinal
- 11-cis configuration required to bind scotopsin
vitamin A
precursor for all-trans retinal
what catalyzes conversion to retinal?
retinol dehydrogenase
vitamin A deficiency
causes decrease all-trans retinal and decrease in rhodopsin cuasing night blindess- poor senstitivty in dim light
how do rods depolarize in dark?
cGMP- gated channels open which allow sodium influx (‘dark current’)
glutamate released
What closes the cGMP-gated channels?
rhodopsin photoactivation
sodium influx stops causing hyperpolarization
What is the mechanism for phototransduction?
- rhodopsin bleaching activates transducin
- transducin activates cGMP PDE to degrade cGMP to 5’-GMP
- [cGMP] decreaases
- cGMP channel closes
- Rod hyperpolarizes and neurotransmitter terminated
What allows difference in the 3 scotopsins’ wavelengths?
slight amino acid differences
Tuning allows
each cone to have an absorbance maximum for particular wavelengths of light
What are the normal cone spectral sensitivty peaks for each color?
blue- 437
green- 533
red- 564
What is true color blindess and what is the cause?
a mutation causing complete lack of a cone
X chromsome
What is color deficiency?
less severe mutation causing a shift of absorbance spectrum
Deuteranomaly
green cone peak sensitivity is shifted toward red (5% males)
protanomaly
red cone peak shifted towards green (1% males)
Signals (except GC) are propagated by
electrotonic conduction
photoreceptors synapse on
bipolar cells and HC cells
Bipolar cells synapse on
ganglion cells with axons and ‘true’ action potentials
amacrine cells
retinal sensitivity increases 10,000 at 40 min dark
rods
dark adaption 4x faster; saturation
cones
what phosphorylates rhodopsin producing and adaption mechanism for regulating light sensitivity (light adaption)?
rhodopsin kinase
lateral inhibition enhances what?
contrast
horizantal cells main mediator of lateral inhibition
pr synapse on adjacent BCs
inhibition by GABA
prevents lateral spread of light excitation on retina
horizantal cells
creates on/off organization; typical feature of sensory systems
horizontal cells
lateral inhibition: color opponency
horizantal cells release of GABA on adjacent PR terminals ( opens Cl- channels) and opposes color signal
off-center BCs via..
typical glutamate receptors
hyperpolarize in light
On-center BCs express..
mGluR6 which maintains TRMP1 in a closed state
What relieves the glutamate inhibition and opens the TRPM1 channels
light
optic nerves are always
ipsilateral
once it reaches optic chiasm, optic nerve is refferred to as..
optic tract
What are the two pathways that optic radiation LGN axons project to?
parietal and temporal
superior visual field goes to
inferior retina–> inferior visual cortex
and vis versa
Functions of LGN
- projects visual scene to primary visual cortex via optic radiation or geniculocalcarine tract
- LGN ‘gates’ transmission of signals to visual cortex
- LGN may receive input from superior colliculus
2 innermost layers of the LGN
M-cells
4 outermost layers of LGN
P-cell
What is adjacent tot the primary visual cortex (area 17)?
calcarine fissure
Area 18 is responsible for higher order visual informaiton processing; what is this area called?
secondary visual cortex
left visual field projects to…
right primary visual cortex
Ocular Dominance Column
loss of input from the right eye (enucleation)- alternating visual input from left and right eyes causes this specific projection pattern in the right primary visual cortex (area 18 interprets these two field)- appears like a zebra pattern
M path superior termination
area 18/ POT for 3d position, shape, and motion
P path inferior termination
temporal lobe for fine detail and color information
What cells respond to specific orientation or direction and where does it occur?
simple cells
layer IV
Distal to layer VI, some neurons sensitive only to directional movement across the image
complex cells
most distal cortical layers, some neurons only respond to specific line lengths, angles and velocities
hypercomplex cells
specific neuronal polls are sensitive to specific image features
generalization
What fibers from GLN convey color signals and fine details?
P fibers
P fibers form LGN project where?
to sublayers a and cbeta in layer IV
voluntary fixation area (frontal eye field)
bilateral premotor cortex for voluntary control EOM
involuntary fixation area (area 18)
bilateral to automatically ‘lock’ eyes on target
What are the three types of eye movement?
- continuous tremor
- slow drift
- flicking movement
What type of eye movement is lost with lesion to the superiror colliculus?
flicking movement
What is used to test for ‘pursuit movement’?
opticokinetic drum
Saccades are also referred to as …
anticipatory jumps
What are the 6 EOMs and the pairs?
- superior and inferior recti move eyes up and down
- medial and lateral recti move eyes side to side
- superior and inferior obliques rotate the eye
What cranial nerve is the lateral rectus controlled by?
6
What cranial nerve is the superior oblique controlled by?
4
What cranial nerve controls the rest of the EOM (not LR or SO)?
CN 3
PPRF signals what?
signals the right abducens ( command from L frontal eye) nucleus to contrac tthe right lateral rectus
signals (via MLF0) the left oculomotor nucleus to contract the left medial rectus
involves the rostral intersitital nucleus of the MLF
vertical gaze
Where do parasympathitic nerve rest?
Edinger- Westphal nucleus of CN III which projects to ciliary ganglion
Sympathtic nerves are preganglionics; where is this?
lateral horn of 1st thoracic segment
synapse on postganglionic in superior cervical
Sympathtic dilates pupil; what contracts?
radial fibers & contracts superior tarsal muscle (raises eyelid)
What pupils do not respond to light but accomodate?
Argyll-Roberson pupil
Horner’s syndrome
lesion in sympathetic path causes ipsilateral contricted pupil, eyelid droop, flushing, loss of sweating
PArasympathetic-Syphilis
encephalitis
blocks pupillary light reflex
loss of inhibtition results in contricted pupil
Assessment of comatose patient
- doll eyes- head rotation causes eyes to remain fixed, often a CVA affectign brianstem
- Blown pupils- mydriasis- trauma or stroke may cause pupils that are fixed and dilated unilaterally or bilaterally. compression of CN III and brainstem. recreational drug use causes temporary pupillary dilation.
