Resting Membrane Potential: Neurons typically have a resting membrane potential of around -70mV, which is the electrical potential difference across the cell membrane when the neuron is not actively transmitting signals.
Action Potentials and EPSPs: Action potentials are the result of depolarization of the neuron's membrane, and excitatory postsynaptic potentials (EPSPs) are one of the key contributors to this depolarization. EPSPs increase the likelihood of an action potential firing by making the neuron more positive inside relative to the outside.
EEG and Summation of Potentials: Electroencephalography (EEG) detects the electrical activity in the brain, and it reflects the summation of many postsynaptic potentials (EPSPs and inhibitory postsynaptic potentials, or IPSPs) over a relatively large region, often over an area of at least 6 cm² of the cortex.
Superficial EPSPs and IPSPs: Superficial EPSPs (closer to the cortical surface) lead to negative deflections on the EEG, while superficial IPSPs (which inhibit neuronal firing) cause positive deflections. This reflects the direction of current flow: EPSPs depolarize the neuron, causing an influx of positive ions, while IPSPs hyperpolarize it, causing an influx of negative ions or efflux of positive ions.
EEG Waveform Directionality: Negative potentials (such as EPSPs) cause an "upgoing" wave in EEG readings, while positive potentials (such as IPSPs) lead to a "downgoing" wave. This is a result of how EEG electrodes pick up voltage differences across the scalp.
Dipole and Localization: The orientation and strength of the dipole (the difference in voltage between two regions, such as between the inside and outside of a neuron or between different layers of the cortex) play a crucial role in how well a neural discharge can be localized. The closer the dipole is to the EEG electrodes, the more accurately its source can be detected, and the stronger the dipole, the clearer the signal.