Lecture 22 Flashcards
Photoreceptors
light-sensitive neurons that convert light energy into electrical energy in cells and are located in the retina
Phototransduction
The conversion between light energy to electrical energy
What are the 2 main types of photoreceptors in our retinas?
cones and rods
Each retina contains about ___________ cones and __________ rods.
6 million
120 million
How do rods and cones respond to stimuli?
with graded membrane potentials (they DO NOT fire action potentials)
True/False: rods and cones are neurons
True
What segments are rods and cones made up of?
outer segment and inner segments
Outer Segments
- contain the membrane (which folds into disk-like layers which contain the visual pigments that respond to light)
- These segments face the epithelial cells close to the back of the eye
Inner Segments
- are the nucleus and organelles (ER, mitochondria etc) for protein synthesis; and in a basal layer
- contain the synaptic terminal that releases glutamate.
What is the dominant neurotransmitter in the retinal network?
Glutamate
(Has both excitatory and inhibitory effects based on what receptors it binds to)
Ie: excites on-Center bipolar cells when it binds the the photoreceptors in the Center rather than the surrounding
What 2 cells are affected by glutamate binding?
OFF-bipolar cells
ON-bipolar cells
What happens to bipolar cells when there is no light?
- photoreceptors continuously release glutamate which
- HYPERpolarizes ON-bipolar cells (making them less likely to fire) BUT
- DEpolarizes OFF-bipolar ones (making them more likely to fire).
What happens to bipolar cells when there is light?
- photoreceptors stop releasing glutamate; this causes
- DEpolarization of ON-bipolar cells (they start firing) and
- HYPERpolarization of OFF bipolars (they stop firing).
What is Rhodopsin?
a transmembrane visual pigment protein
How many pigments does each type of photoreceptor contain?
one
What is the name of the pigment in the rod photoreceptor
rhodopsin
What pigments are in the 3 cones?
blue
green
red
(protein- iodopsin)
What are more sensitive to light; rods or cones?
cones
What are cones responsible for?
they are responsible for vision in bright light and for distinguishing colors
What type of photoreceptors don’t operate in dim conditions?
cones
What type of photoreceptors only operate in low light?
Rods in daylight they are “bleached out”- their rhodopsin is broken down so they can’t sense light.
Why can’t rods operate in normal light
their rhodopsin is broken down so they can’t sense light and in daylight, they are “bleached out”
How do rods adapt to dim light?
hey rebuild their stores of rhodopsin over ~30 minutes
Where are photoreceptors most densely packed?
The Macula a central disk and especially in its central pit called the fovea
What de we use the fovea for?
deatiled vision
What is the blind spot?
- a region with no photoreceptors
- also known as the optic disc or optic nerve head
- it is the point of exit for ganglion cell axons leaving the eye
The fovea contains almost exclusively __________. Whereas more-peripheral retina contain mainly _________.
cones
rods
What are the 3 layers of neurons in the retina?
(front of the eye/light) Ganglion cells > Bipolar cells > receptors (back of the eye)
Up to ____ photoreceptors may converge on a single bipolar cell
45
Bipolars in turn converge on ganglion cells, so in each eye, the signal from 126 million receptors is condensed into ______________.
1 million ganglion cells
Convergence is greatest in the _________________ and least in the ______
peripheral retina
fovea
What are the 2 types of bipolar-cell receptive fields?
On center
Off center
On-center cells
are excited by light in the center of their field, and inhibited by light in the surround.
These cells respond most when a light spot fills their center and the surround is dark
Off-center cells
are inhibited by light in the center, and excited by light in the surround.
They respond best when a dark spot fills their center and the surround is light.
What happens to bipolar receptive fields when the lighting is uniform
neither type of bipolar cell responds, because the effects of the center and surround cancel, leaving the cell at its resting level of activity (only weakly active)
the Chevreul illusion
a brightness illusion that consists of adjacent homogeneous grey bands of varying luminance that appear inhomogeneous.
the Hermann grid
an optical illusion in which the crossings of white grid lines appear darker than the grid lines outside the crossing
Large, magnocellular ganglion cells, (M cells)
provide information that is used by the brain to infer the movement of objects. These cells are phasic. ~10% of retinal ganglion cells are M.
Small, parvocellular ganglion cells, (P cells)
provide information that is used to infer form and fine detail, such as texture. ~70% of retinal ganglion cells are P.
What are the least common retinal ganglion cells?
melanospin cells
A ganglion cell near the _____ gets input via bipolars from only a few photoreceptors, mostly _____
fovea
cones
Farther from the _____, each ganglion cell gets input from many receptors (up to 75,000), mostly ____
fovea
rods
And near the fovea, ganglion cells are _____ sensitive to light but have better ______ because each one gets input from just a few densely packed cones
less
spatial resolution
The cranial nerve II
- Also known as the optic nerve
- responsible for transmitting visual information.
- contains only afferent (sensory) fibers, and like all cranial nerves is paired
Chiasm
an intersection or crossing of two tracts in the form of the letter X
Fibers from the _____ half of each retina cross; those from the _______ retinas do not.
nasal
temporal
What are the optic tracts?
The nerve bundles emerging from the chiasm
How do signals make their way to the cortex?
- through the chiasm to the optic tracts
- The to the LGN in the thalamus
- Then project via the optic radiations to the primary visual cortex
In the eye, the right side of the scene (the right visual hemifield) projects onto the _________ of each retina
left side
retinotopic organization
the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream.
neurons close to each other in ______ get information from close-together parts of ________
the brain
the retina
