EEG Readings --> Why don't they behave like generic action potentials? Options

[Emotive_Adamantium]

As an action potential propogates between neurons and amidst neural ensembles, the basic membrane potential change is always supposed to occur from circa -70 mV to circa +40 mV with a subsequent ion channel refractory period of ~5 ms (please do correct me if this is too much of an idealized statement).

So my question is, when we place electrode caps on folks, why do the wave forms we see have such chaotic amplitudes as seen here [please see: wikipedia --> Beta_wave].

Is it because of the EPSP and IPSP variance of membrane potentials?

I mean, what happens when the same electrode picks up an axonal signal at, say, gamma frequency traversing one direction while an overlaying axonal signal at, say, alpha frequency traverses the opposite direction? Why don't multiple concurent signals muck up the reading? Or is that the reason why it graphs so variably?

Most of all I'd like to know what causes an EEG wave amplitude to raise or lower to such extremes when seemingly there's a built in cap and floor to mV across membrane proteins.

I think I'm just generally confused as to what, specifically, in terms of neural physiology and node-to-node signal propogation, EEG waves are supposed to be showing us.

[Emotive_Adamantium]

I see in Theta Waves and Delta waves especially that the waves picked up by EEG's are perfectly curved and smooth flowing. But action potentials rise suddenly as positive Na and Ca ions flood into the cell soma.

How is it that these very smooth wave rhythms can be detected from a system that's so jerky on the lowest level?

[maximus242]

Think of an eeg like a sound meter, sound meters are also crude but can show the change in sounds at minute levels. Alls an eeg is - is a glorified gauss meter, the electromagnetic signal is too weak for conventional means of measuring the waves. So instead they have electrodes that pick up the signal and relay them back to the EEG.

Remember, your just measuring electromagnetic waves.

[Casey]

How is it that these very smooth wave rhythms can be detected from a system that's so jerky on the lowest level?

Are you really surprised? Your body's motor functions use the same action potential system, yet, clearly, we can make very smooth, precise body movements.

Neurons rarely (if ever) fire alone, but they don't fire simultaneously either. They form complex patterns of firing due to spatial and temporal summation (among other factors).

Here's a simple example:
Neuron A fires, but it alone cannot trigger Neuron E.
Neurons A, B, C, D fire - this potential is large enough to fire neuron E. Neuron E, can then fire neurons F & G.
Depending on the initial stimulation, a different number of total neurons may fire. They don't behave like light switches (simple on and off).

In case 1, one firing would be measured. In case two, 8 firings would be detected.

That's grossly over simplified, but you should get an idea why the overall pattern becomes complex.

[maximus242]

Actually Casey it depends on what the weight is on neuron A and what the threshold is on neuron E. You simply need the electrical signal from A to be greater than the resistance of E.

[Casey]

Actually Casey it depends on what the weight is on neuron A and what the threshold is on neuron E. You simply need the electrical signal from A to be greater than the resistance of E.

This is a common misconception. Intensity of a signal is transmitted as frequency, not "weight" (or AP voltage, etc).

The threshold of a neuron increases after firing (relative refractory period). It can only fire again if it can overcome this higher threshold.

However, this threshold is not overcome by a stronger action potential, per se. It can be overcome by a signal from multiple neurons (spatial summation) or by generating signals so quickly that they "add" (temporal summation).

This mechanism can lead to varied recruitment of neurons across the spectrum of stimulus intensity. This leads to the patterns which can be detected outside the body.


I know a few people who would claim that neurons can also be fired by God...but, that's another story...

Of course, other factors play a role too, but it would take way to long to go into detail. Neurons are fascinating but very complex.

[lcsglvr]

Agree with Casey. Except, the threshold doesn't change for the relative refractory period, but yet the hyperpolarization becomes greater, therefore just the depolarization has to become greater, not the physical threshold. Potassium wants to reach it's Ek (equilibrium potential for K+).

This may have been what you meant, I'm just trying to clarify for Emotive.

[project-2501]

I know a few people who would claim that neurons can also be fired by God...but, that's another story...

I for one wouldn't mind hearing that story

[Casey]

Yes, lcsglvr is right on the money.

I know a few people who would claim that neurons can also be fired by God...but, that's another story...

I for one wouldn't mind hearing that story

LOL Haven't you ever heard someone say that? Maybe I'm just around too many religious people. I really need to move to a big city

[project-2501]

Yes, lcsglvr is right on the money.

I know a few people who would claim that neurons can also be fired by God...but, that's another story...

I for one wouldn't mind hearing that story

LOL Haven't you ever heard someone say that? Maybe I'm just around too many religious people. I really need to move to a big city

Of course I have heard the principle many times, I was only expressing my interest

[maximus242]

This is a common misconception. Intensity of a signal is transmitted as frequency, not "weight" (or AP voltage, etc).

The threshold of a neuron increases after firing (relative refractory period). It can only fire again if it can overcome this higher threshold.

However, this threshold is not overcome by a stronger action potential, per se. It can be overcome by a signal from multiple neurons (spatial summation) or by generating signals so quickly that they "add" (temporal summation).

This mechanism can lead to varied recruitment of neurons across the spectrum of stimulus intensity. This leads to the patterns which can be detected outside the body.


I know a few people who would claim that neurons can also be fired by God...but, that's another story...

Of course, other factors play a role too, but it would take way to long to go into detail. Neurons are fascinating but very complex.

Actually Casey, no.

In Modern Neuroscience Theory its common practice to switch between the biochemical relation of neurons
and the computer science simulations. A Weight is the Computer Science term for the signal that is
sent by a neuron. The Threshold Function is used to determine when the neuron will fire, once the
threshold is bypassed, it fires the neuron.

Why do you switch between the two? Because A, in the computer simulations you can test theories and B
neuroscience is very mathematical, computers are still just extremly efficent calculators and so they
are excellent at figuring out large mathmatical equations.

Right now, scientists are using computers to predict human neural behavious patterns, this is simply
using the computer science terms. The accuracy of using computers to simulate neurons is getting
better every day. Neurons can be calculated mathmatically and weight is one of the terms that are used
when doing said calculations.

Now time to give you an explanation on how neurons work.

To simplify, we can divide the neuron into 3 parts

The Cell Body - Nucleus and large portion of cytoplasm reside in here
Dendrities - Takes electrical signals to the cell body, in computer science terms, its the input
Axon - Only one axon per neuron for most of the time, carries signals away from the cell body, the output
or weight in CS terms. Axons are covered by a Myelin Sheath.

Electrical Potential is caused by a diffrence in charges. Electrical Activity takes the form of Ions, Na+ and K+. To make a long story short on how a threshold is formed, positive ions move out of the cell where the
negative ones remain in the cell. This causes Polarization in the cell, created by the resting potential of the negative ions in the cell body and the positive ones on the outside (of the cell membrane).When a neuron becomes 'excited' this causes the release of Na+ and then Depolarization occurs. After Depolarization, K+ is released and Repolarization occurs. Then we have the Refractory Period in which the Cell returns to its resting potential.

Neurons are essentially in a state of constant Depolarization and then being Repolarized. This is not a classical electrical current because A, the electrical signal is always the same and B, its a series of stop and go. An electrical current usually flows constantly. A neuron becomes stimulated when the positive impulse is greater than the negative ions in the cell membrane.

I think maybe now you will understand that it is the diffrence in ions, causing polarization or depolarization. Whether or not a Neuron becomes depolarized is dependant on the amount of positive Ions in the impulse. While its quite common to have a large network of neurons sending signals up to one neuron to overcome said threshold, you need to keep in mind that weights are how a neural network 'learns' and the learning rate is of course defined by the variation in weight per trial, usually 0.7 I believe. The network
adjusts its signals until it can overcome the negative ions.

To explain on a motor function. Lets say there is a cup of water in the middle of the table, you try to grab it but go too far right, so the nerves in charge of pulling the Pectoralis Major on the right arm adjust their signals to overcome more neurons so they can cause a greater pull of the Pectoralis Major and cause the arm to swing further to
the left. The signals from the eye and the nerve tissue in the hand send error signals to the motor nerves to tell whether or not it has succeded. Now keep in mind this is a grossyly oversimplified explanation and a wee bit innacurate so that it can be more easily understood. To elborate on the nerves affecting the Pectoralis Major, we have the

Neuro-Tendious Spindles - The nerves Supplying tendons have special modification of termination fibers, the tendon bundles become enlarged and the nerve fibers - penetrate between the fasiculi of the tendon and spread out between the fibers to end in irregular discs or varicosities.

Neuro-Muscular Spindles - The majority of voluntary muscles there are special end organs consisting of a small bundle of peculiar muscular fibres (Intrafusal Fibers) invented by a capsule within which nerve fibres, experimentally shown to be sensory in origin, terminate. The majority of the rest of the details concerning Neuro-Muscular Spindles are to do with Organs of special sense and are irrelivant to our discussion.

Motor Nerves - Are to be traced either into unstriped or striped muscular fibers. In the unstripped (involuntary) the nerves are derived from the sympathetic, and are composed of non-medullated fibers. I will cut out some details in this to make it faster to read. In the stripped or vouluntary muscle, the nerves supplying the muscular fibers are derived from the cerebral spinal nerves and are composed mainly of medullated fibres. The nerve, after entering the sheath of the muscle, breaks up into fibres or bundles of fibres, which form plexusus and generally divide until, as a rule, a single nerve fiber enters into a single muscular fiber. However if the muscular fiber is too long, more than one nerve fiber may enter it.

Now I believe if we trace the Branchial Plexus, we can find the arm nerves are controlled by it. Tracing the Articulations of the Ulnar Nerve, we can see how the hand can be guided to the glass and grabbing the glass. Then we get nitty gritty to Motor nerves again. Its actually several nerves of the Branchial Plexus that can be held responsible
for the movement of the hand, towards the glass. However, going into this deeper is the equivilant of preparing for neurosurgery. If this is a conscious act then one can trace some of the nerves to the Cerebral Spinal Nerves, although things get so damn complicated when you start tracing nerves like this, I would have to go far more indepth to give a truely accurate picture of whats going on.

Anyways, this gives an idea of how the nervous system works.

[Emotive_Adamantium]

Arigato gozaimashita for those most thought provoking responses, especially the in depth treatment of motor signaling. That reminds me of an article I was just reading about Neurally Controlled Animat. You've heard of them? Where electrodes connect a monkey's pre-motor area to an on-screen mouse cursor and "emergence" occurs, wherein the monkey adapts to learn how to control the cursor by taking mental note of which neurons affect the electrodes and thusly propelling the mouse cursor. Worthy of a few ripe yellow bananas I'd say.

Hmm, perhaps I could tap out more than just a bit of remaining knowledge from these wisdom trees? I'm going to drop off all my neural questions that books have so far not given me answers for. Bear with me, it will be a new topic.

[lucid_dream]

EEG picks up signals from millions of neurons, so you can't really compare it to the activity of a single neuron since it is a measure of population activity.

[maximus242]

Yes, naturally, lucid this discussion got a bit off topic. EEG's essentially measure the brains electromagnetic frequency as a whole. Measuring individual neurons frequencies would require almost direct contact since the magnetic field on it is so weak. In fact, you need a special, highly sensitive magnometer to measure it.

Emotive, I saw many of the files regarding manual stimulation of neurons, some of it is quite freaky if you ask me. They are using sharks to patrol US water borders and there have been human chips developed. However, I believe on careful anaylsis of the chip and its placement, it may be possible to over-ride the chip by allwing the Dendrities connected to the chip to be destroyed. If the chip has no connectivity, its useless.