lect19-08 - Reading for lectures 17 20 pp129 131 158-160...

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Reading for lectures 17 - 20: pp129 – 131, 158-160, 166-169, 171-173, 206 - 224 Zoology 523 Lecture 19 2008 The difference between quantal content and quantal size. It is very important to get this straight in your mind, otherwise you will be floundering. The quantal content (given the symbol m) is the average number of quanta in a PSP. Experiments have shown pretty convincingly that each quantal event is the fusion of a synaptic vesicle with the presynaptic membrane and the release of its payload of transmitter. So m is the average number of such release events when the presynaptic nerve is stimulated and actives the terminal. Quantal size is the amplitude of each m.e.p.p. I didn't present you with the Poisson distribution JUST to torture your delicate minds, delicious as that prospect is. One of the very important features of the way the nervous system works is that synaptic transmission is subject to modulation. A wide variety of mechanisms have been evolved to change the size of PSP's. Among other things, such mechanisms have been shown to be important in memory and learning - we'll talk about this in more detail in a couple of weeks. One of the most important ways of sorting out which mechanism is at work in a particular case is to examine quantal content and quantal size. A change in a synapse can be in either of the two cells involved, the presynaptic neuron or the postsynaptic neuron. Looking at quantal content and quantal size helps you figure out which is changing. Quantal size can depend on postsynaptic changes like altering the number or sensitivity of postsynaptic receptor for the neurotransmitter, or by changing the input resistance of the postsynaptic cell, and these are the most common ways postsynaptic cells change. Though this is rare, quantal size can also depend on how many transmitter molecules are packed into a vesicle (this is the one people often confuse with quantal content) - in this case, this is a presynaptic change. Quantal content, on the other hand, is a purely presynaptic property, depending on the release mechanisms of the presynaptic terminal. (A common source of changes in quantal content is a change in the calcium current in the presynaptic terminal). If you observe a change in m while the quantal size stays constant, you safely conclude that the effect is presynaptic. The number of ACh molecules in a quantum. Quantal events at the neuromuscular junction are not due to the action of single molecules of transmitter (see notes from last lecture for the evidence): quanta are due to the action of multimolecular packets of ACh. How many molecules of ACh are there in each quantum? This determination was carried out by Doju Yoshikami and Stephen Kuffler. Their idea was to use an
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lect19-08 - Reading for lectures 17 20 pp129 131 158-160...

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