methods and strategies of research Flashcards
reading 6
Explain what researchers can learn from studies that involve lesioning:
The goal is to discover what functions are performed by different regions of the brain and then to understand how these functions are combined to accomplish particular behaviours. The distinction between brain function and behaviour is an important one. Circuits within the brain perform functions, not behaviours. No one brain region or neural circuit is solely responsible for a behaviour; each region performs a function (or set of functions) that contributes to performance of the behaviour.
Compare various methods of producing brain lesions:
Experimental ablation: The removal or destruction of a portion of the brain of a laboratory animal; presumably, the functions that can no longer be performed are the ones the region previously controlled.
Excitotoxic lesion: A brain lesion produced by intracerebral injection of an excitatory amino acid, such as kainic acid- does cause some damage to the area even before turning on the lesion maker or starting the infusion.
To control for this, sham lesions are used: A placebo procedure that duplicates all the steps of producing a brain lesion except the one that actually causes the brain damage.
Describe the process of stereotaxic surgery:
Brain surgery using a stereotaxic apparatus to position an electrode or cannula in a specified position of the brain
A stereotaxic apparatus holds the head in a standard position and has an arm that moves an electrode or a cannula through measured distances in all three axes of space. Before performing stereotaxic surgery, a researcher or surgeon will consult a stereotaxic atlas which is a book containing images that correspond to the frontal sections of the brain taken at various distances rostral and caudal to the bregma.
The bregma is the junction of the sagittal and coronal sutures of the skull meaning “front of the head.”
1. Drill a hole immediately over the section to place the tip of the wire here
2. To place the tip of the wire in the fornix, a researcher would drill a hole over the target and then lower the electrode through the hole until the tip was at the correct depth, relative to the skull height at bregma.
3. We always have to try out a new set of coordinates, slice and stain the animal’s brain, see the actual location of the lesion, correct the numbers, and try again.- atlas only hsowing approximate locations.
4. A stereotaxic apparatus is a device that includes a head holder, which maintains the animal’s skull in the proper orientation, a holder for an electrode or cannula, and a calibrated mechanism that moves the electrode/cannula holder in measured distances along the three axes: anterior– posterior, dorsal– ventral, and lateral– medial.
What is deep brain simulation?
another procedure that requires the use of a stereotaxic apparatus. Deep brain stimulation is used to treat chronic pain, movement disorders (including Parkinson’s disease), epilepsy, depression, and obsessive-compulsive disorder. Deep brain stimulation utilizes a stereotaxic apparatus to implant a permanent electrode into the brain of patients. Rather than produce a lesion, electrical current passed through the electrode is used to stimulate brain regions and reduce symptoms
Summarise the general steps of histological methods:
- Fixation and sectioning: To achieve both of these objectives, neural tissue is placed in a fixative . The most commonly used fixative is formalin , an aqueous solution of formaldehyde, a gas. Formalin cross links proteins to strengthen the very soft and fragile brain
- Microtome: contains 3 parts- a knife, a platform on which to mount the tissue, and a mechanism that advances the knife the correct amount after each slice so that another section can be cut.
- Staining: Methylene blue and cresyl violet are two examples of dyes that stain cell bodies. All cells are stained, neurons and glia alike. It is up to the investigator to determine which cell type is which— by size, shape, and location. Other types of staining techniques can be used to identify and label individual cells by their chemical composition.
○ immunocytochemical methods: method for producing antibodies to any peptide or protein. - Electron microscopy: The microscope scans the tissue with a moving beam of electrons. The information from the reflection of the beam is received by a detector, and a computer produces a remarkably detailed three-dimensional view.
- Confocal laser scanning microscopy: makes it possible to see detail inside thick sections of tissue or in the upper layers of tissue in the exposed living brain.
Differentiate between techniques for tracing efferent and afferent axons:
- Efferent: include identifying where cells in a brain region send their axons and terminals. Use anterograde labelling method to trace these axons. Anterograde labelling methods employ chemicals that are taken up by dendrites or cell bodies and are then transported through the axons toward the terminal buttons.
- Afferent: will tell us only part of the story about the neural circuitry involved in a behaviour. we need to find the inputs to a structure, to do this we use a retrograde labelling method. After injecting a retrograde tracing chemical into a brain region, the chemical is taken up by terminal buttons in the region and transported backward toward the cell bodies by means of retrograde axoplasmic transport to fill the afferent neurons.
What are transneuronal tracing methods?
Trans neuronal tracing methods identify a series of two, three, or more neurons that form serial synaptic connections with each other. Together, anterograde and retrograde labelling methods – including trans neuronal methods – enable us to discover circuits of interconnected neurons. Thus, these methods help to provide us with a “wiring diagram” of the brain.
A histological method that labels cell bodies that give rise to the terminal buttons that form synapses with cells in a particular region.
Contrast the methods used to study the structure of the living human brain?
- computerised tomography (CT) The use of a device that employs a computer to analyse data obtained by a scanning beam of X-rays to produce a two-dimensional picture of a “slice” through the body. The beam scans the head from all angles, and a computer translates the information it receives from the detector into pictures of the skull and its contents.
- magnetic resonance imaging (MRI) A technique whereby the interior of the body can be accurately imaged; involves the interaction between radio waves and a strong magnetic field. The MRI scanner is tuned to detect the radiation from hydrogen atoms. Because these atoms are present in different concentrations in different tissues, the scanner can use the information to prepare pictures of slices of the brain.
- Diffusion tensor imaging (DTI) takes advantage of the fact that the movement of water molecules in bundles of white matter will not be random but will tend to be in a direction parallel to the axons that make up the bundles. The MRI scanner uses information about the movement of the water molecules to determine the location and orientation of bundles of axons in white matter.
Compare recording neural activity using microelectrodes, macroelectrodes, and magnetoencephalography.
- microelectrode A very fine electrode, generally used to record activity of individual neurons. This technique is usually called single-unit recording- wires are insulated so only their tips are bare.
- macroelectrode An electrode used to record the electrical activity of large numbers of neurons in a particular region of the brain; much larger than a microelectrode. - through EEG scans
- magnetoencephalography A procedure that detects groups of synchronously activated neurons by means of the magnetic field induced by their electrical activity; uses an array of superconducting quantum interference devices, or SQUIDs.
Compare the types of metabolic and synaptic activity that can be revealed by autoradiography, staining for immediate early genes, positron emission tomography, and functional magnetic resonance imaging.
Autoradiography is a post-mortem technique that involved measuring the radioactivity from cells that have taken up radioactive 2-DG while they are metabolically active prior to tissue collection. Staining for immediate early genes reveals the neurons that we recently activated prior to tissue collection. PETR scans visualise activity in brain regions that are metabolically active in a living brain (uses radioactive 2-DG). fMRI visualises brain regions that have increased blood flow and local blood oxygen levels in a living brain.
Compare electrical and chemical neural stimulation, transcranial magnetic stimulation, and optogenetic methods.
Electrical and chemical stimulation of neurons are accomplished by passing an electrical current or injecting a chemical into a specific brain region through a cannula. Optogenic methods are used to stimulate or inhibit particular types of neurons in a specific brain region. These neurons are genetically modified to express a protein that is wavelength sensitive. Researchers implant LEDs into the brain that can emit the corresponding wavelengths of light. Turing the light on and off allows the researcher to control the activity of the neurons that are light sensitive. TMS uses a magnetic signal to stimulate or inhibit neurons beneath the TMS device. Unlike other methods of stimulating neural activity, TMS is non-invasive.
Describe how a researcher could identify a neuron that produces a particular neurochemical.
Using immunocytochemical methods, a researcher could localise the chemicals themselves or localise the enzymes that produce the neurochemicals of interest, with specific neurons.
Compare autoradiography and immunocytochemistry for localizing particular receptors.
Autoradiography to identify specific receptors involves immersing brain tissue in a solution containing a radioactive ligand for the receptor of interest. Slides of the tissue are then developed to expose the locations of the receptors for the ligand. Immunocytochemistry for specific receptors involve exposing brain tissue to antibodies that are selective for a protein on the receptor of interest,
Summarise the steps involved in measuring brain chemicals using micro dialysis:
- A micro dialysis is place unto the brain region of interest using stereotaxic surgery
- A small amount of a solution similar to extracellular fluid is pumped into the brain through one of the small metal tubes into the dialysis tubing.
- The fluid circulates thorough the dialysis tubing and passes through the second metal tube, from which it is taken for analysis.
- As the fluid passes through the dialysis tubing, it collects molecules from the extracellular fluid of the brain, which are pushed across the membrane by diffusion.
- A researcher can analyse the brain chemical contents of the fluid that have passes through the dialysis tubing by an extremely sensitive analytical method.
Describe how concordance rates in twins can be used to assess the genetic contribution to a behaviour.
Concordance rates help researchers understand the contributions of genetic differences to variations in behaviour. E.g.: if a disorder has a strong genetic basis, the percentage of MZ twins who are concordant for the diagnosis will be higher than that for DZ twins.
