Mechanism behind electrically induced neuron firings?

[shoestring]

Electricity affect neurons. For example, in 1791 Galvani made the legs of a dead frog twitch by exposing them to a spark, and more recently in history, electric currents through the brain has been used to deliberately induce "therapeutic" epileptic seizures. What I wonder about is the exact mechanism behind this effect. What is it about for example the electric current through the brain that causes the nerve cells to fire uncontrollably and thereby cause a seizure?

The current itself must be quite a slow drift of ions, because even in low resistance copper wires in a typical electric circuit, the drift velocity of the electrons is surprisingly small. The conductivity of the brain must be many orders of magnitude smaller than that of copper, so the current used in electroconvulsive treatment can't be anything but a really weak drift of ions through the brain, that nevertheless manages to cause a clonic seizure.

I looked around on the web but couldn't find anything about the exact mechanism of how currents affect neurons. Does anyone know?

(Btw, I know more about physics than about nerve cells. :) )

 

[Pythagorean]

nerve cells are like little amplifiers. They have voltage gated ion channels that, once activated with a sufficient voltage potential, will open up and let a bunch of positive sodium ions, raising the potential of the neuron inside with respect to outside.

In nature, the axon hillock (at the start of the neurons action potential) generally initiates this series of channel openings, but if you can change the potential across the neuron externally, you can bring it closer (or across) threshold.

 

[shoestring]

Thanks, I'll have to read up on voltage gated ion channels! :)

Another example of electricity affecting nerves I just though of is when people test a 9V battery by touching the poles to the tongue. (Se for example the discussion http://www.instructables.com/answers/Have-you-ever-touched-your-tongue-to-a-9-volt-batt/")

The averge field (calculated as voltage divided by distance) between the poles of the battery is of order 1 kV/m, and the field across a neuron membrane of order 10 MV/m, 10 000 times stronger. It doesn't look as if the field from the battery should have any obvious effect on the neuron, which makes me wonder if some other process is involved, or if it is the voltage gated channels that initiate the firing.

 

[Pythagorean]

Well, now you're going onto sensory neurons, so it's not just a matter of membrane potential.

Sensory neurons have receptors specific to the kind of information being transmitted by the environment (mechanoreceptors sense vibrations and sound and inertia, for instance)

chemoreceptors, like on your tongue, sense the chemical makeup of the surface of your tongue. I don't know what a 9V battery does to your tongue, but there's all kinds of things it could do: deform the chemoreceptors themselves; if the chemoreceptors detect pH, putting a current through a solution will change the pH.

I don't know a lot about the tongue's chemoreceptors, but I've always wondered if metal and 9V batteries taste the same because of the same kind of "short-circuiting" operation they perform on the tongue. I notice both make my jaw tense slightly, like sour foods.

 

[shoestring]

Ok, that's a different thing. But I imagine the voltage applied to the head during ECT can't be high enough to compete with the field strenght across neuron membranes. According to http://hypertextbook.com/facts/2005/GinaCastellano.shtml" page, the voltage may be up to 450V. Scary thing, makes me a quite uneasy to read about it.

 

[Pythagorean]

Well, even if you don't overcome the potential, you change the threshold of the neurons in the region, making them more (or less) susceptible to firing on noise and small signals (which they occasionally already do). So you're increasing the chance of a whole population of neurons firing. Additionally, the neurons are coupled in a complex network of gap and chemical junctions, so you may have damping and driving effects. Remember that it's a feedback system; if your dablings cause any kind of synchronization, you could potentially set the whole sub-system into synchronization (i.e. in physics language, resonance causes super-synchronization). Again though, it depends on both topology and the functional role of the neurons involved. It's a complex problem, not something where the behavior scales with a single variable. Bifurcations can sometimes occur as a result of small changes.

Can external electric fields have similar effects on the brain? "This is an interesting question," Anastassiou says. "Indeed, physics dictates that any external field will impact the neural membrane. Importantly, though, the effect of externally imposed fields will also depend on the brain state. One could think of the brain as a distributed computer -- not all brain areas show the same level of activation at all times.
"Whether an externally imposed field will impact the brain also depends on which brain area is targeted. During epileptic seizures, pathological fields can be as strong as 100 millivolts per millimeter¬ -- such fields strongly entrain neural firing and give rise to super-synchronized states." And that, he adds, suggests that electric field activity -- even from external fields -- in certain brain areas, during specific brain states, may have strong cognitive and behavioral effects.

from http://www.sciencedaily.com/releases/2011/02/110202132617.htm

Associated Journal:

Costas A Anastassiou, Rodrigo Perin, Henry Markram, Christof Koch. Ephaptic coupling of cortical neurons. Nature Neuroscience, 2011; 14 (2): 217 DOI:

 

[shoestring]

Thanks, that was an interesting article.

An electric field acting on living tissue can set ions in motion. Even if it's only a slow drift, this could cause changes in ion concentrations at boundaries between areas of different conductivity, like near a neuron membrane. Potentials and ion concentrations are so closely related that I can't help wondering if ion gated channels also could be affected by electric fields. Any ion in the brain will be a carrier of electric charge as well as having chemical properties, so the situation is a different from a "normal" electric circuit with metal wires and electrons as the only charge carrier.

Edit:

Hmm ... perhaps there's no such thing as ion gated channels. There are ligand gated channels but those ligands are not necessarily ions, of course. It's the voltage gated channels that could just as well be called ion gated, because the voltage is created by the concentrations of specific ions. I guess what I wondered was if any change in concentrations due to currents induced by the field could be of any matter.

Anyway, the article shows that weak fields can affect neurons, but doesn't explain by what mechanism. That's still a mystery perhaps. :)

 

[Pythagorean]

The two major receptors are ligand and voltage gated.

Voltage-gated obviously are triggered open by voltage.

Ligand gating relies on protein conformations between the ligand and the binding sites for that ligand on the receptor. You can think of the protein as changing state from closed to open in the presence of the ligand binding, because the binding changes the energy landscape of the protein, causing it to find a lower state by readjusting itself in space.

It's conceivable that a strong enough field can further manipulate the energy landscape of the protein, bringing it closer to, or farther away from the proper conformation, or possibly causing a whole new formation (or distortions in the canonical conformation).

Anyway, the article shows that weak fields can affect neurons, but doesn't explain by what mechanism. That's still a mystery perhaps. :)

The physics of biology will be a frontier for a long time. Today, you can still read five different reviews explaining away different mechanisms for the same phenomena.

 

[shoestring]

Thanks, I feel enlightened. Great answers!