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137 Cards in this Set
- Front
- Back
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What kind of cells are neurons and what is it's function
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Excitable cells; able to generate electrical signals as ions move across their cell membranes
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Membrane potential
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Results from the uneven distribution of ions across the cell membrane
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What is the membrane potential influenced by?
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Ion concentration gradients in a resting neuron and membrane permeability to the ions
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What will cause a result in a more positive charge outside the neuron
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A higher concentration of Na+, Cl-, and Ca2+ in the ECF
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What will cause a result in a more positive charge inside the neuron
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A higher concentration of anions and K+ in the ICF
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What is the difference in charge in the ion concentration gradients called?
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Membrane's potential (voltage)
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Resting neuron's membrane
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More permeable to K+ than to Na+ or Ca2+ (K+ leaks slowly out of K+ ion channels)
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Importance of K+ to membrane permeability
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K+ is the major ion that contributes to the resting membrane potential
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What is the resting membrane potential in an average neuron
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Resting membrane potential is about -70 mV
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How are electrical currents in the body created?
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By the flow of ions (esp. Na+ and potassium K+) through ion channels in the plasma membrane
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What does it mean if a membrane is polarized
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A cell with a difference in charge across its membrane
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How does a membrane become depolarized?
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If the neuron membrane increases its permeability to Na+, Na+ moves into the cell
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How does a membrane become hyperpolarized?
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If the membrane becomes more permeable to K+, K+ moves out of the cell
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Compare the potential between depolarized and hyperpolarized
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Depolarized = potential becomes more positive; hyperpolarized = more negative
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What are the three types of gated channels
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Mechanically, chemically, and voltage gated ion channels
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What are the function of the three types of gated channels
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Controls neuronal ion permeability
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Mechanically gated
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Ion channels in sensory neurons open due to pressure or stretch
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Chemically (ligand) gated
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Ion channels in most neurons open in response to ligand (e.g., neurotransmitter) binding
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Voltage gated
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Ion channels open or close in response to changes in membrane voltage
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Graded potential
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Local; has variable strength signals that travel short distances and gradually lose strength
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Action potentials
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Strong, uniform depolarizations that travel from the cell body to the axon terminals without losing strength
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What causes graded potentials
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Depolarizations or hyperpolarizations that usually occur in dendrites or the cell body
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How can the neurons be stimulated for graded potentials
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By chemicals, light, heat, voltage, or mechanical distortion of the plasma membrane
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What happens once the neurons are stimulated for graded potentials
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Neuronal ion channels open or close, causing ions to enter or leave the neuron
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After the neurons are stimulated, describe the resulting change in membrane potential of graded potentials
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It is directly proportional to the strength of the initial stimulus
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What will a weak stimulus do to the graded potential
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It will initiate a weak graded potential that will not travel far
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What will a strong stimulus do to the graded potential
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It will stimulate a strong graded potential that may be converted to an action potential
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What are the two types of graded potentials
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Inhibitory and excitatory post synaptic potentials
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Inhibitory post synaptic potential (IPSPs)
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Result from hyperpolarization of the post synaptic neuron’s membrane
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Excitatory post synaptic potential (EPSPs)
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Result from depolarization of the post synaptic neuron’s membrane
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Compare how IPSPs and EPSPs are likely to fire an action potential
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IPSPs make the neuron less likely to fire an action potential, while EPSPs is more likely
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How does the graded potential (EPSP) die out
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If the depolarization is not strong enough to reach threshold
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How will an action potential be generated by EPSP
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If the depolarization does reach threshold
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How do action potentials differ from graded potentials
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All have the same voltage, do not diminish in strength as they travel, and are 'all or nothing'
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What does it mean that action potentials are 'all or nothing'
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They will only be generated if threshold is reached in the neuron’s trigger zone
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What does action potentials result from
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The movement of Na+ and K+ through voltage-gated ion channels in the membrane
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Where are voltage-gated ion channels found?
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In the highest concentration in the trigger zone of a neuron
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What happens if an excitatory graded potential is strong enough when it reaches the trigger zone?
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It can open these ion channels and generate an action potential down the axon
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Describe the resting potential and the ion channels prior to the action potential (process that causes an action potential)
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Neuron has a resting potential of about -70mV, and voltage-gated ion channels are closed
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What does a stimulus in the neuron initiate? (process that causes an action potential)
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A graded potential, which travels toward the axon hillock and may generate an action potential
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What happens if enough Na+ ions enter the axon hillock to reach threshold? What is the threshold voltage? (process that causes an action potential)
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Voltage-gated Na+ channels open, and more Na+ ions rush into the neuron; about -55mV
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How is an action potential is generated down the axon after it has reached the threshold? (process that causes an action potential)
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As more voltage-gated Na+ channels are opened (domino effect), as the depolarization wave rises to about +30 mV
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What happens as the voltage-gated Na+ channels close? (process that causes an action potential)
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The voltage-gated K+ channels open causing K+ to move out of the neuron, and repolarization occurs
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Repolarization
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Voltage moves back towards the resting potential
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Hyperpolarization
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A decrease in membrane voltage slightly below the resting potential (-90 mV)
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Why does hyperpolarization occur? (process that causes an action potential)
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Because voltage-gated K+ channels are slow to close
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What does the Na+/K+ pump restore after an action potential has occurred?
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Restores the ion balance of the neuron by pumping Na+ ions out and K+ ions into the neuron
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Refractory period
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A period of neuronal insensitivity to restimulation during and after the action potential
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Absolute refractory period
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Occurs from start to end of the action potential, when the Na+ channels are still open
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What happens to the neuron during the absolute refractory period
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The neuron cannot be further stimulated, regardless of the stimulus strength
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What does the absolute refractory period ensure?
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Ensures one way transmission of the action potential
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Relative refractory period
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Lasts until hyperpolarization ends, when K+ channels are still open, but Na+ channels are closed
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What happens during the relative refractory period
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An exceptionally strong stimulus can trigger another action potential
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Stimulus intensity
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Coded by the frequency of action potentials
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Compare the frequency of action potentials between a stronger and weaker stimulus intensity
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A stronger stimulus will generate a greater frequency of action potentials in a neuron than a weaker stimulus
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Why do neurons with larger diameters conduct faster action potentials
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Neurons with larger diameters have decreased resistance to electrical current flow
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Unmyelinated axons
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Have voltage-gated Na+ channels along their whole length
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Describe the unmyelinated axons depolarization of one part of the neuron membrane
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It will propagates the action potential from axon hillock to axon terminal like a domino effect
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Myelinated axons
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Have the majority of their voltage-gated Na+ channels in the nodes of Ranvier between the myelin sheaths
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Describe the diffusion of Na+ in myelinated axons
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Na+ ions diffuse inside the membrane between the nodes of Ranvier
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What happens after the diffusion of Na+ in myelinated axons
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Open the voltage-gated Na+ channels at the nodes and saltatory conduction occurs
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What is saltatory conduction of the myelinated axons
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Propagation of action potentials along myelinated axons from one node of Ranvier to the next node
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What does saltatory conduction of the myelinated axons result in?
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Faster nerve conduction than in unmyelinated axons
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How does some chemicals alter the action potential conduction
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By binding to Na+, K+, or Ca2+ in the neuron membrane
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What binds to and blocks Na+ channels
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Neurotoxins (e.g., tetrodotoxin from puffer fish) and local anesthetics
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How can the electrical activity of the nervous system be altered?
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Changes in ECF concentrations of K+ and Ca2+
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Define Hyperkalemia
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Shifts the resting membrane potential toward threshold causing neurons to fire action potentials in response to smaller graded potentials
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Define Hypokalemia
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Hyperpolarizes neurons, making them less likely to fire action potentials
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What does hyperkalemia and hypokalemia cause?
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Hyperkalemia can cause heart failure; hypokalemia causes muscle weakness
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What does the specificity of neural communication depends on
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Signal molecules secreted by neurons, Target cell receptors for the chemicals, and Synapses
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Synapse
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he place where a neuron meets its effector (another neuron, muscle or gland cell)
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What are the parts of each synapse
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Presynaptic neuron axon terminal, Synaptic cleft, and Postsynaptic cell membrane
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Synaptic cleft
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Space between the presynaptic and postsynaptic endings
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Electrical synapses
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Electrical synapses are those in which adjacent cells are joined by gap junctions
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What does electrical synapses allow?
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Allow ions to diffuse directly from cell to cell
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Where are electrical synapses found?
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Found in glia, cardiac & smooth muscle
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Chemical synapses
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Chemical synapses are those in which neurons communicate with effectors via neurotransmitters.
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Where are chemical synapses found?
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These synapses are the most common in the nervous system
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What is the signal for neurotransmitter release at the synapse
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Calcium
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How can neurotoxins block nerve transmission
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Neurotoxins (e.g., tetanus and botulinum) can inhibit certain proteins (e.g., voltage-gated Ca2+ channels) in the neuron’s exocytotic apparatus
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What may signal molecules released from neurons function as
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neurotransmitters, neuromodulators, or neurohormones
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Neurocrines
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Convey information from neurons to other cells
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What act as paracrines and/or autocrines, with target cells near by
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Neurotransmitters and neuromodulators
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Neurotransmitters
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Acts at a synapse and elicit rapid responses
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Neuromodulators
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Acts at synaptic and nonsynaptic sites, and are slower acting
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Where are neurohormones are secreted, and where does it go
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Secreted into the blood where they travel to distant target cells
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How are neurotransmitters grouped into classes, and what are the classes
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According to their structures; classes are ACh, amino acids and amines, polypeptides, purines, gases, and lipids
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What is acetylcholine synthesized from and where is it used?
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Synthesized from acetylCoA and choline, and used in the PNS
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Cholinergic
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Neurons that secrete ACh and their receptors
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Where is Acetylcholine excitatory in?
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Skeletal muscle and neurons
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Where is Acetylcholine inhibitory in?
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Cardiac muscle
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Where may Acetylcholine be excitatory or inhibitory in? What does it depend on?
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Smooth muscles and glands, depending on their types of receptors
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What are the excitatory amino acids in the CNS
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Glutamate and aspartate
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What are the inhibitory amino acids in the CNS
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Glycine and gamma-aminobutyric acid (GABA)
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Where are amino acid derived Amines are mostly made? What does it also function as?
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Made from tyrosine; functions as neurohormones
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List some amino acid derived amines
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Epinephrine (adrenaline), norepinephrine (catecholamines), and dopamine
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Adrenergic
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Neurons that secrete norepinephrine and the NE receptors
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What is the main neurotransmitter of the Sympathetic NS.
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Norepinephrine
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Catecholamines
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Used in the PNS; they are excitatory in cardiac muscle and excitatory or inhibitory in smooth muscle & glands
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Function of Dopamine
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Inhibits involuntary skeletal muscle movements
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What disease will a lack of dopamine cause?
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Parkinson’s disease
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What does indolamines include?
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Seratonin and histamine
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Where is Seratonin made? What is it involved in?
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Made from tryptophan in the CNS; is involved in sleep, alertness, thermoregulation and mood
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Function of histamine
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Vasodilator
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What does the polypeptide class of neurotransmitters include?
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Substance P and Opoid peptides
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Substance P
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Involved in some pain pathways
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Opoid peptides
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(endorphins & enkaphalins) are analgesic (suppress pain)
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What are the two types of neurotransmitter receptors
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Either ligand-gated ion channels or G protein-linked receptors that stimulate 2nd messengers
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Cholinergic receptors and the 2 subtypes
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ACh binds to receptor; subtypes are nicotinic receptors and muscarinic receptors
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Nicotinic receptors and where are they found
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(nicotine is an agonist) ligand-gated ion channels found on skeletal muscle, in the PNS and in the CNS
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What are inotropic
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Ligand-gated ion channels are inotropic – they open ion channels
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Where are Muscarinic receptors found
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(fungal muscarine is an agonist) Found in the parasympathetic NS and CNS
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What are muscarinic receptors linked to?
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They are linked to G proteins with 2nd messengers
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What happens when ACh binds to the G protein-linked receptor and how does response vary
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Activates a G protein, which activates a 2nd messenger; vary according to the receptor subtype
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What does Adrenergic receptors include
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alpha (α) and beta (β), which are also G protein-linked receptors and initiate 2nd messenger cascades
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What does both Amines and neuropeptides have
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Metabotropic effects that use a second messenger, such as cAMP, to change the metabolism of the postsynaptic cell
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What is ACh is broken down by and what happens to choline after
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Acetylcholinesterase; choline is actively transported back into the presynaptic axon to make more ACh
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Where is Norepinephrine (NE) actively transported to? For what purpose?
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transported back into the presynaptic axon to be used again or broken down by monoamine oxidase (MAO)
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What happens to Amines, polypeptides, and amino acid neurotransmitters in the CNS when the neurotransmitter activity is terminated
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Moves into the blood or is actively transported back into the presynaptic axon to be metabolized or used again
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Neuronal circuits
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Pathways among neurons, like a circuit board of an electrical appliance
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What are the two types of neuronal circuits
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Diverging and converging circuit
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Diverging circuit
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One nerve fiber branches and synapses with several postsynaptic cells
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Function of the diverging circuit
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Allows one neuron to control many other cells (e.g., 1 motor neuron controls many skeletal muscle cells)
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Converging circuit
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Several neurons synapse with one neuron or a neuronal pool
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Function of the converging circuit
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allows input from several body regions (e.g., eyes, inner ear) to another area (e.g., brain area for equilibrium)
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Summation and inhibition
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A neuron may receive input from thousands of presynaptic neurons at one time
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Summation
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the process of “adding up” postsynaptic potentials (+EPSPs and –IPSPs) and responding to their net effect
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What happens if the EPSPs overrides the IPSPs during summation
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A nerve impulse may be generated
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Temporal (time) summation
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Occurs when one presynaptic neuron generates EPSPs so rapidly that each is generated before the other decays
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What can result from temporal summation
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Can add up to threshold and cause an action potential
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Spatial (space) summation
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Occurs when EPSPs from several presynaptic neurons add up to threshold at the axon hillock of one postsynaptic neuron
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When does Postsynaptic inhibition occur
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when one neuron suppresses the effect of a another neuron
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Example of Postsynaptic inhibition
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A neuron that generates IPSPs prevents neurotransmitter release by another neuron
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What does Presynaptic modulation allow
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Selective modulation of axon collaterals and their target cells
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When does Presynaptic inhibition occur
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When a presynaptic neuron decreases neurotransmitter release by a postsynaptic cell
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When does Presynaptic facilitation occur
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When the presynaptic neuron increases neurotransmitter release by the postsynaptic cell
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When does Postsynaptic modulation occur
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When a modulatory neuron, usually inhibitory, synapses with the dendrites or cell body of a postsynaptic cell
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