Lecture 9: Vision l Flashcards
sensation
How cells in the nervous system detect stimuli in the environment and how they transduce these signals into a change in membrane potential and neurotransmitter release
perception
the conscious experience and interpretation of sensory information
sensory receptors/neurons
Specialized neurons that detect a specific category of physical events. They accomplish this task with receptor proteins that are sensitive to specific sensory stimuli
do sensory receptors have axons and release action potentials?
some do, but not all of them
do sensory neurons release neurotransmitters?
yes
what are the sensory neurons that do not release action potentials?
small cells that release neurotransmitters in a graded fashion dependent on their membrane potential
the more depolarized a cell is, the ____ neurotransmitters it releases
more
sensory transduction
the process by which sensory stimuli are converted into receptor potentials
receptor potential
graded change in the membrane potential of a sensory neuron caused by sensory stimuli
opsins
receptor proteins that are sensitive to light
4 types of opsin proteins
rhodopsin, and the red, green, and blue cone opsins
4 types of photoreceptor cells
rod cell, red cone cell, green cone cell, blue cone cell
what do rod cells express?
rhodopsin
what do red cone cells express?
the red cone opsin
what do green cone cells express?
the green cone opsin
what do blue cone cells express?
the blue cone opsin
photoreceptors function
the sensory neuron responsible for vision. transduces the electromagnetic energy of photons into receptor potentials
how many opsin proteins does each photoreceptor cell contain
1
where are photoreceptors located?
in the retina
what type of receptors do opsins have?
inhibitory metabotropic receptors
what are retinals?
small molecules that bind to opsin proteins. absorb the energy of photons
how are retinals made?
synthesized from vitamin a
what type of light do retinals interact with
depends on the opsin protein that the retinal is bound do
configuration 1 -> 2 of retinal
when light hits the retinal molecule, it triggers an intracellular G protein cascade that causes a change in the membrane potential
configuration 2 -> 1 of retinal
An enzyme binds to ATP to turn the retinal back into its original configuration that can detect light
visible light
electromagnetic energy that has a wavelength from 380-760 nm
how many photoreceptor cells do we use to detect light
4 (3 cones & 1 rod)
what type of waves are 400 nm
small wavelength gamma rays
what type of waves are 760 nm
large wavelength radar/infrared rays
what type of opsin evolved first?
red cone opsin
what type of opsin evolved second?
blue cone opsin
what type of waves have a rapid alternation between electromagnetic energy
small wavelength (gamma)
what type of waves have a slow alternation between electromagnetic energy
large wavelength (radar)
what kind of light are blue cone opsins sensitive to?
short wavelengths
what kind of light are green cone opsins sensitive to?
medium wavelengths
what kind of light are red cone opsins sensitive to?
long wavelengths
colour perception is a function of ____
the relative rates of activity in the three type of cone cells
the amount any cone will be activated depends on
its wavelength of light and its intensity/ amount
if someone is shown three colours at the same time at the same intensity, they will say ___ is brighter and why
green because green cone opsins are the most sensitive to light
additive colour and its primary colours
creates light by adding light to black. thus, the primary colours are red, green, and blue
subtactive colour and its secondary colours
subtracts colour from white light. thus, the primary colours are yellow, cyan, and magenta because each only absorbs one colour from white light
brightness
intensity, luminance, amount
saturation
purity of the colour (in terms of composite wavelengths)
hue
dominant wavelength (colour)
if the brightness of an image is 0%, it will be
black
if the saturation of an image is 0%, it will be
black and white because you are in the middle of the colour cone where there is no colour
protanopia
absence of the red cone opsin, resulting in trouble distinguishing colours in the green-yellow-red section of the spectrum
how common is protanopia
occurs in 1% of males
is visual acuity affected by protanopia
Visual acuity is normal because red cone cells get filled with green cone opsin
deuteranopia
Absence of the green cone opsin resulting in trouble distinguishing colours in the green-yellow-red section of the spectrum
how common is deuteranopia
occurs in 1% of males
is visual acuity affected by deuteranopia?
Visual acuity is normal because green cone cells get filled with red cone opsin
tritanopia
absence of the blue cone opsin
is visual acuity affected by tritanopia?
it’s not noticeable affected because the blue cone opsin is not that sensitive to light
how common is tritanopia?
occurs in 1% of the population
achromatopsia
true colour blindness
what causes achromatopsia?
Typically caused by mutations in the g protein signalling cascade that is similar in all cone cells
conjunctiva
mucus membranes that line the eyelid
cornea
the outer, front layer of the eye that focuses incoming light
iris
a pigmented ring of muscles that can contract or relax to determine how much light gets into the eye
lens
consists of several transparent layers.
accommodation
the process by which the shape of the lens changes to allow the eye to focus
pupil
the opening in the iris that regulates the amount of light entering the eye
sclera
opaque and does not permit the entry of light. Six extraocular muscles are attached to the sclera that holds the eye in place and moves it around
retina
the interior lining of the eye. Contains photoreceptor cells that are classified as rod or cone cells
vitreous humour
a clear gelatinous fluid that light crosses after passing through the lens
fovea
the central region of the retina that contains a high density of photoreceptors and thus mediates acute vision. it connects photoreceptors to bipolar and ganglion cells
optic nerve
where visual information leaves the eye to be processed in the brain. the site of the blind spot
foveal vision
clear colour vision that is sensitive to detail because the fovea contains cone cells that each connect to a downstream neuron, allowing them to register the exact location of the input
peripheral vision
vision that is sensitive to dim light and has low spatial resolution. rod cells converge onto multiple downstream neurons, meaning that the precise location and shape of the input are impeded
differences between rods and cones
- Cones are more prevalent in the central retina; rods are more prevalent in the peripheral retina
- Cones are found in the fovea; rods aren’t
- Cones are sensitive to moderate to high levels of light; rods are sensitive to low levels of light
- Cones provide information about hue; rods provide only monochromatic information
- Cones provide excellent acuity; cones provide poor acuity
photoreceptors that are found in the fovea are
cones
photoreceptors that are prevalent in the peripheral retina are
rods
photoreceptors that are prevalent in the central retina are
cones
photoreceptors that are sensitive to high levels of light are
cones
photoreceptors that are sensitive to low levels of light are
rods
photoreceptors that are monochromatic are
rods
photoreceptors that provide acuity are
cones
saccadic movements
rapid, jerky shifts in your gaze from one point to another
pursuit movements
allow us to maintain an image of a moving object
orbits
bony sockets in the front of the skull
eyes are suspended…
in orbits
photoreceptor cells
neurons responsible for the transduction of light. They project to bipolar cells
bipolar cells
neurons that relay information from photoreceptor cells to ganglion cells
ganglion cells
the only neurons in the retina that send axons out of the eye. They receive information from bipolar cells and project it to the rest of the brain. Their axons give rise to the optic nerve.
horizontal cells
neurons that interconnect and regulate the excitability of adjacent photoreceptors and bipolar cells
amacrine cells
neurons that interconnect and regulate the excitability of adjacent bipolar and ganglion cells
do photoreceptors have action potentials?
no
do photoreceptors release glutamate?
They release glutamate in a graded fashion dependent on their membrane potential
the more depolarized a photoreceptor is, the more
glutamate it releases
what types of ion channels do photoreceptors express
leak sodium channels that are open in the dark (when the cell is at rest)
what happens when photoreceptors are in the dark (at rest)
Sodium continually enters through these channels which depolarizes the photoreceptor cell membrane to -40 mV. At this depolarized membrane potential, they release glutamate
what happens when photoreceptors are in the light?
When the retinal portion of the retinal-opsin complex absorbs light, it causes a change in shape in the opsin receptor protein. This launches a g-protein signalling cascade that closes the sodium ion channels. The closing of these channels hyperpolarizes the membrane to -70 mV, stopping the photoreceptor from releasing glutamate
do bipolar cells have action potentials?
no
do bipolar cells release glutamate?
Release glutamate in a graded fashion dependent on their membrane potential
two types of bipolar cells
OFF bipolar cells and ON bipolar cells
OFF bipolar cells
express excitatory ionotropic glutamate receptors so they are depolarized by glutamate. More active (depolarized) in the dark
ON bipolar cells
only have inhibitory metabotropic glutamate receptors, so they’re uncommonly inhibited by glutamate. More active (depolarized) in the light
do ganglion cells have action potentials?
yes
ganglion cells and glutamate
ganglion cells are excited by glutamate
receptive fields
The area of visual space where the presence of light influences the firing rate of a neuron
identifying the receptive field of a neuron
record from the neuron as an animal maintains focus on a central fixation point. Then shine a light in different areas of the visual space and see where a change in the light alters the spiking activity of a neuron
simple mutations in the red cone opsin occur in
1% of males
simple mutations in the green cone opsin occur in
6% of males
sensation is composed of
detection and transduction
perception is composed of
experience and sensory interpretation
photoreceptors release more glutamate ____
in the dark
retinal
the molecule that absorbs photon energy
why are opsins sensitive to light?
from binding to retinal
what types of visual agnosia are more common in males?
Protanopia and Deuteranopia
why are Protanopia and Deuteranopia more common in males?
they’re x-linked
the fovea only contains
cone cells
OFF bipolar cells in the dark
sodium channels are open so they’re depolarized and release more glutamate
OFF bipolar cells in the light
sodium channels are closed so they’re hyperpolarized & release less glutamate
ON bipolar cells in the dark
they’re hyperpolarized & release less glutamate
ON bipolar cells in the light
they’re depolarized & release more glutamate
what type of receptors do ganglion cells have?
on-off receptor fields for light and colour
