Nervous Tissue II_Chp 12
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| Propagation of Action Potential | spreads over the surface of the axon membrane
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| Deploration Na+ | fows into the cell; affects voltage of adjacent areas; voltage-gated Na+ channels open;
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| Action Potentional | self propagated along membrane; can be progagated via continuous or saltatory conduction, depending on myelin
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| Continuous Conduction | occurs in unmyelinated axons; AP is propagated b step-by-step depolarization of each portion of the length of the axon membrane "domino effect"
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| Saltatory Conduction | occurs in myelinated axons; deplorization occurs only at nodes of ranvier (where ion channels are); current is carried through ECF and "jumps" from node to node
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| myelinated axons (saltatory conduction) | ions only have to diffuse at nodes not across the entire membrane; this allows for more rapid conduction of action potentials
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| factors that affect speed of Impulse Conduction | fibers myelinated or not; diameter of fiber (thick fibers transmit AP's faster; presece/absence of calcium ions
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| Calcium ions | are needed to close Na+ channels
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| If Na+ keeps diffusing in | impulses are transmitted repeatedly (muscle spasms); fetus may take up to much calcium and cause mother to cramp a lot.
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| Electrical Synapses 1 | AP's conduct directly between adjacent cells through gap junctions; AP spreads from cell to cell via cytosol
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| Electrical Synapses 2 | common in visceral smooth muscle, cardiac muscle, and developing embryo; conduction is faster than chemical synapse; allows synchronization-a large number of neurons can produce AP's in unison-coordinated contraction
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| Gap Junctions | in cardiac and visceral smooth muscle allow the action potential itself to spread from cell to cell, enabling coordinated contraction of muscle fibers
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| Chemical synapse 1 | AP is transferred from teh presynaptic ("sending") neuron to the postsynaptic ("receiving") neuron across a synaptic cleft
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| Chemical synapse 2 | current generates AP, then AP is transmitted; requires neurotransmitters; takes longer than electrical synapse
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| Excitatory (effects of neurotransmitters) | if the neurotransmitter causes opening of Na+ channels; Na+ diffuses in - membrane depolarizes; if enough Na+ channels open to reach threshold nerve impulse will be generated
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| Inhibitory (effects of neurotransmitters) | if the neurotransmitter causes opening of other channels (Cl- or K+, hyperpolarizing membrane; Cl- can diffuse in, or K+ will leave neuron; inside of neuron will be even more negatice and will be less likely to reach threshold
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| Summation | each neuron in brain and spinal cord may receive neurotransmitters from hundreds of nerve fibers; some neurotransmitters may be excitatory or inhibitory; sum of excitatory+inhibitory messages determines whether AP will be generated in postsynaptic neuron
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| Removal of Neurotransmitter | after AP is generated in postsynaptic neuron, the neurotransmitter must be removed
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| Diffusion (Removal of Neurotransmitter) | NT simply diffuses out of synaptic cleft, losing contact with postsynaptic neuron and NT receptors
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| Enzymatic degradation (Removal of Neurotransmitter) | enzyme from postsynaptic neuron breaks down NT after AP has been generated; Acetycholine & acetylcholinesterase; epinephrine and monoamine oxidase
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| Uptake into cells (Removal of Neurotransmitter) | NT may be actively transported into neighboring neuroglial cells (uptake) or into presynaptic neuron (re-uptake)
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| drug effects on re-uptake (cocaine) | re-uptake of blocks dopamine (produces continued stimulation pleasure
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| drug effects on re-uptake (SSRI's-selective serotonin re-uptake inhibitors) | keeps deficient serotonin in cleft as long as possible (elevates mood, relieves depression)
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| drug effects on re-uptake (MAO-monamine oxidase inhibitors) | prevent MAO from breaking down epinephrine and norepinphrine; EPI and NE do a lot of stuff, including elevating mood; preventing breakdown may help treat depression
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| Acetylcholine (ACh) (representative neurotransmitters) | released by many PNS neurons & some CNS; excitatory in skeletal muscle; inhibitory in cardiac muscle
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| Amino Acid (representative neurotransmitters) | Glutomate=released by most excitatory neurons in the brain; inactivated by re-uptake; GABA=inhibitory NT for 1/3 of all brain synapses, & 1/2 of inhibitory synapses in spinal cord
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| Amino Acid (representative neurotransmitters) | Glycine is inhibitory NT for other 1/2;
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| Valium | enhances binding GABA=increasing inhibition=decreases anxiety
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| Huntington's disease | causes progressive GABA deficiency=thrashing, dementia
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| Strychnine | blocks glycine receptors on skeletal muscles=no inhibition of excessive contraction=can't relax muscles, including diaphragm=can't breathe
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| Norepinephrine (representative neurotransmitters) | affects mood, dreaming, awakening from a deep sleep
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| Dopamine (representative neurotransmitters) | affects skeletal muscle tone, emotion, pleasure, addictive behavior (Parkinson's disease=tremors)
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| Serotonin (representative neurotransmitters) | involved in control of mood, appetie, temp regulation, induces sleep
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| Endorphins and enkephalins (representative neurotransmitters) | pain reliever, may be released in response to stress; morphine, heroin, opium fit same receptors; blocked endorphins acupunture don't work
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| Substance P (representative neurotransmitters) | when released in sensory nerves, transmits pain related info to CNS; endorphins and enkephalins suppress release; 200x stronger than morphine
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| chemical synapse (AP process step 1) | AP reaches synaptic end bulb and depolarizes membrane causing voltage-gated calcium channels to pen in the synaptic end bulb
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| chemical synapse (AP process step 2) | calcium ions in ECF diffuse into the presynaptic neuron
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| chemical synapse (AP process step 3) | calcium ions trigger exoctosis of vesicles containing neurotransmitters (NT)
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| chemical synapse (AP process step 4) | NT is released into the synaptic cleft and diffuses across to the postsynaptic neuron; receptors in the membrane of the postsynaptic neuron bind NT
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| chemical synapse (AP process step 5) | Ligand-gated ion channels open, ionsdiffuse in and membrane depolarizes
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| chemical synapse (AP process step 6-7) | if membrane potentional reaches threshold, AP in the postsynaptic neuron and AP/nerve impulse is sent on.
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