"Come now, let us reason together," says the Lord."
~ Isaiah 1:18
A neuron has the unique ability to transmit electrical signals across long distances along its axon. This is achieved by the propogation of an action potential – a localized, momentary reversal of the cell’s transmembrane potential. What are these “potentials” and how do they allow neurons to transmit information?
A neuron at rest has a slightly negative charge inside compared to the extracellular environment. This is due primarily to the abundance of negative proteins and phosphates inside the cell and an abundance of positively-charged sodium ions (Na+) outside the cell. This uneven distribution of charges produces a measurable voltage difference across the cell membrane called the transmembrane potential. At rest, a typical neuron has a transmembrane potential of -70 mV (millivolts).
An action potential is a localized “flip-flop” of charges (negative-to-positive) across the cell membrane moving down the axon of the neuron. The progression of the action potential down an axon can be visualized as falling dominoes; at the point where the dominoes are falling the action potential is switching from -70 mV to +30 mV, an event called depolarization. In an axon, however, the membrane potential quickly goes back to -70 mV following the action potential (as if the dominoes automatically set themselves back up after falling!). The return to -70 mV is a process called repolarization.
Figure 1 depicts a portion of an axon at rest and three snapshots of the axon as an action potential passes through. As stated above, the resting potential is established by negative ions inside the cell and positive sodium ions outside. There are also positive potassium ions (K+) inside the cell but not enough to overtake the negative charges. An action potential begins as a neuron is stimulated and sodium channels are opened at the site of stimulation. If enough sodium enters to lower the transmembrane potential to -60 mV (i.e., threshold potential), then an action potential will begin. Sodium ions rush into the cell by the millions (driven by a large concentration gradient). As the sodium ions rush in, they bring with them positive charge and the membrane depolarizes. Eventually, the inside of the cell becomes positive. At +30 mV, the sodium channels close and potassium channels open, allowing potassium ions to move out of the cell (also driven by a concentration gradient). Enough potassium leaves the cell to swing the positive charge outside the cell again. Indeed, so much positive charge leaves the cell that it "overshoots" to -90 mV. When the K+ channels close, the membrane potential is reset to -70 mV (the resting potential). Although the resting transmembrane potential is restored to -70 mV, the sodium is inside the cell and potassium is outside. An ATP-driven pump called the Na+,K+-exchange pump (or, Na+,K+ ATPase) pumps sodium out and potassium in to restore the ions to their proper locations.
An action potential is depicted graphically in figure 2.