Question

Describe how an action potential is generated across the axon. Make sure to explain the role of Na+ and K+ channels and pump in this process.

Answer 1

Action potentials are produced by voltage-gated ion channels in the plasma membrane.These channels are inactive when the membrane potential is near the (negative) resting potential of the cell, but they suddenly begin to open if the membrane potential increases to a level of threshold voltage, depolarising the transmembrane potential.

As the membrane potential is increased, sodium ion channels open, allowing the influx of sodium ions into the cell. This is followed by the opening of potassium ion channels that permit the efflux of potassium ions from the cell. The influx of sodium ions in to the cell increases the concentration of positively charged cations in the cell and produce depolarization, where the potential of the cell is higher than the cell's resting potential.

The sodium channels shutdown at the peak of the action potential, while potassium continues to leave the cell. The outwzrd flow of potassium ions decreases the membrane potential or hyperpolarizes the cell. For small potential increases from rest, the potassium current higher than the sodium current and the voltage returns to its normal resting value, typically −70 mV

if the voltage increases over a critical threshold, typically 15 mV more than the resting value, the sodium current dominates over the other. This create a condition whereby the positive feedback from the sodium current activates even more and more sodium channels. Thus, the cell fires, producing an action potential. The frequency at which a neuron elicits action potentials is often called as a firing rate.

Currents produced by the opening of voltage-gated channels in the route of an action potential are typically significantly more than the initial stimulating current. the , duration,amplitude and shape of the action potential are determined largely by the features of the excitable membrane and not by the applied stimulus.

The important ions involved in an action potential are sodium and potassium cations; sodium ions enter the cell, and potassium ions leave, restoring potential. some ions need to cross the membrane for the membrane voltage to change drastically. The sodium and potassium ions exchanged during an action potential, therefore, make a small change in the inside and outside ionic concentrations. The few ions that do pass are pumped out again by the continuous action of the sodium–potassium pump, which, with other ion transporters, balance the normal ratio of ion concentrations across the membrane