Methods in brain research 1.Structure a. Morphology b. Pathways 2. Function.
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Transcript of Methods in brain research 1.Structure a. Morphology b. Pathways 2. Function.
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Methods in brain research
1. Structurea. Morphologyb. Pathways
2. Function
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1. Sacrificing the animal / or post-mortem studies
2. Replace the blood with saline (physiologic water)
3. Fixation – injection of formaline
Dissection
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Grey matterWhite matter
frontal section horizontal section
ventricles
Thalamus
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Nissl stain
Staining of cell bodies
Histology
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Staining of axon fibers
Hard to see individual cells
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Golgi stain
Individual cells
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Microscopy
1. Light (up to x1500 magnification)
2. Electron Microscopy (EM; magnification of ~108 in tissue slices)1. Transmission EM (high resolution 2d information)2. Scanning EM – lower resolution but provides 3d information
3. Confocal microscopy – Uses laser and fluorescent dyes. Higher resolution than light microscopy Thicker slices (3d) in living tissue
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Neural pathways
Anterograde labeling – chemicals that enter the cells (through dendrites), and travel along the axons to the terminal buttons
1. Injection of PHA-L
2. Substance is transported throughout the cell
3. Animal is sacrificed and brain is perfused
4. Histology – PHA-L is made visible through use of immunocytochemical methods
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Autoradiography – using radioactive markers for labeling
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Retrograde labeling – chemicals that enter the terminal buttons, and travel through the axons to the cell bodies
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Auditory pathway
Tracer in the thalamus
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Imaging techniques to study structure
Computerized Tomography (CT)
Magnetic Resonance Imaging (MRI)
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Computed Tomography
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Computed TomographyX ray
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Computed TomographyX ray
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Computed Tomography X ray
Advantages of CT
• Accessible (and cheap)
• Efficient in detecting
• Stroke•
Hemorrhages• Tumors
• Uses X-Ray• No separation between white/grey
matter• Can only take horizontal sections
Drawbacks of CT
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Good spatial resolution (~1mm)Separation between grey/white matter
Magnetic Resonance Imaging
Very strong magnetic field.Earth is ~0.5 Gauss10,000 Gauss is 1 TeslaTypical scanner today has a magnetic field of 3 Tesla
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Magnetic resonance angiography (MRA)
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Measuring proton density (from water molecules) with MRI
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Advantages:
Non-invasive (and can be used in the living brain)
High spatial resolution (~1mm)
Safe (Does not damage tissue)
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Diffusion Tensor Imaging (DTI)
Can NOT tell from _X_ to_Y_ - but gives the general direction of the fibers
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Tools to study brain function
1. Manipulating brain function (e.g. by lesions) and inferring a region’s role in behavior/cognition
2. Manipulating behavior and measuring changes in brain activity
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Naturally occurring lesions (stroke) or accidents (e.g. gun wounds etc.)
Broca
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Phineas Gage’s lesion reconstructed
(H. Damasio and R. Frank, 1992)
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Experimental ablation (removal of brain tissue)
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1. Suction of brain tissue
2. Destruction of brain tissue Electrically (general)Chemically (more specific)for example – use of kianic
acid to over-excite cells until they die
3. Temporary lesions (cooling)
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Recording of electrical activity / electrical stimulation of brain tissue
Spike trains
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Free recall demo for hippocampal re-activation
Gelbard-Sagiv, Mukamel, Harel, Malach & Fried. Science 2008
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Quiroga, Mukamel, Isham, Malach & Fried. PNAS 2008
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Positron emission tomography (PET) – Enables tracking metabolic processes in various brain
regionsInvolves injection of a radio-active tracerEnables scanning in multiple planes
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1. Seeing words2. Listening to words3. Saying words4. Generating verbs
PET-Positron Emission tomography
1 2
3 4
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Functional Magnetic Resonance Imaging
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What does the BOLD fMRI signal measure?
• Hemoglobin has two states: Oxygenated (diamagnetic), andDe-Oxygenated (paramagnetic)
• The BOLD fMRI signal is sensitive to the ratio between Oxy and De-oxy hemoglobin in a manner that an increase in Oxygenated blood results in an increase in the fMRI signal.
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Following neural activation:
Consumption of oxygen
Overcompensation with fresh oxygenated blood
Time
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fMRI signal
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Advantages of fMRI over PET
• Higher temporal resolution (seconds vs min)• No radioactive radiation• cheaper