POSTSYNAPTIC POTENTIALS (PSPs): THE SOURCE OF EEG SIGNALS

Introduction

EEG signals are generated by a complex series of synchronized post synaptic potentials. In order to understand how EEG picks up postsynaptic potentials, we need to dive deeper into how these are electrical signals are generated.

Postsynaptic Potentials

When a neuron receives input from other neurons, it happens mainly at its dendrites and cell body. This input generally comes in the form of neurotransmitters, which are chemical signals that either excite or inhibit the neuron. When neurotransmitters bind to receptors on the dendrite, they may open ion channels, allowing charged particles (ions) to flow across the neuronal membrane. The movement of ions creates an electrical change called a postsynaptic potential (PSP).

Excitatory Postsynaptic Potentials (EPSPs)

EPSPs occur when positive ions enter the neuron, making the inside more positive and bringing the neuron closer to firing an action potential. When the charge inside of the neuron becomes more positive, this process is called depolarization.

Inhibitory Postsynaptic Potentials (IPSPs)

IPSPs occur when negative ions enter or positive ions exit the neuron, making the inside of the neuron less positive and reducing the likelihood of the neuron firing an action potential. When the inside of the neuron becomes less positive, this process is called hyperpolarization. 

Synchronized Activity and Large-Scale Field Potentials

Like an action potential, postsynaptic potentials are quite small and on their own can not be detected by an electrode on the scalp. However, when populations of neurons in the cortex have synchronized PSPs, their combined PSPs produce electrical fields large enough for detection by an EEG electrode.

Pyramidal neurons are especially important for creating an EEG signal. This is due to their alignment and structure. Pyramidal neurons have long vertical dendrites that extend from the cell body near the deeper layers of the cortex up toward the surface of the cortex. When a large group of these particular neurons are activated by PSPs, the electric fields can be detected by EEG.

Volume Conduction

The electrical fields generated by PSPs travel through the surrounding tissue in a process called volume conduction. The surrounding tissue and the skull act as a filter before the electrical fields can propagate to the electrode. These physical barriers will block out higher-frequency signals which is why EEG doesn’t typically pick up cortical activity greater than 100 Hz.

Detection by EEG Electrodes

EEG electrodes on the scalp are measure voltage of the extracellular space. This means, that if a neuron is depolarized and the inside of the cell becomes more positive, the area surrounding the neuron will become more negative. In this case, it would be this more negatively charged area that is being detected by the electrode.

As described above, pyramidal neurons in the cortex have dendrites deep in the cortex that extend near the surface of the cortex. These dendrites essentially form a long line from deeper in the brain to the surface. When PSPs occur on these dendrites, the signal they produce can very depending on where along the dendrite they occur.

Apical Dendrites (closer to the surface)

These dendrites are located closer to the cortical surface and thus the charge of the extracellular space is more easily detected by the scalp electrode. If there is an EPSP that occurs at these apical dendrites (Remember this means the neuron has become more positive and the extracellular space around it has become more negative) the negative extracellular space that is close to the surface of the cortex is picked up by the electrode causing an upward deflection on EEG. The opposite is true if an IPSP occurs at these dendrites, as the extracellular space becomes more positive this is detected by the electrode resulting in a downward deflection.

Basal Dendrites and Soma (deeper in the cortex)

Because these dendrites are located deeper in the cortex, the way their signal is detected by EEG is a bit  more tricky. An EPSP that occurs deeper at these basal dendrites will still cause the extracellular space in this region to be negative, however this negative charge is too far away from the electrode to be directly detected. Because the extracellular space at the basal dendrite is more negative, relatively the extracellular space closer to the cortex (closer to the electrode) is now more positive. It is this relatively positive voltage that is detected by the electrode resulting in a downward deflection on EEG. If an IPSP occurs at the basal dendrites, this will make the surrounding extracellular space more positive which in turn makes the extracellular space closest to the electrode more negative and the deflection on the EEG will be upward. 

Remember:

EPSPs at the apical dendrite and IPSPs at the basal dendrite result in a negative voltage and upward deflection. 

EPSPs at the basal dendrite and IPSPs at the apical dendrite result in a positive voltage and downward deflection.

H5P