Excitatory Postsynaptic Potential - Field EPSPs

Field EPSPs

EPSPs are usually recorded using intracellular electrodes. The extracellular signal from a single neuron is extremely small and thus next to impossible to record in the human brain. However, in some areas of the brain, such as the hippocampus, neurons are arranged in such a way that they all receive synaptic inputs in the same area. Because these neurons are in the same orientation, the extracellular signals from synaptic excitation don't cancel out, but rather add up to give a signal that can easily be recorded with a field electrode. This extracellular signal recorded from a population of neurons is the field potential. In studies of hippocampal LTP, figures are often given showing the field EPSP (fEPSP) in stratum radiatum of CA1 in response to Schaffer collateral stimulation. This is the signal seen by an extracellular electrode placed in the layer of apical dendrites of CA1 pyramidal neurons. The Schaffer collaterals make excitatory synapses onto these dendrites, and so when they are activated, there is a current sink in stratum radiatum: the field EPSP. The voltage deflection recorded during a field EPSP is negative-going, while an intracellularly recorded EPSP is positive-going. This difference is due to the relative flow of ions (primarily the sodium ion) into the cell, which, in the case of the field EPSP is away from the electrode, while for an intracellular EPSPs it is towards the electrode. After a field EPSP, the extracellular electrode may record another change in electrical potential named the population spike which corresponds to the population of cells firing action potentials (spiking). In other regions than CA1 of the hippocampus, the field EPSP may be far more complex and harder to interpret as the source and sinks are far less defined. In regions such as the striatum neurotransmitters such as dopamine, acetylcholine, GABA and others may also be released and further complicate the interpretation.