Action Potential
Fall 2002

An action potential is a rapid change in the voltage across a cell membrane caused by the movement of ions. An AP occurs when Na+ channels open. Na+ is driven into the neuron by concentration and electrical forces. Moving Na+ into the cell makes the intracellular space less negative. These channels are voltage gated. That is, if the neuron is depolarized to -55 mV, then Na+ channels open and Na+ rushes in. This is an all or none process.
 

Current flow is shown by a Voltage graph (Figure 4.1). The graph shows the change in potential (current flow) over time. Voltage changes with the movement of ions (Figure 4.4 & 4.5). For example:

1) Depolarization can be caused by moving Na+ or K+ in, or Cl- out
2) An action potential is caused by opening channels that lets Na+ in (Figure 4.8)
3) Return to the Resting Potential is caused by moving K+ out (Figure 4.9)
4) Refractory period is the time necessary for the neuron to recover before firing another AP. Neural communication requires two things:
  • Message travels within a neuronócalled an action potential
  • Message travels between neuronsó(next week)


    • So far, all we have talked about is ion flow in a single place in a neuron. The AP moves down the membrane like a wave (Figure 4.10). There are ion channels throughout the neuron. A voltage change in one part causes neighboring channels to open. This cause the AP to be regenerated along the membrane. The refractory period prevents the AP from moving backwards.
     

    An AP can move along neuron at about 2 m/s. This is very slow especially in long neurons like the ones that run from your toe to your brain (i.e., 2 meters).

    One way to make the AP move faster is to wrap the neuron in insulation. This insulation is called myelin (Figure 4.11 & 4.12). Myelin is formed by glia (Figure 2.22). Because of the myelin, current flows inside the neuron to nodes of Ranvier where APs are generated. The "jumping" of the AP from one node to the next is called Saltatory conduction. It is very fast (120 m/s).
     

    Multiple Sclerosis is caused by the destruction of myelin surrounding a neuron. This causes the AP to short out (i.e., the AP does not travel the length of the neuron because the current leaks out across the membrane).
     

    Return to Psych 372 Syllabus