Electrophysiology


The following are bits of writing from many sources such as personal correspondence, posts to on-line discussion groups, notes, and occasionally even some journaling. All of this is informal in nature, but contains some interesting and/or useful information.

An electroencephalophone
[Posted to the neuroscience group on MySpace.com]
>Your brain waves don't really create any ordered rhythms, no?
Yes and no. If they're really ordered you are having an epileptic seizure. However, the fact that these are "waves" implies a pattern already, doesn't it?
The lab I'm in right now looks at small groups of cells in culture. The frontal cortex networks in particular have a definite rhythm that holds up for long periods. Of course, pharmacological manipulations can modify this and make it more or less regular. It's hardly a drum machine though.
If anyone has any links to music produced with these devices, I would be very interested. My own aesthetic interests aside, there has been some success at using music to identify patterns in naturally occurring sequences. Specifically, motifs in proteins were found more quickly by converting the amino acid sequence to notes than by inspecting the same information visually. Who knows what this could do for neuroscience?

Random firing of neural/cortical cells
[Posted to the neuroscience group on MySpace.com]
>random firing of neural/cortical cells ...Any thoughts on what might trigger this?
Well, it depends on what you mean my "random." To begin with, this is hard to define in part since, just because we don't see a pattern or a cause for the firing sequence, that doesn't mean it is as purposeless as it implies.
All neurons are believed to have some inherently spontaneous activity. Since they are connected to other neurons, that activity is propagated throughout the system. What is all "means" in any given system is the subject of great debate. I've heard the problem referred to as the cracking the "neural code." This isn't my area, but I am in electrophysiology, so I am interested in anything anyone can elaborate on this topic.
The lab I am in works with cultured neuronal networks, usually of frontal cortex or spinal cord tissue (although we have tried other tissue types). These are spontaneously active, and we use this activity as a baseline for comparing effects of various neuroactive compounds. For example, in my research, I am testing the effects of a couple different calcium channel blockers. These limit the amount of calcium entering the cells when they fire. As a result, the "bursts" of activity are shortened proportional to the amount of drug applied. The entire network can be effectively silenced with enough activity.
This "silencing" can be achieved through a number of other compounds as well (e.g., tetrodotoxin, aka TTX) or you can just slow things down by hyperpolarizing neurons (this makes it harder for them to fire). Drugs like Valium do this. Conversely, you can increase activity in parts of the brain (or elsewhere) with still other classes of drugs like stimulants (e.g., amphetamines). And one more thing; although it isn't often acknowledged as a separate class, you can also "disinhibit" the brain with still other compounds like strychnine. These drugs inhibit the inhibitory neurons, such that you are driving without brakes. Curiously, rather than "random," the firing pattern becomes very organized and rhythmic, one burst after another. This is what is going on in the brain during an epileptic seizure.
One theory about why this pattern is so organized is that the neurons become "coupled oscillators." I don't fully understand this concept, but the idea is that they balance one another into a stable pattern. One real-world example of this phenomenon is that when two clocks with pendulums are placed next to each other, the motion of these pendulums will synchronize. Both will go left, right, left, right in the same instant. You can probably find a lot more about this on the web, although I haven't looked myself.
Anyway, getting back to firing patterns, one thing you tend to find is that the various tissue types have a characteristic firing pattern. It isn't so neatly organized that you can understand it intuitively, but after you see each a few times, you start to recognize them at a glance. For example, frontal cortex tends to have somewhat rhythmic bursts and fewer random spikes, whereas spinal cord tissue tends to be a lot more "spiky" without as much of a pattern.
Incidentally, during embryonic development, a lot of places in the central nervous system go into rhythmic firing patterns. For example, the retina becomes spontaneously active long before it ever "sees" anything. This activity helps organize the visual cortex so that it will be able to properly process information when the eyes are actually used somewhere down the line. In this case, it helps the brain keep up with which eye is seeing what as you would require for depth perception. There is a similar pattern of activity in the developing spinal cord that contributes to proper development of locomotion, although I don't know as much about that.
I think that's pretty much everything I can say on this topic. Sorry about the long responses, but I would rather give too much information than just be ambiguous.

The neural code
[Posted to the neuroscience group on MySpace.com]
>What is this 'neural code' you are talking about?
This is probably an oversimplification of it, but there was/is a persistent idea that you could tell a lot about what a cell was "saying" by the sequence of spikes. The ideal end of this would be that you could decipher the information encoded in a stream of action potentials the way you could a series of dots and dashes in Morse Code or bits of computer code into ASCII (or whatever) or DNA into proteins (and so on). That's a tempting idea in the digital era.
But it also seems pretty far fetched if you've ever looked at spike trains. Of course, you could say that, well, we know that certain firing patterns (i.e., high frequency long-lasting bursts) result in LTP, and we know that the opposite, LTD, can be achieved through its own characteristic firing pattern. So maybe you *could* tap into a retinal ganglion cell and be able to "see" what it is seeing. That would be cool.
I'm just skeptical of extending the idea too far beyond the LTP/LTD examples... largely because I think you have to take into account how things are wired. I very much doubt there is a universal code like our genetic code in which every cell reads things similarly at the level of transcription (and even there the story is more complicated than that). Complicated as they are, our neurons don't know a whole hell of a lot by themselves. Everything seems to indicate that the mind is much more of an emergent phenomenon than that.

Coupled oscillators
Here's a primer on coupled oscillators (admittedly, it is pretty dry): http://en.wikipedia.org/wiki/Oscillation#Coupled_oscillations Probably one of the best-known examples is how individual heart cells (cardiac myocytes) beat independently, but will synchronize once they touch. There are a lot of thing in that do this at every level from quantum particles to solar/planetary/lunar systems. You know how women who live together will synchronize their cycles? That's an example. Another real-world example is that a pair of pendulum clocks in the same room will eventually synchronize their ticks and tocks so that they fall on exactly the same beat. My interest comes out of the fact that the neurons in almost every part of our central nervous system are influencing one another in this way. Somehow this makes you and I who we are. How? We don't know yet.




Copyright Alexplorer.

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