NSCI 201 Lecture Notes - Hurlburt Field, Rela, Visual Cortex
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LECTURE NOTES: BRAIN METABOLISM
• Axis: Temporal resolution, spatial resolution (how large or small a region of the brain that the technique can study), how invasive the techniques are
• EEG and scalp ERP; Human optical imaging; MEG; fMRI > least invasive
• Transcranial magnetic stimulation (TMS); PET (invasive b/c use radioactive tracers) > medium invasive
• Drug manipulations; lesions > invasive
• Non-electrical techniques (PET and fMRI) show the activity of cells and the role of the circuitry system (not responsible for names on right diagram)
• At a local scale there are tiny capillaries and veins that feed local grps of neurons, the neurons that ppl are studying
• CT imaging is basic: X-ray source shot through structure you want to image, and detectors on the other side. Dpding on composition of the, obect the image produced
will be different
• Not hard tissue: X-rays pass through, detect many events
• Hard tissue: X-rays don’t pass through. X rays absent from that area
• Changing position of emitter and detectors, and taking shots from around the object, combine these images to decide where the ‘shadow’ (hard tissue) is produced from
• Take a molecule you’re interested in (e.g sugar, major energy source for brain): sugar taken up is proportional to energy used. Use radioactive tag on the sugar, put in
bloodstream
• Sugar is radioactive thus emits radioactive particles. Common isotope uses Flu-18, which emits radioactive particles (positrons) which hit nearby electron, causing
2 gamma rays to shoot out at 180 degrees from each other.
• Circle of crystal detectors fluoresce when hit by gamma rays. Rays emitted in proportional to amt of sugar taken up (and thus the activity of brain)
• Computer looks for 2 detectors 90 degrees apart that have flashes occur simultaneously; computer draws a line btwn the two. Continue for all the flashes, and the result
is the lines all cross at the source of the radioactivity
• These techniques are very impt for detecting cancers
• A special form of sugar was created in which one oxygen from the ring was removed. Its shape is changed so it can enter cell regularly and undergo first step of
metabolic processing, but then its shape changes so it can’t interact with enzymes that would process to extract energy
• This molecule is called 2-DG
• Improvement in high speed electronics enable researchers to get more accurate determinations of when real, coinciding events occur (differentiate from scatter or
random events)
• This is an improvement in time resolution
• Detectors made several layers thick, and off-set in space in their rows to increase spatial resolution
• MRI uses magnetic fields to manipulate characteristics of molecules already in the brain
• to control magnetic fields, the tube is very small
• Auxiliary coil used to boost signals coming from the brain
• MRI looks at protons: (left) a moving particle in a magnetic field generates a current
• analogy: when charge moves in space, a magnetic field is generated
• MRI machine has strong magnetic field; net effect is to take spinning protons and line up the axis on which they are spinning, onto the lines of the magnetic field
• 2 orientating states the protons can move
• protons parallel to magnetic field, in same direction: doesn’t require energy for protons to stay in this state
• (more rare) protons parallel to magnetic field, in opposite direction: high energy
• protons destabilized by pulse of energy from magnetic field at 90 angle to it; this changes alignment of protons. Then removing their energy causes the protons to be
influenced by the magnetic field kept constant (the one you applied)
• as proton loses energy (like a spin top), the proton slows down and wobbles in a circle and emits a signal. Signal is called net magnetization and its value dpds on ratio
of protons in energy parallel state to high energy anti parallel state
• (B) apply energy from a right angle to the protons to change their orientation. When energy pulse is then removed, the protons emit energy
• frequency of emitted energy dpds on strength of magnetic field