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Sakai
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NSO Test

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<p>Bernard Katz and his colleagues were pioneers in investigating mechanisms of synaptic transmission at the neuromuscular junction. They suggested that the channel opened by ACh was one that had equal permeability to both Na<sup>+</sup> and K<sup>+</sup>. Because it was equally permeable to Na<sup>+</sup> and K<sup>+</sup>, Katz suggested that, as a result of the opening of these channels, the membrane potential would move toward 0 mV. (A value of alpha in the <a title="Membrane Potential Laboratory (Lab)" href="https://uth.instructure.com/courses/349 ... ratory-lab" target="_blank" rel="noopener" data-api-endpoint="https://uth.instructure.com/api/v1/cour ... ratory-lab" data-api-returntype="Page">GHK equation</a> equal to one, which when substituted into the equation, yields a potential of about 0 mV.)</p>
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<td style="text-align: center;">Figure 4.9</td>
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<p>The experiment shown in the figure on the left tests that concept. The muscle cell has been penetrated with a recording electrode as well as another electrode that can be connected to a suitable source of potential in order to artificially change the membrane potential. Normally, the membrane potential is about -80 mV [Skeletal muscle cells have higher (more negative) resting potentials than most nerve cells.] Again, a small amount of curare is added so that the EPP is small. Katz noticed in these experiments that the size of the EPP changed dramatically depending upon the potential of the muscle cell. If the membrane potential is moved to 0 mV, no potential change is recorded whatsoever. If the membrane potential is made +30 mV, the EPP is inverted. So three different stimuli produce endplate potentials that are very different from each other.</p>
<p>The lack of a response when the potential is at 0 mV is particularly informative. Consider why no potential change is recorded. Presumably, the transmitter is being released and binding to the receptors. The simple explanation for a lack of potential change is that the potential at which the opening of ACh channels are trying to reach has already been achieved. If the membrane potential is made more positive than 0 mV, then the EPP is inverted. No matter what the potential, the change in permeability tends to move the membrane potential towards 0 mV! If the resting potential is more negative than 0 mV, there is an upward deflection. If it is more positive, there is a downward deflection. If it is already at 0 mV, there is no deflection.</p>
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<td style="text-align: center;">Figure 4.10</td>
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<p>This potential is also called the reversal potential, because it is the potential at which the sign of the synaptic potential reverses. The experiment indicates that, as a result of ACh binding to receptors, specific channels become equally permeable to Na<sup>+</sup> and K<sup>+</sup>. This permeability change tends to move the membrane potential from wherever it is initially towards a new potential of 0 mV.</p>
<p>Why does the normal endplate potential never actually reach 0 mV? One reason is that the sequence of permeability changes that underlie the action potential "swamp out" the changes produced by the EPP. But even if an action potential was not triggered, the EPP still would not reach 0 mV. This is because the ACh channels are only a small fraction of the total number of channels in muscle fibers. The K<sup>+</sup>&nbsp;channels that endow a muscle cell with its resting potential are present as well. Their job is to try to maintain the cell at the resting potential.</p>
<p>The channel opened by ACh is a member of a general class of channels called ligand-gated channels or ionotropic receptors. As illustrated in Figure 4.10, the transmitter binding site is part of the channel itself. As a result of transmitter binding to the receptor (generally two molecules are necessary), there is a conformational change in the protein allowing a pore region to open and ions to flow down their electrochemical gradients. Additional details of the channel are presented in <a title="S1.11.1 Introduction" href="https://uth.instructure.com/courses/349 ... troduction" target="_blank" rel="noopener" data-api-endpoint="https://uth.instructure.com/api/v1/cour ... troduction" data-api-returntype="Page">Chapter 11</a>.</p>
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~S~

~Sakai
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Re: NSO Test

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<p class="pageText"><font size="-1">Figure 1.1</font><br />
<font size="-1">Tap the colored circles (light stimulus) to activate.</font></p>
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~S~

~Sakai
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