IOA2 Exam 2 - Retinal Physiology Pt 2

In dim light, _______ predominate.

rods

How sensitive are rods to light and what type of vision do they facilitate?

Rods are extremely sensitive in poorly lit conditions, facilitating scotopic vision ("night vision")

The light-sensitive retina allows detection of objects at ________ levels of illumination

low (ability to recognize details is poor)

What colors are perceived in scotopic vision?

Shades of gray

What is the peak wavelength sensitivity of rods?

507 nm

What is the peak lumen/watts of rods?

1700 lumen/watts

What type of lighting conditions produce mesotopic vision?

Conditions intermediate between day (photopic) and night (scotopic)

Which photoreceptors are active during mesopic vision?

Rods and cones

Which photoreceptor activity dominates in photopic vision?

Cone activity

What level of illumination is required for photopic vision?

Bright illumination

What visual functions are enhanced in photopic vision?

Sharp visual acuity and color discrimination

What are the three types of cones, and what wavelengths do they absorb?

Blue (S-cones) - 420 nm

Green (M-cones) --> 531-534 nm

Red (L-cones) --> 564-588 nm

Way to remember the types of cones

SML = small medium large

BGR (burger) = Blue Green Red

What are the two main components of visual pigments in rods and cones?

1) Opsin

2) Chromophore

Opsin

A membrane protein bound to a light-absorbing pigment molecule

What does opsin form?

a long helix that loops back and forth across the membrane seven times

What is the role of opsin in visual pigments?

Determines the wavelength absorption

Chromophore

Molecule that absorbs the photon of light

What is the chromophore in both rod and cone visual pigments?

11-cis-retinal (derivate of vitamin A)

How do cones differ from rods in terms of opsin?

Cones have three different types of opsins, while rods have only one (rhodopsin)

How do the three opsin photopigments in cones differ?

Differ in amino acid composition that binds 11-cis retinal

What color is the S-cone cell sensitive to?

Blue

What color is the M-cone cell sensitive to?

Green

What color is the L-cone cell sensitive to?

Red

What is the result of these different opsins in cones?

They cause the chromophore's absorption to diferent wavelengths

What % of amino acids are identical between all cones and rhodopsin?

~40%

What % of amino acids are identical between blue cones vs. green and red cones?

~40%

What % of amino acids are identical between green and red cones?

~95%

Where is rhodopsin pigment located?

Disc membrane

What induces the constant replacement of photoreceptors?

Damage of the photoreceptors in the outer segment due to light absorpition induces a constant replacement

When are rod outer segments shed and what removes them?

In the morning, removed by RPE phagocytosis mechanisms

When are cone outer segments shed and renewed?

In the evening

What happens to Vitamin A in the RPE?

Oxidized in the RPE to give retinal access to the RPE via diffusion thru the large capillary fenestrations in the choriocapillaris

Nerve transduction of information between retinal neurons occurs by:

ion channel activity at gap junctions

What is a gap junction and how does it work in the retina?

An electrical synapse that allows current to pass directly between cells for rapid signal transmission

In gap junctions, no _______ is necessary.

chemical mediator

Between which types of retinal cells are gap junctions (electrical synapses) found?

1) Photoreceptor and photoreceptor

2) Photoreceptors and horizontal cells

3) Horizontal cells and horizontal cells

4) Bipolar axon and amacrine cells

Between which types of retinal cells are chemical synapses found?

-Amacrine and ganglion cells

-Bipolar and ganglion cells

How do chemical synapses transmit signals in the retina?

Neurotransmitter binds to specific sites at the postsynaptic membrane, eliciting an excitatory or inhibitory change in that neuron

Excitatory neurotransmitter in the retina

Glutamate

Which types of retinal cells release glutamate?

All photoreceptors, bipolar cells, and most ganglion cells

Inhibitory neurotransmitter in the retina

GABA and glycine

Which types of retinal cells release GABA and Glycine?

Most amacrine cells and horizontal cells

Chemicals that alter neuron transmission

Neuromodulators

Name the neuromodulators of the retina

Dopamine

Acetylcholine

What is the role of dopamine in retinal function?

changes the conductance of gap junctions between horizontal cells

Acetylcholine is synthesized by

starburst amacrine cells (SACs)

How does acetylcholine affect visual function?

Acetylcholine, along with GABA, helps modulate visual function in the inner plexiform layer (IPL)

Process by which a photon of light is. changed to an electrical signal

Phototransduction

Phototransduction occurs in the

photoreceptors

What triggers phototransduction and the start of the process of vision?

The absorption of light (outer segment) by rhodopsin

During phototransduction, a series of biochemical changes follow light absorption, causing the cell to

hyperpolarize

What is the immediate result of photoreceptor hyperpolarization?

It starts an electrical current flow through the retina

After the photoreceptors, which cells receive the signal in the retina?

Bipolar and horizontal cells

After bipolar and horizontal cells, where is the signal transferred to?

Amacrine and ganglion cells

Which retinal cells' axons carry the visual message to the brain cortex upon activation?

Ganglion cell

What is the resting membrane potential of photoreceptors?

-40 mV

Why do photoreceptors hyperpolarize instead of generating action potentials?

Dependent on potassium influx

Photoreceptors maintain a _________ electrical charge, of about ________mV in the dark.

slight negative electrical charge = -40 mV

What maintains the negative electrical charge of photoreceptors in the dark?

Active transport of cations (sodium and calcium) from within to outside the cell

CGN channels

Cyclic Gated-Nucleotide Channels

in the outer segment; are repsonsible for the light-induced changes in the electrical activity of photoreceptors

Cyclic GMP gated-channels

What pump is located on the inner segment plasma membrane, and what does it do?

Na/K ATPase pump; it uses ATP to pump Na+ out of the inner segment while moving K+ inside

Na+ then re-enters the photoreceptor via ________ in the __________.

Na channels; outer segment

What is the dark current in photoreceptors?

The flow of sodium and other cations into and out of the cell while in the dark