Lecture 9a Flashcards
Computerized Tomography (CT) Scans
Computer assisted x ray image
lie withheld in the center of a large cylinder
X ray beam goes through head to a detector
Computer makes this into a series of pictures of brain
X rays absorbed by dense tissue like bone
Good for finding lesions
Cheap and fast
MRI
Magnets polarize hydrogen atoms to spin a certain way
Radiofrequency wave passed through body, knocks H atoms out of spin. When they return to it, they emit radiofrequency of their own
Detected
Scanner estimates the density of hydrogen. Most prevalent in fat and water.
High spatial resolution, 3d image of brain
Expensive
Diffusion tensor imaging DTI
MRI technique that measures the direction and speed of the diffusion of water molecules
Biased to see H in water molecules every few seconds
Shows where axons are in brain
Colors indicate the direction of water molecule diffusion
fmri
Detection of oxygenated blood by iron in blood. Oxygen in blood influences its magnetic properties.
Wo work, neurons need oxygenated blood therefore within 5ms of activation, there is an infusion of oxygenated blood
With a series of fMRIs you can detect changes in blood oxygenation which reflects blood flow. This correlates with neural activity.
Non invasive
Now using it to measure signaling molecules but using enzyme activated magnetic resonance contrast agents. The goal is to develop molecules that differentially affect the magnetic properties of water when they undergo specific chemical (enzymatic) reactions
Positron Emission Tomography (PET)
Radioactive compound injected
2-DG. Similar to glucose in that it is taken up by energy consuming cells but is not metabolized as easily so stays for hours
Radioactivity of 2dg captured by scanner. Computer determines which region has taken up radioactive 2dg and makes a picture of brain showing its activity.
Expensive. Must make radioactive 2dg on site as it is designed to break down fast
Now used with radioactive L dopa to see where it is taken up
Also useful to see changes in expression of receptor proteins across days. Use radioactive ligands (receptor agonists).
EEG
Macroelectrodes
measure net effect of APs of millions of cells around the electrodes
Records gross activity cross the cortex
Can be used to diagnose as certain states of consciousness and atrophy have characteristic patterns
Experimental ablation
Remove or destroy a part of the animals brain
Presumably the functions that can no longer be performed after were mediated by this brain region
Radiofrequency lesions
Pass redifrequency current through a wire at this location
heats up and burns the area
Size of lesion = duration and intensity of current
Axons also passing through get burned
Chemical lesions
Excitotoxic lesion
Glutamate agonist injection
Causes so much calcium influx = apoptosis
Axons passing through spared as lack receptors
Sham lesion
Placebo - use saline
Reversible lesion
Voltage gates sodium channel blockers (all APs stopped)
GABA receptor agonists (hyperpolarize cell bodies
Recording neural activity
Microelectrodes are thin wires that can record the activity of single neurons
Implant with stereotaxic surgery
Plugs into recording system
Chronic electrical recordings are made over an extended time period
Acute are short time (often during surgery when animal anesthetized
Electrical stimulation
Pass a current through a wire implanted in a brain
Affects everything in the area including fibers of passage
Some stimulation patterns (high f)( produce the same behavior as lesioning the area
Chemical stimulation
Use drugs
Guide cannula
Infuse drug
Optogenetic principles
Make brain cells express light activated proteins like ion channels
Take a light sensitive protein from a species (eg algae)
Take the gene for the protein
Inset into specific neurons - match it with a promotor section only that cell reads eg one for an uptake channel to that cells neurotransmitter (such as dopamine uptake channel)
Cell reads promotor and makes proteins
Can now cause the neuron to fire by shining blue light on it
ChR2 makes membrane permeable to sodium when light shone
IC++ increases chlorine when light shone
Optogenetic viral delivery
Find light sensitive protein eg algae
take genes for it
insert into hollowed virus
Insert virus into brain
infects all cells but only the ones that reads the right promotor make the proteins
Target it to
Where axons are located
Where the soma are located
The proteins they express
Whether they recently had more APs than normal
Usually pair with a florescent protein eg GFP so you can identify infected cells
Tracing neural networks
Once you know a particular region is involved in a particular function, you can ask what structures provide inputs and what receives outputs
Retrograde labeling - tracing afferents
Use chemicals like flurogold
Taken up by axons and transported back to the soma
Anterograde labelling - tracing efferent
PHLa - an anterograde tracer
Transported down towards axon terminals
In reality these days we don’t use these chemicals and instead use DNA to make the whole cell green
