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Suzanne Erb

PSYC62: LECTURE 3: NEURONAL CONDUCTION & TRANSMISSION Review material on intranet for midterm Writing assignment  Two edged swords – amphetamines o Drug for focus, treatment and abusive o Positive and negative aspects o Morphine  Powerful analgesic, high abuse liability o Alcohol  Moderation – health benefits, abuse liability  Cirrhosis of the liver  Check online powerpoint for instructions!!! o Can be in groups 2-3 students by Feb 8 via email to prof  Reference list is additional th 4-5 pages double spaced  Hardcopy and turnitin by April 5 4:30pm*** Cell types in the CNS  Neurons and glia look different and function differently  Neurons – primary messengers in central nervous system o Long process that extends from cell body is the axon – main conducting unit of neuron Estimating neuron numbers in CNS  Billions of neurons  Density varies from one region to another  Check figure*  Cerebellum involved in balance and movement o Extremely densely packed neurons  Not so densely packed in cerebral cortex  Glia – microglia, astrocytes and oligodendrocytes o 10-15 times as many glia cells as neurons o Metabolic support o Protection – blood brain barrier (endothelial cell wall) o Myelin sheath o Endocytosis within cells o Scavenge debri after cell death or damage/injury o Vesicle support at synaptic cleft o Guide migration for neurons during development o Provide neutrogen functions? o Communication within CNS now o Genesis of diseased states  Alzhiemer’s, chronic pain, depression, paralysis, schizophrenia, chronic pain The other brain  How understanding has advanced with respect to role of what glia cells play in communication Main features of nerve cells  Presynpatic cell synapsing on several post synaptic cells  Presynaptic – main messengers of info  Post synaptic – receives info from presynaptic cell  Any neuron can be presynaptic or postsynaptic cell relative to its adjacent neurons  Every neuron has a soma (cell body) main metabolic centre  Long axon – main conducting unit of cell o Longer than dendrites o Distinguishes from glia morphologically o Peripheral nervous system can be 1m long  Dendrites from cell body o Main apparatus for receiving input to the cell o Receptors located  Axon hillock o Specialized region of soma giving rise to axon o All signals converge and decivision made by cell whether or not to become activated or to fire an action potential  Electronic signal that travels from one cell to another when neuron fires  Myelin sheath – insulating barrier around axon o Speeds up rate of propogation of action potential  Nodes of ranvier o Gaps in myelin sheath o Electrical signal jump from one node to the other o Increase speed of propagation of action potential  Terminal branches  Terminal button/ending o Chemicals/transmitters synthesized and stored into synaptic cleft (space)  Synapse- presynaptic membrane, post synaptic and cleft that chemical crosses  Info released from presynaptic to influence postsynaptic activity Two major processes involved in communication between neurons Conduction  What occurs when AP fired  Processes are electrical o Involve changes in elective potential as signal moves from one cell to another  All or none o Once there’s a minimal threshold reached, AP will fire at same rate, no matter how much the threshold is exceeded o Analogy of gun firing  Minimal force needs to be applied for bullet to be shot from gun, but doesn’t matter how much force is applied because bullet will fly at same speed with same time every time Transmission  Release of neurotransmitters from one neuron and what they do to adjacent neurons  Primarily chemical o How does chemical released after….  Graded o Once bound to post-synaptic cell, temporary changes in electrical component o Chemicals that act on post synaptic cells cause variable changes in cell membrane at their point of site of action o Variable changes in membrane potential converge at axon hillock to determine if that post synaptic cell will fire an AP o Dynamic interplay? Neuronal Conduction  Primarily an electric event, also electrochemical  Distribution of 4 ions across membrane  Relative concentration of charged molecules/ions o Shifts when AP occurs  Small uncharged molecules, oxygen, urea, nitrogen? Can move freely across membrane  Charged ions need protein channels to cross over membrane o Represented by gaps in line  In absence of disturbance, when cell is at rest o Membrane potential inside of neuron is more negative -70mv more negative than extracellular space o Resting potential arises from large negatively charged proteins o High sodium concentration extravellularly Forces keeping intracellular more negative than extracellular  Selective permeability o When cell at rest, potassium and chloride channels permit ions to pass slowly, but easily into and out of cell o 1 moves in (cl) and other moves out(k) = little effect o Sodium channels are closed, cannot enter cell across concentration or electrical gradient  Sodium-potassium pump o Specialized mechanism in membrane wall o At rest, pumping sodium out and potassium comes in o Net movement is positive charge outside relative to inside cell  Electrical and concentration gradients for potassium o 2 opposing gradients o Concentration – potassium flows outside of cell o Electrical – potassium flows inside because inside of cell is more negative  At rest – 2 forces nearly balanced, but electrical gradient wins  Net inflow of positive charge  If resting potential of neuron is negative and net movement of potassium is inside then what accounts for negative membrane potential o Potassium is moving in to increase the charge to keep Vm at -70mv o If it moved out then inside of neuron would be even more negative o Neuron wants to be kept at -70mv Slide 11  At rest cell maintains resting potential of -70mv inside relative to outside with minor fluctuations  Membrane can increase to 10mv in positive direction o Depolarization  Inhibitory inputs arriving to cell o Hyperpolarization – decrease in membrane potential  Resting potential can tolerate minor fluctuations in 10mv range in either direction  If cell membrane becomes depolarized in 10 mv or more o -60mv or more then AP will fire o Will fire every time with same rate, frequency and same series of events o Sodium channels open, cell membrane becomes depolarized o Inside of cell becomes more positive to outside o Membrane potential reaches +30mv inside to outside  Sodium channels close  Potassium opens and rushes outside cell along its concentration and electrical gradient  Cell becomes hyperpolarized  Sodium channels close  Forces maintaining resting Vm kick into action to restore it o All events happen in less than 3 milliseconds o Refractory period – hyperpolarization o Absolute refractory period – trough  Any further excitation to neuron will not cause an AP to occur o Relative refractory period – cell comes out of abs
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