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• Stimulus must be sufficient to make the neuron respond and cause complete depolarization. __________ stimuli do not cause depolarization. • “_________________________ principle” - neuron depolarizes to its maximum strength or not at all • Strong stimuli cause numerous sodium channels open, which creates a WAVE OF DEPOLARIZATION • Conduction of the Action Potential —spreading wave of opening Na+ channels in sufficient numbers to allow sodium influx and depolarization • Wave of Depolarization = Conduction of Action Potential = Nerve Impulse Threshold Stimulus Neuron Function: Action Potential Propagation • Time period when sensitivity of neuron is reduced —Cell is still in depolarization/early repolarization • _____________ refractory period - during Na+ influx and early K+ outflow • No amount of stimulus can cause depolarization • ____________ refractory period - during end of repolarization period • Strong stimulus can cause depolarization Refractory Period Saltatory Conduction • RAPID means of conducting an action potential (more rapid than in unmyelinated axons) • Depolarization in myelinated axons can only take place at _______________________________ • Myelin sheath prevents sodium ions from entering the cell. • The depolarization wave is able to skip from one NOR to the next, accelerating the rate at which the wave moves from the cell body to the end of the axon http://www.blackwellpublishing.com/matthews/actionp.html Local Anesthetics • Work by blocking the propagation of the action potential • Blocks _______ channels • Sodium cannot flow into the cell, so threshold is not achieved • Example: Lidocaine • ____________ – junction between two neurons or a neuron and target cell • ___________ ________– gap between adjacent neurons/effector cell • Presynaptic neuron - neuron bringing the depolarization wave to the synapse • Releases _______________________ • Postsynaptic neuron - contains receptors for the neurotransmitter Synaptic Transmission Neurons: synapses • The axon of the presynaptic neuron ends as a branched structure (________________). • Synaptic knob/synaptic end bulb/terminal bouton- slightly enlarged bulb at end of axon • Vesicles contain neurotransmitter (a chemical) • When depolarization wave reaches axon terminal, calcium channels open and cause vesicles to fuse with cellular membrane and release neurotransmitter into the synapse Synaptic Transmission Synaptic Transmission • Neurotransmitters diffuse across synaptic cleft toward postsynaptic membrane • Receptors on postsynaptic membrane bind neurotransmitter • Receptors are VERY specific for each neurotransmitter (similar to a lock and key) Types of Neurotransmitters • ______________ neurotransmitters (acetylcholine, norepinephrine, dopamine) • Usually cause an influx of Na+; postsynaptic membrane moves toward threshold (more positive) • _______________ neurotransmitters (acetylcholine, norepinephrine, dopamine, GABA, glycine) • Move the charge of postsynaptic cell farther away from threshold (more negative) • May open K+ channels/Cl- channels Recycling the Neurotransmitter • Acetylcholinesterase - found on postsynaptic membrane; breaks down acetylcholine • Monoamine oxidase (MAO) - breaks down epi/norepinephrine • Catechol-O-methyl transferase (COMT) - breaks down epi/norepinephrine that is not reabsorbed