Transcript Methods of Studying The Nervous System - U

Methods of Studying The
Nervous System
Ch. 5
Pinel's Biopsychology, 5th Ed.
Outline
(1)
(2)
(3)
Summary of Previous Lecture
Methods of Visualizing the Living Human Brain
a. Contrast X-rays
b. Computerized Axial Tomography
c. Magnetic Resonance Imaging
d. Positron Emission Tomography
e. Functional MRI
f. Magnetoencephalography
Recording Psychophysiological Signals
a. Scalp Electroencephalography
b. Measures of Somatic Nervous System Activity
c. Autonomic Nervous System Activity
Pinel's Biopsychology, 5th Ed.
Methods of Visualizing the
Living Human Brain
Pinel's Biopsychology, 5th Ed.
Contrast X-rays
• To take an X-ray photograph of an object, a
beam of x-rays is passed through it onto a
photographic plate; any part of the object
that absorbs X-rays differently than does the
surrounding medium will be distinguishable
Pinel's Biopsychology, 5th Ed.
Contrast X-rays
• Standard X-rays can’t be used for studying the
brain because the brain is composed of many
overlapping structures that all absorb X-rays to
about the same degree
• Contrast X-rays solve this problem in some
cases; a radio-opaque material is introduced into
the structure of interest to make it “stand out”
from the others on an X-ray photograph
Pinel's Biopsychology, 5th Ed.
Contrast X-rays
• For example, in cerebral angiography a
radio-opaque dye is injected onto the
carotoid artery; it reveals displacement or
enlargement of blood vessels
Pinel's Biopsychology, 5th Ed.
Computerized Tomography
(CT)
• Provides a 3-dimensional view of structure
• The X-ray gun and the X-ray detector rotate
in apposition around the brain at one level
taking a series of measurements from which
an image of one section is constructed; this
is repeated at multiple levels
• The CT-scan image of the brain is not high
spatial resolution
Pinel's Biopsychology, 5th Ed.
Magnetic Resonance Imaging
(MRI)
• It has highest spatial resolution
• The images are created from measurements
of the waves emitted by hydrogen atoms
when they are placed in a magnetic field
• Its clarity stems from the fact that neural
structures differ considerably in their
density of hydrogen atoms
Pinel's Biopsychology, 5th Ed.
Positron Emission Tomography
(PET)
• A method of highlighting brain areas that are
active, rather than equally showing all brain
structures
• The patient is injected with radio-active 2deoxyglucose; because 2-DG is structurally
similar to glucose, it is taken up by neurons as if it
were glucose (This is somewhat invasive,
although not as invasive as lesion surgery, etc.)
Pinel's Biopsychology, 5th Ed.
Positron Emission Tomography
(PET)
• More active neurons need more energy and
take up more 2-DG; unlike glucose, 2-DG
cannot be metabolized by neurons and it
accumulates in them
Pinel's Biopsychology, 5th Ed.
Positron Emission Tomography
(PET)
• The patient is injected with radio-active 2-DG and
then engages in the activity under study (e.g.,
reading) while a PET scan of the brain is being
taken
• The PET scan reveals on a series of images of
horizontal sections where radio-activity has
accumulated, and thus it indicates what areas were
particularly active during the test
Pinel's Biopsychology, 5th Ed.
Functional Magnetic Resonance
Imaging (fMRI)
• Allows brain activity to be measured by
imaging the increase in oxygen (blood flow)
that occurs to brain areas that are active
• Surplus of blood occurs are active sites
Pinel's Biopsychology, 5th Ed.
Functional Magnetic Resonance
Imaging (fMRI)
• It’s four advantages over PET include:
(1) nothing must be injected into the subject
(2) one image provides structural and
functional information
(3) the spatial resolution is better
(4) changes can be measured in real time
(although, the temporal resolution is
poor compared to ERP)
Pinel's Biopsychology, 5th Ed.
Magnetoencephalography (MEG)
• Measure brain activity in terms of changes
in magnetic fields measured on the surface
of the scalp
Pinel's Biopsychology, 5th Ed.
Recording Psychophysiological
Signals
Pinel's Biopsychology, 5th Ed.
Scalp Electroencephalography
(EEG)
• An EEG signal is measured through an
array of scalp electrodes
• EEG waves reflect the sum total of all the
electrical events in the head (action
potentials, eye movements, blood flow, etc.)
thus, the EEG reveals little about the nature
of the underlying neural activity
Pinel's Biopsychology, 5th Ed.
Scalp Electroencephalography
(EEG)
• Its value lies in the fact that particular EEG
wave forms are associated with particular
states of consciousness; generally lowamplitude, fast EEG activity is associated
with alert aroused state; and highamplitude, slow EEG activity (alpha waves)
is associated with a relaxed but awake state
Pinel's Biopsychology, 5th Ed.
Scalp Electroencephalography
(EEG)
• EEG can be used to study brain activity in
real time, in response to specific events.
These are called evoked potentials.
• Usually many evoked potentials are used to
generate an averaged evoked potential in
order to reduce the noise of the background
EEG activity
Pinel's Biopsychology, 5th Ed.
Scalp Electroencephalography
(EEG)
• A subset of evoked potentials are eventrelated potentials, which are time-locked
evoked potentials, meaning that the EEG in
response to an event is always measured
during a specific interval of time
• ERPs have good temporal resolution, but
poor spatial resolution
Pinel's Biopsychology, 5th Ed.
Scalp Electroencephalography
(EEG)
• EEG recording are a valuable diagnostic
tool; for example the presence of highamplitude spikes in the EEG (i.e., epileptic
spikes) is the primary criterion for
diagnosing epilepsy
Pinel's Biopsychology, 5th Ed.
Measures of Somatic Nervous
System Activity
• Muscle tension:
– An electromygram (EMG) is the changing difference
in the voltage between two large electrodes placed on
the skin above a large muscle; the amplitude of EMG
signals indicates the combined level of tension in the
underlying muscle
– The raw signals are usually integrated so that the data
are easier to work with; the height of the curve of
integrated EMG activity indicates the number of spikes
on the EMG signal per unit of time
Pinel's Biopsychology, 5th Ed.
Measures of Somatic Nervous
System Activity
• Eye movement:
– in an electrooculogram (EOG) eye movements are
recorded by placing four electrodes around the eye; the
signals result from the fact that the front of the eye is
more positively charged than the back
– The direction of movement can be inferred from the
relation between the activity recorded on two channels:
(1) above vs. below and (2) left vs. right
Pinel's Biopsychology, 5th Ed.
Autonomic Nervous System
Activity
• Skin conductance:
– Skin conductance level (SCL) is the general
level of skin conductance associated with a
particular situation
– A skin conductance response (SCR) is a rapid
change in skin conductance in response to a
particular event; one application is the lie
detector test
(break)
Pinel's Biopsychology, 5th Ed.
Websites
• A Primer for CAT and MRI:
http://www.med.harvard.edu/AANLIB/hms1.
html
• The Electroencephalogram:
http://www.medfak.uu.se/fysiologi/lectures/E
EG.html
Pinel's Biopsychology, 5th Ed.
Methods of Studying The
Nervous System
Ch. 5 cont’d
Pinel's Biopsychology, 5th Ed.
Outline
(1)
(2)
Invasive Physiological and Pharmacological Methods
a. Stereotaxic Surgery
b. Lesion Methods
c. Electrical Stimulation
d. Electrical Recording Methods
e. Pharmacological Methods
Genetic Engineering
a. Knockouts
b. Gene replacement
Pinel's Biopsychology, 5th Ed.
Invasive Physiological and
Pharmacological Methods
• In most cases, laboratory animals serve as
the subjects when invasive procedures are
required to directly manipulate or measure
the brain
Pinel's Biopsychology, 5th Ed.
Stereotaxic Surgery
• The first step in may invasive
biopsychology experiments is stereotaxic
surgery; it allows accurate placement of
lesions, probes, electrodes, and other
devices into the brain
Pinel's Biopsychology, 5th Ed.
Stereotaxic Surgery
• The method employs a stereotaxic atlas
and a stereotaxic instrument (head holder
and electrode holder)
• The reference point is often bregma (the
point where two main plates of the rat skull
naturally fuse together)
Pinel's Biopsychology, 5th Ed.
Lesion Methods
• The aspiration method is often used to
remove cortical tissue
• The radio-frequency electrolytic lesion is
the most common subcortical lesion; the
tissue is destroyed by the heat of the current
Pinel's Biopsychology, 5th Ed.
Lesion Methods
• Small knife cuts are often used for severing
tracts
• Cryogenic blockade is like a reversible
lesion; the tissue is temporarily cooled to
the point that all neural activity in the
vicinity of the probe stops; it produces little
permanent damage
Pinel's Biopsychology, 5th Ed.
Lesion Methods
• Lesion studies must be interpreted with
caution; a lesion inevitably damages
structures other than the one that the
surgeon targeted, and lesions seldom
completely remove a structure that is
targeted
Pinel's Biopsychology, 5th Ed.
Electrical Stimulation
• The effects of electrical stimulation are
often opposite to those of a lesion to the
same brain site
• Electrical stimulation research is done prior
to any lesioning
Pinel's Biopsychology, 5th Ed.
Electrical Recording Methods
• Intracellular unit recording
– Measures changes in the membrane potential of
a neuron over time; it requires a microelectrode
positioned inside a neuron
– It is next to impossible to record intracellularly
in a freely moving animal because it is difficult
to keep the microelectrode inside the neuron
Pinel's Biopsychology, 5th Ed.
Electrical Recording Methods
• Extracellular unit recording
– A microelectrode is positioned near a neuron
– The signal is a series of spikes; each spike
indicates an action potential from a nearby
neuron; spikes of the same amplitude are
assumed to come from the same neuron
Pinel's Biopsychology, 5th Ed.
Electrical Recording Methods
• Multiple unit recording
– Multiple unit recording provides an indication of the
rate of firing of many neurons in the general vicinity of
the electrode tip
– An electrode larger than a microelectrode picks up the
action potentials from many nearby neurons
– The signal is integrated so that the height of the curve
indicates the number of action potentials in the vicinity
per unit of time
Pinel's Biopsychology, 5th Ed.
Electrical Recording Methods
• Invasive EEG recording
– Implanted electrodes are used to record EEG in
laboratory animals because the scalp electrode
do not allow as clear or finite position
recording
Pinel's Biopsychology, 5th Ed.
Pharmacological Methods
Pinel's Biopsychology, 5th Ed.
Pharmacological Methods
• Routes of drug administration include
intragastrically (stomach), intraperitonally
(abdomen), intramuscularly, subcutaneuously, or
intravenously
• These peripheral routes all suffer from th fact that
many drugs cannot pass the blood-brain barrier;
• This problem can be overcome administering drug
via stereotaxically positioned cerebral cannula
(microinjection of drugs directly into brain tissue)
Pinel's Biopsychology, 5th Ed.
Pharmacological Methods
• Sometimes, drugs can be injected into the
brain that produce chemical lesions that are
more selective than electrical lesion might
be; for example, 6-OHDA is a neurotoxin
that selectively destroys dopaminergic and
noradrenergic neurons in the vicinity of the
injection site
Pinel's Biopsychology, 5th Ed.
Locating Neurotransmitters and
Receptors in the Brain
• Immunocytochemistry begins by injecting
antigens (foreign proteins) into an animal
such that the animal will create and bind
antibodies to the antigen to remove or
destroy it
Pinel's Biopsychology, 5th Ed.
Locating Neurotransmitters and
Receptors in the Brain
• Antibodies for the most of the brain’s
peptide neurotransmitters and receptors
have been created; these can be labeled with
a dye or radioactive element and then used
to identify specific neuroproteins in slices
of brain tissue
Pinel's Biopsychology, 5th Ed.
Locating Neurotransmitters and
Receptors in the Brain
• In situ hybridization also allows peptides and
proteins in the brain to be located; labeled hybrid
RNA strands with a base sequence complementary
to the mRNA for synthesizing the target
neuroprotein;
• The hybrid RNA binds to the complementary
mRNA in the target cells and allows the target
neuroproteins’s location to be marked
Pinel's Biopsychology, 5th Ed.
Genetic Engineering
Pinel's Biopsychology, 5th Ed.
Gene Knockout Techniques
• Involve the creation of organisms that lack a
specific gene; any measurable neural or
behavioral anomalies are then noted
Pinel's Biopsychology, 5th Ed.
Gene Knockout Techniques
• PROBLEMS:
(1) Most behaviors are determined by
multiple genes
(2) Eliminating one gene usually alters the
expression of other genes
(3) Gene expression dependent upon
experience, which may be altered by the
absence of the missing gene
Pinel's Biopsychology, 5th Ed.
Gene Replacement Techniques
• Involves the replacement of one gene with
another; useful implications for the
treatment of genetically related diseases
• Sometimes, genetic information from a
different species is implanted, creating a
transgenic subject
Pinel's Biopsychology, 5th Ed.
Website
• In vivo Microdialysis:
http://www.microdialysis.se/techniqu.htm
Pinel's Biopsychology, 5th Ed.