midterm 1 Maya Nicole

What is the optimal test for dark adaptation?

method of ascending limits

Dark adaptation used what size test light and where?

uses a 10º white test light 20º off fixation

When are cones more sensitive than rods during dark adaptation?

before the rod-cone break

What does the rod-cone break signify?

where rods and cones are equally sensitive to light

When do the rods vs cones start dark adapting?

as soon as the bleaching light is turned off

Why do cones respond first in dark adaptation?

because cones regenerate photopigment faster

How does the dark adaption curve change with a weak bleach?

rod-cone break happens earlier (rods take over earlier)

What is the photochromatic interval?

area between rod + cone plateau, aka the difference in threshold between the two

How does the photochromatic interval change if the test light is red?

smaller interval (later rod-cone break)

What happens at the rod-cone break for the observer?

where the light goes from color to grey to the observer

What pattern does recover of visual sensitivity follow?

exponential increase (not directly correlated to the # of photopigment molecules available)

Why do cones regenerate their chromophore faster?

they have alternate methods of regeneration other than the RPE, can use muller cells or even self

Why does ARMD affect dark adaptation?

drusen forms at @ Bruch's impairs Vit A transport from choroid into RPE, since rods can only regenerate via RPE, they are primarily affected

What are the early symptoms of ARMD?

decreased night vision, trouble driving at night

With ARMD, where is the rod cone break when testing dark adaptation?

after 6.5 minutes (the more severe the AMD, the longer it takes for R/C break to occur)

Longer wavelength lights are what color

red

What color are shorter wavelength lights?

violet

Is the optic nerve (CN II) more similar to the CNS or the PNS?

CNS

What is the optic nerve more similar to the CNS?

1. retina + ON come from same embryological tissue as CNS

2. myelin on ON is produced by oligodendrocytes

3. specialized BRB

4. ON does not regenerate naturally

5. retinal synapses are narrow and completely ensheathed by glial cells

What are the three principal functions of the retina?

1. transduce light into electrochemical signals

2. process the signal

3. convert the signal into one that can be sent + interpreted by the brain

What layers of the retina does synapse occur?

plexiform layers

What are the 10 layers of the retina?

RPE

PR layer

OLM

ONL

OPL

INL

IPL

GCL

NFL

ILM

What makes up the OLM?

outer end plates of muller cells

What makes up the ILM?

inner end plates of muller cells

What are muller cells?

specialized glial cells that span thru almost all of the retinal layers

What is the main (vertical) pathway of the retina?

PRs-->bipolar cells-->ganglion cells

What do the PRs do?

convert light entry into an electric signal

What do bipolar cells do?

receive visual info from PRs + transmit it to RGCs

What do RGCs do?

project signal to brain and the axons make up the ON

What cells are involved in the horizontal pathway of the retina?

horizontal cells

amacrine cells

When do the RGCs become myelinated in majority of people?

as they pass thru lamina cribrosa

What happens in the people whos RGCs get myelinated before reaching ONH?

usually no affect on vision, may increase physiological blind spot

What is the visual pathway in the brain?

RGC --> LGN (thalamus) --> primary visual cortex (occipital lobe)

What do you call the RGCs axons before the chiasm?

optic nerve

What do you call the RGCs axons after the chiasm?

optic tract

What axons cross at the chiasm?

axons from nasal retina (temporal visual field)

What axon forms the knees of wilebrand?

axons from inferior nasal retina (superior temporal visual field)

What are the nerve fiber traveling from the LGN to the visual cortex called?

optic radiations

Where do the optic radiations from the superior retina (inferior VF) go?

directly to occipital lobe

Where do the optic radiations from the superior retina (inferior VF) synapse?

above calcarine fissure

Where do the optic radiations from the inferior retina (superior VF) go?

loop into temporal lobe (Myers loop) before entering occipital lobe

Where do the optic radiations from the inferior retina (superior VF) synapse?

below calcrine fissure

What do the right optic radiations contain?

temporal retinal fibers from the right eye and the nasal retinal fibers from the left eye

therefore the LEFT visual field

What do the left optic radiations contain?

temporal fibers from the left eye and nasal fibers from the right eye

therefore the RIGHT visual field

What fibers does the left Myers loop contain?

inferior temporal fibers from left eye

inferior nasal fibers from the right eye

therefore the superior right visual field

What fibers does the right Myers loop contain?

inferior temporal fibers from the right eye

inferior nasal fibers from the left eye

therefore the superior left visual field

What fibers do the right optic radiations heading straight to the occipital lobe contain?

superior temporal fibers from the right eye

superior nasal fibers from the left eye

therefore the left inferior visual field

What fibers do the left optic radiations heading straight to the occipital lobe contain?

superior temporal fibers from the left eye

superior nasal fibers from the right eye

therefore the inferior right visual field

What does a lesion at the optic nerve (pre-chasmal lesion) cause?

monocular VF defect

Where does a lesion @ON close to the chiasm cause?

full VF loss in ipsilateral eye

superior temporal VF loss in contralateral eye

What does a lesion at the chiasm cause?

bitemporal VF defect

What does a lesion post chiasm (before optic radiations split) cause?

homonymous hemianopsia of contralateral VF

ex: lesion of right optic tract=loss of left VF in both eyes

What does a lesion in Meyers loop cause?

contralateral superior quandrantinopsia OU

(ex: left Myers loop--> right superior VF defect OU)

What does a lesion in the straight optic radiations cause?

contralateral inferior quadrantinopsia OU

(ex: right optic radiations --> left inferior VF defect OU)

What does a lesion at the 1º visual cortex cause?

contralateral homonymous hemianopsia with macular sparing

(ex: left 1º--> right VF loss w/ macular sparing OU)

What is the range of light intensities that humans can see?

~12 log units

How does pupil constriction affect the light intensity that reaches the retina?

pupil constriction decreases light reaching retina however only by ~1 log unit

What does mesopic vision use?

both rods and cones

Where does light capture happen?

in the OS

Which PR have disks that are individual/distinct from the plasma membrane?

rods

What is the cone mosaic?

when taking a cross section at the level of the inner segments, the cones are packed tightly together similar to a honeycomb

Where is cone peak density?

fovea

Where are there no rods?

at the fovea

Are there more rods or cones in the retina?

rods

How are we able to pack in so many cones at the fovea?

cones take on a rod-like shape so more can be packed in

Where is rod density the greatest?

15-20º from fovea

How do we maximize acuity at the fovea?

cones only

high density of cones

inner retinal elements pushed to side (less layers)

no retinal vasculature

macular pigment to minimize chromatic aberration

almost 1:1:1 ratio of cone:bipolar:RGC

What is Henle's Fiber Layer?

cone axons that run horizontal as they are pushed to the side at the fovea

Where does phototransduction take place?

in the OS of photoreceptors

What contains the visual photopigment?

disks in the outer segment

What is the resting membrane potential (in dark) of a photoreceptor?

-40mV (more positive than usual)

What happens to photoreceptors when exposed to light?

they hyperpolarize

PRs have graded potentials, what does this mean?

amount hyper polarization is proportional to the amount of light stimulus

What is the max hyper polarization of a photoreceptor?

-65mV

What is the dark current?

PRs at rest (in dark) have a steady flow of Na+ and Ca2+ into cell

Why is the resting potential of photoreceptors less negative/more positive than usual?

dark current

What happens to the dark current when PRs are exposed to light?

light stimulates the Na+ channels to close (no more dark current), resulting in hyperpolarization

What is the mechanism of phototransduction (in rods)?

1. light is captured by the photopigment disks in the OS

2. 11 cis-retinal --> all trains retinal, creating metarhodopsin (R*)

3. R* stimulates transducin (Gt)

4. transducin converts GDP --> GTP

5. GTP activates PDE

6. activated PDE hydrolyzes cGMP --> GMP

7. Na+ channels close as cGMP levels drop

8. K+ channels not affected and continue to leave cell, no Na+ coming into cell --> hyperpolarization

What is rods photopigment?

rhodopsin

What is the chromophore in photoreceptors?

11-cis-retinal/all-trans-retinal

Where are opsins (photopigments) made?

made in the PR inner segment

Are opsins reused/photopigments?

yes, for 10-14 days

How are old opsins/photopigments removed?

after 10-14 days, phagocytosed by RPE

What is rhodopsin peak absorption?

500-510nm

What is s-cone peak abs?

430nm

What is M-cone peak abs?

530nm

What is L-cone peak abs?

560nm

What is the principle of univariance?

every photon (no matter the wavelength) captured by a photopigment has the same effect --> phototransuction cascade

What happens to a photon once it is absorbed?

all info about the wavelength is lost

What dictates the probability of absorption?

spectral sensitivity

What is this an example of:

An S-cone that absorbs 100 photons of 450nm light will have the same response as an S-cone that absorbs 100 photons of 520nm light.

Principle of univariance

What is the method of constant stimuli?

predetermined range of stimuli are presented in random order + data is collected

What is the method of adjustment?

patient turns knob until stimulus is seen

What is the method of limits?

examiner adjusts knobs until criterion is obtained

What is the ascending method of limits?

start at a stimulus too low and then raise it until seen

What testing is ascending method of limits used for?

dark adaptation

What is the descending method of limits?

start at a stimulus too high and the lower it until just seen

What is the method of descending limits used for?

infant testing

What is the staircase method of testing?

examiner turns stimuli up if patient cannot see and down if patient can see, threshold is an average of stimulus at reversal points

What psychophysical method does automated visual fields use?

fancy staircase