BIO 311D 1st Edition Lecture 25 Outline of Last Lecture I Neurons II Information Processing III Neuron Structure and Function IV Ion Pumps V Resting Potential Outline of Current Lecture I Action Potential II Hyperpolarization and Depolarization III Graded Potentials and Action Potentials Current Lecture Action potentials are the signals conducted by axons Changes in membrane potential occur because neurons contain gated ion channels that open or close in response to stimuli When gated K channels open K diffuses out making the inside of the cell more negative This is hyperpolarization an increase in magnitude of the membrane potential Hyperpolarization and Depolarization Opening other types of ion channels triggers a depolarization a reduction in the magnitude of the membrane potential For example depolarization occurs if gated Na channels open and Na diffuses into the cell At ZERO msec on the graph it is likely that there was A a localized opening of K channels B A localized opening of some Na channels C A rapid opening of most K channels D A rapid opening of most Na Channels Graded Potentials and Action Potentials Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus These are not the nerve signals that travel along axons but they do have an effect on the generation of nerve signals If a depolarization shifts the membrane potential sufficiently it results in a massive change in membrane voltage called an action potential Action potentials have a constant magnitude are all or none and transmit signals over long distances They arise because some ion channels are voltage gated opening or closing when the membrane potential passes a certain level At step four in the graph it is likely that A Most Cl channels closed B Most VG Na channels opened C Most VG K channels closed D Most VG K channels opened E Na K pumps were inactivated Generation of Action Potentials A Closer Look An action potential can be considered as a series of stages At resting potential 1 Most voltage gated sodium Na channels are closed most of the voltage gated potassium K channels are also closed When an action potential is generated 1 Voltage gated Na channels open first and Na flows into the cell 2 During the rising phase the threshold is crossed and the membrane potential increases 3 During the falling phase voltage gated Na channels become inactivated voltage gated K channels open and K flows out of the cell 4 During the undershoot membrane permeability to K is at first higher than at rest then voltage gated K channels close and resting potential is restored During the refractory period after an action potential a second action potential cannot be initiated The refractory period is a result of a temporary inactivation of the Na channels Conduction of Action Potentials At the site where the action potential is generated usually the axon hillock an electrical current depolarizes the neighboring region of the axon membrane Action potentials travel in only one direction toward the synaptic terminals Inactivated Na channels behind the zone of depolarization prevent the action potential from traveling backwards Evolutionary Adaptation of Axon Structure The speed of an action potential increases with the axon s diameter In vertebrates axons are insulated by a myelin sheath which causes an action potential s speed to increase Myelin sheaths are made by glia oligodendrocytes in the CNS and Schwann cells in the PNS
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