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Summary EEG Principles and Understanding Epilepsy
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  • Course
  • Physiology
  • Institution
  • Humanitas University

Explore the principles of electroencephalography (EEG) and epilepsy in this comprehensive guide. Learn how EEG recordings reflect brain activity and uncover the mechanisms behind epileptic seizures. Discover the different brain waves observed during wakefulness and sleep, and understand how epil...

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Document information

  • April 3, 2024
  • 6
  • 2022/2023
  • Summary

Subjects

  • ultrafiltrat
  • eeg
  • epilepsy
  • hippocampus
  • petit mal
  • grand ma
  • grand mal
  • tonic clonic seizu
  • tonic clonic s
  • tonic clonic seizures
  • alpha band
  • neuronal hyperexcitability
  • paroxysmal depolarizing shifts

Written for

  • Humanitas University
  • Physiology
All documents for this subject (33)

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Enrico Tiepolo


The Electro-encephalo-gram
When an excitatory synapse is activated on a cortical neuron,
intracellular recording shows an inward current and a depolarization
(EPSP) that electrotonically propagates to a certain distance. A
positive recording will be registered as we have positive charges
moving away from the positive electrode. Extracellular recording will
correspondingly show current that from the bulk of the extracellular
fluid (considered zero) reaches the electrode to enter the cell (as
intracellularly the opposite is happening, so positive charges are
moving towards the positive electrode); thus, negative potential is
transiently measured at the electrode.
An electrode positioned on the surface of the cortex, or on the skull,
will not necessarily capture underlying electrical events, because they
tend to be inconsistent and desynchronized. You don’t have a neat
signal, because your neurons should be depolarizing all at the same
time (wide areas of depolarization), but (luckily) this doesn’t happen.
However, whenever pyramidal neurons below the electrodes fire an
action potential, which back propagates electrotonically from the
soma in the dendritic tree toward the surface of the cortex, the
approaching depolarizing wavefront will produce a positive signal in
the skull electrode (extracellularly positive charges are moving away
from the positive electrode à positive signal).




In a fully awake person, each neuron is therefore going to produce its own signal, since neurons fire in a
discriminative way (each neuron is processing something different from the surrounding neurons). Neat
depolarization waves cannot be seen, but instead small and fast waves create “noise”.
Instead, when a person is in deep sleep, the cortex processes information in a non-discriminative way,
and therefore wide areas of the cortex will process the same type of information and will be in phase;
wide and slow waves will be recorded (synchronization).

These are the principles of EEG recording: activation of neurons below the electrode may produce a
signal; the higher the number of neurons concurrently activated, the larger the signal; repetitive
activation of the neuron may produce a more prolonged signal: as a general consequence, coordinated
and repetitive activation of neurons produces slow and large waves (low-frequency, high amplitude),
while desynchronized activity, such as it is observed when each neuron is elaborating a specific
information, tends to give rise to low- amplitude, high frequency activity.


203 Body At Work II

, Enrico Tiepolo

EEG signals are typically analyzed by considering a number of distinct frequency bands and “splitting”
the signal accordingly.

The predominant band in a resting
awake subject – eyes closed – is the 8-13
Hz band, called alpha band. Activity
at about 10 Hz also predominates in
primary neuronal cultures, when they
reach a stage of maturation such that
network activity is observed; this suggests
that ≈10 Hz is the “default” firing
frequency of most neurons.
Alpha rhythm is particularly evident in
occipito-parietal regions, in an awake
person with his eyes closed.
When the subject opens their eyes, the
cortex (mainly the occipito-parietal) will
receive many more inputs from the
external environment and therefore the
alpha rhythm tends to be superseded by
a much less synchronized activity which gives rise to a higher frequency, lower amplitude signal: the
dominant band under these conditions is the so-called beta band, between 13 and 30 Hz. This
activity predominates in awake individuals, alert and attentive to external stimuli, or exerting a mental
effort (they are present in REM sleep as well).

The amplitude of the beta rhythm is lower than alpha rhythm: this is not due to a decrease in electrical
activity of the brain, but rather to its being less synchronized.




N.B.: In the derivations 1à2, 9à10, 9à14 (etc.) the big downwards waves describe a blinking
movement or a possible quick movement of the eye and are not related to the activity of neurons.

When the thalamus shifts toward a bursting activity (when we go to sleep), coordinated activity of large
numbers of cortical neurons ensues (as they stop processing information in a discriminative way and sum
inputs that arrive not synchronously), so that slower and larger waves (theta band, 4-8 Hz) or even
larger and slower (delta waves, 1-4 Hz) appear.

These low-frequency rhythms increase during non-REM sleep (also called slow wave sleep, SWS). They
can be recorded in awake individuals during emotional, affective responses, during frustrating


204 Body At Work II

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