Common Traits of Action Potentials

• Action potentials synchronize the contraction of microscopic fibers within muscle cells.Comstock Images/Comstock/Getty Images
  
  An action potential is ultimately a wave of membrane depolarization (and subsequent repolarization) that passes from one end of a cell to the other. This is caused by the flow of positively-charged sodium and potassium ions across the cell's membrane via specialized ion channels. An influx of sodium depolarizes the membrane, making it more positive. This causes an efflux of potassium, which repolarizes (and, in fact, hyperpolarizes) the membrane, making it more negative.
  
  Once initiated, an AP is an all-or-nothing event; either the AP fires or the cell remains silent with no state in-between. All APs are identical in a given cell because the pattern of ion flow is a property of ion channels rather than the initial cause of the AP. Therefore, it is impossible to determine the cause of an AP simply by measuring the membrane potential of the cell. Rather, the cell must be carefully analyzed to see what stimuli it reacts to.
  
  

Electrochemical Action Potentials

• Several neurotransmitters are derived from a class of molecules called amino acids.Jupiterimages/BananaStock/Getty Images
  
  The most common initiator of action potentials is electrochemical activity. This is caused when a cell is depolarized (excited) by a chemical signal. In neurons, this happens at special junctions between nerve cells, known as synapses.
  
  In the synapse model, a transmitting (presynaptic) neuron releases a chemical signal called a neurotransmitter, which activates the receiving (postsynaptic) neuron by opening special neurotransmitter-gated ion channels, causing depolarization. If enough depolarization occurs, the change in voltage causes a population of special voltage-gated ion channels to open near the base of the neuron. This is what initiates the AP.
  
  Thus, an initial depolarization is triggered by neurotransmitters opening ion channels, which in turn triggers an action potential via voltage-gated ion channels. This form of AP is ideal for neurons because they can be readily strengthened or weakened, which is the basis for learning in the brain.
  
  

Electrical Action Potentials

• Direct electrical stimulation can cause action potentials in neurons that normally only respond to chemical stimuli.Hemera Technologies/AbleStock.com/Getty Images
  
  While electrodes can certainly be used to induce action potentials in brain cells, the nervous system already has a system of electrical activation between some neurons. Although most neurons are driven by chemical synaptic activity, some are activated by gap junctions. These are large pores that join two cells.
  
  At these junctions, the AP moves seamlessly from one cell to the next due to electrical current travelling uninterrupted within the cells' shared fluid. If this current induces enough depolarization in the downstream cell, the AP continues unabated. Some cells may require concurrent excitation by multiple gap junctions (or by a combination of gap junctions and synapses) to activate.
  
  

Intrinsic Action Potentials

• Ganglion cells of the retina encode varying degrees of color or brightness by firing at variable rates.Jupiterimages/Photos.com/Getty Images
  
  Certain cells, such as "bursting" neurons of the hippocampus, do not require external activation to express action potentials. These cells have a unique phenotype (physical state) that causes them to fire in periodic spikes or bursts. Rather than causing APs per se, the electrochemical activity of presynaptic cells causes the intrinsic activity of the neuron to speed up or slow down. This type of signaling allows neurons to transmit complex signals encoded by the rate or pattern of firing rather than the all-or-nothing firing of a single AP.