Lecture 17 Outline of Last Lecture I. Resting Membrane Potential Outline of Current Lecture II. Action Potential III. Synapses !Current Lecture !Action Potential !**Clicker Question** !I. Action potentials can happen in many types of cells II. Membrane Potentials exist in every cell III. action potentials involve a hyper polarization followed by a depolarization IV. Only happen once the membrane has reached the threshold potential !Answer = II & IV so D (I isn’t true because its not “many”) -muscle cells can have an action potential, a certain kind of algae can (so very few cells really) -action potential approach Nernst potentials, they don’t change them !Action potential — the membrane potential has to depolarize a little bit (reaching the threshold) or the dashed line in the diagram. When it reaches the threshold potential, voltage grated sodium channels open, causing the membrane potential to move towards the Nernst potential for sodium (white line in diagram) = depolarization, becoming more positive -as it rapidly depolarizes and sodium channels open, some ions of Na go through which moves to Na Nernst potential -when it reaches the Nernst potential, the sodium channels inactivate (not just closed, means that they can’t be opened for a little while) and the potassium voltage channels open — some potassium moves through (inside to outside) of cell -membrane potential falls and it is hyper-polarizing (so going towards negative) — and then it over shoots or goes beyond the normal resting potential -then the leak channels for potassium restore the normal resting potential BIO 152 1st Edition!!!These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute. !Edited with the trial version of Foxit Advanced PDF EditorTo remove this notice, visit:www.foxitsoftware.com/shopping!-refractory period is mostly due to the sodium channels being inactivated, meaning that they are unable to respond to a new action potential (longer period than hyper polarization) !**Clicker Question** !During the falling phase of an axon action potential - K+ channels are open, and the Na channels are inactivated (cannot be reactivated, closed can be reopened so they’re NOT closed) !Simulation: -shows action potential coming in, positive outside, negative inside (shows concentrations of both Na and K) -red channels open first (sodium voltage gated channels) — sodium comes in **notice the amount of change in concentration of the Na** = almost nothing -then the green, potassium gated channels open and K goes out (repolarization) — nano molar change (NOTHING) !**Clicker Question** What significant facet of an action potential is missing from this simulation? -There is no threshold shown -do have hyperpolarization, voltage-gated channels do inactivate !**Clicker Question** Giant squid axon has the following ion concentration **see slide** How would the action potential be affected if half of the Na were replaced by choline? -the depolarization wouldn’t get as positive -the Nernst potential would be lower, so when you open the sodium gated channels -choline doesn’t directly change the Nernst potential, but half of the sodium has been replaced — so the sodium concentration has been lowered (it’s not the choline itself, just know that the sodium itself now is a lower concentration which is why the Nernst potential would be lower and it would not get as positive) !**Clicker Question** What is likely the molecular problem with these shaker flies? -B would mean that the sodium channels close or inactivate, but you get very slow re polarization because the K voltage channels are slower to open, leading to a much more gradual re-polarization as seen in the graph on the bottom (the graph of the shaker flies) -molecular reason is actually B, but C could be correct as well !**Clicker Question** Tetrodoxoxin TTX added to a nerve cell will cause effect seen at right. The pre is without the TTX and is offset (so normalized to a different resting potential) What does TTX do? -Blocks the sodium voltage gated channel — C is the correct answer -doesn’t block the Na/K ATPase because the membrane potential isn’t changed so A is wrong!-this is the toxin in pufferfish — they don’t get poisoned because they have different Na gated channels K+/NA+ ATPase channel pumps sodium out of the cell and potassium into the cell in order to create the gradient of the membrane (using ATP to use the pump mechanism) and is one of the primary uses of ATP in all of the cells in your body !TAKE HOMES !Synapses !-Once the signal gets to the end of the axon it needs to move on to the next cell -can synapse to a cell body, can also connect to a dendrite -synapse includes both the pre and post synaptic cell and the synaptic space/cleft -signal goes from electrical signal (action potential) to a chemical signal (neurotransmitter) and back to electrical -benefit of doing this is that you can bring the signal back up again or make it stronger, picks up the signal and “starts it over” or brings it back up -it allows a change in meaning — a single cell could synapse to 100 different neuron and integrate all that information -by allowing multiple neurons to come in and synapse means that the neuron can integrate all that info and “decide” whether or not to fire As membrane potential reaches axon terminal, the depolarization thats coming in opens voltage gated calcium channels that allow calcium in - causes exocytosis of neurotransmitter which dumps tons of that n. transmitter into the synaptic cleft -can start a standard signaling cascade (signaling cascade) -can also do ionotropic things -n. transmitter binds to channel on post synaptic neuron, which then depolarizes the postsynaptic membrane !**Clicker Questions** What would be the effect on the membrane potential of opening an Na channel in the postsynaptic membrane? -The membrane should depolarize = very quick way of starting the action potential (well sends info the the cell body which then decides if there will be an action potential) What would be the effect on the membrane potential of opening Cl- channels in the postsynaptic membrane? -The membrane would be hyper polarized -makes the cell less likely to fire !Any given neurotransmitter can be excitatory in some instances, and