In this lesson, we explore the nervous system and share notes as part of the study guide series. We will explore the awesome brain and nerves! Topics include the Early Methods vs Modern Ways of Studying the Brain, Brain Imaging – CT vs MRI vs EEG vs fMRI vs PET Scans.
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Early Methods of Studying the Brain
- Old study of phrenology thought brain areas were divided into different tasks / characteristics, and this created bumps on the skull. By studying bumps, they thought they could learn about a person. Wrong.
- Autopsies — told scientists a lot about different structures of the brain, but was limited in that it can’t show how the brain functions or controls the body.
- Wait until someone has some kind of brain injury and then study the effect it has on the
- Ex: Phineas Gage in 1848 got a metal rod from the railroad through his head but survived! Actually walked away from the accident despite losing brain matter and lots of blood. However, the injury completely changed his personality (for the worse).
- Scientists learned cerebral localization from these types of studies — the idea that specific areas of the brain control specific aspects of behavior and emotion, even personality.
- We didn’t have much control over these types of studies though. Because strokes / accidents typically cause a lot of damage, it’s hard to tell what area is responsible for what behavioral change. But there are some areas around it.
- Paul Broca studied a patient he called “tan” who lost the ability to speak (except for that word), yet didn’t seem to suffer any other type of mental impairment. When the patient died, Broca discovered he had damage in a very particular part of the left frontal lobe. Broca then studied autopsied brains for a number of patients with speech impairment and while the type of damage varied, it was all in this particular region. He discovered this area must be involved in speech production.
- We now call this region in the left frontal lobe Broca’s area,
- Aphasia = loss of ability to understand or express speech.
- Broca’s aphasia = trouble with speech production because of damage to Broca’s area.
- Problem with these methods is the long time span (waiting until someone dies means they may outlive doctor, may sustain other brain injuries by then, may lose touch with doctor, etc) and the lack of control over the types of injuries / damage done.
Lesion Studies and Experimental Ablation
- Ablation studies — method of deliberately destroying tissues (making lesions) in order to see what effect this will have on an animals’s behavior. (This research obviously not done with humans.)
- Functions that can no longer be performed after the damage must be the ones that were controlled by those damaged regions.
Different methods of creating lesions:
- Surgical removal, with a scalpel or aspiration (literally sucking out brain tissue).
- This is limited in that it can only remove structures on the surface of the brain.
- Also, scientists aren’t always interested in actually removing tissue, but instead damaging the tissue in place (e.g. through the following methods… less invasive)
- Severing the nerve with a scalpel — inhibits signals from the nerves so it can’t do its job
- Radio frequency lesions — can destroy tissue both on the surface and deep inside
- A wire that’s insulated except at the very tip, is inserted to a pre-determined area in the brain. Then a high frequency current is passed through the wire, which heats up and destroys tissue just around the wire’s tip.
- This allows scientists to vary the intensity and duration of current to control size of resulting lesion. However, it destroys everything in the area — not just the cell bodies of neurons in that area, but also the axons of other neurons just passing through. Hard to determine which is responsible for any behavior change.
- Neurochemical lesions — very precise, and can be created through many different methods, including excitotoxic lesions… Excitotoxins are chemicals that bind to glutamate receptors and cause such an influx of Ca2+ ions that it kills the neuron, essentially exciting it to death.
- Ex: kainic acid. This method destroys cell bodies of neurons but not those of axons passing by, so you don’t need to worry about severing connections like in radio frequency lesions or knife cuts.
- Ex: Oxidopamine (6-hydroxydopamine) — selectively destroys dopaminergic neurons (release dopamine) and noradrenergic neurons (release norepinephine or adrenaline).
- Say you have a presynaptic cell that’s releasing dopamine to the synaptic cleft between cells. After dopamine binds with post-synaptic cell, the body wants to get rid of it or recycle it. It does this through re-uptake, where basically a little vacuum on the pre-synaptic cell sucks all the neurotransmitter back in. Oxidopamine looks a lot like dopamine, so when released into an area, it’s also taken up by re-uptake channels.. then it kills those cells.
- This is extremely useful because it gives us a lot of control, allowing us to destroy cell bodies (not axons) and to target specific populations of neurons in specific areas of the brain.
- Ex: researchers use this to model Parkinsons disease in lab animals (by targeting and destroying neurons in substantia nigra)
- Cortical cooling (cryogenic blockade) — cools down neurons until they stop firing / functions. Can be done many different ways, including with use of a cryoloop. This device is surgically implanted between skull and brain, and then a chilled liquid is circulated through the loop.
- Unlike other ablation techniques mentioned thus far, this is reversible!!
- Temporary lesions can also be created through neurochemical means…
- Ex: A drug called muscimol temporarily binds with GABA receptors and winds up temporarily inhibiting those neurons so they can’t fire.
Modern Ways of Studying the Brain: Imaging: CT vs MRI vs EEG vs fMRI vs PET Scans
- Can be divided into two types of studies: those that tell us structures and those that tell us function.
- CAT scans or CT scans (Computerized Axial Tomography) — Uses X-rays to create images of brain.
- Can show us if there’s a tumor or abnormal swelling in the brain.
- Can’t tell us what areas are active at a given time.
- Magnetic Resonance Imaging (MRI) — Uses radio waves to get picture of brain.
- A person’s head is exposed to a strong magnetic field, which aligns the atoms in their brain in a certain direction. Then a radio wave is added to the magnetic field, which disrupts that alignment.
- As the atoms then move back to realign with the magnetic field, they emit a signal. Different types of atoms emit different signals!
- This allows for a creation of a (much more) detailed picture of the brain.
- Still can’t tell us anything about brain function, though.
- Electroencephalography (EEG) — measures electrical activity generated by neurons in the brain.
- Done by placing electrodes on someone’s scalp at predetermined positions (usually by using a cap with electrodes that are filled with a conductive gel.
- Pro: non-invasive.
- Con: Because it’s non-invasive, EEGs can’t really tell us anything about specific neurons or groups of neurons; just looks at sum total electrical fields generated from the brain.
- Unlike structural methods (CT scans, MRIs), we don’t get a picture of the brain- just lots of squiggly lines that show if a person is awake/asleep or if they’re engaged in certain cognitive tasks
- Magnetoencephalography (MEG) — Records the magnetic fields produced by electrical currents in the brain.
- These fields are measured using SQUID (superconductive quantum interface devices).
- Gives better resolution than EEG, but this method is also more rare (especially in social sciences)… It needs a much bigger and more expensive set-up.
- fMRI (functional MRI) — gives the same structural images from the MRI, but can also look at which structures are active.
- Does this by measuring relative amounts of oxygenated / deoxygenated blood in the brain, because neurons that are firing a lot require more oxygen than those that aren’t active.
- fMRIs thus tell us what parts of the brain are active, what parts we’re using to do a certain task.
- Positron Emission Tomography (PET scans) — On their own can’t give us a detailed structure of the brain, but are combined with CAT scans and MRIs.
- Involves radioactive glucose that’s injected into a person.
- Since active cells naturally use more glucose because they need more energy, we can see (with CAT or MRI scans) what areas of the brain are more active at a given point in time.
- fMRI is more popular (at least in social sciences), probably because PET scans are more invasive.
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