Chapter 8 Flashcards
What is the bonus of a CT scan? a Negative?
They are relatively cheap and fast. But the resolution is not great for soft tissue.
What is a CT scan?
A computer assisted X-ray procedure used to take a “photograph” of the brain.
How does a CT scan work?
- Patient lies with his or her head positioned in the centre of a large cylinder
- An X ray-beam (i.e. high energy electromagnetic raditiation) is projected through the head to an X-ray detector
- The X-ray is delievered from all angles
- A computer translates the information received from the X-ray detector into a series of pictures of the skull and brain
What does an MRI use instead of X rays?
Strong magnetic fields
Explain the process of an MRI..
Patient lies in the middle of a large cylinder
A strong magnetic field is applied to the body, the spin of every hydrogen atom proton assumes a particular direction inline with the magnetic field.
Then, radiofrequency waves are administered to the body. This energy is asborbed by protons, changing their direction of their spin, These protons then emit their own radio waves when they immeditaely flip back to that determined by the magnet
How can the scanner of an mRI provide an estimate of the relative density of protons in each area of the body?
By triangulating where the emitted radio waves came from
What is the result of an mRI?
A high spatial resolution three dimensional image of the brain
What do MRI scans primairly reveal? Why?
How can this be changed?
Reveal the density of lipid molecules.
Because the settings of the magnet and the radiofrequencies delievered to the brain are optimized to detect the hydrogen atom protons of lipid molecules.
This can be changed by the magnet settings to detect hydrogen atom protons(DTI)
What’s a DTI?
Diffusion tensor imaging.
An MRI technique that measures the direction and speed of the diffusion of water molecules
What’s a DTI used for?
To identify axon tracts
What do the colours of a DTI indictate?
The direction of water molecule diffisuion
what do fMRI scans use?
A rapid series of MRI scans
What is possible with an MRI scan?
To detect changes in blood oxygenation, which reflects blood flow and correlates neural activity
What does the amount of oxygen in blood do?
Distorts the local magnetic field
What happens when a brain area is active?
Blood flow to that region quickly increases
Why is the fMRI popular?
It doesn’t involved needles, surgery or radioactivity and it produces both structural and functional information with decent spatial resolution
What are PET scans?
Positron Emission Tomography
What do PET scans involve?
Injecting a person with a radio active compound (usually radio active sugar molecules like 2-DG because it is not broken down easily)
What does the scanner of a PET scan identify?
Where radioactive 2-DG molecules are located over time
What is the main disadvantage of PET scanners
Their operating cost. The radioactive molecules need to be made on site due to safety reasons so they are made on site the morning of the experiment
PET scans are also used to measure changes in expression levels of what?
The expression levels of neurotransmitter receptors across weeks
What is an EEG?
An electroencephalogram.
A measure of electrical activity in the brain that uses macroelectrodes (metal discs) attached to the scalp.
What does an EEG record?
The summed population level activity of millions of neurons
What can an EEG be used for?why?
as a diagnostic tool, since specific patters of EEG activity are associated with different stages of consciousness, stages of sleep and types of cerebral atrophy
What do you do if you want to know what an area of the brain is good for?
Lesion it
What is experimental ablation?
Lesion study… involves the removal of destruction of a portion of the brain.
What is the assumption of experimental ablation?
The functions that can no longer be performed following the surgery are the ones the brain region normally controls
What is the technique of creating small lesions through passing radiofrequency currency though a metal wire that is insulated everywhere but the tip?
A radiofrequency lesion
What determines the size and shape of the lesion of a radiofrequency lesion?
The duration and intensity
What is the downside to a radiofrequency lesion?
Axons just passing through will also be burned
What are the different types of lesions?
Excitotoxic lesion
Sham lesion
Reversible lesion
What’s an excitotoxic lesion?
Brain lesion producd by the intracerebral injection of a glutamate receptor agonist.
The drugs cause so much excitation and calcium influx that the affected neurons undergo apoptosis but axons passing through are usually spared
What’s a sham lesion?
A place procedure that duplicates all steps of producing brain lesion except for one that actually causes extensive brain damage
What’s a reversible lesion?
A temporary brain lesion can be acheived by injecting drugs that block or reduce neural activity in a given region
What are the most direct measurements of neural activity?
Metal wires placed in the brain
What are microelectrodes?
Thin metal wires with a fine tip that can record the electrical activity of individual neurons
What are chronic electrical recordings?
Recordings made over an extented period of time
What are acute recordings?
Made over a relatively short period of time
What’s electrical stimulation?
What does this affect?
It involves passing an electrical current through a wire inserted into the brain.
Everything in the area.. some electrical stimulation patters tend to produce the same behvaioural effects as lesioning the brain area
How is chemical stimulation acheived?
With drugs administered through a guide cannula implanted in a particular brain region
What can anesthetics be injected to do?
Shut down all neural activity
What is optogenetics?
The use of light to control neurons which have been made sesnitibe to light through the introduction of foreign DNA. This foreign DNA encodes light sensitive proteins known as opisins. Opsins are proteins that are sensitive to light
What type of receptors are the opsins in our eyes?
Metabotropic with a 30 millisecond delay
What type of channels are the opsins we use for optogenetics?
Ion channels that open and close instantly in rsponse to light
Where were the original opsins discovered?
In bacteria in different parts of the world
Where did different photosensitive ion channels evolve
?
In bacteria and algea
In optogenetics what do excitatory opsins (ChR2) do?1
Either pulse light or leave it on to drive action potential activity
What is a virus a type of?
DNA delievery system
How do viruses normally replicate?
By injecting viral DNA into host organism
What does Virus DNA include?
Instructions on how to make more virus
What is a virus?
A small infectious agent that replicated inside the cells of other oragnisms.
What happens to the virus when we remove the DNA from it?
It becomes “replication deficient”
What happens when a modified virus is injected into the animal’s brain?
It infects the cells it comes into contact with
What happens when a virus gets its DNA into the infected cell’s nucleus?
That cell will start to transcribe it and make the foreign particle
Accortding to what can we target opsin expression to specific neurons in the brain?
According to :
1. Where the soma are located
2. Where their axons are located
3. The proteins they express
4. Whetherthey recently had more action potenials than normal
What happened when researchers modified the flurorescent protein GFP?
What’s this protein called?
What’s it good for?
Caused it to bind calcium and fluorescence much brighter.
This protein is called GCamP
It’s a good way to measure neural activity
What is retrograde labelling?
Tracing afferent axons
What is retrograde labelling used for?
To label the cells that innervate to a given region
What is anterograde labelling?
Tracing efferent axons
What is anterograde labelling used for?
Labelling where axons from a particular location go to
What is stereotaxic surgery?
A surgical intervention that uses a stereotaxic apparatus
What is a stereotaxic apparatus?
A device that permits a surgeon to put something into a very specific part of the brain
What is a stereotaxic apparatus used for?
To inject things into the brain such as drugs viruses or tracer molecules
OR
To permanently implant things like cannula, electrodes or fiber optic cable
What is the Bregma?
The junction where pieces of skull fuse together
What is bregma often used for?
A reference point for stereotaxic surgery
What are common reasons for stereotaxic surgery?
Used for one time injections of drug or virus to :
- lesion a bain area
-lesion a specific type of cell in a particular brain area to change gene expression
and
implant guide cannulas for later infusion of drugs
implant microelectrodes for stimulation or recording experiments
implant fiber optic cables to allow for imaging or stimulation using optogentic techniques
How do we measure fluctuations in neurotransmitter levels in behaving animals?
Microdialysis
What is dialysis?
The use of a semipermeable membrane to either deliever molecules to or measure the amount of molecules in some solution or brain are
What is the microdialysis probe?
A small metal tube that holds dialysis tubing
What is the sampling rate? Why?
1/ 1min - 1/10 mins
Because it takes time for the concentration of molecules to equilibrate across a dialysis membrane
What is immunohistochemistry?
A histological method used to label proteins and peptides in biological tissue
How do we often classify neurons?
By the proteins they express
What are we interested in regards to specific neurotransmitter receptors?
Where they are located
What does immunohistochemistry take advantage of?
Antibodies
What are antibodies?
proteins made by the immune systems of mammals, designed to selectively bind to a single type of protein
What have researches developped that are designed to selectively bind to all different types of proteins?
Fluorescent antibodies
What is often not possible in regards to immunohistochemisty?
To make an antibody selectively bind to a small molecule
What does research often use to identify cells that make and release classical neurotransmitter?
Antibodies againt the enzymes that make these neurotransmitter
What do immediate early genes tend to be expressed following?
Periods of elevanted spiking activity
What is c-Fos?
An immediate early gene
What happens when a neuron experiences a sharp increase in spiking activity?
Levels of C-Fos protein become elevated in the nucleus within minutes
How can c-fos protein levels be measured?
In brain slices using immunohistochemisty to identify neurons that were highly active the hour or two before the animal died
