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251Ch12-1

55mv is reached voltage gated k channels open k

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-55mV is reached, voltage-gated K+ channels open K+ channel opening is much slower than Na+ channel opening which caused depolarization When K+ channels finally do open, the Na+ channels have already closed (Na+ inflow stops) K+ outflow returns membrane potential to -70mV If enough K+ leaves the cell, it will reach a -90mV membrane potential and enter the after-hyperpolarizing phase K+ channels close and the membrane potential returns to the resting potential of -70mV
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Refractory Period of Action Potential Period of time during which neuron can not generate another action potential Absolute refractory period even a very strong stimulus will not initate another AP inactivated Na+ channels must return to the resting state before they can be reopened large fibers have absolute refractory period of 0.4 msec and up to 1000 impulses per second are possible Relative refractory period a suprathreshold stimulus will be able to initate an AP K+ channels are still open, but Na+ channels have closed
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The Action Potential: Summarized Resting membrane potential is -70mV Depolarization is the change from -70mV to +30 mV Repolarization is the reversal from +30 mV back to -70 mV)
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Propagation of Action Potential An action potential spreads (propagates) over the surface of the axon membrane (axolemma) as Na+ flows into the cell during depolarization, the voltage of adjacent areas is affected and their voltage-gated Na+ channels open self-propagating along the membrane The traveling action potential is called a nerve impulse
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Local Anesthetics Prevent opening of voltage-gated Na+ channels Nerve impulses cannot pass the anesthetized region Novocaine and lidocaine
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Continuous versus Saltatory Conduction Continuous conduction (unmyelinated fibers) step-by-step depolarization of each portion of the length of the axolemma Saltatory conduction depolarization only at nodes of Ranvier where there is a high density of voltage-gated ion channels current carried by ions flows through extracellular fluid from node to node
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Saltatory Conduction Nerve impulse conduction in which the impulse jumps from node to node
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Speed of Impulse Propagation The propagation speed of a nerve impulse is not related to stimulus strength. larger, myelinated fibers conduct impulses faster due to size & saltatory conduction Fiber types unmyelinated sensory & autonomic motor
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Chemical Synapses Action potential reaches end bulb and voltage-gated Ca+ 2 channels open Ca+2 flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft & binds to ligand-gated receptors the more neurotransmitter released the greater the change in potential of the postsynaptic cell Synaptic delay is 0.5 msec One-way information transfer
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55mV is reached voltage gated K channels open K channel...

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