Physiologic Basis of the EEG Signal

Abstract

Listen

Translate article

The EEG is the recording of the spontaneous electrical activity generated by cerebral neurons. Like all other cells in the body, neurons have high concentrations of potassium (K+) and chloride (Cl−) ions inside, while high concentrations of sodium (Na+) and calcium (Ca2+) ions are kept outside. This leads to a voltage difference of about −60 to −70 mV with respect to the outside of the cell membrane. Such a voltage difference is modified by the flux of ions depending on the opening and closing of ion channels induced by electrical or chemical stimuli. A reduction of charge separation across the membrane, due to an influx of positive charged ions into the cell, results in a less negative membrane potential and is termed depolarisation, whereas an increase in charge separation leading to a more negative membrane potential is called hyperpolarisation. When a critical amount of Na+ enters the cell and the membrane potential reaches a threshold level, the opening of additional Na+ channels is facilitated leading to a sudden marked increase of depolarisation. This fast depolarising event corresponds to the rising phase of the action potential, while the falling phase is related to an outflux of K+ ions which, combined with a decrease of Na+ influx, induces a repolarisation of the cell. After an action potential, there is a transitory inactivation of Na+ channels that causes a refractory period during which another action potential cannot be generated.