The nerve signal, or action potential, is a coordinated movement of sodium and potassium ions across the nerve cell membrane. Here's how it works:
- As we discussed, the inside of the cell is slightly negatively charged (resting membrane potential of -70 to -80 mV).
- A disturbance (mechanical, electrical, or sometimes chemical) causes a few sodium channels in a small portion of the membrane to open.
- Sodium ions enter the cell through the open sodium channels. The positive charge that they carry makes the inside of the cell slightly less negative (depolarizes the cell).
- When the depolarization reaches a certain threshold value, many more sodium channels in that area open. More sodium flows in and triggers an action potential. The inflow of sodium ions reverses the membrane potential in that area (making it positive inside and negative outside -- the electrical potential goes to about +40 mV inside)
- When the electrical potential reaches +40 mV inside (about 1 millisecond later), the sodium channels shut down and let no more sodium ions inside (sodium inactivation).
- The developing positive membrane potential causes potassium channels to open.
- Potassium ions leave the cell through the open potassium channels. The outward movement of positive potassium ions makes the inside of the membrane more negative and returns the membrane toward the resting membrane potential (repolarizes the cell).
- When the membrane potential returns to the resting value, the potassium channels shut down and potassium ions can no longer leave the cell.
- The membrane potential slightly overshoots the resting potential, which is corrected by the sodium-potassium pump, which restores the normal ion balance across the membrane and returns the membrane potential to its resting level.
- Now, this sequence of events occurs in a local area of the membrane. But these changes get passed on to the next area of membrane, then to the next area, and so on down the entire length of the axon. Thus, the action potential (nerve impulse or nerve signal) gets transmitted (propagated) down the nerve cell.
There are a few things to note about the propagation of the action potential.
When an area has been depolarized and repolarized and the action potential has moved on to the next area, there is a short period of time before that first area can be depolarized again (refractory period). This refractory period prevents the action potential from moving backward and keeps everything moving in one direction.
- The action potential is an "all-or-none" response. Once the membrane reaches a threshold, it will depolarize to +40 mV. In other words, once the ionic events are set in motion, they will continue until the end.
- These ionic events occur in many excitable cells besides neurons (like muscle cells).
- Action potentials are propagated rapidly. Typical neurons conduct at 10 to 100 meters per second. Conduction speed varies with the diameter of the axon (larger = faster) and the presence of myelin (myelinated = faster). The rapid nerve conductions throughout the neural circuitry enable you to respond to stimuli in fractions of a second.
- The channels can be poisoned and prevented from opening. Various toxins (puffer fish toxin, snake venom, scorpion venom) can prevent specific channels from opening and distort the action potential or prevent it from happening altogether. Similarly, many local anesthetics (e.g. lidocaine, novocaine, benzocaine) can prevent action potentials from being propagated in the nerve cells in an area and temporarily prevent you from feeling pain.
- The propagation of the action potential is also sensitive to temperature in experimental settings. Colder temperatures slow down the action potential, but this usually doesn't happen in an individual. However, you can use cold-block techniques to temporarily anesthetize an area (like putting ice on an injured finger).
So, if the size of the action potential does not vary, how does an action potential code information? Information is encoded by the frequency of action potentials, much like FM radio. A small stimulus will initiate a low frequency train of a few action potentials. As the intensity of the stimulus increases, so does the frequency of action potentials.
On the next page, we'll learn about how nerves communicate with each other.