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Nervous Tissue II

Nervous Tissue II_Chp 12

Propagation of Action Potential spreads over the surface of the axon membrane
Deploration Na+ fows into the cell; affects voltage of adjacent areas; voltage-gated Na+ channels open;
Action Potentional self propagated along membrane; can be progagated via continuous or saltatory conduction, depending on myelin
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"
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
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
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
Calcium ions are needed to close Na+ channels
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.
Electrical Synapses 1 AP's conduct directly between adjacent cells through gap junctions; AP spreads from cell to cell via cytosol
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
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
Chemical synapse 1 AP is transferred from teh presynaptic ("sending") neuron to the postsynaptic ("receiving") neuron across a synaptic cleft
Chemical synapse 2 current generates AP, then AP is transmitted; requires neurotransmitters; takes longer than electrical synapse
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
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
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
Removal of Neurotransmitter after AP is generated in postsynaptic neuron, the neurotransmitter must be removed
Diffusion (Removal of Neurotransmitter) NT simply diffuses out of synaptic cleft, losing contact with postsynaptic neuron and NT receptors
Enzymatic degradation (Removal of Neurotransmitter) enzyme from postsynaptic neuron breaks down NT after AP has been generated; Acetycholine & acetylcholinesterase; epinephrine and monoamine oxidase
Uptake into cells (Removal of Neurotransmitter) NT may be actively transported into neighboring neuroglial cells (uptake) or into presynaptic neuron (re-uptake)
drug effects on re-uptake (cocaine) re-uptake of blocks dopamine (produces continued stimulation pleasure
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)
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
Acetylcholine (ACh) (representative neurotransmitters) released by many PNS neurons & some CNS; excitatory in skeletal muscle; inhibitory in cardiac muscle
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
Amino Acid (representative neurotransmitters) Glycine is inhibitory NT for other 1/2;
Valium enhances binding GABA=increasing inhibition=decreases anxiety
Huntington's disease causes progressive GABA deficiency=thrashing, dementia
Strychnine blocks glycine receptors on skeletal muscles=no inhibition of excessive contraction=can't relax muscles, including diaphragm=can't breathe
Norepinephrine (representative neurotransmitters) affects mood, dreaming, awakening from a deep sleep
Dopamine (representative neurotransmitters) affects skeletal muscle tone, emotion, pleasure, addictive behavior (Parkinson's disease=tremors)
Serotonin (representative neurotransmitters) involved in control of mood, appetie, temp regulation, induces sleep
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
Substance P (representative neurotransmitters) when released in sensory nerves, transmits pain related info to CNS; endorphins and enkephalins suppress release; 200x stronger than morphine
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
chemical synapse (AP process step 2) calcium ions in ECF diffuse into the presynaptic neuron
chemical synapse (AP process step 3) calcium ions trigger exoctosis of vesicles containing neurotransmitters (NT)
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
chemical synapse (AP process step 5) Ligand-gated ion channels open, ionsdiffuse in and membrane depolarizes
chemical synapse (AP process step 6-7) if membrane potentional reaches threshold, AP in the postsynaptic neuron and AP/nerve impulse is sent on.
Created by: sweetlatrece



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