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Neurons, Synapses, and Signaling Overview Neural Signals Organization of Nervous Systems
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Functions of Nervous Systems Sensory input-conduction of signals from receptors to integration centers (CNS) Integration-interpretation of signals and generation of proper response Motor output-signals to effector cells (muscles, glands), carry out response
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Composition of Nervous System Neurons cells specialized for transmitting electrical and chemical signals Composed of cell body, dendrites, and axons Three types: sensory neurons, interneurons, and motor neurons Arranged into circuits of two or more kinds of neurons: convergent, divergent, and reverberating circuits
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Composition of Nervous System con’t. Supporting Cells (glial cells) Structurally reinforce, protect, insulate and assist neurons Do not conduct impulses Outnumber neurons 10- to 50- fold Types: astrocytes (blood-brain barrier), oligodendrocytes, Schwann cells
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The Nature of Neural Signals Signal transmission along a length of a neuron depends on voltages created by ionic fluxes across neuron plasma membranes All cells have an electrical membrane potential: range from –50 to –100 mV; outside the cell is 0, inside is negatively charged Neurons have about –70mV at rest
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Ion Concentrations: [Na+] 10x greater outside (main cation outside) [K+] 30x greater inside (principal cation inside) [Cl-] 12x greater outside Other anions-100x inside Selective permeability of the membrane maintains these differences
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Action Potential-Nerve Impulse Four phases Resting phase-no channels open Depolarizing phase-brief reversal of polarity, Na+ gates open Na+ in, result: -70mV to +30 mV Repolarizing phase-Na+ gates close, K+ gates open, K+ out, +30 mV to –70 mV Undershoot phase (refractory period)-K+ and Na+ are returned to proper concentrations, neuron insensitive to depolarizing stimuli
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Action Potentials “all-or-none” require a minimum stimulus or threshold Self-propagating Saltatory conduction-action potential “jumps” from one node of Ranvier to the next, skipping the myelinated regions of the membrane
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Synapses Tiny gap between terminal of an axon and beginning of a dendrite on the next neuron Presynaptic cell-transmitting cell Postsynaptic cell-receiving cell Two types of synapses: electrical and chemical
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Electrical Synapses Electrical synapse Less common Travel through gap junctions Example-giant neuron in crustaceans
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Chemical Synapses Separated by synaptic cleft Ca 2+ enters cell stimulating cytoskeleton to move vesicles to the surface of the axonal knob Neurotransmitter from the vesicle is released into the cleft Neurotransmitter binds to receptor sites on the postsynaptic membrane, reestablishing the action potential
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Web sites and Videos http://outreach.mcb.harvard.edu/animatio ns/actionpotential.swf http://outreach.mcb.harvard.edu/animatio ns/actionpotential.swf http://outreach.mcb.harvard.edu/animatio ns/actionpotential.swf http://people.eku.edu/ritchisong/301notes 2.htm http://people.eku.edu/ritchisong/301notes 2.htm http://people.eku.edu/ritchisong/301notes 2.htm
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