Lecture 10 + 9b Flashcards
Microdialysis
Use an artificial semipermeable membrane to either deliver molecules or measure them
Microdialysis probe is a small meatal tube that holds the membrane. Places in the animals brain
It takes time for the conc or molecules to equilibrate across the membrane so the fastest sampling rate possible is once a min. More common to see once every 10
Can then correlate the release of the detected neurotransmitter with the activity the animal was doing when it was detected
Immunohistochemistry
Histological = label proteins and peptides
Make antibodies that are attracted to the receptor proteins of a specific target
Antibodies are florescent and when washed over a brain slice, bind to their targets making them labeled and easy to ID
Tells us which proteins the cell expresses and hence, what transmitters they released and what neurotransmitters they have receptors for
Cannot make an antibody for a neurotransmitter as too small but CABN target proteins associated with it
eg a protein that makes them like an enzyme is feasible
Immunohistochemistry and expression of immediate early genes
Immediate Early Genes are expressed following periods of elevated spiking
c-Fos is one - rises in nucleus minutes after an increase in neural activity
Use immunohistochemistry to determine what neurons were highly active in the hour two before they died
Sensation vs perception
Sensation - how cells of nervous system detect stimuli and how they transduce these signals
Perception - the conscious experience and interpretation of sensory information
Sensory transduction
Transduction - sensory stimuli converted to receptor potentials
Receptor potential
receptor potentials are graded change in the membrane potential of a sensory neuron caused by sensory stimuli
Sensory neuron
Sensory neuron - specialized neuron that detects a particular category of physical event (eg photoreceptor cell)
Do all sensory neurons make action potentials
NO
All release neurotransmitter
Some don’t make APs but release neurotransmitter in a graded fashion, depending on their membrane potential
The more depolarized, the more the release
4 types of light detectors protein
Receptor proteins sensitive to light are called opsins
4 types of opsins
rhodopsin, and red, blue, green cone opsins
Each photoreceptor only has one
Photoreceptors
Transduce EM energy of photons into receptor potential
There are 4 of these, one for each opsin
Opsin
They are responsible for transduction. They are all INHIBITORY metabotropic, g-coupled receptors
Cone cells express one cone opsin
rods, rhodopsin
Retinal
Small molecules (from vit A) the bind to opsin proteins
Retinal absorbs light
Type of light absorbed depends on protein it is bound to
Two configurations of retinal
Is in opsin protein which is embedded in the cell membrane of photoreceptor cells
One electron absorbs energy from light
The high energy electron changes the shape of the retinal
This causes, the opsin to initiate intracellular g-protein signaling cascades cascades that change the membrane potential
An enzyme plus atp restore the original shape of retinal
What the eye detects
Red = 580nm peak
Blue 530nm peak
Visible light 380-760nm
Each cone cell is sensitive to s spectrum. With the color the brain perceives coming from the average of all 3 and the intensity of the light in all three.
Hence, more than one rod type is needed for color vision
Color perception & Trichromatic Coding
Is a function of the relative rates of activity in these three cell types codes for the color
Also these cells have different sensitivity for light and hence this sensitivity is also a factor
Green cells have the highest sensitivity for light
Additive and Subtractive colour
Light, adds together
Paint adds together absorption therefore subtracts
Magenta - pure removal of green
Yellow - pure removal of blue
Cyan - pure removal of red
Perceptual dimensions of Color and Light
Brightness - intensity (amount of light)
Saturation - purity (in terms of composite wavelengths)
Hue - dominant wavelength (color)
If saturation is 0%, there is equal contributions from all wavelengths which means you have a black and white image
Color blindness
2 types coded for by x chromosone
Genes on X chromosome
Protanopia
1% men
No red opsin. Get filled with green so normal acuity but green-red-yellow spectrum deficits
Mutation in the opsin instead produces milder symptoms (also 1% of males)
Deuteranopia
1% men
No green opsin. Get filled with red so normal acuity but green-red-yellow spectrum deficits
Opsin mutation SIX PERCENT OF MEN - most common color blindness
Less pronounced issue
Blue blindness
Tritanopia
1% if population - not on X chromosome
Blue cells do not compensate but acuity preserved as blue cone opsin is not sensitive to light anyway
Anatomy of eye
Cornea at front - focuses the incoming light a fixed amount
Iris is pigmented muscles
Lens consists of several transparent layers
Can change shape to focus - accommodation
Pupil regulates the amount of light entering the eye
Sclera is opaque and does not permit entry of light
At back, retina - contains photoreceptors
Central region - fovea, cones and high acuity vision
Blind spot/optic disk
Fovea vs periphery
Fovea 1:1:1
receptor:bipolar:ganglia
Periphery 1:X:XY
Sensitive to dim light
Compression of info
Cones vs rods
CONES Prevalent in fovea Sensitive to moderate/high levels of light Provide info about hue Excellent acuity
RODS Peripheral retina, not found in fovea Sensitive to low light Monochromatic only Poor acuity (cannot read with this)
Saccadic movement
6 eye muscles
rapid, jerky movement
Scan scene by one of these to another
Hides blind spot
Pursuit movements allow us to maintain an image of a moving object
Photoreceptor cells
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 sends axons out of the eye. They receive information from bipolar cells and project to the rest of the brain; their axons give rise to the optic nerve, which leaves the retina through the optic disc (i.e., the blind spot of the retina).
Horizontal cells – neurons that interconnect and regulate the excitability of adjacent photoreceptor and bipolar cells. They adjust the sensitivity of these neurons to light in general.
Amacrine cells – neurons that interconnect and regulate the excitability of adjacent bipolar and ganglion cells. There are many different types of amacrine cells, and each have different functions
Transduction of light into receptor potentials
Photoreceptor cells do not have action potentials.
They release glutamate in a graded fashion dependent on their membrane potential: the more depolarized they are, the more glutamate-filled vesicles they release.
In addition to the regular leak potassium ion channels that all neurons have, photoreceptor cells express “leak” sodium ion channels which are open in the dark (when the cells are at rest).
In the dark, sodium continually enters through these ion channels, which depolarizes the photoreceptor cell membrane to -40 mV. At this depolarized membrane potential, photoreceptor cells continuously release glutamate.
When the retinal portion of the retinal-opsin complex absorbs light, it causes a conformational change (change in shape) in the opsin receptor protein. This launches a g-protein signaling cascade that closes the open sodium ion channels. The closing of these channels hyperbolizes the membrane to -70 mV, at which point the photoreceptor cell largely stops releasing glutamate.
So, photoreceptor cells are more depolarized and release more glutamate in the dark than in the light.
Bipolar cells
Like photoreceptor cells, bipolar cells also do not have action potentials and release glutamate in a graded fashion dependent on their membrane potential.
There are two main types of bipolar cells: OFF bipolar cell and ON bipolar cellsOFF bipolar cells express ionotropic glutamate receptors, so they are depolarized by glutamate. Because photoreceptor cells constantly release glutamate in the dark, OFF bipolar cells are more active (more depolarized) in the dark than in the light.
ON bipolar cells (depicted to the right) only have inhibitory metabotropic glutamate receptors, so they are uncommonly inhibited by glutamate. Thus ON bipolar cells are more active (more depolarized) in the light vs the dark.
And finally, retinal ganglion cells (RGCs) are typical neurons. They have action potentials and are generally excited by glutamate.
