Nadya is running an experiment for her neurophysiology class. She incubates neur
ID: 3478547 • Letter: N
Question
Nadya is running an experiment for her neurophysiology class. She incubates neurons with different characteristics in a petri dish with different compounds present, stimulates the initial segment of the axon with an electrode and looks at action potential conduction and neurotransmitter release. In the petri dish, the solution mimics the extracellular fluid.
Her results are summarized in the table above. Assume that a depolarization of 15 mV will reach threshold for all of neurons.
A. List all the experiments where you would expect an action potential to be propagated. Identify in which experiment the action potential propagation would be the fastest and in which experiment it would be the slowest and explain why.
B. Explain why action potentials wouldn’t be propagated in the other experiments.
C. Identify the experiment where you would expect the lowest and highest frequency of action potential generation and why.
D. List all the experiments where you would expect neurotransmitter to be released at the axon terminal and state whether this neurotransmitter release would be normal, increased or decreased. Explain why neurotransmitter would not be released when appropriate.
E. Methylphenidate (also known as Ritalin®) is a medication prescribed to treat ADHD (attention-deficit & hyperactivity disorder) and works as a dopamine reuptake inhibitor. Knowing that dopamine is an excitatory neurotransmitter, what effects would this drug have on action potential generation in the post-synaptic neuron and why.
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Explanation / Answer
It is important to analyse the effect of various factors involved in these experiments to find the correct solutions.
Considering all these factors, the answers for the questions are as follows:
A.The action potentials will be propagated in Experiment no. 2, 5, 6, 7.
Experiment no. 7 will have the fastest conduction. The stimulus intensity of 40mV will help the neuron to quickly overcome the threshold of 15mV. In addition to that, the neurons of EXP.7 have heavy myelination as compared to neurons in EXP.6. Thus, the thickness of myelin sheath will help in fast conduction of action potential.
Experiment no. 2 will have the slowest conduction of action potential. First, the stimulus intensity is low but enough to overcome the threshold potential. However, the neurons are demyelinated (i.e no myelin sheath), so the action potential will travel throughout the length of the axon. As a result, even though an action potential will be propagated, it will be slowest of all.
B.The action potentials will not be propagated in following experiments:
C.Frequency of action potential generation depends directly on stimulus intensity. Therefore, highest will be Experiment 6 and lowest in Experiment 7.
D.Neurotransmitter release can be expected in Experiment 2, 5 and 7. The absence of calcium ions in the ECF environment of Experiment 6 will result in lack of neurotransmitter release from the neurons of Experiment 6.
E.Dopamine reuptake inhibitors act to stop the action of dopamine transporter. This leads to increase in dopamine concentrations in the ECF. Now, since, dopamine is an excitatory neurotransmitter, it will cause the postsynaptic neurons to depolarise and carry the nerve signal forward. Thus, Methylphenidate acts by producing EPSP (Excitatory Post Synaptic Potentials).
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