Extracellular Recording

Extracellular electrophysiology measures the strength of the electric field at the site of the electrode. Movement of charged particles, such as during synaptic inputs (mainly Na+ through AMPA receptors and Ca2+ through NMDA receptors) and action potentials, contribute to the strength of the electric field. Electrodes pick up these changes as deflections in a baseline signal. We can distinguish between slower changes (the local field potential, LFP) and for instance action potentials from a single cell (single unit activity or spikes). 

When measuring individual action potentials, the deflection caused by an action potential will look different depending on the location of the electrode, as the various transmembrane currents have different directions and contribute with varying weights to the electric field at any point. The amplitude of the spike decreases with distance and filtering causes it to widen. Because small differences in the location of the electrode relative to the cell lead to large differences in spike amplitude or waveform, multiple cells measured by one electrode can be separated.