Download Parts of a Neuron…… Neuronal Communication….

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
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Parts of a Neuron……
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Neuronal Communication….
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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
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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
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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
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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)
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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
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After Averaging
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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
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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)
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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
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most likely explanation:
nuclear has bad connotations
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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
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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.
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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.
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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.
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History of fMRI
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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
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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
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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.
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