lect17-08 - Zoology 523 Lecture 17 2008 Readings: pp 129...

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Zoology 523 Lecture 17 2008 Readings: pp 129 – 131; pp 158 – 160; pp 166 – 169 IONIC BASIS OF SYNAPTIC POTENTIALS For the rest of the lectures on synapses we will focus on chemical synapses. In this lecture I will tell you about the currents that flow in postsynaptic cells that receive synaptic input. In lecture 3 we discussed two sorts of post synaptic potentials (PSPs) - excitatory or EPSPs and inhibitory or IPSPs, and at that point we talked of the EPSPs as being depolarizing and the IPSPs as being hyperpolarizing. By the end of this discussion you will see that this was an oversimplification and that some IPSPs can be depolarizing. This paradox will be resolved before you have your next cup of coffee. Let's start by pointing out some obvious ways in which PSPs differ from action potentials (APs), expanding the list you already learned. 1.) First, PSPs have a longer duration than APs. APs are about 2 msec long and PSPs are about 10 msec long. 2.) PSPs propagate only passively. They are generated locally. 3.) APs and PSPs are pharmacologically distinct. e.g., APs are blocked by TTX, not by curare, whereas certain PSPs are blocked by curare, not by TTX. There are many different types of synapse, with different transmitters and each is pharmacologically distinct, but none of them are blocked by TTX, and none of the compounds that block the action of neurotransmitters block APs. I will give you some more information about this about a week from now. 4.) PSPs involve only a single phase of conductance change whereas APs involve 2 -- first a sodium conductance change to depolarize and then a potassium conductance change to repolarize. There is no active repolarization of the membrane in PSPs and that explains item (1.) PSPs have one phase of current flow. APs have 2. In PSPs the potential changes are much longer than the current flows. This is because the current, the inflow of positive charges in the case of an EPSP, depolarizes the membrane just as in an AP. However, the falling phase of the membrane potential is passive, unlike the AP. This gives the PSP a slower time course, because the falling phase is determined only by the passive properties of the membrane, described by Rm, Cm, and the internal resistivity, Ri. This
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This note was uploaded on 03/26/2010 for the course ZOOLOGY 523 taught by Professor Stretton during the Fall '08 term at Wisconsin.

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lect17-08 - Zoology 523 Lecture 17 2008 Readings: pp 129...

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