3) Neuronal Signaling and Recording (10 points): The ionic concentrations inside

Question

3) Neuronal Signaling and Recording (10 points): The ionic concentrations inside and outside of a neuron in the brain are given in the table below Extracellular Concentration (mM Intracellular Concentration (mM) 13 100 15 Ion Chloride Potassium Sodium Relative Permeability 0.45 1.00 0.04 150 a) You insert a sharp pipette filled with 1 M sodium chloride solution and approach the neuron at the axon hillock. Once you are next to the axon hillock, you release the pipette solution with a brief pressure pulse. How will the neuron respond to the released ions? Assume that released pipette solution mixes 50:50 with the extracellular solution around the axon hillock. (5 points) b) If the pipette had 0.1 M potassium chloride solution, what would happen to the neuron? Assume that released pipette solution mixes 50:50 with the extracellular solution around the axon hillock. (5 points)

Explanation / Answer

a)

It is given that the intracellular and extracellular sodium concentration increases equally (50:50). Note that it does not matter if the extracellular sodium concentration increases; this is because the concentration is already high and it contributes to the resting membrane potential. Also according to ‘all or none law’, extra ionic concentration does not contribute in any way to the action potential; only the threshold value is enough.

Now, an increase in the intracellular sodium concentration disturbs the resting membrane potential. There is a slight positive charge on the inside and a slight negative charge on the outside of the axon hillock. This will further result in an action potential; which will start travelling downhill.

b)

If there were potassium ions instead of the sodium ions, then the outside and inside concentration of potassium will be increase. This will result in return of the resting membrane potential; this phenomenon is known as repolarization.

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