Neurological Control

The Action Potential

Neurons  have an elaborate signalling mechanism based on their selective permeability to certain ions and their flow through channels and pumps in the plasma membrane. Resting neurons have a negative membrane potential, caused by a steady outflow of potassium ions and an impermeability to sodium ions, and the action potential  represents transient changes in this resting membrane potential.

For most types of axons , depolarisation initiates the action potential and causes a transient change in the membrane that briefly switches its permeability from that allowing the passage of potassium ions to that allowing the passage of sodium ions. The opening of voltage-sensitive channels in the membrane allows sodium ions to move down the concentration gradient to enter the cell. This produces the rising phase, or upshoot, of the action potential, and means that the membrane potential becomes positive for a short time. The falling phase of the action potential is caused by the subsequent closing of the sodium channels, which reduces the sodium influx, and by the opening of the voltage-gated potassium channels which allows increased efflux of potassium ions from the cell, to restore the negative resting membrane potential. In most nerve cells, action potentials are followed by a transient hyperpolarisation. During this time, the efflux of potassium ions from the cell is greater than during the resting state and, as a consequence, the membrane is hyperpolarised with respect to its normal resting value.