Embedded Files
WAVES
WAVES
The following rhythms have been distinguished:
delta (0.5–4Hz)
theta (4–8Hz)
alpha (8–13Hz)
beta (13–30 Hz)
gamma (above 30 Hz).
However, by scalp electrodes frequencies higher than 45Hz cannot be recorderd, whilst the EcoG can go up to 100Hz and more. [1]
Delta: predominantly recorded during deep sleep, the waves have large amplitudes (75–200 microV) and show strong coherence over the whole scalp
Theta: are infrequently observed in adults. They are commonly present in rodents, where they are thought to act as a gating mechanism for information transfer between brain areas. In humans, they may appear during certain emotional states or as a pathological slowing of alpha rhythms.
Alpha: predominant during wakefulness, they are best observed when the subject is in a resting state, with closed eyes. They are blocked mostly by visual attention and mental effort. Mu rhythms have a frequency band similar to alpha, but are blocked by motor functions.
Beta: states of increased alertness and intense focus
Gamma: information processing and onset of voluntary movements
ACTION POTENTIALS
ACTION POTENTIALS
To understand how the EEG signals are captured and transmitted, it is first important to understand how the signals are transmitted through neurons. This is via action potentials.
Around 1/5 of the Energy consumed by the brain is used to create an action potential. To find an equilibrium with its environment, the cell must create a dynamic equilibrium. This equilibiurm is not only between the concentration gradient inside and outside of the cell, but also considering the electrical gradient. For this reason, one speaks of electrochemical equilibrium.
Ion Flow
Ion Flow
Being the membrane semipermeable, the flow of concentration also depends on the ions involved (for example how much is available in the body) and what type of channels are on the membrane surface.
average concentrations inside and out of a cell:
[Ki] = 140mM [K]= 5mM; [Nai]= 10mM [Na]= 150mM; [Cli]= 10mM [Cl]= 100mM
i = inside
If we consider 10mM of K+ out of the cell and 1mM inside, at a tension of 58mV we do not have a flux of K+ ions, this can also be expressed by saying that 58mV is the slope for the tenfold change in K+ gradient.
This explanation was using a 10:1 gradient of K+, however in human cells the concentrations are different for each ion and to find the real equilibriums one uses the Nerst equation.
Membrane Potential and the Goldman Equation
Membrane Potential and the Goldman Equation
The membrane potential of a cell, typically ranging from -60 mV to -80 mV, represents the electrical potential difference across the cell membrane. This potential is determined by the movement of various ions, particularly potassium (K⁺) and sodium (Na⁺).
To calculate the overall membrane potential by considering all contributing ions simultaneously, the Goldman equation was introduced.
It takes into account the concentration of each ion inside and outside the cell, as well as the membrane's permeability to each ion.
Em is the membrane potential,
R is the gas constant,
T is the temperature (in Kelvin),
F is the Faraday constant,
P represents the permeability of the membrane to each ion, and
[ion]in and [ion]out represent the concentration of each ion inside and outside the cell, respectively.
Action potential propagation in unmyelinated axon - charge distribution
Experimental Evidence on Ion Contributions to Membrane Potential
Experimental Evidence on Ion Contributions to Membrane Potential
To clarify the specific roles of Na⁺ and K⁺ ions in generating the cell’s membrane potential, an experiment was conducted:
Changing Extracellular K⁺ Concentration: When the concentration of K⁺ outside the cell was increased, the membrane potential shifted closer to 0 mV from an initial value of around -61 mV (as calculated by the Goldman equation). This shift happened because the typical high concentration of K⁺ inside the cell contributes to its negative potential. Adding more K⁺ outside the cell reduces this concentration difference, leading to a depolarization toward 0 mV.
Changing Extracellular Na⁺ Concentration: When the concentration of Na⁺ outside the cell was increased, there was little to no change in the cell's membrane potential, which remained around -61 mV. This result indicates that K⁺, rather than Na⁺, plays the dominant role in maintaining the cell’s negative resting potential.
Animation of action potential generation
Animation of action potential generation
This animated GIF shows how action potentials propagate in an axon through the activity of three ion channels: potassium "leak" channels (blue), voltage-gated sodium channels (red), and voltage-gated potassium channels (green). At rest, potassium leak channels maintain a negative-inside membrane potential by allowing potassium ions to leave the cell.
Action potentials begin at the axon initial segment when excitatory signals depolarize it to the threshold, triggering voltage-gated sodium channels to open. Sodium ions flow in, making the inside of the membrane positive. This positive shift spreads to adjacent regions of the axon, propagating the action potential. Voltage-gated potassium channels then open, allowing potassium ions to exit, restoring the membrane potential to negative-inside. Once these channels close, the axon returns to its resting state.
AP graph
AP graph
With these findings it was possible to recognize the three phases of an Acton potential. In a resting state, there is more [Na+] outside the cell and the cell has a negative voltage (-61mV). When a neurotransmitter opens the sodium channel, one will see a flow of Na+ inside the cell, causing a depolarisation (the membrane potential becomes positive).
K+ channels are also activated, but they take longer to open, leaving time for the Na+ to first enter the cell and cause a depolarisation. When the K+ channels are activated, the ions inside will quickly follow the gradient out of the cell to try and make the membrane potential negative again (repolarising phase). [2]
PYRAMIDAL CELLS AND DIPOLES
PYRAMIDAL CELLS AND DIPOLES
The signal of the EEG mostly detects dipoles created by pyramidal neurons in the cortex. These neurons are aligned so that their dendrites (branch like structures) point towards the surface of the cortex. The activation of a pyramidal neuron leads to an excitatory post synaptic potential (EPSPs).
This works thanks to neurotransmitters, which bind to receptors on the dendrites (the input region of the neuron), and cause the ion channels to open. In this case, Na+ ions flow into the neuron at the dendrites (causing the positive charge). This intracellular inflow reduces the membrane potential locally. As already explained before, if a positive charge is created, there also must be an outflow to reach the equilibrium charge again. In the cell of a neuron, this outflow happens at the axon hillock, through an outward flow of K+ ions.
Due to the movement of the ions, there is a separation of charges between two regions, and a dipole is formed. The sink is created by the inflow of Na+ ions, wich leaves the area outside the neuron more negative than before. The source is located at the cell body, where a positive charge is formed as K+ ions flow out, making the sorrounding area more positive. When enough neurons are synchronised, the dipole can be detected on the surface of the scalp using the EEG.
The major signal that the EEG recollects are made by extended patches of grey matter, polarised by synchronous synaptic input either in an oscillatory manner or as transient evoked activity. The patches contain thousands of cortical columns, and perpendicular to these cortical surfaces lie large pyramidal cells. The different layers correspond to different synaptic connections of different structures. [5]
references images:[1] Wiley Encyclopedia of Biomedical Engineering, Copyright & 2006 John Wiley & Sons, Inc.(https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=d0e3a5e418f4ba8ff3ba117d45f9cc6b1d89ee7dI)
[2] Network dynamics and connectivity of in vitro neurons derived from human induced pluripotent stem cells of subjects affected by neurodevelopmental or mitochondrial disorders - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Characteristic-profile-of-the-action-potential-in-which-the-three-phases-of-AP-evocation_fig1_348772678 [accessed 14 Nov 2024][3] Germis, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons[3] John Schmidt, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons[3] Imaging Brain Slices - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Histological-reconstruction-of-neurons-using-biocytin-and-Neurolucida-A-Biocytin_fig3_251137802 [accessed 14 Nov 2024]
[2] Network dynamics and connectivity of in vitro neurons derived from human induced pluripotent stem cells of subjects affected by neurodevelopmental or mitochondrial disorders - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Characteristic-profile-of-the-action-potential-in-which-the-three-phases-of-AP-evocation_fig1_348772678 [accessed 14 Nov 2024][3] Germis, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons[3] John Schmidt, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons[3] Imaging Brain Slices - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Histological-reconstruction-of-neurons-using-biocytin-and-Neurolucida-A-Biocytin_fig3_251137802 [accessed 14 Nov 2024]
Page updated
Google Sites
Report abuse