The relationship between action potentials and EEG (electroencephalogram) signals provides insight into how neuronal communication contributes to large-scale brain activity. Here's an overview:

Action Potential

  • An action potential is a rapid electrical event in a neuron, consisting of:
    • Depolarization: Voltage-gated Na⁺ channels open, allowing Na⁺ ions to rush into the cell. The membrane potential becomes positive (e.g., from -70mV to +20mV).
    • Repolarization: Voltage-gated K⁺ channels open, allowing K⁺ ions to flow out, restoring the resting potential.
    • Hyperpolarization: A temporary overshoot where the membrane potential becomes more negative than the resting state (e.g., -70mV to -90mV).
  • Action potentials are all-or-nothing events and are key to transmitting signals within a single neuron.

Action Potentials and EEG

  1. EEG Reflects Population-Level Activity:

    EEG does not directly measure individual action potentials. Instead, it captures the summed extracellular voltage changes of many neurons firing together, particularly in the cortex.

  2. Depolarization and Extracellular Voltage:

    During depolarization, Na⁺ ions enter the cell, leaving the extracellular space relatively negative. These voltage shifts contribute to the signals detected by EEG.

  3. Synchrony is Key:

    For action potentials to influence EEG, large groups of neurons must fire in a synchronized manner, creating detectable oscillatory patterns.

  4. Relation to EEG Rhythms:

    Different EEG rhythms (e.g., alpha, beta, theta waves) are associated with the synchronized activity of neuronal networks, which involves not just action potentials but also postsynaptic potentials.

Key Distinction

While action potentials are essential for signal transmission within neurons, EEG signals are more influenced by postsynaptic potentials (excitatory and inhibitory) in cortical pyramidal neurons, as these create the sustained electrical fields EEG can detect.

In summary, action potentials are critical for individual neuronal signaling, while EEG captures the collective electrical activity of neuronal networks, shaped by both action potentials and synaptic events.