Impulse Propagation

Action Potential Propagation. Three stages illustrate the propagation of an action potential past a point on an axon at which microelectrodes have been positioned to record intracellular and extracellular potentials, each with respect to “ground” (the bath fluid). The intracellular electrode records the transmembrane potential, while the extracellular electrode records the much smaller voltage changes produced by the flow of current through the extracellular fluid.

Stage 1. The nerve impulse is approaching the recording point from the left. Inward current flow at the active region gives rise to compensatory outward current flow through a section of axonal membrane on either side of the active region. (The inward flow of sodium ion [Na⁺] current in excess of that required to charge the membrane capacitance must be balanced by the outward flow of other ionic currents.) The outward current flow is passive in that it is not initiated by a change in membrane permeability, as is the inward Na⁺ current. According to Ohm’s law, such a passive flow of outward current through the membrane resistance causes a voltage drop that depolarizes the axonal membrane at the recording point. The intracellular electrode therefore records depolarization of the membrane, and the extracellular electrode records a positive voltage shift caused by the outward flow of current away from the recording point.

Stage 2. As the activity approaches, the transmembrane depolarization at the recording point becomes greater, until it reaches the threshold for action potential initiation. At this point, the membrane becomes active. The passive outward current flow shifts to an active inward flow of Na⁺ current. In accordance with this reversal in the direction of current flow, the voltage recorded by the extracellular electrode shifts from positive to negative. Rather than changing sign, however, the intracellular potential moves farther in the depolarizing direction. This happens because the inward current flow is caused by a change in membrane permeability to Na⁺, which shifts the membrane potential toward E_Na+ (+50 mV).

The strong flow of inward current at the recording point gives rise to a passive flow of outward current through the axonal membrane to the right and to the left. Depolarization caused by this current triggers an action potential in the axon to the right. Re-excitation of the axon to the left does not occur immediately because the membrane is temporarily refractory as a result of the passage of the nerve impulse.

Stage 3. The axon to the right has become active, while the potential at the recording point has fallen to −75 mV. This takes place because Na⁺ inactivation has returned Na⁺ permeability to a low level, and potassium ion (K⁺) permeability has increased, thus moving the potential toward E_K⁺ (−90 mV). The increase in K⁺ permeability and the active zone to the right give rise to an outward current flow, which is revealed by a final positive extracellular voltage. Because of the altered permeability of the membrane to K⁺ and Na⁺ inactivation during the refractory period, this outward current cannot give rise to another action potential.