Methods of Studying The Nervous System

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Methods of Studying The Nervous System

• Ch. 5

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Outline

• 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
• (3) Recording Psychophysiological Signals
• a. Scalp Electroencephalography
• b. Measures of Somatic Nervous System Activity
• c. Autonomic Nervous System Activity

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Methods of Visualizing the Living Human Brain
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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

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Contrast X-rays

• Standard X-rays cant 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

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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

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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

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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

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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
  2-deoxyglucose 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.)

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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

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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

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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

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Functional Magnetic Resonance Imaging (fMRI)

• Its 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)

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Magnetoencephalography (MEG)

• Measure brain activity in terms of changes in
  magnetic fields measured on the surface of the
  scalp

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Recording Psychophysiological Signals
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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

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Scalp Electroencephalography (EEG)

• Its value lies in the fact that particular EEG
  wave forms are associated with particular states
  of consciousness generally low-amplitude, fast
  EEG activity is associated with alert aroused
  state and high-amplitude, slow EEG activity
  (alpha waves) is associated with a relaxed but
  awake state

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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

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Scalp Electroencephalography (EEG)

• A subset of evoked potentials are event-related
  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

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Scalp Electroencephalography (EEG)

• EEG recording are a valuable diagnostic tool for
  example the presence of high-amplitude spikes in
  the EEG (i.e., epileptic spikes) is the primary
  criterion for diagnosing epilepsy

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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

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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

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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)

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Websites

• A Primer for CAT and MRI
• http//www.med.harvard.edu/AANLIB/hms1.html
• The Electroencephalogram
• http//www.medfak.uu.se/fysiologi/lectures/EEG.htm
  l

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Methods of Studying The Nervous System

• Ch. 5 contd

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Outline

• (1) Invasive Physiological and Pharmacological
  Methods
• a. Stereotaxic Surgery
• b. Lesion Methods
• c. Electrical Stimulation
• d. Electrical Recording Methods
• e. Pharmacological Methods
• (2) Genetic Engineering
• a. Knockouts
• b. Gene replacement

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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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

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Pharmacological Methods
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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)

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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

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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

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Locating Neurotransmitters and Receptors in the
Brain

• Antibodies for the most of the brains 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

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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
  neuroproteinss location to be marked

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Genetic Engineering
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Gene Knockout Techniques

• Involve the creation of organisms that lack a
  specific gene any measurable neural or
  behavioral anomalies are then noted

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Gene Knockout Techniques

• PROBLEMS
• Most behaviors are determined by multiple genes
• Eliminating one gene usually alters the
  expression of other genes
• Gene expression dependent upon experience, which
  may be altered by the absence of the missing gene

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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

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Website

• In vivo Microdialysis
• http//www.microdialysis.se/techniqu.htm