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Chapter 3

Ch. 3 notes - Neuroscience Part 2.docx

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McMaster University
Christopher Teeter

The Anatomy of the Nervous System Terminology • Neuraxis – line drawn along the sc and through to the front of the brain – straight line in many animals but curves forward as the spine enters the brain in humans • Rostral – front of the line (towards the head) • Caudal – back of the line (towards the tail) • Dorsal – top and back of head (above neuraxis) • Ventral – surfaces face the ground • Medial – closer to the center of the brain • Lateral – further away from the center Studying the Brain • Franz Joseph Gall’s theory of phrenology – different brain functions located in different areas – details of his proposal were not based on evidence – different mental attributes (ex. benevolence, hope, intelligence) were located in different sub-organs of the brain and the sub-organs would grow with use of that facility – growth of sub-organs could be measured by looking at the shape of the skull • Ext. shape of skull has almost nothing to do with internal conformation of the brain – his theory was discredited • Brought the attention of more reputable scientists to the idea of the localization of function Lesion Studies • If a certain area of the brain is damaged and certain function is lost, that structure can be associated with that brain function • Unethical to deliberately damage an area of the brain on a human but can be done to animals • Pierre Florens (1825) – surgically removed/ablated parts of pigeon’s brains and studied the results (ablation studies) • Removal of cerebellum affected bird’s motor coordination • Removal of medulla interfered with vital functions such as HR and resps. • In humans, patients are examined with accidental brain damage to determine the site of the damage (often done during autopsy) and the functional changes in their behaviour • One of the most famous cases: Phineas Gage (1848) – railroad construction foreman who survived having a large iron rod driven through his skull in an accident – destroyed a large part of his left frontal lobe – after recovery, his personality changed – before the accident he was a capable and efficient foreman/after he was fitful, irreverent, and grossly profane – appeared to be unable to plan for his future actions and was constantly changing from one idea to the next • Suggested that the frontal lobes were responsible for functions like planning and impulse control – later studies confirmed these ideas Electrical Stimulation and Single Cell Recording • Alternative to removing part if the brain is to stimulate part of the brain and try to determine what function is elicited – used by Wilder Penfield (1951) • Performed brain surgery on epileptic patients in order to surgically remove the focus of the epileptic activity from the brain – anxious to avoid cutting into the eloquent areas (now called the eloquent cortex – areas where damage would lead to paralysis, loss of language ability, or loss of sensory processing) • These areas were not yet mapped out – patients were under a local anesthetic – Penfield used an electrical probe to stimulate various parts of the brain and asking the patients what they were feeling – mapped out the sensory and motor areas of the brain • Was also able to identify other areas (ex. where stimulation led to the vivid recall of past memories) • General features of these brain maps are very similar across patients but exact details can vary quite a bit between people – this type of mapping still used on each patient during brain surgery – other functional brain studies are performed prior to surgery • In animal studies we can get more detailed info about the operation of the living brain by recording brain activity with microelectrodes • Much of the info we have learned about APs and membrane dynamics was obtained by inserting microelectrodes into the axon of a giant squid, stimulating the axon, and recording the activity – in squids and other marine animals, the axon diameter is large (up to 1mm) – an adaptation that speeds up transmission of the AP – neurons in these animals lack myelin • Alan Hodgkin and Andrew Huxley performed experiments that uncovered most of what we know about how ions flow during the AP using microelectrodes inserted into these squid axons – won the Nobel Prize in 1952 – their electrodes were relatively crude so they required the big axons • Today we have microelectrodes with tip diameters less than a micrometer that can be inserted through the cell membrane of a single cell of a living animal without causing undue damage – can obtain intracellular recordings – used to measure electrical potential and ion flow • Responses recorded by these electrodes are known as single-unit recordings • Groups of larger electrodes can be inserted into the brain tissue of an awake, behaving animal to obtain extracellular recordings – these recorded responses from the array of electrodes is then processed by computer in order to separate the individual APs from any cell bodies that are near the electrode tips – activity form several cells can be simultaneously recorded with this technique • Recording the activity of single cells in rats led to the discovery of place cells in the hippocampus – by letting the rats wander around in their environment while recording the activity of individual cells, it was discovered that some cells would fire only when the rat was in a certain area of the cage (ex. by the door) while another cell might fire when the rat was near the food dish – it was hypothesized that a cognitive map of the environment is located in the hippocampus of the rat Structural Neuroimaging • Clinical lesion studies often rely on the autopsy of the patient after they die – drawbacks: after an extensive behavioural study of an interesting patient he may live for many years/relatives may decline permission to do an autopsy when he dies • Many neuroscientists may make use of other techniques for studying the structure of the living human brain (noninvasively – without surgery to open the skull) • X-RAY CT: • Computerized x-ray tomography • Patient’s head is placed in a ring containing an x-ray emitter and detector on opposite sides of the patient’s head – the ring rotates while the emitter passes x- rays through the patient’s head to the detector – detector’s response is recorded from all angles and a computer is used to reconstruct an image of the brain • Able to distinguish different tissue types to some extent – able to show areas of damage due to stroke if image is taken early enough • Will not show tumours unless they are big enough to distort the underlying structure of the brain – tumour tissue and normal brain tissue absorb the same amount of x-rays • Subject individuals to a relatively high dosage of radiation and thus pose a level of cancer risk • MRI: • Magnetic resonance imaging • Also involves placing the patient’s head in a giant ring • Uses a very strong magnetic field to image the brain • When tissue is placed inside a strong magnetic field and is excited by a radio frequency pulse, the molecules in the body will vibrate at a certain rate, emitting their own frequency waves • An antenna called a head coil picks up these radio waves and records them for computer analysis • Different molecules will produce slightly different radio waves and the computer will decode this info, producing very detailed pictures of different slices of the brain • Able to tell the difference between different tissue types (ex. grey and white matter) Functional Neuroimaging • POSITRON EMISSION TOMOGRAPHY/PET: • When a particular part of the brain is particularly active, those neurons fire APs at a faster rate – this area requires more energy than less active areas • One of the main sources of energy for a cell is glucose carried into the cell via the blood – PET scanning uses the idea that if you could measure the amount of glucose consumed by a particular area of the brain, we would know how active it is • In order to find where the glucose is being consumed, the patient is injected with a mildly radioactive form of glucose (radionuclide) – an isotope with a very short half-life (half the radioactive particles will decay back into a non-radioactive form in a very short time (typically less than 1 hour) • Each time a radioactive particle decays, it emits 2 positrons that shoot out in opposite directions • A ring of detectors around the patient’s head detects the positrons and a computer calculates the exact location of the positron emission – amount of glucose being used by each brain area can be calculated – gives an image of the amount of brain activity in a particular area – typically overlaid on a picture of the individual’s brain anatomy recorded from an MRI scan • Disadvantages: the production of radionuclides is expensive (requires an atomic particle accelerator and, since half-life is short, they must be used almost immediately), radionuclide must be injected into an artery (relatively safe but not completely non-invasive), and the person being scanned is exposed to a small dose of radiation (3-4 times of a CT scan) – minimal cancer risk • One problem is that all areas of the brain are at least a little active all of the time and would emit some radioactive particles – ex. if we wanted to find out which areas were active while the participant listened to music, the subject would still likely have their eyes open and stimulation of the visual areas of his brain would also occur • PET scan can only provide limited specific functional localization info • THE FUNCTIONAL MRI/fMRI: • Another requirement of highly active neurons is O2– when an area of the brain is activated, capillaries supplying the area will dilate and blood supply will inc. – takes about 3-5 seconds – O 2olecules attached to the Hb are then used up – slightly changes the magnetic properties of the blood and these responses are recorded by the MRI • Rapidly scanning an area of the brain and searching for changes in the magnetic properties of the blood can measure the amount of activity in a particular area of the brain • Very detailed images of brain activation with a resolution of 1mm can be produced in a completely non-invasive way • Disadvantages: blood O responses take some seconds to build up so very short 2 brain events are difficult to measure, fMRI is an indirect measure of neural activity – measurements provided may be influenced by non-neural activity, and it can only provide limited specific functional localization info • THE ELECTROENCEPHALOGRAM/EEG: • When a neuron fires an AP, it produces a small amount of electrical current that can be measure from outside the cell – if enough neurons fire at the same time, this electrical activity can easily be detected from outside the brain at the surface of the scalp • An EEG is the record of this electrical activity collected form an arra
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