8 - Visual System: 2

Question 1

What are the characteristics of the rods/rod system (scotopic)? About how many rods do we have?

Answer 1

More sensitive (ampllification)
Slow (integrate 100 msec)
Poor spatial resoulation 
More pigment 
Not directionally selective 

We have about 100 million rods

Question 2

What are the characteristics of the cones/cone system (photopic)? About how many cones do we have?

Answer 2

Less sensitive 
Fast (integrate 25 msec)
High spatial resolution 
Less pigment 
Directionally selective 

We have about 5 million cones

Question 3

What is an important difference between rods and cones? What does this tell us about vision in normal light?

Answer 3

Rods saturate and cones do NOT.

This means that as the amount of light that reaches the retina is increased, the rods response increases. But when very moderate light levels are reached, the rod reaches it’s maximum response.

This saturation level is quite low (normal room temp), which means most of your vision is based on ~5% of your photoreceptors–your cones

Question 4

Photoreceptors ______ in response to light.

Answer 4

Hyperpolarize!

Question 5

What are photopigments made up of? Where is this located? At what wavelength does it absorb light?

Answer 5

Chromophore and a protein called opsin.

embedded in the lipid membrane of the outer segment of rods and cones.

Absorbs light at 375 nm (opsin “red shifts” the abs spectrum)

Question 6

Describe the absorption spectrum for rods and cones? What are the photopigments characterized according to?

Answer 6

Rods absorb most strongly in the blue-green

Three types of cones and the spectrums are broad and overlapping.

Characterized according to the efficiency with which they absorb light of different wavelengths.

Question 7

What is the principle of Univariance?

Answer 7

Photoreceptors cannot register the wavelength of the photons they catch ie the output depends the amount of light presented to it and not the specific wavelength of the light presented to it.

Question 8

Knowing that photoreceptors cannot register wavelength (rule of univariance) how do we perceive color?

Answer 8

Based on varying sensitivities of photoreceptors. This is why we have cones with overlapping sensitivity.

By comparing outputs, you can infer which color is being presented.

Ie you can decouple univariant only by decoding the outputs.

Question 9

What begins the excitatory cascade in phototransduction?

Answer 9

Visual pigments are GPCRs, but they are activated by light instead of a chemical ligand.

Question 10

What occurs in phototransuction in vertebrates?

Answer 10

1. Isomerization of chromophore to all trans.
2. Opsin- activates transducin
3. PDE- convertscGMP to GMP
4. Low cGMP closes cyclic nucleotide channels ion channels
5. Cone hyperpolarizes - inward flux of Ca and Na blocked

Question 11

Will using viagra make you blind? How does it work? How does this relate to the visual system?

Answer 11

Phosphodiesterases (PDE) inhibitors selectively inhibit cGMP-PDE5 which is present in all vascular tissues and leads so vasodilation.

It also has an inhibitory effect on PDE6 which is present in photoreceptors
-increase in cGMP and an opening of cyclic-nucleotide gated ion channels and thus a depolarized cell.

Question 12

What does viagra interfere with in the visual system? What visual symptoms may be experienced? What about at high doses?

Answer 12

Can interfere with basic phototransduction and cause transient changes in rod and cone OS function.

Effects:

• impaired blue-green color discrimination
• decreased rod and cone driven ERG amplitudes

High doses:

• blue tinged vision
• increased apparent brightness of lights
• blurred vision

Question 13

What is the role of RPE in phototransduction?

Answer 13

Makes the 11-cis-RDH into an all trans retinyl ester in a reaction catalyzed by lecithin.

Then it gets transported back into the outer segment.

Critical to phototransductiotn.

Question 14

Why is understanding phototransuduction important clinically?

Answer 14

Some inherited and degenerative defects have their origin at the level of molecules involved in phototransuduction.

Since this is the first step of vision, even small defects can have major consequences.

Question 15

Describe the concept of a receptive field?

Answer 15

Every neuron that is higher order than the photoreceptors has a “field” of photoreceptors that provide input to it.

Light falling within the receptive field of a neuron influences the activity of that neuron.

Question 16

All photoreceptors act the same way in response to light. What is this response and what does it cause?

Answer 16

HYPERPOLARIZE: as they hyperpolairze, they release LESS nts.

Ie light turns photoreceptors OFF.

They hyperpolarize in a graded fashion and release nts in a graded fashion and DO NOT produce APs.

Question 17

All photoreceptors are termed “OFF CENTER”, why is this?

Answer 17

Because light that is centered on the photoreceptor turns if off.

Question 18

Typically, when a cone is depolarized is releases more of what? What is the effect of this?

Answer 18

The excitatory nts glutamate.

Glutamate is not always excitatory though.

Question 19

What can the activity of a single cone give rise to? What are properties of this cell type?

Answer 19

2 parallel pathways, by connecting 2 different bipolar cell types.

The bipolar is an ON CENTER type because light in the center of the receptive fields of these cells turn them ON (depolarized).

Question 20

Describe the “sign reversing” synapse with cones?

Answer 20

The off center character of the cone output is REVERSED in sign to on-center.

When light goes on in the center, it turns the cone off, turning the on-center bipolar ON.

Question 21

Describe the contacts onto cones? What impact does glutamate have on ON-center bipolar cells?

Answer 21

They are invaginating and act like classical inhibitory synapses (ie nt release hyperpolarizes the postsynaptic ON-center bipolar ells).

Glutamate (released by cones when theres no light) has a hyperpolarizing effect on ON center bipolars. (this is the exception)

Question 22

Other than on-center, what is the other type of bipolar cells? What type of synapses do these have?

Answer 22

Off center: light in the center of the receptive field turns them off (hyperpolarized)

These have sign conversing synapses: the cone output is conserved in sign as OFF-center
-when light goes ON in the center, it turns the cone off, turning the off-center bipolar off as well

Question 23

What are the contacts of off-center bipolar neurons like? What is the action of light onto the central photoceptots of the off-center bipolar cells receptive field?

Answer 23

Flat/basal contacts on to cones.

Causes hyperpolarization of the off-center bipolar cells.

Question 24

What type of receptors do off-center and on-center cells use?

Answer 24

Off-center: ionotrophic receptors and flat contacts

On-center: metabotrophic receptors and evaginating contacts.

Question 25

What is the purpose of having parallel pathways?

Answer 25

Improved sensitivity.

One channel provides information concerning stimuli that are brighter than the background (ON-center), while the other provides information regarding stimuli dimmer than the background (OFF-center).

Question 26

What cells types can bipolar cells talk to?

Answer 26

Amacrine cells

Ganglion cells

Question 27

What is the function of amacrine cells?

Answer 27

Provide lateral connections and many produce transient depolarizing responses.

Question 28

What is the function of ganglion cells? What are the physiological types?

Answer 28

Only neuron in the retina that produces APs;
Contain photosensitive melanopsin RPC that sample light in your environment

Physiological types: ON center and OFF center sustained /transient responses

Question 29

What are the anatomical types of ganglion cells? Which type are there more of? Where do they project? What is the size of their receptive field?

Answer 29

1. Parasol ganglion cells: (105) project to magno laterals (M).
   - large cells with large receptive fields (more transient)
2. Midget ganglion cells (90%) project to parvo layers (P)
   - smaller cells with small receptive fields (more sustained)

Question 30

Even though cones respond the same to light, activity of a single cone gives rise to 2 parallel pathways by connecting 2 different _____ cell types, which each connect to like-minded _______. What is this called?

Answer 30

Bipolar cell types

Like minded ganglion cells

Called sign-conserving

Question 31

Photoreceptors are completely synaptically interconnected by an elaborate system of inhibitory interneurons called _________ cells.

Answer 31

Horizontal cells.

Question 32

What is the structure and function of horizontal cells?

Answer 32

They laterally interconnect cones with other cones.

Every cone has a reciprocal inhibitory synaptic relationship with all of its neighboring cones.

When one cone is excited (depolarized) it excites horizontal cells that it synapses with and inhibits them via GABA and hyperpolarizes them. (center is hyperpolarized while the cones on either side are depolarized.)

Question 33

What is the purpose of lateral inhibition in horizontal cells? What does it allow us to do?

Answer 33

Improved resolution.

This greatly increases the visual system’s ability to respond to the edges of a surface.

This occurs because neurons at the edge of a stimulus respond more strongly than those in the middle due to the inhibition only from neighbors on one side while those in the middle get inhibition from both sides.

Question 34

Darkness falling on the central photoreceptor turns it ______.

Answer 34

ON

Question 35

Light falling on the surrounding cones turns them _____. What does this result in?

Answer 35

OFF

Releasing inhibitory action that the surrounding cones have on their central neighbor (further turning the central cone ON).

Question 36

What is Retinitis Pigmentosa (RP)? How common is it and what is a common symptom? How is it inherited?

Answer 36

Affects 1 in 4,000; characterized by progressive loss of rod and cone photoreceptors with accompanying vision loss.

Night blindness is earliest symptom, with progressive constriction of visual field.

Inherited X-linked, aut. dominant, or aut. recessive.

Question 37

What is Usher’s syndrome?

Answer 37

Retinitis Pigmentosa (RP) with hearing loss - “deaf-blindness”

Progressive vision and hearing loss assocaited with reduced odor identification, vestibular dysfunction, and reduced sperm motility.

Defects in sensory cilia proteins.

Question 38

What is Achromatopsia? How common is it? What is it caused by?

Answer 38

Aut recessive- 1 in 33,000

True colorblindness; pts have no cone function. Cones cannot hyperpolarize in response to light.

Casued by mutation in CNGB3, CNGA3, PDE6C, or GNAT2.

Question 39

What are the characteristics of congenital color vision defects? Do they worsen?

Answer 39

Red-green.

Present at birth, severity stays the same throughout life.

Can be classified precisely; both eyes effected equally.

Visual acuity is often unaffected. Higher incidence in males.

Question 40

What are the characteristics of acquired vision defects? Do they worsen?

Answer 40

Blue-yellow

Type and severity fluctuates; type may not be easily classified.

Monocular differences often occur.

Visual acuity is often reduced. Equal incidence in men and women.