Action potentials of neurons

2021-06-04 03:28 PM

To generate nerve signals, action potentials travel along with the nerve fibre cell to its termination point.

Action potentials that move along the neuronal membrane transmit nerve signals. Each action potential begins with the sudden change of the negative membrane potential to a positive potential and ends with an equally rapid rate of change back to the negative potential. To generate nerve signals, action potentials travel along with the nerve fibre cell to its termination point.

The upper part of the figure shows the change that occurs in the membrane during the generation of action potential with the transport of a positive charge inside the membrane at the beginning and then returning the positive charge out at the end. The lower part of the figure shows the subsequent change in membrane potential in only a few thousandths of a second, demonstrating the explosive emergence of the action potential and the equally rapid return to the initial state.

Consecutive phases of action potentials.

Figure. Record a typical action potential using the method shown in the upper panel of the figure.

Rest period

The resting phase is the membrane potential before the action potential appears. The membrane is said to be in a depolarized state during this period because the -90mV membrane potential proves it.

Depolarization phase

At this point, the membrane is suddenly very permeable to sodium ions, allowing large amounts of sodium to rush inside the neuron axon. The -90mV polarization state is immediately lost by the positively charged sodium current causing a rapid change in the potential to the positive side. This phenomenon is called depolarization. In large nerve fibres, too much sodium moves inward causing the membrane potential to rise excessively past zero to some positive value. In some smaller neurons, as well as those of CNS cells, the measured potential is only close to zero but does not spike to a positive value.

Repolarization phase

Even within a few thousandths of a second after the membrane has spiked in permeability to sodium, the sodium channel begins to close, and the potassium channel opens wider than usual. The potassium ion then diffuses rapidly outward to re-polarize at rest, which is referred to as the repolarization phase.

To more fully explain the factors that cause depolarization and repolarization, we will describe the characteristics of two types of transport channels across the neuronal membrane: the potential sodium gate and the potassium channel.