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Systems Neuroscience > Vision > Flashcards

Vision Flashcards

1
Q

Macular degeneration:

A

progressive loss of central vision

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2
Q

most common cause of vision loss in people over 55 and most _____ is age-related
Why?
First noticed as things being kinda blurry in the _____ when you are ____
Can be detected with the _____

A

Age-related macular degeneration (AMD):
Why? Not really sure. Lots of things (lifestyle, genetics…) may contribute
First noticed as things being kinda blurry in the middle when you are reading
Can be detected with the Amsler grid

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3
Q

Age-related macular degeneration (AMD):

A

two main types- wet and dry

Wet AMD (“exudative-neovascular”): 10% of AMD
Abnormal blood vessel growth under the macula
Blood vessels leak blood and fluid  “wet”
Progresses rapidly and can cause severe damage (to vision) and may easily lose central vision
Treatment: laser therapy or lasers + light-activated drug to destroy these leaky vessels

Dry AMD (“nonexudative”): the other 90%
Involves a gradual loss of the retinal pigment epithelium, which is a layer of cells right next to the photoreceptor cells  photoreceptor loss
Much slower to progress, which is good because there is no effective treatment for it as of now

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4
Q

most people that have AMD have wet or dry AMD?

A

Dry

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5
Q

Retinitis Pigmentosa

A

Group of hereditary eye disorders
Photoreceptors die by apoptosis (programmed cell death)
Choosing to die
Symptoms: night blindness (rods seem to go first), loss of peripheral vision, migration of pigment from the epithelium to other places in the retina
No real cure, but there are some treatments that can help for a while

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6
Q

the eye functions

A
  • The eye functions
  • Let in light
  • Let in the right amount of light
  • Focus the light/image on the retina
  • Be flexible to permit seeing things at different distances
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7
Q

• Retina-

A

at the back of the eye; part of the CNS; contains the light sensitive neurons that send the visual information to the brain
• (Vision, itself, actually occurs in the retina)
• The retina is actually an extension of the brain,
•

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8
Q

T/F one of the roles of the eye is assist in vision

A

FALSE this is a role of the reina

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9
Q

Uveal tract

A

3 parts: choroid, ciliary body, iris
• Choroid: lots of capillaries that supply the photoreceptor cells; contains melanin (absorbs light)
-We need these capillaries to supply
-The pigment is not what is contributing to sight
• Ciliary body: encircles the lens
-Muscular component refractive power of lens
-Vascular component: Because things need blood
• Iris: the colored portion of the eye
- Muscles to adjust the size of the pupil

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10
Q

can the pupil be adjusted?

A

yes
• Adjustable pupil lets you try to get the best of both worlds- reduce aberrations while still letting in enough light to see

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11
Q

the aberrations of the pupil

A

Spherical aberration due to the difficulties in focusing planar images with spherically lenses; causes the specimen image to appear hazy or blurred and slightly out of focus
Chromatic aberration different wavelengths of light have different refractive indices, i.e. are bent to different degrees by a lens surface.

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12
Q

Cornea:

•

A

• in the front; lets light into the eye
Cornea does most of the refraction (of the eye and in comparison to the lens) and is adapted for air
• Put cornea in water, which has almost the same refractive index –> hazy, unfocused vision

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13
Q

Vitreous humor:

A

thick, gelatinous substance that fills the space between the lens and the retina
• 80% of eye’s volume
• Maintains shape, removes debris

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14
Q

• Floaters:

A

bits of debris that are stuck

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15
Q

chambers of the eye

A

• Anterior chamber- between cornea and lens
-Contains aqueous humor
• Posterior chamber- between the lens and the iris
-Aqueous humor is produced here and flows out through the pupil

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16
Q

Vitreous humor:

A

thick, gelatinous substance that fills the space between the lens and the retina
• 80% of eye’s volume
• Maintains shape, removes debris

thick, gelatinous substance that fills the space between the lens and the retinal surface, which accounts for 80% of the volume of the eye.

  • maintains shape of eye
  • contains phagocytic cells that remove blood and other debris that might interfere with light transmission.
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17
Q

aqueous humor

A

aqueous humor (watery liquid that supplies nutrients to the cornea and lens)

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18
Q

(zonule fibers)

A

• Lens connected to muscle by little bands of connective tissue (zonule fibers)
- tension wants to flatten the lens

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19
Q

Conditions/Problems with vision: what are they are and what causes them ***
-what are they

A

• Glaucoma and cataracts

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20
Q

refraction (of the eyeball)

A
  • cornea and lens
  • Cornea does most of the work and is adapted for air

*Lens does help some and is adjustable
Helps with objects at different distances

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21
Q

Ametropia:

A

having some sort of refractive error

Almost everyone has some sort of imperfect vision

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22
Q

zonule fibers

A

• Lens connected to muscle by little bands of connective tissue (zonule fibers) –> tension wants to flatten the lens

radially arranged connective tissue bands that are attached to the ciliary muscle and hold the lens in place.

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23
Q

 Accommodation:

A

 Accommodation: dynamic changes in shape of the lens

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24
Q

what causes glaucoma

A
  • Genetic predisposition common

* Drainage failure = increased intraocular pressure

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25
Q

role of zonule fibers in accommodations

A
  • to see things far away: Zonule fibers create more tension=> Flatter, thinner lens
  • To view things close up: the zonule fibers have to relax and let the elasticity of the lens take over =>Thicker, rounder lens
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26
Q
  • *myopia

- what in the eye causes this? (2 possible reasons)

A
  • Near-sighted: unable to bring distant objects into focus (But can still see up close very well)
  • caused by: Corneal surface too curved or eyeball is too long. Basically, cannot flatten the lens out enough to see distant objects
  • Lines converge too early
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27
Q

Why are so many people near-sighted?

A

myopia
• might be because of learning to read and write at an early age
• The growth of the eyeball is influenced strongly by light falling on the retina
• Reading and writing may change this, leading to an overly elongated eyeball

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28
Q

how to correct myopia ***

A

• Near-sighted correction: concave lenses (thicker at the edges)

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29
Q

 Accommodation:

A

 Accommodation: dynamic changes in shape of the lens

dynamic changes in the refractive power of the lens due to the activity of the ciliary muscle that surrounds the lens attached by the zonule fibers.

  • for distant object viewing the lens is made relatively thing and flat and has the least refractive power.
  • for near vision the lens is made round and thick and has the most refractive power.
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30
Q

Most people age into _______ (hyperopic//myopic) … why?

A

• Most people age into hyperopic

  • Not as common for younger people to have
  • Lots of people eventually become somewhat far-sighted as they age

Why? The lens loses some of its elasticity (presbyopia)
• Therefore, the maximum curvature of the lens is reduced
• Need a curvy lens to see things close up. Thus, the need for reading glasses

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31
Q

how do glasses work

A
  • Essentially, glasses work by helping focus the light that is getting bent in the wrong direction
  • Contact lens do the same thing, but they can be much smaller since they are right on your eye
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32
Q

Far-sighted correction:

A

convex lenses (thicker in the center)

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33
Q

• 5 basic neuron types in retina:

***

A
  • 5 basic neuron types in retina:
  • Photoreceptors
  • Bipolar cells
  • Ganglion cells
  • Horizontal cells
  • Amacrine cells
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34
Q

phototransduction

A

*Light travels through the retina to the pigment epithelium-> -> -> changes membrane potential of photoreceptor cells–> neurotransmitter release

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35
Q

inner vs outer segment of rods and cones

A

both rods and cones have an inner segment and outer segment
-outer segment: is adjacent to the pigment epithelium. contains membranous disks that contain light-sensitive pigments

-inner segment contains the nucleus and gives rise to the synaptic terminals (i think syapse onto bipolar cells

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36
Q

• Fundus:

A
  • Fundus: surface of the retina
  • Blood vessels arising from the opthalamic artery and vein
  • Lots of vascular
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37
Q

• Optic disk

-clinical use

A

(optic papilla): blood supply enters and exits from here; also where retinal axons exit
• This region has no photoreceptors = blind spot (scotoma)
• Basically of tunnel of cables
• clinical use: as a way to detect an increase in intracranial pressure (pressure on the inside of the skull)
• Subarachnoid space of brain continuous around the optic nerve
• Increased pressure will make the disk bulge
•

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38
Q

Macula lutea:

-pigment:

A

circular; near center of the retina
• Contains yellow pigment (xanthophyll), which filters UV light (protective)
• Supports high visual acuity
• Acuity highest in the center of the macula, which is a small pit called the fovea

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39
Q

fovea

A

small pit in the center of the macula with the highest acuity

-only has cones

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40
Q

• 5 basic neuron types in retina:

***

A
  • Photoreceptors
  • Bipolar cells
  • Ganglion cells
  • Horizontal cells
  • Amacrine cells
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41
Q

phototransduction

A

** Light travels through the retina to the pigment epithelium-> -> -> changes membrane potential of photoreceptor cells–> neurotransmitter release

  • light activation causes graded change in membrane potential of photoreceptors and a corresponding change in NT (glutamate) release rate. (add light= hyper polarization up to -65mV, dark= depolarized, resting at 40mV)
  • huge signal amplification capability.

In the dark:
1) Inward Na+ and Ca2+ flow into the outer segment though cGMP gated membrane channels (cGMP-bound=open) acts to depolarize the cell. Outward current of K+ through K+ selective channels in the inner segment acts to hyper polarize the cell. In dark, large quantity of cGMP in outer segment=many open cGMP-gated channels= inward>outward= depolarized= high # synaptic terminal Ca2+ channels open=high transmitter release.

Addition of light:

1) Photon absorbed by the photopigment in the receptor disk by the chromophore retinal coupled to one of several opsins.
2) When light is absorbed by retinal, one of the double bonds between the carbon atoms in the molecule breaks, causing a configuration shift from the T1-cis isomer to the all-trans retinal.
3) Series of changes in the opsin component triggered.
4) opsin change triggers activation of transducin, which activates phosphodiesterase which hydrolyzes cGMP thus causing channel closure, leading to hyper polarization, closing of synaptic terminal Ca2+ channels and decrease in transmitter release.

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42
Q

for rods and cones: light causes? dark causes?

A
  • Light => hyperpolarization (more negative)

* Dark => depolarization (less negative)

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43
Q

• Fundus:

A
  • Fundus: surface of the retina
  • Blood vessels arising from the opthalamic artery and vein
  • Lots of vascular
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44
Q

• Optic disk

-clinical use ?

A

(optic papilla): blood supply enters and exits from here; also where retinal axons exit

  • This region has no photoreceptors = blind spot (scotoma)
  • Basically of tunnel of cables
  • can use it (clinically) as a way to detect an increase in intracranial pressure (pressure on the inside of the skull)
  • Subarachnoid space of brain continuous around the optic nerve
  • Increased pressure will make the disk bulge

the site where optic veins and arteries enter the eye (to then fan out over the fundus of the retina) and the site where retinal axons leave the eye.

  • contains no photoreceptors and is responsible for the blind spot phenomenon.
  • its appearance is a useful gauge of intracranial pressure because it is continuous with the subarachnoid space of the brain (swelling of papilla=increased intracranial pressure).
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45
Q

cones : location

A

• are present at a low density throughout the retina, but are concentrated in the center of the fovea

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46
Q

Macula lutea:

-contains:

A

circular; near center of the retina
• Contains yellow pigment (xanthophyll), which filters UV light (protective)
• Supports high visual acuity
• Acuity highest in the center of the macula, which is a small pit called the fovea

oval spot containing yellow pigment near the center of the retina which is responsible for high visual acuity.
- acuity is greatest in the fovea which is the small depression at the very center of the macula.

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47
Q

fovea

-vascularization?

A

small pit in the center of the macula with the highest acuity

  • highly specialized region in the center of the macula.
  • increased density of cones and sharp decrease in density of rods (density of rods is far greater than that of cones throughout the rest of the retina)
  • called the foveola: at very center, there are no rods at all. has the highest level of visual acuity ( due to high density of cones + one-to-one neural relationships in the cone system)
  • displacement of inner layers of retina at this region which decreases photon scattering also adds to the superior acuity of the area.
  • devoid of blood vessels means higher acuity.
  • high acuity limited to this small region is why humans move their eyes and heads so much depending on the object they want to bring into focus.
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48
Q

Role of pigment epithelium***

A

• Cells in the pigment epithelium have processes that extend to and surround the tips of the outer segments of each photoreceptor

• Two roles:
1) Eat (phagocytose) old membranous disks filled with pigment (lifespan of disk= 12 days)
2) Regenerate photopigment molecules after exposure to light
• Pigment is continuously cycled between the outer segment of the photoreceptor and the pigment epithelium
• Disks migrate from innermost portion of the outer segment to the outermost portion of the outer segment (get closer to pigment epithelium) as they age
• The disks then shed and are eaten by the pigment epithelium

functions:
1) Removal of the expended (old ~12 day lifespan) photoreceptor disks which house the light sensitive photo-pigment, via engulfment of the tip of the photoreceptor outer segment into the pigment epithelium.
2) Regeneration of photopigment molecules after they have been exposed to light– photopigment is cycled continuously between photoreceptor outer segment and the pigment epithelium. (retinoid cycle, IRBP brings retinol to the pigment epithelium, where it is converted back to 11-cis-retinal, and then transported back to the outer segment by IRBP)

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49
Q

How do we “return things to normal” after light hits a photoreceptor
• **

A

We would like some control over this wild cascade
• arrestin:
-Arrestin blocks the ability of rhodopsin to activate more transducin
-Activated rhodopsin is phosphorylated by rhodopsin kinase
-This allows arrestin to bind it

  • All-trans retinal dissociates from the opsin and goes into the cytosol of the outer segment
  • Converted to all-trans retinol
  • All-trans retinol gets escorted to the pigment epithelium by interphotoreceptor retinoid binding protein (IRBP)
  • Gets converted back to 11-cis retinal –> IRBP sends it back
  • Restore retinal to stop the cascade
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50
Q

Arrestin

A

Arrestin blocks the ability of rhodopsin to activate more transducin

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51
Q

interphotoreceptor retinoid binding protein (IRBP) *

A

• All-trans retinol gets escorted to the pigment epithelium by interphotoreceptor retinoid binding protein (IRBP)
IRBP carries the 11-cis retinal back to the photorecptor

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52
Q

Who sends axons from the retina to the brain?

***

A

Ganglion cells are the only way out
• Axons of the retinal ganglion cells exit through the optic disk and bundle together for form the optic nerve
• The optic nerves from each side of the head meet at the optic chiasm
• Here, some fibers cross and some stay on the same side
• In humans, 60% of fibers cross over and the rest stay on the same side
• 40% stay on the same side
• As the axons continue on to their targets, they are now called the optic tract
• Optic tract contains information from both eyes. Optic nerves only contain information from the ipsilateral eye

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53
Q

color vision

-wavelength:

A

Rods contain a single photopigment –> not useful for color vision

(Human) Cones have three kinds of photopigments, each sensitive to a different wavelength of light
Trichromatic- blue, green, and red (short, medium, and long wavelength)
The actual color information is extracted higher up in the visual processing pathways

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54
Q

• Opsin

A

Opsins are multipass transmembrane proteins and they form a pocket for the retinal to sit in

There are a variety of opsins.
• Example: rhodopsin
• Different opsins tune the absorption of light by the molecule based off the wavelength of light in the spectrum

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55
Q

No response in either pupil when shining light in the left eye, but normal responsiveness in both when shining it in the right eye …. leads you to the conclusion that

A

there is sensory problem on left

56
Q

Pupillary light reflex**

A
  • Prectum : Coordination center for the pupillary light reflex
  • Receives bilateral input (info from both eyes)
  • Pretectal neurons project to the Edinger-Westphal nucleus
  • Helping us to control our actual pupil size
  • This is where the preganglionic parasympathetic neurons that terminate in the ciliary ganglion by way of the oculomotor nerve
  • Neurons of the ciliary ganglion innervate the constrictor muscles in the iris
  • Decrease pupil diameter when activated
  • Leave pupil more open when not activated
  • shine light in eye –> optic nerve and tract –> pretectum –> Edinger-Westphal nucleus –> ciliary ganglion –> muscle constriction and decrease pupil diameter
  • Useful diagnostic tool
  • Use to test the function of the eye, the motor outflow to the pupil, and the paths in between
  • Normal: both pupils will respond the same even if you shine the light in one eye
  • Bilateral input to the pretectum
  • Comparing differences in the response between the two eyes can help figure out where you might have a lesion
57
Q

Prectum*

A

: Coordination center for the pupillary light reflex

-a targets of the optic tract

58
Q

o Optic nerve damage-
o Optic chiasm damage-
o Optic tract damage-
o Optic radiation damage-

A

o Optic nerve damage- no vision from that eye
o Optic chiasm damage- involves both eyes (peripheral usually)
o Optic tract damage- both eyes, but generally from the same side of field
o Optic radiation damage- more specified field of view missing, but can depend on where it is

59
Q

mesopic vision

A

Mesotopic: twilight, both rods and cones used in vision.

60
Q

scotopic vision

A

: mediate vision in low light conditions (scotopic vision); not responsible for color vision; low spatial acuity

  • no color vision
  • poor acuity

Scotopic: lowest illumination, only rods because cones are not sensitive enough to detect light, poor resolution.

61
Q

(photopic vision)

A

: mediate vision in normal or higher lighting conditions (photopic vision); responsible for color vision; high spatial acuity
Spatial acuity due to high number of cones in central vision and their 1:1:1 connections

Photopic: high levels of illumination, rods no longer contribute because all membrane channels are closed and illumination changes no longer change the membrane potential (“saturated”), thus cones are entirely used for vision.

62
Q

receptive fields of M vs P-cells

cell bodies, axons

A

• M ganglion cells have larger receptive fields, larger cell bodies, and larger diameter axons (faster) than P ganglion cells

63
Q

MT

A
  • medial temporal area
  • part of the Extrastriate Cortex

-neurons that are selective for direction of a moving edge without regard to color
Detection of direction of motion

64
Q

Dorsal Stream *

A

• Dorsal stream, which includes MT, leads from striate cortex towards the parietal lobe. Responsible for spatial aspects of vision (motion, position)

65
Q

Ventral stream*

A
  • Ventral stream, which includes V4, leads from the striate cortex (primary) into the inferior part of the temporal lobe. Responsible for high resolution vision and object recognition.
  • Cares about color
66
Q

V4

A

-part of the Extrastriate Cortex

V4- lots of neurons that respond selectively to the color of a visual stimulus, but don’t care about movement

67
Q

Pupillary light reflex**

A
  • Prectum : Coordination center for the pupillary light reflex
  • Receives bilateral input (info from both eyes)
  • Pretectal neurons project to the Edinger-Westphal nucleus
  • Helping us to control our actual pupil size
  • This is where the preganglionic parasympathetic neurons that terminate in the ciliary ganglion by way of the oculomotor nerve
  • Neurons of the ciliary ganglion innervate the constrictor muscles in the iris
  • Decrease pupil diameter when activated
  • Leave pupil more open when not activated
  • So, shine light in eye –> optic nerve and tract à pretectum à Edinger-Westphal nucleus à ciliary ganglion à muscle constriction and decrease pupil diameter
  • Useful diagnostic tool
  • Use to test the function of the eye, the motor outflow to the pupil, and the paths in between
  • Normal: both pupils will respond the same even if you shine the light in one eye
  • Bilateral input to the pretectum
  • Comparing differences in the response between the two eyes can help figure out where you might have a lesion
68
Q

glaucoma

A

failure of adequate drainage by the specialized network of cells lying at the junction of the iris and the cornea, which causes high levels of intraocular pressure which reduce the blood supply to the eye and eventually damage retinal neurons.

69
Q

cataracts

A

alterations in the composition of the cornea of lens that reduce transparency and thus have serious consequences on visual perception (necessity of transparent optical media). corrective surgery is available.

70
Q

Targets of the optic tract **

A
  • Dorsolateral geniculate nucleus (aka lateral geniculate nucleus) of the thalamus
  • Pretectum
  • Suprachiasmatic nucleus
  • Superior colliculus
71
Q

two opposing forces that determine the shape of the lens

A

1) the elasticity of the lens, which tends to keep it rounded.
2) the tension of the zonule fibers which flattens the lens.

distant objects: force of zonule fibers > lens elasticity
near objects: force of zonule fibers < lens elasticity

72
Q

Pupillary light reflex**

A
  • Prectum : Coordination center for the pupillary light reflex
  • Receives bilateral input (info from both eyes)
  • Pretectal neurons project to the Edinger-Westphal nucleus
  • Helping us to control our actual pupil size
  • This is where the preganglionic parasympathetic neurons that terminate in the ciliary ganglion by way of the oculomotor nerve
  • Neurons of the ciliary ganglion innervate the constrictor muscles in the iris
  • Decrease pupil diameter when activated
  • Leave pupil more open when not activated
  • So, shine light in eye –> optic nerve and tract –> pretectum –> Edinger-Westphal nucleus –> ciliary ganglion –> muscle constriction and decrease pupil diameter
  • Useful diagnostic tool
  • Use to test the function of the eye, the motor outflow to the pupil, and the paths in between
  • Normal: both pupils will respond the same even if you shine the light in one eye
  • Bilateral input to the pretectum
  • Comparing differences in the response between the two eyes can help figure out where you might have a lesion
73
Q

o Elasticity of the lens tends to keep it _____

A

rounded

74
Q

role of pupil in visual acuity (clarity)

A
  • narrowing of the pupil reduces spherical and chromatic aberrations that blur the retinal image, and increases the depth of field (distance within which objects are seen without blurring)
  • -> this however limits the amount of light that reaches the retina, and limits visual acuity under dim light situations due to a lack of photons reaching the retina.

“Adjustable pupil provides an effective means of reducing optical aberrations, while maximizing depth of field to the extent that different levels of illumination allow”

75
Q

in the general population (or really in younger people) which is more common: myopia or hyp

A

myopia

76
Q

Fundus

A

inner surface of the retina which is covered by both arteries and veins which fan out over its surface, entering from the optic disk (papilla).
- macula lutea near center of the retina supports high visual acuity

77
Q

is the retina part of the peripheral or CNS

A

CNS. develops from an out pocketing of the diencephalon called the optic vesicle which undergoes invagination to form the optic cup whose inner wall gives rise to the retina.
The outer wall gives rise to the retinal pigment epithelium

78
Q

retinal pigment epithelium structure

A
  • thin, melanin-containing structure that develops from the outer layer of the optic cup, and reduces backscattering of light that enters the eye and plays a vital role in maintaining the phototransduction machinery of retinal photoreceptors
79
Q

the outer segment of the rods and cones are sitting in the _____

A

pigment epithelium

80
Q

structure of the retina

A

1) pigment epithelium is outermost layer
2) the photoreceptors’ membraneous disks containing light- sensitive photopigments in the photo-receptor outer segment.
3) the photoreceptor cell bodies/nucleus in the outer nuclear layer.
4) synapses between photoreceptors and bipolar cells/horizontal cells in the outer plexiform layer.
5) bipolar, horizontal and amacrine cell bodies in the inner nuclear layer.
6) synapses between bipolar cells, ganglion cells and amacrine cells in the inner plexiform layer.
7) ganglion cell bodies in the ganglion cell layer.
8) nerve fiber later where ganglion cell axons lead to the optic disk where they form optic nerve.

light flows from 8–>1
information flows from 1–>8
there is lateral information flow that occurs via the horizontal and amacrine cells in the outer and inner plexiform layers respectively.

81
Q

most direct path of information flow to the optic nerve

A

three neuron path.

1) photoreceptor
2) bipolar cell
3) ganglion cell –> optic nerve

82
Q

T/F Photoreceptors do exhibit action potentials

A

FALSE
• Photoreceptors do not exhibit action potentials
• Why no action potentials? Do not really need them since it is such a short distance to the postsynaptic target

In the retina, however, photoreceptors do not exhibit action potentials; rather, light activation causes a graded change in membrane potential and a corresponding change in the rate of transmitter release onto postsynaptic neurons.

83
Q

horizontal cells

A
  • cell body in the inner nuclear layer and processes in the outer plexiform layer
  • enables lateral interactions between photoreceptors and bipolar cells that maintain the visual systems sensitivity to contrast over a range of luminance.
84
Q

amacrine cells

A
  • cell body in the inner nuclear layer and processes in the inner plexiform layer
  • processes are post-synaptic to bipolar terminals and presynaptic to ganglion dendrites of ganglion cells, with different classes believed to make distinct contributions to visual functions increasing diversity of pathways.
85
Q

light induced phototransduction cascade- limiting mechanisms

A

1) Activated rhodopsin (photopigment) is rapidly phosphorylated by rhodopsin kinase, permitting arrestin to bind and block rhodopsin’s ability to activate transducin–truncating the phototransduction cascade.
- Arrestin blocks the ability of rhodopsin to activate more transducin
2) The retinoid cycle

86
Q

the retinoid cycle

-steps and why is it important

A

1) After light excitation and conformational change, all-trans retinal dissociates from opsin and diffuses into the cytosol of the outer segment
2) it is converted to retinol in the cytosol
3) it is transported into the pigment epithelium via a chaperone protein, inter photoreceptor retinoid binding protein (IRBP)
4) In the pigment epithelium, enzymes convert it back to 11-cis retinal
5) It is transported back to the outer segment by IRBP where it recombines with opsin in the receptor disks.
- critically important for maintaining photoreceptors/photopigments light sensitivity by maintaining a sufficient number of active photopigment molecules even under intense illumination.

87
Q

the retinoid cycle

A

1) After light excitation and conformational change, all-trans retinal dissociates from opsin and diffuses into the cytosol of the outer segment
2) it is converted to retinol in the cytosol
3) it is transported into the pigment epithelium via a chaperone protein, inter photoreceptor retinoid binding protein (IRBP)
4) In the pigment epithelium, enzymes convert it back to 11-cis retinal
5) It is transported back to the outer segment by IRBP where it recombines with opsin in the receptor disks.
- critically important for maintaining photoreceptors/photopigments light sensitivity by maintaining a sufficient number of active photopigment molecules even under intense illumination.

88
Q

adaption

A
  • the variation in magnitude of phototransduction amplification due to level of illumination.
  • photoreceptors are most sensitive to light at low illumination levels, and sensitivity decreases as illumination increases.
  • prevents the receptors from saturating thus extending the range of light intensities over which they operate.
  • mainly dependent on amount of Ca2+ in outer segment of photoreceptor:
    1) Light induced closure of cGMP channels leads to a net decrease in the internal Ca2+ concentration.
    2) Decrease triggers a number of changes in the phototransduction cascade, which tend to reduce the sensitivity of receptors to light.

-also dependent on neural interactions between horizontal cells and photoreceptor terminals.

89
Q

examples of how Ca2+ _____( increase or reduction) triggers light adaption

A

calcium reduction triggers light adaption

1) Causes increased gaunylate cyclase activity (cGMP synthesizing) and thus leads to higher cGMP levels (less hyper polarization due to light)
- in the light cGMP REDUCTION causes hyperpolarization
2) Increases the activity of rhodopsin kinase, permitting more arrestin to bind to rhodopsin, slowing signal amplification of the phototransduction cascade.
3) Increases the affinity of the cGMP channels for cGMP, reducing the impact of the light-induced reduction of cGMP levels.

90
Q

rods

-what vision works best in

A
  • low spatial resolution, extremely sensitive to light (produce response to a single photon), no color vision (contain a single photopigment)
  • at lowest illumination levels, vision is entirely rods called “scotopic vision” – difficulty making fine visual discriminations due to rod’s poor resolution.
91
Q

cones

A
  • very high spatial resolution, specialized for acuity at the expense of sensitivity to light, allow for color vision.
  • begin to contribute to visual perception at the level of starlight, become more dominant as illumination level increases.
  • at highest illumination levels, vision due entirely to cones cones called “photopic vision” (rods response to light saturates because all membrane channels are closed)
  • normal daylight seeing is mainly mediated by the cone system
92
Q

differences in transduction mechanism of rods v cones (light sensitivity and adaption)

A
  • major factor in the ability of rods and cones to respond to different ranges of light intensity.

1) Rods:
- produce reliable response to a single photon of light
- response of an individual cone saturates at high levels of steady illumination due to less effective light adaptation mechanism with less than 1/4 the recovery speed after activation by a light flash.
2) Cones:
- require more than 100 photons to produce a response comparable to that produced by a single light photon in a rod.
- more effective light adaption mechanism means that the response of a cone does not saturate at high levels of illumination. cones recover more than four times as fast from a response to a light flash than do rods.

93
Q

differences in rods v cones circuits to ganglion cells (information flow and convergence)

A
  • in most parts of retina, rod and cone signals converge on the same ganglion cells.
  • differences occur in the early stages of the pathway between photoreceptors and ganglion cells.

Rods:

1) rods–> distinct rod bipolar cells–>synapse with the dendritic processes of a specific class of amacrine cells–> gap junctions and chemical synapses with the terminals of cone bipolars–> synaptic contacts on the dendrites of ganglion cells in the inner plexiform layer.
2) very convergent pathway: each rod bipolar cell is contacted by a number of rods and many rod bipolars contact an amacrine cell.
- –> convergence increases low light detection but decreases spatial resolution, as the signal could have come from anywhere within a relatively large area of retinal surface.

Cones:

1) cones–> cone bipolar cells–> retinal ganglion cells. 2)much less convergent pathway than rods: each retinal ganglion cell receives input from a single cone bipolar cell which receives input from a single cone.
- –> this one-to-one relationship decreases light sensitivity but is required to maximize acuity.

94
Q

foveola

A
  • Rods are present at a high density throughout most of the retina, but are at a low density in fovea
  • In the very center of the fovea, called the foveola, there are no rods
95
Q

cones and color vision

A
  • there are three types of cones which contain different photopigments, each of which is differentially sensitive to light of different wavelengths: short (S) which sees blue, medium (M) which seems green, and long (L) which sees red.
  • S cones are only ~5-10%, M and L cones are majority, however ratios vary between individuals and different ratios of M to L appear to have no affect on color perception.
  • normal human vision is trichromatic, based on the relative levels of activity in three sets of cones that have different absorption spectra.
  • impossible to tell whether the change in the membrane of a particular cone is due exposure to few photons of a wavelength it is sensitive to, or many photons of wavelengths it is not sensitive to.
96
Q

ganglion response to light stimulation

A

each ganglion cell responds to stimulation of a small circular patch of the retina, which defines the cell’s receptive field.

  • can be “on-center” or “off-center” ganglion
  • -> present in equal numbers
  • -> have overlapping distributions, so that several on-center and off-center ganglion cells analyze every point on the retinal surface.
  • -> information about increases or decreases in luminance is carried separately to the brain by the axons of the two types of ganglion cells, which provides unambiguous information about both increments and decrements in luminance.
97
Q

on-center cell pathway :

addition of light

A

turning on a spot of light in the receptive field results in a burst of action potentials. turning off light results in a reduction in the rate of discharge to a base spontaneous level.
–> increase discharge rate in response to luminance increase.

  • Their dendrites form synapses selectively with on-center bipolar cells in the inner ganglion layer which respond to luminance increases.
    1) Light removal from photoreceptors leads to an in glutamate release at their synapses.
    2) Glutamate binds to G-protein-coupled metabotropic glutamate receptors of the on-center bipolar cells.
    3) Once bound, the receptors activate an intracellular cascade that closes cGMP-gates Na+ channels, reducing inward current and hyper polarizing the cell.
    4) No glutamate released into synapse to ganglion.
    5) No ganglion action potentials.

** conversely, addition of light–>decrease in glutamate from photoreceptor–> no glutamate bound to mGluR6 –> no intracellular cascade in bipolar cells –> depolarization of bipolar cells –> glutamate release to retinal ganglion cells –> burst of action potentials.

98
Q

off-center pathway

A

turning on a spot of light reduces the rate of discharge to a base spontaneous level. turning off the light on the receptive field causes a burst of action potentials.
–> increase discharge rate in response to luminance decrease.

  • Their dendrites form synapses selectively with off-center bipolar cells in the inner plexiform layer which respond to luminance decreases.
    1) Light removal form photoreceptors causes an increase in glutamate release at synapse.
    2) Glutamate binds to ionotropic (AMPA and kainate) receptors in dendrite of off-center bipolar cells, that cause the cells to depolarize.
    3) Bipolar cell depolarization causes glutamate release to retinal ganglion cell = burst of action potentials due to light removal.

** conversely: light activation of photoreceptors –> reduction of glutamate release to bipolar cells–> no binding to AMPA kainate receptors of off-center bipolar cell dendrites–> hyper polarization –> no glutamate release to ganglion cell and no action potentials.

99
Q

Retina emphasizes the regions where …..

A

retinal ganglion cells are sensitive to differences between the level of illumination that falls on the receptive field center and the level of illumination that falls on the surround –> retina does not give equal weight to all regions of the visual scene, but emphasizes the regions where there are differences in luminance.

100
Q

light adaption and retinal circuits

A
  • response rate of an on-center ganglion cell to a spot of light in its receptive field center varies as a function of the spot’s intensity.
    • the intensity of the spot illumination required to evoke a given discharge rate is dependent on the background illumination–> the more illuminated the background illumination, the greater the stimulus intensities required to achieve the same response.
  • Horizontal cells are thought to exert their influence via the release of neurotransmitter directly onto photoreceptor terminals, regulating the amount of transmitter that the photoreceptors release onto bipolar cell dendrites.
  • horizontal cells synapse with both photoreceptors in the center of the receptive field and in the surround
  • –> when light is shone in the center of the receptive field only, there is little effect on the horizontal cell and little opposition to the hyperpolarization and decrease in glutamate release of the photoreceptor.
  • -> when light is shone in the center AND the surround of the receptive field, the horizontal cell becomes depolarized and decreases GABA release to the photoreceptors in the receptive field center, opposing the hyper polarization and reducing light sensitivity and increasing the intensity necessary for a response.

1) Glutamate release from photoreceptors due to light reduction depolarizes the horizontal cells.
2) Horizontal cells decrease GABA (which has hyper polarizing effect on photoreceptors) release, which depolarizes the photoreceptor, thus increasing glutamate release and opposing the phototransduction.
3) Ultimately reduces the firing rate of the on-center ganglion cell.

101
Q

overall pathway***

A

ganglion cell axon in retina –> optic disk (papilla) in nasal section, where they form optic nerve–> optic chiasm at the base of the diencephalon (site of decussation) –> ganglion cell axons from both sides form the optic tract (40% of axons from same side optic nerve, 60% axons from opposite optic nerve) –> (some to hypothalamus and pretectum –> dorsal lateral geniculate nucleus of thalamus –>

102
Q

dorsal lateral geniculate nucleus

A
  • in the thalamus
  • the major target of ganglion cell axons in the diencephalon.
  • neurons here send their axons to the cerebral cortex via the internal capsule, through the optic radiation, and terminate in the primary visual cortex (V1) or striate cortex (brodmann’s area 17) which lies largely along the calcimine fissure in the occipital lobe.
103
Q

the primary visual pathway

A

“retinogeniculostraite pathway”

  • conveys information that is essential for most of what is thought of as seeing.
  • projections to this region arise from ganglion cells whose visual response properties are appropriate for mediating the richness of visual perception (high acuity, color, motion)

(optic nerve–> optic chiasm in thalamus of diencephalon–> lateral geniculate nucles–> internal capsule through optic radiation–> cerebral cortex [primary visual cortex (V1) or striate cortex (brodmann’s area 17)].

104
Q

pretectum

A
  • area between the thalamus and the midbrain
  • second major target of retinal ganglion cell axons
  • coordinating center of the pupillary light response
  • project to the edinger-westphal nucleus which controls nerves which innervate constrictor muscles in the iris.
  • projections to this region arise from ganglion cells that lack visual response properties appropriate for mediating the richness of visual perception, and instead are highly suited form detecting luminance flux.
105
Q

pupillary reflex and visual sensory damage detection

A
  • light in one eye should produce a direct response in the same eye and a consensual response in the opposite eye.
  • comparing response of two eyes can help to localize a lesion.

EX:

1) direct response in left eye without consensual response in right eye suggests problem with motor outflow to right eye, likely the oculomotor nerve.
2) Both eyes respond normally to right eye stimulation, but neither eye responds to left eye stimulation signifies damage to left eye input, likely the retina or optic nerve.

106
Q

Suprachiasmatic nucleus

A
  • retinal ganglion cell axon target
  • in the hypothalamus at the base of the diencephalon
  • important in regulation of circadian rhythms.
  • projections that go here arise from ganglion cells that are highly suited for detecting luminance flux rather than the richness of visual perception.
  • “retinohypothalamic pathway” is the route by which variation in light levels influences the broad spectrum of visceral functions that are entrained to the day-night cycle.

(target of the optic tract)

107
Q

Superior colliculus*

A
  • prominent structure on dorsal dorsal surface of diencephalon.
  • coordinates head and eye movements to visual targets.
108
Q

unique property of ganglion cells that project to the pretectum and hypothalamus

A

They express their own light detecting pigment, melanopsin, and are capable of modulating their response to changes in light levels in the absence of signals from other photoreceptive cells.
- explains why the normal circadian rhythms are maintained in animals that have completely lost form vision as a result of degeneration of rod and cone receptors

109
Q

visual field perception

A
  • information from the left half of the world, whether it originates in the left or right eye, is represented in the right half of the brain, and vice versa.
  • visual field is divided into nasal and temporal, and superior and inferior.
  • these meridians intersect at the point of fixation (which falls on the fovea), located nasally on the visual field of each eye.
  • images in the visual field are inverted and left right reversed on the retina –> objects in the temporal part of the visual field are seen in the nasal part of the retina and vice versa.
110
Q

binocular vision

A
  • with both eyes open, the two foveas align on a single target, causing the visual fields of both eyes to overlap extensively.
  • binocular field consists of two symmetrical visual hemifields. (left binocular hemifield includes the right eye nasal and left eye temporal visual fields
  • because temporal field is larger than nasal field, images in the periphery of the field of vision are viewed monocularly by the most medial portion of the nasal retina– most of the rest of the field can be seen by both eyes
111
Q

retinal field

A

The image projected on the retina (retinal field) is the reverse of the visual field= inverted form of the visual field

112
Q

Tunnel vision (bitemporal hemianopsia) can be associated with lesions of the

a. Visual cortex
b. Lateral geniculate body
c. Optic radiation
d. Optic chiasm
e. Optic nerve
Optic chiasm

A

Optic chiasm

113
Q

does the retina have more rods or cones

A

more than 3x more rods

114
Q

optic chiasm

A

In humans, 60% of fibers cross over and 40% stay on the same side

115
Q

rods in the light:

A
  • In light, cell membrane becomes hyperpolarized as Na+ close, release of
    neurotransmitter (glutamate) declines
116
Q

Instant blindness in one eye would occur from transection of destruction of the

a. Optic nerve
b. Optic tract
c. Right half of the optic chiasm
d. Superior colliculus
e. Temporal lobe

A

Optic nerve

117
Q

Inhibition of phosphodiesterase will interfere with transduction in which systems?

a. Vestibular and Visual
b. Auditory and Visual
c. Visual and Gustatory
d. Olfactory and Auditory

A

Inhibition of phosphodiesterase will interfere with transduction in which systems?

Auditory and Visual

118
Q

The space between the cornea and the lens is separated into two chambers by the

a. Ciliary body
b. Aqueous humor
c. Zonules
d. Iris
e. Pigment epithelium

A

iris

119
Q

The pupillary reflex is initiated in

a. Rod photoreceptors
b. Cone photoreceptors
c. Suprachiasmatic Nucleus
d. Photosensitive ganglion cells in peripheral retina
e. Ciliary body

A

The pupillary reflex is initiated in

Photosensitive ganglion cells in peripheral retina

120
Q

Blockage of aqueous humor circulation causes

a. Cataracts
b. Glaucoma
c. Decreased intraocular pressure
d. Retinal detachement

A

Glaucoma

121
Q

the fovea of the eye

a. Has the lowest light threshold
b. Is the region of highest visual acuity
c. Contains only cones
d. Contains only rods
e. Is situated over the head of the optic disc

A

the fovea of the eye

Is the region of highest visual acuity

122
Q

does the fovea have rods

A

in the fovea:

  • increased density of cones and sharp decrease in density of rods (density of rods is far greater than that of cones throughout the rest of the retina)
  • at very center, called the foveola, there are no rods at all
  • high density of cones + one-to-one neural relationships in the cone system allows for highest levels of visual acuity in this region.
123
Q

scotoma

A
  • i think it means blind spot ?

The exit point of the optic nerve

124
Q

Damage to the sixth cranial nerve will produce which deficit in eye movement?

a. Inability to move the eyes in a vertical up-and-down motion
b. Inability to rotate the eyes within the eye socket
c. Inability to move the eyes laterally toward the midline
d. Inability to move the eyes laterally away from the midline
e. Vertical strabismus

A

d. Inability to move the eyes laterally away from the midline

125
Q

Light entering the eye passes through which retinal layer first?

a. Inner nuclear layer
b. Outer nuclear layer
c. Outer plexiform layer
d. Photoreceptor layer
e. Retinal ganglion layer

A

Light entering the eye passes through which retinal layer first?

Retinal ganglion layer

126
Q

ciliary muscles

A

dynamic changes in the refractive power of the lens due to the activity of the ciliary muscle that surrounds the lens attached by the zonule fibers.

127
Q

Which muscle is contracted as part of the pupillary light reflex?

a. Ciliary muscle
b. Pupillary dilator muscle
c. Pupillary sphincter muscle
d. Radial fibers of the iris
e. Superior oblique muscle

A

Which muscle is contracted as part of the pupillary light reflex?

Pupillary sphincter muscle

128
Q

A female patient is unable to close her left eye tightly. Which nerve is most likely affected?

a. Facial (VII)
b. Maxillary (V2)
c. Oculomotor (III)
d. Opthalmic (V1)
e. Trochlear (IV)
f. Abducens (IV)
g. Vagus (X)

A

a. Facial (VII)

129
Q

Which of these is the prominent cell collection above the oculomotor nucleus (III) that controls pupil constriction and lens accommodation?

a. Edinger-Westphal nucleus
b. Ciliary ganglion
c. Red nucleus
d. Medial geniculate nucleus

A

Edinger-Westphal nucleus

130
Q

Most neurons are permeable to which of these ions at rest?

a. Sodium (Na+)
b. Potassium (K+)
c. Calcium (Ca2+)
d. Chloride (Cl-)

A

Potassium (K+)

131
Q

List the structures and fluids through which light passes as it travels from the cornea to the retina
**

A

cornea, aqueous humor, pupil of iris, lens, vitreous humor, retina

132
Q

Which of the following is a somatosensory modality?

nociception

touch

vision

hearing

proprioception

temperature

taste

Itch

smell

equilibrium

A

A, B, E, F, and H

nociception

touch

proprioception

temperature

Itch

133
Q

in the light and the dark, K+ is effluxing out: explain the hyperpolarization in light

A

in the light and the dark, K+ is efluxing out. however, in the light, getting a hyperpolarization, as Na+ influx is reduced and in the dark getting a depolarization since Na+ influx increases

134
Q

shine light in left eye: direct response in left eye without consensual response in right eye suggests problem:

A

shine light in left eye: direct response in left eye without consensual response in right eye suggests problem with motor outflow to right eye, likely the oculomotor nerve.

135
Q

Both eyes respond normally to right eye stimulation, but neither eye responds to left eye stimulation signifies:

A

Both eyes respond normally to right eye stimulation, but neither eye responds to left eye stimulation signifies damage to left eye input, likely the retina or optic nerve.

136
Q

One light-activated opsin can lead to ____ activated transducins molecules

Each PDE molecule can break down ___ cGMPs

A

One light-activated opsin can lead to 800 activated transducins molecules
Amplification
Each PDE molecule can break down 6 cGMPs

  • i think this has to do with amplification?
137
Q

do rods or cones have a more effective light adaption mechanism

A

cones
more effective light adaption mechanism means that the response of a cone does not saturate at high levels of illumination. cones recover more than four times as fast from a response to a light flash than do rods.

Systems Neuroscience (24 decks)

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