Figure 1. Formation of current loops in the extracellular space between pyramidal cell dendrites. Dendrites receive many simultaneous synaptically transmitted signals and will initiate an action potential when the net balance of the afferent excitatory traffic exceeds the inhibitory traffic by a margin that allows a localized patch of its membrane to depolarize to the critical value. If the excitatory activity is somewhat separated physically from the inhibitory activity, the resulting difference in membrane potential will cause a current to flow both inside and outside the membrane acting. The extracellular component of this current, when magnified by summation with the currents from neighboring parallel pyramidal dendrites, creates voltages that are detectable on the scalp as the EEG.