Sunday, March 23, 2008

Oscillations Part VI: Origins and Interactions

(This is a continuation of notes taken from Ginger Campbell's podcast #31 with György Buzsáki, and a continuation from Part V):

How do oscillators get themselves synchronized?

It's simple to link oscillators together:
- every neuron is an oscillator - the issue is how they get synchronized into an oscillation activity
- one can make an oscillator where none of the elements individually serve as an oscillator - e.g., a mechanical clock is a perfect example - a mechanism that ties all the elements together to make the clock tick is all that is needed
"...if you record from single neurons, you can record from them for long long long times, and you will never see that there is an underlying oscillation. And this is the main reason perhaps, this explains the skepticism of many investigators in neuroscience that deal usually with one neuron at a time. For example, the ripple oscillations that I just mentioned is a perfect illustration of that. If we would be looking at single neurons for five years, there would be no moment in time, perhaps, when we would say, aha - there is a population output here. But if we begin to look at 50 or 100 together, then it becomes absolutely obvious, and we won't miss any of these events. So, in this case, none of the neurons oscillate, but their cooperative activity produces a perfect sinusoid oscillator."

Is there a pacemaker, something in the brain that controls these oscillations?

- there are pacemakers: e.g., our respiration rhythm is determined by a group of neurons in the brainstem responsible for maintaining or pacing respiration
- in hippocampal theta activity it has been thought that there is a pacemaker in the medial septum
- many neurons in the medial septum, cholinergic and gabenergic neurons can be recorded when in slice preparation, in in vitro conditions, when they are disconnected from the hippocampus and every part of the brain, they maintain oscillating activity
- David McCormick has shown that isolated neurons can oscillate perfectly
"Now, calling many of these or some of these 'true' pacemakers comes with some burden, because in many cases, like in the thalamus, even in the medial septum, it turns out that yes, individual neurons have the propensity to oscillate, and under certain circumstances they do, but when they are embedded into a physiological substrate or physiological activity their timing is coordinated by various feedbacks. And even if we have an independent set of neurons, let's say in the medial septum, that can fire at 5 Hz, somehow their activity must be coordinated. And in many cases that kind of coordination comes from their target structures, in our case the hippocampus."

- in terms of energy, it may be cheaper to have neurons that can transform information, affect the firing patterns of neurons, and simultaneously in cooperation with others, produce an oscillation, than it would be to have a set of neurons which did nothing but keep time

How does an isolated neuron (without excitation or inhibition) oscillate independently?

- inhibition and excitation are very important in the brain, but not required for an oscillation
- ocean waves don't have inhibition and excitation in this exposit sense - oscillation always emerges
- it's inevitable when you have opposing forces - a push and a pull, like in the swing, is perfect enough to maintain oscillations
- in the brain opposing forces include potassium and sodium going in opposite directions in the membrane - this is perfectly sufficient to maintain oscillation in a single neuron
- no inhibitory neurotransmitter is needed for this but there is inhibition in the sense that one force tends to counteract the other force

This last part is important because it suggests that no outside agency is required to make a brain or to keep it oscillating, other than energy from ordinary, thermodynamically congruent energy sources, utilized through ordinary metabolic pathways; this is supported by other observations as noted in the post Oscillatory Matters where different concepts regarding "movement" are listed. There is a bit more on this in the next post.

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