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UT BIO 311D - Neurons, Synapses, and Signaling (Part II)
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BIO 311D 1st Edition Lecture 25Outline of Last Lecture I. NeuronsII. Information ProcessingIII. Neuron Structure and FunctionIV. Ion PumpsV. Resting PotentialOutline of Current Lecture I. Action PotentialII. Hyperpolarization and DepolarizationIII. Graded Potentials and Action PotentialsCurrent LectureAction potentials are the signals conducted by axons• Changes in membrane potential occur because neurons contain gated ion channels that open orclose 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 potentialHyperpolarization 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 cellAt ZERO msec on the graph, it is likely that there wasA. a localized opening of K+ channelsB. A localized opening of some Na+ channelsC. A rapid opening of most K+ channelsD. A rapid opening of most Na+ ChannelsGraded 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 levelAt step four in the graph, it is likely thatA. Most Cl- channels closedB. Most VG-Na+ channels openedC. Most VG-K+ channels closedD. Most VG-K+ channels openedE. Na/K pumps were inactivatedGeneration of Action Potentials: A Closer Look• An action potential can be considered as a series of stages• At resting potential1. Most voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed• When an action potential is generated1. Voltage-gated Na+ channels open first and Na+ flows into the cell2. 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 cell4. 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+ channelsConduction 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 backwardsEvolutionary 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


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UT BIO 311D - Neurons, Synapses, and Signaling (Part II)

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