Nadya is running an experiment for her neurophysiology class. She incubates neur
ID: 3477802 • 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.
Myelin content Stimulus intensity Compounds added or removed from the petri dish Neuron diameter Experiment 1 Experiment 2 Experiment Experiment 4 Experiment 5 Experiment 6 Experiment 7 10 m 10 um 100 m 100 um 100 m 100 um 100 um Light None Medium Heavy None Medium Heavy 10 mV 20 mV 40 mV 40 mV 50 mV 60 mV 40 mV Mg2+ added TTX added Nat removed 3 Ca2+ removedExplanation / Answer
A) and B) all experiments apart from experiment 1,3:and 4 an action potential would be generated. Not in experiment 1 because for each of them the stimulus provided is higher than the threshold(15mV) required to initiate an action potential. Not in 3 because TTX (tetrodotoxin) is a toxin that actively bind and inhibits the voltage gated sodium ion channels thereby ceasing generation of action potential.Not in experiment 4 because action potential is generated by the influx of Na+ that causes depolarization. Removing of Na+ remove all chances of QN action potential.
Fastest = experiment 7 (because it is the most myelinated one; experiment 4 too has similar heavy myelination but Na+ has been removed resulting in no current at all.
Slowest- experiment 2 ( because it has least myelin and thinner axonal diameter.)
NOTE : thinner the axon more resistance the action potential faces during conduction and more is the myelination more would be the shielding along the axon allowing the action potential to jump at regular intervals (at the nodes of ranvier) hence increasing the velocity of conduction. The vice versa holds true.
B) lowest - experiment 7
Highest - experiment.
This is because when myelin sheath is absent the voltage gated sodium channels are no longer insulated and hence firing occurs. While in case of myelinated neurons firing would occurs only at the nodes of ranvier decreasing it's frequency.
D) experiments 2, 5 and 7. Because release of neurotransmitters require an influx of calcium. Not in 1,3 and 4 as no action potential is generated and not in 6 because no calcium is present outside for its inflow.
Normal - 7
Increased - 5 ( because stimulus is higher (50mV))
Increased- 2 ( because Mg2+ causes a delayed decay in calcium current)
E) dopamine needs to be reuptaken or else it will accumulate in the synapse constantly stimulating the post synaptic neuron. Hence when Ritalin is taken dopamine transporters no longer take the dopamine in resulting in a constant dopaminergic stimulation of the post synaptic neuron
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