You’ve Got Some Nerve!

Hint: If it looks anything like this, you’re in a spot of trouble.
Image courtesy BBC.

Studying the physics that lies at the heart of neuroscience means getting an up-close look at what’s actually happening when a thought crosses your mind; it’s a little “meta”, to say the least. But research from the Niels Bohr institute, published this month in Physical Review X, hope to bring about a serious change in how we think about, well, how we think.

Earlier this year, when I reported on emerging research into trans-cranial direct current stimulation, I described the transmission of signals along a nerve as a flow of charge mediated by the motion of ions. In terms of biophysics, this notion is best typified by the “Hodgkin-Huxley” model, which treats the nerve, a string of neurons, like a string of biological fuses (the electronic kind, not pyrotechnic). When someone tickles your arm, it sends an action potential, a voltage pulse, down that string. As the signal passes through each fuse, it “blows” and becomes useless for a few moments while it “repairs” itself during what’s known as the refractory period. This keeps the pulse travelling one direction, preventing a signal from dissipating back along the wire it came down. However, Dr. Alfredo Gonzalez-Perez and his colleagues have reported a finding that appears to contradict some aspects of this model, and propose a modified version more in line with the results of their experiments.

One of the key implications of the Hodgkin-Huxley model is that two action potentials, if they collide travelling opposite directions down a nerve, will annihilate one another. This follows logically from knowledge of the refractory period—a neuron which just fired can’t fire again for a moment, so when those pulses collide the charge should have nowhere to go. The common thinking is that, in such a scenario, the energy involved would dissipate as mechanical energy, i.e. sound or heat. But prior research has failed to show the kind of heat we’d expect from theoretical models, and there was scant evidence that pulse annihilation has even been observed, prompting these investigators to dig deeper and examine the phenomenon as directly as possible: extracting live nerve fibers from earthworms and lobsters, and using them as a sort of action potential collider track.

While invertebrates such as these may not be perfect analogues of a human system, I’m glad that they stuck to bugs and crustaceans; the paper describes in great detail the process of dissecting recently-dead specimens for their sensory tissues, but I don’t feel so bad about the grisly practice when they look like aliens.
Image courtesy

By sending pulses from each end of the nerve simultaneously, the researchers were able to observe the phenomenon of action potential collision directly. What they found was that, rather than annihilating, the pulses appeared to pass through each other with near-perfect fidelity. To explain this, they suggest a model of the action potential as an electromechanical soliton.
Since we can detect neural signals as spikes in voltage, it’s obvious that they have an electrical component to them. However this result suggests that, rather than treating the nerve as a string of fuses with a current running through it, it is more appropriate to think of action potentials as longitudinal pulses in a charged slinky.
 In this model, the rarefaction accompanying a compression is responsible for the refractory period.
Image courtesy
Imagine a slinky kept at such a high voltage that it began to stretch out; since alike charges repel one another, an excess of charge could cause its coils to separate. A longitudinal compression in such a slinky could be created either by pushing one end forward with your hand, or by dropping the voltage for a moment at that same end, relaxing the repulsive force. An analogous symmetry in neurons means that, while we can think of nerves as wires, it’s important to remember that the actual electrons which pass through a light switch when you flip it on will bump and jostle around for minutes before they finally reach the bulb; the light comes on immediately because the first electron to pass through the switch will force one out the other end of the wire and into the filament nearly instantaneously (the “pressure” propagates at almost exactly light speed!)
This model might help to shed light on how something as simple as putting electrodes on a person’s head can help them learn faster (assuming recent tDCS results stand the tests of time and scientific rigor)—immersing a neuron in an external electric field would change how those pulses propagate through it.  This study implies a picture of the brain somewhere between the common notion of “organic supercomputer” and a vast, tangled array of tin cans on quivering strings. However, both of these systems are troublesomely inanimate—perhaps a more apt comparison would be the switchboard of an impossibly complex telephone network, with the conscious mind, the self, in the operator’s seat. 

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