Describe the ionic basis of an action potential, and how this is reflected on the cell's membrane potential (6 marks)

At rest (steady state) there is a greater concentration of Na+ ions and a smaller concentration of K+ ions on the outside compared to the inside of the cell, and the membrane potential is about -70mV. Upon depolarisation, voltage-gated Na+ channels open, allowing Na+ ions to move inside the cell (under the influence of a strong electrochemical gradient). Once enough Na+ ions have entered, and a threshold potential has been reached (-55mV), a positive feedback loop is set up whereby the more the membrane depolarises, the more voltage-gated Na+ channels open, allowing more Na+ ions to enter and cause further depolarisation. This carries on until the voltage-gated Na+ channels close, in a time-dependent manner. This is usually at a time when the membrane has reached a potential of +35mV. This wave of depolarisation carries on across the length of the entire membrane of the cell in a similar manner.
The depolarisation that caused the Na+ channels to open and thereby induce further depolarisation, also causes voltage-gated K+ channels to open. However, said channels are slower to open, and their effect is only seen when the Na+ channels close. The electrochemical gradient acting on K+ is such that it causes K+ ions to move out of the cell once the K+ channels open. This causes the membrane potential to drop. K+ channels close in a voltage-dependent manner, and given how slow they are to respond to voltage stimuli, they only close after the membrane has been hyperpolarised (-90mV).
Then, when both the Na+ channels and K+ channels have been closed, there is no further movement of ions and the membrane potential is at -90mV, the Na+/K+-ATPase acts to restore the chemical gradients of the ions. It pumps 3Na+ ions out of the cell and 2K+ into the cell, restoring their concentrations to their original values and taking the membrane potential back to -70mV.