Lecture3_action_potential_wide - Lecture 3 Action...

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Lecture 3 Action Potentials and Synaptic Transmission
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Gated ion channels Leak channels are ungated (always open). Ligand-gated channels are opened (or closed) when a chemical molecule bind to them (such as a neurotransmitter, hormone, drug, second messenger, etc.). Voltage-gated channels are opened (or closed) when V m falls into some target range. Mechanically gated channels are opened (or closed) by a mechanical force such as physical touch, vibration, etc. Optically gated channels are opened (or closed) by light.
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nucleus Multipolar neuron compartments LEAK POTASSIUM CHANNEL LIGAND-GATED SODIUM CHANNEL VOLTAGE-GATED SODIUM CHANNEL DENDRITES SOMA AXON VOLTAGE-GATED POTASSIUM CHANNEL Na+/K+ ATPase PUMP Active zone Ion Channels Ion Pumps
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Classification of Neurons into Three Principal Types Multipolar Bipolar Unipolar All three neuron types have an “integration zone” (or “active zone”) where the action potential usually originates
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Dorsal root ganglion (DRG) neuron } } SKIN SPINAL CORD DRG cells are unipolar neurons in the peripheral nervous system (PNS) Their job is to relay somatosensory information from the skin to the spinal cord (CNS)
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DRG Cell Membrane Compartments LEAK POTASSIUM CHANNEL MECHANICALLY GATED SODIUM CHANNEL (mechanoreceptor) AXON VOLTAGE-GATED POTASSIUM CHANNEL VOLTAGE-GATED SODIUM CHANNEL Na+/K+ ATPase PUMP Dendritic Branch (Nerve Ending) Compartment Mechanoreceptor Na+ channels Leak K+ channels Na+/K+ ATPase pump Integration Zone Compartment Voltage-gated Na+ channels Voltage-gated K+ channels Leak K+ channels Na+/K+ ATPase pump Active zone Ion Channels Ion Pumps
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AXON A touch to the skin generates a receptor potential by opening mechanoreceptor Na+ channels The receptor potential passively propagates (like electricity flowing through a wire) to the integration zone Since the cytoplasm is a resistive electrical conductor, the receptor potential is attenuated (gets smaller) as it propagates Touch Stimulus Active zone
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AXON Touch Stimulus If receptor potentials can depolarize the axon hillock’s membrane from its resting potential of -75 mV up to the spike threshold of around - 55mV, then… Rest = -75 mV Spike threshold = -55 mV TIME MEMBRANE VOLTAGE Active zone
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AXON TIME Rest = -75 mV Spike threshold = -55 mV Peak Voltage = +60 mV Activation of voltage-gated Na+ channels Subthreshold receptor potentials do not trigger an action potential The action potential is called an “all -or- none” phenomenon because it either occurs or it doesn’t. There is no such thing as half of an action potential! If the spike threshold is crossed, then a FULL ACTION POTENTIAL is guaranteed to occur. WHY????? Because opening voltage-gated sodium channels will allow sodium to enter and depolarize the membrane.
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