Ionic Mechanism + Propagation of Action Potentials
The Ionic mechanism and propagation of action potentials.
The action potential is the result of a large, sudden increase in sodium permeability of the membrane. The resulting rush of sodium ions into the membrane and accumulation of positive charge on its inner surface drives the potential towards Ena. This is followed by repolarisation, whereby there is a large increase in the membranes permeability to potassium ions, hence the membrane returns to Ek. Explanation of the (ionic) mechanisms underlying generation of the action potential leads directly to understanding impulse propagation. The main feature underlying the ion currents associated with the action potential is that both the sodium and potassium conductances are voltage dependent. Depolarisation increases the membrane’s conductance to sodium ions and, after a delay, potassium. Hence it becomes more likely for sodium ion channels to open with depolarisation. Following a small depolarisation, the number of sodium ion channels open increases and sodium enters the membrane down its electrochemical gradient. This produces further depolarisation, opening more sodium ion channels, with more rapid sodium entry. This effect of sodium conductance is regenerative and known as positive feedback. At the peak of the action potential there is an overshoot during which the membrane potential becomes positive on the inside. This is because sodium entry across the membrane continues beyond zero membrane potential until sodium equilibrium potential is reached Conversely, the voltage dependence of potassium conductance is self-limiting and involves negative feedback. Depolarisation increases the number of potassium ion channels open and there is an efflux of potassium down its electrochemical gradient. Rather than causing more sodium channels to open, as is the case with sodium, the efflux leads to repolarisation and return of potassium conductance to its resting level. The depolarisation inactivates sodium channels;
The action potential is the result of a large, sudden increase in sodium permeability of the membrane. The resulting rush of sodium ions into the membrane and accumulation of positive charge on its inner surface drives the potential towards Ena. This is followed by repolarisation, whereby there is a large increase in the membranes permeability to potassium ions, hence the membrane returns to Ek. Explanation of the (ionic) mechanisms underlying generation of the action potential leads directly to understanding impulse propagation. The main feature underlying the ion currents associated with the action potential is that both the sodium and potassium conductances are voltage dependent. Depolarisation increases the membrane’s conductance to sodium ions and, after a delay, potassium. Hence it becomes more likely for sodium ion channels to open with depolarisation. Following a small depolarisation, the number of sodium ion channels open increases and sodium enters the membrane down its electrochemical gradient. This produces further depolarisation, opening more sodium ion channels, with more rapid sodium entry. This effect of sodium conductance is regenerative and known as positive feedback. At the peak of the action potential there is an overshoot during which the membrane potential becomes positive on the inside. This is because sodium entry across the membrane continues beyond zero membrane potential until sodium equilibrium potential is reached Conversely, the voltage dependence of potassium conductance is self-limiting and involves negative feedback. Depolarisation increases the number of potassium ion channels open and there is an efflux of potassium down its electrochemical gradient. Rather than causing more sodium channels to open, as is the case with sodium, the efflux leads to repolarisation and return of potassium conductance to its resting level. The depolarisation inactivates sodium channels;