BIo 330: General Physiology 5) Neuron A and Neuron B form on EPSP or IPSP, not a
ID: 3477351 • Letter: B
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BIo 330: General Physiology 5) Neuron A and Neuron B form on EPSP or IPSP, not an action Several measurements action potential, and both the duration and magnitude of the EPSP in t Grim Fall 2017 m synapses on the same postsynaptic cell as shown in Figure 1. When there is a uron B (indicated by the vertical arrow in Figure 1), a postsynaptie potentia ential) results. Several measurements were made, including the equilibrium cell, postsynaptic reversal potentials when either Neuron A or Neuron B fired ai he postsynaptic cell. These data are presented below: . Resting Vm of the postsynaptic cell-.75 EN +50 mV EK =-80 mV Ea -60 mV E- +50 m V (when Neuron A fires) E--80 mV (when Neuron B fires) Post EPSP duration when Neuron A fires 30 msec EPSP duration when Neuron B fires 80 sec EPSP magnitude when Neuron A fires- 1 mV . EPSP magnitude when Neuron B fires-1.5 mV Answer the following questions. Identify the type (not the fon involved) of receptor that binds the NTs from Neuron A. Justify your answer by explicitly referring to ata in addition to your knowledge of neurotransmitter receptorsExplanation / Answer
The muscarinic acetylcholine receptor is a G proteincoupled receptor whose activation leads to opening of K+ channels and subsequent hyperpolarization of the plasma membrane. Like other G proteincoupled receptors, the muscarinic acetylcholine receptor has seven transmembrane helices. Binding of acetylcholine to the receptor activates a trimeric transducing G protein; the released G subunit then directly binds to and opens a particular K+ channel protein. That Gdirectly activates the K+ channel has been shown by single channel recording experiments in which purified G was added to the cytosolic face of a patch of heart muscle plasma membrane. Potassium channels opened immediately on addition of G in the absence of acetylcholine or other neurotransmitters. The K+ channels coupled to muscarinic acetylcholine receptors are tetrameric proteins similar in structure to those that maintain the resting membrane potential.
Unequal distribution of ions across the membrane creates what is called an ionic battery. There are two equations that describe how the different ionic batteries contribute to the membrane potential. They say the same thing in a different way. One is in terms of permeability and the other is in terms of conductance. The Goldman equation is in terms of permeability (Pion). Vm = 58*log((PK[K]o + PNa[Na]o + PCl[Cl]i) / (PK[K]i + PNa[Na]i + PCl[Cl]o)). This is the Nernst equation for each major ion combined into one formula with one additional term, permeability, which determines each ionic battery’s relative contribution to the membrane potential. Permeability is a measure of the relative ease with which an ion can pass through a membrane and is expressed as a value between 0 and 1. Looking at the Goldman equation you can see that the permeability factor gives different weights to each of the three ionic batteries in the equation. So, the ionic battery corresponding to the ion that the membrane is most permeable to will contribute the most to the membrane potential. Permeability and conductance vary in proportion to each other so the Goldman equation can be expressed in a different way when it is rearranged in terms of conductances and equilibrium potentials for each ion: Vm = (ENa*gNa + EK*gK + ECl*gCl) / (gNa + gK + gCl). This equation clearly says that the contribution of each battery to the membrane potential is weighted based on the conductance for that ion. If there is no conductance (g = 0) for an ion then it contributes nothing to the membrane potential. Vrest (-75mV) is much closer to EK (-80 mV) than ENa (+50 mV) because, at rest, the conductance (and permeability) of the membrane to K is much greater than that of Na. At the peak of the action potential waveform the membrane potential is positive (perhaps around + 40 mV) because at this point in time the membrane is more permeable to Na due to the opening of voltage-gated Na channels.
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