PSB2000 Test 2 Resting Potential and Action Potential 2 2 1 What ions are important in the action potential When the cell is at rest which ions are most highly concentrated inside of the cell and which ones are most highly concentrated outside of the cell a The action potential is an electrical impulse down an axon b It depends on sodium Na and potassium K moving thru channels in the axon s membrane 2 Understand the forces working on the ions electrical gradient and concentration a Concentration gradient ions flow from areas of high concentration to gradient low concentration i Example b Electrical Gradient ions flow to areas of opposite charge think about putting sugar in water i Negative to positive positive to negative ii Example remember opposites attract 3 What is a voltage gated channel Where are they a The voltage gated channel is a sodium channel structure i Created from a single long polypeptide ii 4 domains I IV that form the pore iii 6 transmembrane alpha helices S1 S6 iv The sodium channel has a pore loop b t S5 S6 that functions as a selectivity filter making it 12X more selective for Na than K Similar to the potassium channel v The channel is gated by a change in voltage across the membrane vi The voltage sensor resides in the S4 segment vii Positively charged amino acid residues are regularly spaced along the coils of the helix viii Depolarization pushes the S4 away from the inside of the membrane causing conformational change that opens the gate ix S4 is lined with amino acids that have positively charged side chains so the channel is sensitive to voltage x 4 Understand the steps of the action potential and how one leads to the next How is an action potential started and propagated What ion enters first Thru what type of channel does it enter What forces drive it inside Why does that channel close What channel opens next What ion moves thru that What forces drive that ion Etc etc a Propagation of the action potential depends on i Diameter of axon ii Insulation myelin b Steps of propagation i Threshold is reached at the axon hillock ii Voltage gated Na channels open iii Na enters the cell iv Now Voltage Gated Na Channels close and Voltage Gated Potassium channels open v K leaves the cell vi So much K exits the cell that is becomes hyperpolarized vii When it reaches resting membrane potential again it can conduct another action potential viii That period of hyperpolarization is called the refractory period ix Refractory period the neuron cannot fire another action potential until it is back up to resting potential and repolarization c The action potential is the signal that conveys information over distances in the nervous system The action potential is a rapid reversal of the situation at rest for an instant the inside of the membrane becomes positively charged relative to the outside Action potentials are all or none The frequency and pattern of action potentials is the code used by neurons to transfer information from one location to another While rate can change speed cannot All of this occurs at the nodes of ranvier between the segments of myelin 5 Terms to know and understand with regard to neurophysiology polarized depolarization hyperpolarization repolarization resting potential threshold a With regard to those last 2 terms What voltage is resting potential What voltage is threshold Where must threshold be reached for an action potential to occur i The membrane has a resting potential of 70 mV ii The membrane is polarized iii Polarized iv Resting Potential v Hyperpolarized difference in voltage across the membrane of a at rest an electrical gradient is maintained across the plasma membrane negative charge is greater inside the cell cell 70 mV at rest At rest means not conducting an action potential hyperpolarized more polarized More negative than 70 is hyperpolarized less polarized Less negative than 70 is depolarized as cell becomes less negative it is depolarized as cell becomes more negative it is the critical level of depolarization that must be achieved to vi Depolarized b Threshold trigger an action potential 6 What is the sodium potassium pump What does it do What purpose does it serve potential a Acts to maintain proper concentrations of Na and K b Needed for maintaining resting potential and for recovery from action c 3 Na out for every 2 K in so more positive on outside d This pump uses 7 of all ATP in the brain e 7 What is the purpose of myelin What happens at the nodes of Ranvier What is saltatory conduction a Voltage gated Na channels are concentrated at the Nodes of Ranvier b In myelinated axons action potential can jump down axons much faster It allows long distance rapid communication 8 What does it mean that an action potential is all or none a Action potentials are generated in an all or none fashion That is in most but not all neurons subthreshold depolarizations have no effect but once the threshold is crossed an action potential is generated b The rate frequency of action potentials depends on the magnitude and duration of the depolarizing stimulus c There is great variability in the firing frequency of neurons some can only fire a single action potential even to a sustained depolarization other can fire up to a limit of about 1000 Hz Synaptic Potential 3 1 3 2 1 Know the different parts of a synapse presynaptic cell postsynaptic cell synaptic cleft neurotransmitters receptors vesicles 2 Know what happens at a synapse chemical transmission 3 Understand different types of ion channels ligand and voltage gated Where are ligand gated channels located What is a ligand a Voltage gated channel VGC i On the axon ii Open when a cell reaches a certain voltage iii Allows ions in or out 1 Changes voltage of cell b Ligand gated channel LGC i At synapses ii Opens when a ligand neurotransmitter binds iii Allow ions thru 1 Changes voltage of cell 2 May also start biochemical cascade within cell 4 What is an excitatory synapse What is an inhibitory synapse For each one what ion enters the cell What does that do to the cell Does it make an action potential more or less likely a b Excitatory Neurotransmitter allow chloride a negatively charged ion Inhibitory Neurotransmitter into the postsynaptic cell and lead to an inhibitory postsynaptic potential allow sodium and or calcium positively charged ions into the postsynaptic cell and lead to an excitatory postsynaptic potential 5 Know what EPSP and IPSP are How do these differ from an action potential a
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