2.1 photoreceptors

which photoreceptor has more elongated neurons

rods

which photoreceptor is shorter and thicker

cones

as the photoreceptor axons terminate at the outer plexiform layer, what cells do they make synapses with?

horizontal and bipolar cells

as the axons of the bipolar cells terminate at the inner plexiform layer, what cells do they make synapses with?

amacrine and ganglion cells

the axons of the ganglion cells form the nerve fiber layer exit the eye via what structure

optic disc

which photoreceptor cell have a higher count?

rods

which photoreceptor cell have a lower count?

cones

how many rods are there in our eye

120 million

how many cones are there in our eye

6 million

what is the peak density of rods

20 degrees

what is the peak density of cones

0 degrees

definition of pea density in our eye

where on the retina each cell type reaches its maximum concentration

how far away from the fovea does the rods reach its peak density

6mm

how is time response of rods

slow

how is time response of cones

fast

is there directionality for rods?

no because it is sensitive to scattered light

is there directionality for cones?

no because it is sensitive to directional light

what is the photopigment of rods

rhodopsin

what is the photopigment of cones

S, M, L cones

how is the light sensitivity of rods

high

how is the light sensitivity of cones

low

why is the light sensitivity for rods so high

because they are slow and can accumulate weak light signals

why is the light sensitivity for cones so low

because they require a lot of photons simultaneously to generate an equivalent neural signal

what is the temporal resolution of rods

low, around 12 Hz

what is the temporal resolution of cones

high, around 60 Hz

how is the spatial sensitivity of rods

low because they sacrifice fine spatial detail to combine weak signals across a wide area to detect dim spatial targets in the dark

how is the spatial sensitivity of cones

high due to high acuity and low summation

how is the dark adaption for rods (time)?

30-40 mins

how is the dark adaption for cones (time)?

10 mins

main function of rods

low light and motion detection

main function of cones

colour vision and high detail central vision in bright light

definition of temporal resolution in terms of rods and cones

visual system's ability to track changes over time, how quickly it can resolve distinct flashes or movements in light)

definition of light sensitivity in terms of rods and cones

ability to detect spatial differences in light and resolve fine details/patterns

which cone is distributed stochastically (randomly) through retina

L and M cones

which cone has moer uniform distribution throughout the retina

S cone

which cone is the most sensitive at shorter wavelength

S cone

which cone is the most sensitive at longer wavelength

L cone

at what wavelength is the S cone most sensitive to

426 nm

at what wavelength is the rod most sensitive to

507 nm

at what wavelength is the L cone most sensitive to

560 nm

If we have photons at 426 nm coming through retina, which photopigment will it land on

S cone

If we have photons at 507 nm coming through retina, which photopigment will it land on

rhodopsin

If we have photons at 530 nm coming through retina, which photopigment will it land on

M cone

If we have photons at 560 nm coming through retina, which photopigment will it land on

L cone

what is the synaptic terminal of rods

spherule

what is the synaptic terminal of cones

pedicle

outer segments of both rods and cones are embedded in what structure

RPE

what does each disk in the outer segment of rods represent?

a photopigment molecule

what is the role of RPE for cones and rods

keep outer segment in good condition and recycle metabolize byproducts

how long does it take for the whole outer segment of single rod to renew

14 days

how long does it take for the whole outer segment of single cone to renew

12 days

what is disk shedding or disk phagocytosis

tips of outer segment of cones and rods regenerated continuously, and are phagocytosized and broken down by RPE. byproducts are then passed to choroid and the vascular system for cleansing

what is retinal eccentricity

angular distance between a specific point on the retina and the center of the fovea in mm or degrees

at a retinal eccentricity of 15-18 degrees, what is expected in our visual field

physiological blind spot

a higher retinal eccentricity corresponds to higher number of what photoreceptor cell

rods

what is the diameter of cones in the fovea

2 μm

what is the diameter of cones near periphery

6 μm

what is the diameter of rods near periphery

2 μm

what is a photopigment

a pigment that chemically reacts when absorbs light (or photons) and contribute to photoreception

what is the receptive Field (RF) of a neuron in the visual system

area that influence neural activity of the cell

what is the receptive field of a cell that is connected to multiple photoreceptor?

sum of the receptive fields of all the photoreceptors that it is connected to

after the photons are absorbed by photopigments, what does the photoreceptors do

they generate an electrical signal caused by ions flowing in and out of cell, then trasmit electrical activity via their connection with the horizontal and bipolar cells

what does it mean when neurons with receptive fields show spatial antagonism

they have a center that responds to light and a surround that repsond to when light is turned off

how does the ON and OFF center differ in spatial antagonism

one area responds differently to light than another area and only one part of the receptive field is active

what does the ON area of the receptive field respond to?

with increment of light (when light turns on)

what does the OFF area of the receptive field respond to?

with decrement of light (when light turns off)

what does it mean when center-surround RFs show spatial antagonism or lateral inhibition

it means light has the opposite effect in the surround than in the center

Based on the principle of univariance, why is a single photopigment unable to distinguish between different wavelengths of light?

The wavelength information is lost once a photon is absorbed, as the receptor only signals the rate of absorption.

The 'photochromatic interval' is closest to zero at which part of the visible spectrum?

long wavelength (red, 650 nm or longer)

A single photon of 680 nm and a single photon of 507 nm both bleach a rhodopsin molecule. Which statement is true regarding the resulting signal?

The effect on the photoreceptor is identical once the photon is successfully absorbed.

What is the primary function of the 'dark current' in photoreceptors?

It keeps the photoreceptor slightly depolarized in the dark by allowing sodium ions to flow in.