Tag Archives: Membrane potential

How the brain really works (maybe)

Stare at the picture just below long and hard. It’s where the brain probably does its calculation — no, not the neuron in the center. No, not the astrocyte just above. Enlarge the picture many times. It’s all those tiny little circles and ellipses you see around the apical dendrite. They all represent nerve and glial processes. A few ellipses have very dark borders — this is myelin (which insulates them allowing them to conduct nerve impulses faster, and which also insulates them from being affected by the goings on of nerve processes next to them). Note that most of the nerve processes do NOT have myelin around them.

Now look at the bar at the lower right in the picture which tells you the magnification. 5 um is 5 microns or 50,000 Angstroms or roughly 10 times the wavelength of visible light (4,000 – 8,000 Angstroms). Look at the picture again and notice just how closely the little circles and ellipses are applied to each other (certainly closer than 1/10 of the bar). This is exactly why there was significant debate between two of the founders of neuroHistology — Camillo Golgi and Ramon Santiago y Cajal.

Unlike every other tissue in the body the brain is so tightly packed that it is impossible to see the cells that make it up with the usual stains used by light microscopists. People saw nuclei all right but they thought the brain was a mass of tissue with nuclei embedded in it (like a slime mold). It wasn’t until the late 1800′s that Camillo Golgi developed a stain which would now and then outline a neuron with all its processes. Another anatomist (Ramon Santiago y Cajal) used Golgi’s technique and argued with Golgi that yes the brain was made of cells. Fascinating that Golgi, the man responsible for showing nerve cells, didn’t buy it. This was a very hot issue at the time, and the two received a joint Nobel prize in 1906 (only 5 years after the prizes began).

The paper discussed below gives a possible reason why the brain is built like this — e.g. it’s how it works !!

Pictures are impressive, but could it be all artifact? To see something with an electron microscope (which this picture is) you really have to process the tissue to a fare-thee-well. One example from way back in the day when I started medical school (1962). Electron microscopy was just coming in, and the first thing we were supposed to see was something called the unit membrane surrounding each cell –two dark lines surrounding a light line, the whole mess being about 60 – 80 Angstroms thick. The dark lines were held to be proteins and the light line was supposed to be lipid. Fresh off 2 years of grad school in chemistry, I tried to figure out just what the chemical treatments used to put tissue in a form suitable for electron microscopy would do to proteins and lipids. It was impossible, but I came away impressed with just how vigorous and traumatic what the microscopists were doing actually was.

To make a long story short — the unit membrane was an artifact of fixation. We now know that the cell membrane has a thickness half that of the unit membrane, with all sorts of proteins going through the lipid.

This is something to keep in mind, for you to avoid being snowed by such pictures. Clinical neurologists and neurosurgeons know quite well that a brain lacking oxygen and glucose swells (a huge clinical problem), and dead brain is exactly that.

Even with all these caveats about electron microscopy of the brain, I think the picture above is pretty close to reality. In favor of tight packing is the following work (along with the staining work of over a century ago). [ Proc. Natl. Acad. Sci. vol. 103 pp. 5567 – 5572 ’06 ] injected spheres of different sizes (quantum dots actually) into the rat cerebral cortex, and watched how far they got from the site of injection. Objects ‘as large as’ 350 Angstroms were able to diffuse freely. This was larger than the width seen on electron microscopy (180 Angstroms) but still quite small and too small to be ‘seen’ with visible light.

What’s the point of all this? Simply that the neuropil of the cerebral cortex (all the stuff in the picture which isn’t the cell body of the neuron or the astrocyte) could be where the real computations of the cerebral cortex actually take place. In my opinion, ‘could be’ should be ‘is’ in the previous sentence.

Why? Because of the work described in a previous post — which is repeated in toto below the line of ****

Briefly, the authors of that paper claim to be able to look at the electrical activity of these small processes in the neuropil. How small? A diameter of 5 microns or less. This had never been done before. It was a tremendous technical achievement to do this in a living animal. What they found was that the frequency of spikes in these processes (likely dendrites) during sleep was 7 times greater than the frequency of spikes recorded next to the cell body (soma) which had been done many times before. During wakefulness, the frequency of spikes in the neuropil was 10 fold greater.

I’ve always found it remarkable that most neurons in the cerebral cortex aren’t firing all that rapidly (a few spikes per second — Science vol. 304 pp. 523 – 524, 559 – 563 ’04 ). Neurons (particularly sensory neurons) can fire a lot faster than that — ‘up to’ 500/second.

Perhaps this work explains why — the real calculations are being done in the neuropil by the dendrites.

Even more remarkably, it is possible that the processes of the neuropil are influencing each other without synapses between them because they are so closely packed. The membrane potential shifts the authors measured were much larger than the spikes in the dendrites. So the real computations being performed by the brain might not involve synapses at all ! This would be an explanation of why brain cells and their processes are so squeezed together. So they can talk to each other. No other organ in the body is like this throughout.

This post is already long enough, but the implications are worth exploring further. I’ve written about wiring diagrams of the brain, and how it is at least possible that they wouldn’t tell you how the brain worked — https://luysii.wordpress.com/2011/04/10/would-a-wiring-diagram-of-the-brain-help-you-understand-it/.

There is another possible reason that the wiring diagram wouldn’t be enough to give you an understanding. Here is an imperfect analogy. Suppose you had a complete map of every road and street in the USA, along with the address of every house, building and structure in it. In addition you could also measure the paths of all the vehicles on the roads for one day. Would this tell you how the USA worked? It would tell you nothing about what was going on inside the structures, or how it influenced traffic on the roads.

The paper below is seminal, because for the first time, it allows us to see what brain neurons are doing in all their parts — not just the cell body or the axon (which is all we’ve been able to look at before).

If these speculations are true, the brain is a much more powerful parallel processor than anything we are able to build presently (and possibly in the future). Each pyramidal neuron in the cortex would then be a microprocessor locally influencing all those in its vicinity — and in a cubic millimeter of the cerebral cortex (1,000 x 1,000 x 1,000 microns) there are 20,000 – 100,000 neurons (Science vol. 304 pp. 523 – 524, 559 – 563 ’04).

Fascinating stuff — stay tuned

A staggeringly important paper (if true)

Our conception of how our brain does what it does has just been turned upside down, inside out and from the middle to each end — if the following paper holds up [ Science vol. 355 pp. 1281 eaaj1497 1 –> 10 ’17 ] The authors claim to be able to measure electrical activity in dendrites in a living, behaving animal for days at a time. Dendrites are about the size of the smallest electrodes we have, so impaling them destroys them. The technical details of what they did are crucial, as much of what they report may be artifact due to injury produced by the way they acquired their data.

First a picture of a pyramidal neuron of the cerebral cortex — https://en.wikipedia.org/wiki/Pyramidal_cell — the cell body is only 20 microns in diameter (the giant pyramidal neurons giving rise to the corticospinal tract are much larger with diameters of 100 microns). Look at the picture in the article. If the cell body is (soma) 20 microns the dendrites arising from it (particularly the apical dendrite) are at most 5 microns thick.

Here’s what they did. A tetrode is a bundle of 4 very fine electrodes. Bundle diameter is only 30 – 40 microns with a 5 micron gap between the tips. This allows an intact dendrite to be caught in the gap. The authors note that chronically implanted tetrodes produce an immune response, in which glial cells proliferate and wall off the tetrode, shielding it from the extracellular medium by forming a high impedance sheath. This allows the tetrode to measure the electrical activity of a dendrite caught between the 4 tips (and hopefully little else).

How physiologic is this activity? Remember that epilepsy developing after head trauma is thought to be due to abnormal electrical activity due to glial scars, and a glial scar is exactly what is found around the tetrode. So a lot more work needs to be done replicating this, and studying similar events in neuronal culture (without glia present).

Well those are the caveats. What did they find? The work involved 9 rats and 22 individually adjustable tetrodes. They found that spikes in the dendrites were quite different than the spikes found by a tetrode next to the pyramidal cell body. The dendritic spikes were larger (570 -2,100 microVolts) vs. 80 microVolts recorded extracellularly for spikes arising at the soma. Of course when the soma is impaled by an electrode you get a much larger spike.

More importantly, the dendritic spike rates were 5 times greater than the somatic spike rates during slow wave sleep and 10 times greater during exploration when awake. The authors call these dendritic action potentials (DAPs). Their amplitude was always positive.

They were also able to measure how the membrane potential of the dendrite fluctuated. The membrane potential fluctuations were always larger than the dendritic spikes themselves (by 7 fold). The size of the flucuations correlated with DAP magnitude and rate.

So all the neuronal spikes and axonal action potentials we’ve been measuring over the years (because it was all we could measure), may be irrelevant to what the brain is really doing. Maybe the real computation is occuring within dendrites.

Now we know we can put an electrode in the brain outside of any neuron and record something called a local field potential — which is held to be a weighted sum of transmembrane currents due to synaptic and dendritic activity and arises within 250 microns of the electrode (and probably closer than that).

So fluctuating potentials are out there in the substance of the brain, outside any neuronal structure. Is it possible that the changes in membrane potential in dendrites are felt by other dendrites and if so is this where the brain’s computations are really taking place? Could synapses be irrelevant to this picture, and each pyramidal neuron not be a transistor but a complex analog CPU? Heady stuff. It certainly means goodbye to the McCullouch Pitts model — https://en.wikipedia.org/wiki/Artificial_neuron.

Now we know why hot food tastes differently

An absolutely brilliant piece of physical chemistry explained a puzzling biologic phenomenon that organic chemistry was powerless to illuminate.

First, a fair amount of background

Ion channels are proteins present in the cell membrane of all our cells, but in neurons they are responsible for the maintenance of a membrane potential across the membrane, which has the ability change abruptly causing an nerve cell to fire an impulse. Functionally, ligand activated ion channels are pretty easy to understand. A chemical binds to them and they open and the neuron fires (or a muscle contracts — same thing). The channels don’t let everything in, just particular ions. Thus one type of channel which binds acetyl choline lets in sodium (not potassium, not calcium) which causes the cell to fire impulses. The GABA[A] receptor (the ion channel for gamma amino butyric acid) lets in chloride ions (and little else) which inhibits the neuron carrying it from firing. (This is why the benzodiazepines and barbiturates are anticonvulsants).

Since ion channels are full of amino acids, some of which have charged side chains, it’s easy to see how a change in electrical potential across the cell membrane could open or shut them.

By the way, the potential is huge although it doesn’t seem like much. It is usually given as 70 milliVolts (inside negatively charged, outside positively charged). Why is this a big deal? Because the electric field across our membranes is huge. 70 x 10^-3 volts is only 70 milliVolts. The cell membrane is quite thin — just 70 Angstroms. This is 7 nanoMeters (7 x 10^-9) meters. Divide 7 x 10^-3 volts by 7 x 10^-9 and you get a field of 10,000,000 Volts/meter.

Now for the main course. We easily sense hot and cold. This is because we have a bunch of different ion channels which open in response to different temperatures. All this without neurotransmitters binding to them, or changes in electric potential across the membrane.

People had searched for some particular sequence of amino acids common to the channels to no avail (this is the failure of organic chemistry).

In a brilliant paper, entropy was found to be the culprit. Chemists are used to considering entropy effects (primarily on reaction kinetics, but on equilibria as well). What happens is that in the open state a large number of hydrophobic amino acids are exposed to the extracellular space. To accommodate them (e.g. to solvate them), water around them must be more ordered, decreasing entropy. This, of course, is why oil and water don’t mix.

As all the chemists among us should remember, the equilibrium constant has components due to kinetic energy (e.g. heat, e.g. enthalpy) and due to entropy.

The entropy term must be multiplied by the temperature, which is where the temperature sensitivity of the equilibrium constant (in this case open channel/closed channel) comes in. Remember changes in entropy and enthalpy work in opposite directions —

delta G(ibbs free energy) = delta H (enthalpy) T * delta S (entropy

Here’s the paper [ Cell vol. 158 pp. 977 – 979, 1148 1158 ’14 ] They note that if a large number of buried hydrophobic groups become exposed to water on a conformational change in the ion channel, an increased heat capacity should be produced due to water ordering to solvate the hydrophobic side chains. This should confer a strong temperature dependence on the equilibrium constant for the reaction. Exposing just 20 hydrophobic side chains in a tetrameric channel should do the trick. The side chains don’t have to be localized in a particular area (which is why organic chemists and biochemists couldn’t find a stretch of amino acids conferring cold or heat sensitivity — it didn’t matter where the hydrophobic amino acids were, as long as there were enough of them, somewhere).

In some way this entwines enthalpy and entropy making temperature dependent activation U shaped rather than monotonic. So such a channel is in principle both hot activated and cold activated, with the position of the U along the temperature axis determining which activation mode is seen at experimentally accessible temperatures.

All very nice, but how many beautiful theories have we seen get crushed by ugly facts. If they really understood what is going on with temperature sensitivity, they should be able to change a cold activated ion channel to a heat activated one (by mutating it). If they really, really understood things, they should be able to take a run of the mill temperature INsensitive ion channel and make it temperature sensitive. Amazingly, the authors did just that.

Impressive. Read the paper.

This harks back to the days when theories of organic reaction mechanisms were tested by building molecules to test them. When you made a molecule that no one had seen before and predicted how it would react you knew you were on to something.