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Chapter 3

Chapter 3 - neuroscience.doc

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Dax Urbszat

Chapter 3 - Neuroscience - Einstein’s brain: wider parietal region, distinct sylvian fissure - Sylvian fissure: groove sperating frontal lobe from temporal lobe - Witelson studied his brain said that he was probably smart for those reasons, and inferior parietal lobules - Neurons: cells in nervous system that receive, integrate, and transmit info. - Soma or cell body contains nucleus and much of chemical machinery common to most cells - Rest of neuron handle info - Dendritic trees  composed of dendrites  specialized to receive info - Receive info from thousands of neurons and need extensive dendritic trees - Info dendrite soma  axon  terminal buttons synapse  dendrite of other cell … - Axon: long, thin fibre that transmits signals away from soma to other neurons, muscles or glands - Long (maybe metres long), branch off to communicate with number of other cells - Myelin: white fatty - Myelin sheath: insulating material derived from glial cells, encases some axon, speeds up transmission of signals (Multiple Sclerosis  degeneration of myelin sheath) - Terminal buttons: small knobs that secrete neurotransmitters - Neurotransmitter: chemical messenger, activate neighbouring neurons - Synapse: point where neurons interconnect, info transmitted from one neuron to another - Glia: cells found throughout nervous system, provide various types of support for neurons, smaller than neurons, outnumber neurons by 10 to 1, over 50% of brain’s volume, supply nourishment to neurons, remove neuron’s waste products, insulation (MS), also orchestrate development of nervous system in human embryo - Glia may also send chemical signals, implicated in parkinson’s disease, amyotrophic lateral sclerosis, memory formation, Alzeimer’s disease, chronic pain, psychological disorders such as schizophrenia - Squid axons’ size 100X human axons, used by Alan Hodgkin and Andrew Huxley - Neural impulse: electrochemical reaction - Na+ and K+ and Cl- flow back and forth across semipermeable cell membration - Difference in flow rates of ions leads to slightly higher concentraton of negatively charged neurons inside the cell  voltage, potential energy inside the cell - Resting potential of a neuron is its stable negative charge when the cell is inactive (-70millivolts) - Neuron stimulated  channels open  Na+ ions rush in  neuron’s charge is less negative/positive creating action potential - Action potential is a very brief shif in neuron’s electrical charge that travels along axon - Absolute refractory period: minimum time after action potential during which another action potential cannot begin (1 or 2 milliseconds) o b/c chanells in cell membrane close up, some time needed before they open to let Na+ in - Relative refractory period: neuron can fire, but threshold for firing is elevated, need stronger stimuli to initiate action potential - All-Or-Non-Principle: neuron fires with the same potency each time, although frequency/rate can vary; it either fires or not, it cannot partially fire - Thicker axons transmit neural impulses more rapidly - Neural impulse not as fast as electricity, ~100m/s - Synaptic cleft: microscopic gap between terminal button of one neuron and cell membrane of another neuron - Presynaptic neuron: sends signal - Postsynaptic neuron: receives signals - Synaptic vesicles: store neurotransmitters within the buttons - Neurotransmitter released when vesicle fuses with membrane of presynaptic cell, contents spill into synapse, diffused across synaptic cleft to membrane of postsynaptic neuron, bind to postsynaptic cell membrane at various receptor sites. - Receptor sites are very specific, recognize and respond to some but not all - Postsynaptic potential, voltage change at receptor site on postsynaptic cell membrane: o Post-synaptic potentials do not follow all or none law o Graded: vary in size, increase or decrease probability of neural impulse in receiving cell in proportion to amount of voltage change - Excitatory PSP: positive voltage shift, increase likelihood that the postsynaptic neuron will fire action potentials - Inhibitory PSP: negative voltage shift that decreases the likelihood that postsynaptic neuron will fire - Whether excitatory or inhibitory depends on which receptor site is activated - PSP lasts only fraction of second, neurotransmitter drifts away + inactivated by enzymes that metabolize them into inactive forms - Reuptake: neurotransmitters are sponged up from synaptic cleft by presynaptic membrane, allows synapses to recycle their materials - State of the neuron is a weighted balance between excitatory and inhibitory influences - Our perception, thoughts and actions depend on patterns of neural activity in elaborative neural networks (interconnected neurons fire together or sequentially) - Elimination of old synapses plays larger role in sculpting neural networks than creation of new synapses - Synaptic pruning (gradual elimination of less active synapses) is key to formation of neural networks that are crucial to communication in the nervous system - Human visual cortex, number of synapses peaks at age one and then declines - Donald Hebb: o linkage of neurons to form networks o analysis of neural basis of behavior o understanding the brain fundamental to understandin behavior o neurons do not act alone in influencting behavior, linked in cell assemblies o Hebbian Learning Rule: when an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growh process or metabolic change takes place in one or both cells such that A’s efficienty as one of the cells firing B, is increased  In Enlgish: one neuron stimulating another neuron repeatedly produces changes in the synapse: learning - Neurotransmitters regulate behaviour - Lock and key mechanism: specific neurotransmitters can deliver signals only at certain locations on cell membranes - Variety and specificity reduces cross-talk between densely packed neurons  nervous system’s communication more precise. - All neurotransmitters act to enhance or inhibit action potentials, by depolarizing, or hyperpolarizing postsynaptic cell membrane - Drugs have very important effects on neurotransmitters o Agonist: chemical mimics the actions of a specific neurotransmitter o Antagonist: chemical that inhibits the action of a specific neurotransmitter Common Neurotransmitter and Their Major Functions Neurotransmitter Functions Acetylcholine Motor control skeletal muscles, attention, Memory, Sleeping, & Dreaming MONOAMINES Norepinephrine Mood and arousal, eating Behaviour Cocaine and amphetamines elevate activity at NE synapses Dopamine Reward, pleasurable emotions & Motivation, Motor control over voluntary movement Cocaine and amphetamines elevate activity at DA synapses Serotonin Sleep and wakefulness, dreaming, eating, aggression Depression and obsessive-compulsive disorder Prozac and similar antidepressant drugs affect serotonin circuits AMINO ACIDS GABA (gamma- Inhibition of action potentials, Anxiety & intoxication aminobutyric acid) Valium and similar antianxiety drugs work at GABA synapses PEPTIDE MODULATORS Endorphines Pain reduction, Reward pain Resemble opiate drugs in structure and effects Acetylcholine (ACh) - Only transmitter between motor neurons and voluntary muscles - Attention, arousal, memory - Amplifying Ach drug to slow down progress of Alzeimer’s disease - Nicotine is Ach agonist - It binds to the receptors on muscle ; makes them contract or relax - It both excites skeletal muscles and inhibits heart muscles - Toxins that mimic ACh can cause temporary paralysis o E.g. curare, an herbal poison compete with ACh for the receptor site and inhibits the mechanisms that produce muscle movement, leads to paralysis, used by some south America natives. - Nicotine excites ACh receptors o People wearing the nicotine patch have vivid dreams o Smoking can heighten attention, improve problem solving, and facilitate memory (only for the current smokers, not for non- smokers/ former smokers) Monoamines (dopamine, norepinephrine, and serotonin) - Michael J. Fox, Muhammad Ali  parkinson’s disease  causes in decline in the synthesis of dopamine - Synthesized within the neuron from single amino acids - Dopamine control voluntary movement - Drug used to treat Parkinsonism is converted to dopamine in the brain, compensates for diminished dopamine - Serotonin: regulation of sleep and wakefulness, eating behavior - Serotonin also part of aggressive behavior in animals - Low levels of activation of norepinephrine and serotonin synapses  depressive disorders - Antidepressent drugs exert their main effects at these synapses - Dysregulation in serotonin also a factor in eating disorders, such as anorexia and bulimia, in obsessive compulsive disorder - Dopamine hypothesis: overactivity at dopamine synapses play crucial role in development of schizophrenia - Schizophrenia symptoms tamed by dopamine antagonists - Amphetamines and cocaine exert most of their effects by creating increased activity at dopamine and norepinephrin synapses - Rewarding effects of most abused drugs depend on increased activity in a particular dopamine pathway - Dysregulation in this dopamine pathway  drug craving and addiction - Ach and NE can be excitatory or inhibitor depending on the synaptic receptors they bind to - GABA and glycine have only inhibitory effects, glutamate only excitatory effects Amino Acids (General Inhibitory/Excitatory Transmitters in the Brain) - GABA: o Widely distributed in brain, present at 40% of all synapses o Responsible for much of inhibition in central nervous system o Regulation of anxiety and central role in expression of seizure o Primary inhibitory transmitter in the nervous system o Hyperpolarize postsynaptic membranes o Without its inhibitory effects , synaptic excitation would get out of control and spread out through the brain in a reverberating system  E.g. Epileptic seizures may happen because of an abnormality in GABA functioning  Drugs affecting GABA are used in treating anxiety disorder  E.g. Valium , assist GABA in bidning with one of its primary receptors, an action that inhibits neuronal connection and reduces symptoms of anxiety • Also treat insomnia /other stress-related disorders  Alcohol is a relaxing experience since it is affecting GABA - Glutamate: o Primary excitatory transmitter in the nervous system o Contributes to learning and memory o Subset of glutamate circuits appears to play a key role in long-term potentiation  durable increases in excitability at synapses along a specific neural pathway o Long-term potentiation  building blocks of memory formation o Features of schizophrenia that aren’t explained by dopamine hypothesis implicated with disturbances in glutamate circuits Peptides Modulate Neurotransmitter - Endorphines o Internally produced chemicals that resemble opiates in structure and effects o Mo
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