PROPAGATION OF ACTION POTENTIAL4. Understand the steps of action potential and how one leads to the next. Howis action potential started and propagated? What ion enters first? Through what type of channel does it enter? What forces drive it inside? Why does that channel close? What channel opens next? What ion moves through that channel? What forces drive that?- At rest: o lots of Na+ and Cl- outside of neurono lots of K+ inside the neurono negatively charged proteins inside the cello restin potential -70mVo maintained by semipermeable nature of the cell membrane and by theNa+/K+ pump- action potentialo occurs when axon hillock depolarizes to threshold (-50mV)o opens Na+ channels Na+ rushes INTO cell (pushed in by concentration and electrical gradients)o Na+ channel causes voltage of the cell to change and actually become positive (~30mV)o Na+ channel close o K+ channel opens K+ LEAVES cell (pushed out by concentration and electrical gradients)o Continues over and over again down the length of the axon- Propagation: transmission of an action potential down an axon by voltage gate ion channels and concentration gradients5. Terms to know and understand: polarized, depolarization, hyperpolarization, repolarization, resting potential, and threshold. (What voltage is resting potential? What voltage is threshold? Where must threshold be reached for an action potential to occur?- Polarized: inside of cell is negatively charged compared to outside of cell due to unequal distribution of ions (-70mV)- Depolarization: inside of cell becomes less negative (>-70mV)o Caused by positive ions coming into cell - Hyperpolarization: inside of cell becomes more negative (<-70mV)o K+ channels let more K+ out during action potential- Repolarization: inside of cell returns to polarized level from hyperpolarized level (-70mV)o Na/K pump- Resting Potential: membrane is polarized (-70mV)o Maintained by semipermeable membrane- Threshold: critical level of depolarization that must be achieved to trigger an action potential (-50mV)o threshold must be reached at axon hillock6. What is the sodium/potassium pump? What does it do? What purpose does it serve?- Na+/K+ pump: acts to maintain proper concentrations of Na+ and K+o Needed for maintaining resting potential and for recovery from an action potentialo 3 Na + out for every 2 K+ in (more positive on the outside)7. What is the purpose of myelin? What happens at the nodes of Ranvier? What is saltatory conduction?- Myelin provides insulation - Voltage-gated Na+ channels are concentrated at the Nodes of Ranviero At some cases the action potential starts at the axon hillock, in others it starts at the nodes of ranvier- Salutatory conduction—energy and speedo The jumping of action potentials from node to nodeo Provides rapid conduction of impulseso Conserves energy: instead of admitting sodium ions at every point along the axon and then having to pump them out via sodium/potassium pump, a myelinated axon admits sodium only at itsnodeso Timing of action potentials is often essential for coding, so speed matters Auditory system and sound localization Opening of Na+ channels in chain reaction down axonso In myelinated axons, action potential can “jump” down axon Faster Allows long distance rapid communication8.What does it mean that an action potential is “all or none”?- Action potentials are generated in an “all-or-none” fashion.o Most (but not all) neurons, subthreshold depolarizations have no effect, but once the threshold is crossed, an action potential is generatedo The rate (frequency) of action potentials depends on the magnitude and duration of the depolarizing stimuluso There is great variability in the firing frequency of neurons—some can only fire a single action potential even to a sustained depolarization others can fire up to a limit of about 1000Hz1. SYNAPTIC POTENTIALKnow the different parts of a synapse (presynaptic cell, postsynaptic cell, synaptic cleft, neurotransmitters, receptors, vesicles)2. Know what happens at a synapse (chemical transmission).Presynaptic terminalsome postsynaptic cells send reverse messages to control the further release of NT by presynaptic cellssome postsynaptic cells send reverse messages to control the further release of NT by presynaptic cellsNT molescules may be taken back into the presynaptic neuron for recycling or they may diffuse awayNT molescules may be taken back into the presynaptic neuron for recycling or they may diffuse awayneurotransmitter molecules separate from their receptorsneurotransmitter molecules separate from their receptorsreleased molecules diffuse across teh cleft, attach to receptors, alter the activity of postsynaptic neuronreleased molecules diffuse across teh cleft, attach to receptors, alter the activity of postsynaptic neuronaction potentials travel down the axonaction potentials travel down the axonpresynaptic terminal: action potential enables Ca2+ to enter cellcalcium releases neurotransmitters from the terminals and into the synaptic cleft (space between pre and post)neuron synthesizes chemicals that serve as neurotransmittersneuron synthesizes chemicals that serve as neurotransmitterssynthesizes the smaller neurotransmitters in the axon terminals and neuropeptides in the cell body3. Understand different types of ion channels (ligand- and voltage-gated). Where are ligand-gated channels located? What is a “ligand”?Voltage-gated channel (VGC) Ligand-gated channel (LGC)- On the axon - At synapses- Open when cell reaches a certain voltage- Open when a ligand (neurotransmitter) binds- Allows ions in or outo Changes voltage of cell- Allows ions througho Changes voltage of cello May also start biochemicalcascade within cell4. 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? - Excitatory synapse:o Depolarization of the membrane of the postsynaptic cello Decreases the negative charge within the cello Na+ flow into neurono Makes action potential more likely—closer to the threshold- Inhibitory synapse:o Hyperpolarization of the membrane of the postsynaptic cello Increases the negative charge within the cello K+ leaves cell, Cl- enters cello Makes action potential less likely—further from the threshold5. Know what EPSP and IPSP are. How do these differ from an action potential?- Excitatory postsynaptic
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