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Download Parts of a Neuron…… Neuronal Communication….
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Neurons ……. Historical Overview • Four major parts to neurons…. – Dendrites 1) Greek philosophers – eye receives “copies” of objects 2) Empiricists – we learn to perceive • “receiving” part – electrical activity from coming from other neurons “stimulate” these parts – Soma (body) • Contains structures to keep the cell alive – Axon • “Sending” part – the electrical signal generated by the neuron travels down the axon in order to “stimulate” a nearby neighboring neuron. – Berkeley: How do we perceive depth with 2D receptors??? – James: “blooming, buzzing confusion” – Synapse • Very end of the axon where chemicals are released to stimulate the next neighboring neuron located nearby • Sensory systems have “specialized” neurons called “receptors” – Specialized to detect and react to external stimulus energy 1/24/2006 1 Parts of a Neuron…… 1/24/2006 1/24/2006 2 Neuronal Communication…. 3 1/24/2006 4 electrical signals…… Synapse…. • Release of chemical (neurotransmitter) at the end of the axon at the synapse causes: – Certain ion channels to open or close in the receiving neuron – These channels allow ions to enter or leave the receiving cell – If enough ions enter or leave the “receiving” cell, than the electrical properties of the cell membrane change and cause it to generate an action potential – The action potential then travels down the length of the axon to the synapse and causes release of neurotransmitter onto the next “receiving cell and the process starts all over 1/24/2006 5 1/24/2006 6 •1 Patterns of neural firing … Action potential…. •Each “tick” seen in the figures below represent an action potential being generated in a neuron •The brain assigns different “meaning” to different patterns or rates of firing Stimulation Causes FAST firing Time Stimulation Causes SLOW firing Time 1/24/2006 7 1/24/2006 Basic structure of the brain…. Basic structure of the brain…. • The brain can be divided into different parts and regions – The cortex has four primary “lobes” • Occipital lobe – primary receiving lobe for vision • Temporal lobe - primary receiving lobe for audition and chemical senses • Parietal lobe - primary receiving lobe for skin senses and chemical senses • Frontal Lobe - primary receiving lobe for chemical senses – This is known as “modular organization” • Different parts or regions are responsible for processing different things • Certain parts of the brain are known to be primarily responsible for processing incoming information from the sensory systems and are located I the very outer layering called the cerebral cortex 1/24/2006 9 8 1/24/2006 10 Studying human brain activity in humans…. Structure of the brain….. • In order to find what areas are responsible for processing the different senses, a number of methods are used: – Evoked potentials – Positron Emission Tomography (PET) – Functional Magnetic Imaging (fMRI) 1/24/2006 11 1/24/2006 12 •2 Evoked Potential images…. Evoked Potentials…. • Evoked Potential – Similar to an EEG ( electroencephalogram) • Electrodes are attached to the scalp and the underlying brain activity is recorded numerous times while the person is performing different sensory tasks • The activity is then “averaged” for each trial and specific “waveforms” are then interpreted Before Averaging 1/24/2006 13 After Averaging 1/24/2006 14 Pet images….. Pet…. • PET is able to measure the metabolic activity in a live brain. • A low level radioactive substance is injected into the bloodstream while the person is engaged in a sensory task • The more active a brain region is the more blood it will use • A special imaging camera detects the low radiation emitted – Brains areas with higher blood flow will emit more low level radiation 1/24/2006 15 1/24/2006 16 Recipe for MRI Basic fMRI Physics 1) Put subject in big magnetic field (leave him/her there) 2) Transmit radio waves into subject [about 3 ms] 3) Turn off radio wave transmitter 4) Receive radio waves re-transmitted by subject – Manipulate re-transmission with magnetic fields during this readout interval [10-100 ms: MRI is not a snapshot] 5) Store measured radio wave data vs. time – Now go back to 2) to get some more data 6) Process raw data to reconstruct images 7) Allow subject to leave scanner (this is optional) 1/24/2006 17 1/24/2006 18 •3 Nuclear Magnetic Resonance Why the Name Change? NMR → MRI Isador Rabi Felix Bloch Edward Purcell nuclear: properties of nuclei of atoms magnetic: magnetic field resonance: interaction between oscillating magnetic fields and atomic nuclei Rabi (1944) and Bloch & Purcell (1952) win Nobel prizes for their contributions to NMR 1/24/2006 19 most likely explanation: nuclear has bad connotations 1/24/2006 20 Current Technology Understanding it: RF First Human MR Image Raymond Damadian • 1971: discovered that cancerous tissue had different MR properties than healthy tissue • 1972: first human NMR scanner patented by Damadian’s company, FONAR • first magnet named “Indomitable” (meaning “impossible to subdue”) • skeptics told him clinical MRI wouldn’t work because you’d have to rotate the scanner 10,000X/s • never won a Nobel for his work less likely but more amusing explanation: subjects got nervous when fasttalking doctors suggested they would need an NMR • The MRI machine applies an RF (radio frequency) pulse that is specific only to hydrogen. The system directs the pulse toward the area of the body we want to examine. The pulse causes the protons in that area to absorb the energy required to make them spin, or precess, in a different direction. • This is the "resonance" part of MRI. The RF pulse forces them (only the one or two extra unmatched protons per million) to spin at a particular frequency, in a particular direction. The specific frequency of resonance is called the Larmour frequency and is calculated based on the particular tissue being imaged and the strength of the main magnetic field. Damadian in 1977 with prototype of first MR scanner 1/24/2006 21 1/24/2006 22 Understanding the Technology: RF • These RF pulses are usually applied through a coil. MRI machines come with many different coils designed for different parts of the body: knees, shoulders, wrists, heads, necks and so on. These coils usually conform to the contour of the body part being imaged, or at least reside very close to it during the exam. At approximately the same time, the three gradient magnets jump into the act. • They are arranged in such a manner inside the main magnet that when they are turned on and off very rapidly in a specific manner, they alter the main magnetic field on a very local level. What this means is that we can pick exactly which area we want a picture of. In MRI we speak of "slices." Think of a loaf of bread with slices as thin as a few millimeters -- the slices in MRI are that precise. 1/24/2006 23 Understanding the Technology: RF • We can "slice" any part of the body in any direction, giving us a huge advantage over any other imaging modality. That also means that you don't have to move for the machine to get an image from a different direction -- the machine can manipulate everything with the gradient magnets. 1/24/2006 24 •4 Understanding the Technology: RF Understanding the Technology: RF • When the RF pulse is turned off, the hydrogen protons begin to slowly (relatively speaking) return to their natural alignment within the magnetic field and release their excess stored energy. When they do this, they give off a signal that the coil now picks up and sends to the computer system. What the system receives is mathematical data that is converted, through the use of a Fourier transform, into a picture that we can put on film. That is the "imaging" part of MRI. • MRI contrast works by altering the local magnetic field in the tissue being examined. Normal and abnormal tissue will respond differently to this slight alteration, giving us differing signals. These varied signals are transferred to the images, allowing us to visualize many different types of tissue abnormalities and disease processes better than we could without the contrast. 1/24/2006 25 1/24/2006 History of fMRI 26 What is fMRI? • 1990: Ogawa observes BOLD effect •blood vessels became more visible as blood oxygen decreased • 1991: Belliveau observes first functional images using a contrast agent • 1992: Ogawa et al. and Kwong et al. publish first functional images using BOLD signal • Functional MRI is based on the increase in blood flow to the local vasculature that accompanies neural activity in the brain. This results in a corresponding local reduction in deoxyhemoglobin because the increase in blood flow occurs without an increase of similar magnitude in oxygen extraction. • Since deoxyhemoglobin is paramagnetic, it alters the T2* weighted magnetic resonance image signal. • Thus, deoxyhemoglobin is sometimes referred to as an endogenous contrast enhancing agent, and serves as the source of the signal for fMRI. Using an appropriate imaging sequence, human cortical functions can be observed without the use of exogenous contrast enhancing agents. Seiji Ogawa 1/24/2006 27 1/24/2006 28 Necessary Equipment Protons Align with Field 4T magnet RF Coil gradient coil (inside) Magnet Gradient Coil RF Coil • Really, only 0.0003% of protons/T align with field • But there are about 1021 protons in a typical voxel so that’s still a lot Source: Bushong, Magnetic Resonance Imaging text Source: Joe Gati, photos 1/24/2006 29 1/24/2006 30 •5 fMRI image…. Other methods… • See link on webpage. This is not meant to be memorized, but rather to be used as an additional source of information about the many methods in use across the brain sciences. 1/24/2006 31 1/24/2006 32 •6