An idea neuron contains the potassium and sodium channels. The intracellular con

Question

An idea neuron contains the potassium and sodium channels. The intracellular concentrations of K+ and Na+ are 139 mM, and 12 mM, respectively. The extracellular concentrations K+ and Na+ are 4mM, and 145mM, respectively. (Assuming K+ -Na+ pumps work continuously to establish and maintain concentration gradients of K+ and Na+ across cellular membrane).

A. Initially, the membrane is only permissible to K+ . An electric impulse triggers an action potential by opening Na+ channel. Assuming both numbers and conductance of Na+ channels are much higher than K+ channels. Use equivalent circuit model to describe how action potential is generated.

B. How will the action potential change during the rising phase assuming conductance of Na+ channels is 10 times less than K+ channels? Will this action potential change sustain action potential propagation assuming minimal change of 50 mV in Vm is required for action potential propagating forward?

C. If gK = gNa, how will the action potential change during the rising phase? Will this action potential change sustain action potential propagation assuming minimal change of 50 mV in Vm is required for action potential propagating forward?

Explanation / Answer

A. The cells of most higher organisms maintain are more negatively charged inside than the cell's exterior. This difference in charge is called the cell's membrane potential. In the process of depolarization, the negative internal charge of the cell temporarily becomes more positive (less negative). This shift from a negative to a more positive membrane potential occurs during several processes, including an action potential. Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane.These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value. When the channels open (in response todepolarization in transmembrane voltage), they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current across the cell membrane, and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential. The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate.

B. Even, if the conductance of Na+ channels is less K+ channels, action potential will be rising till all the voltage gated channels have opened. As the sodium channels close, sodium ions can no longer enter the neuron, and then they are actively transported back out of the plasma membrane. Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state

C. As, it is mentioned, that is the rising phase action potential will increase if all the voltage gated channels wiil open. Then slowly, action potential will decrease coming back to resting phase.

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