Possible midterm 1 questions.docx - 1 The puffer fish toxin tetrodotoxin(TTX completely blocks voltage gated sodium channels thus it blocks the

Possible midterm 1 questions.docx - 1 The puffer fish toxin...

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1. The puffer fish toxin, tetrodotoxin (TTX), completely blocks voltage gated sodium channels , thus it blocks the generation and propagation of action potentials in neurons. Suppose the motor unit describe above were bathed in TTX so that stimulating the neuron did not cause an action potential, thus preventing the neuron from secreting ACh at the synapse. If you added ACh (it is a readily available chemical) to the bathing medium, would this cause the muscle to contract? Why or why not? ANSWER: The muscle will not contract because since the toxin is blocking the sodium channel, the muscle membrane cannot be depolarized , so the muscle cannot contract because depolarization is the first step in muscle contraction. 2. Ouabain is a plant alkaloid that inhibits the Na-K ATPase. What would you predict the effect on action potentials to be if ouabain were applied to a neuron? (Hint: remember what was mentioned early in the course about the relative number of ions that cause electrical changes and that cause changes in ion concentrations.) ANSWER: If Ouabain is applied to a cell, the Na + /K + ATP-ase could not pump sodium out of the cell and potassium into the cell to develop a chemical gradient . This pump is electrogenic and it plays a role in mantaining a negative resting membrane potential. Therefore, when ACh attaches to the sodium/potassium channels to open them to depolarize the membrane, there will be less chemical gradient and electrical potential so sodium will not flow in and potassium flow out. The cell will then be unable to depolarize or create an action potential, stopping the signaling process. 3. You are studying a new drug that has been shown to block voltage-gated potassium channels in motor neurons. Will you be able to measure a depolarization in the synaptic knob if you stimulate the axon over threshold once? Can you stimulate this neuron a second time? Explain your answer Yes, you will be able to measure a depolarization at the synaptic knob. This is because the wave of depolarization associated with an action potential is propagated by the opening of the voltage-gated Na + channels . However, the depolarization seen will not reverse since recovery back to resting membrane voltage levels requires the action of the voltage-gated K + channels (which you have blocked).
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