Tag Archives: axon

The wiring diagram of the brain takes another hit

Is there anything duller than wire? It conducts electricity. That’s about it. Copper wires conduct better than Aluminum wires. So what.  End of story. 

That’s pretty much the way we thought of axons, the wires of the nervous system. Thicker axons conduct faster than thin ones, and insulated axons conduct faster than non-insulated ones. The insulation is made out of fat and called myelin.  Just as fat in meat looks white, a bunch of axons sheathed by myelin looks white, which is how white matter got its name. 

Those of you old enough to remember vinyl records, know just how different a record sounds when played at the wrong speed.  That’s what an MS patient has to deal with.  The disease attacks white matter mostly, which means that when myelin is lost or damaged, nerve impulses slow down.  Information gets through, but it’s garbled. 

So we knew that losing myelin causes trouble, but other than that, it was assumed that myelin, once laid down by the cell producing it (the oligodendrocyte) was stable unless trauma or disease damaged it. 

That was until adaptive myelination came along roughly 10 years ago.  There is an excellent review [ Neuron vol. 109 pp. 1258 – 1273 ’21 ] which is irritating to read if you are looking for solid experimental facts.  This is not the fault of the authors.  They are trying to picture the frontier of a fast moving field.  By nature there is a lot of speculation in such an article, which would be a lot shorter (and duller) without it.  

However the following words occur frequently — could (43), has been understood (3), suggested (6), would (12), may (39) and is thought to (2).

The cells making the myelin are just that: cells.  Since the myelin they make is confined within them, a myelinated axon looks like a string of hot dogs, each dog the province of one oligo.  The space between the hot dogs is called the node (of Ranvier), and this is why myelinated axons conduct faster.  The impulse jumps between the nodes (saltatory conduction). 

Adaptive myelination comes in when you stimulate an axon — the myelin gets thicker, meaning that it conducts faster.   Also neuronal activity is held to alter myelin (the space between nodes gets longer meaning they conduct faster). 

Not all axons are myelinated, and activity ‘is thought to’ increase myelination of them. 

This has extremely profound consequences for how we think the brain works.  At the end of the post you’ll find an older one arguing that a wiring diagram of the brain (how the neurons are connected to each other) is far from enough to understand the brain.  But the article assumes that the wires are pretty much fixed in how they act. The Neuron article shows that this is wrong.  

Imagine if the connections between transistors on a computer chip, grew and shrunk depending on how much current flowed through them.   That appears to be the case for the brain.

Here’s the old post

Would a wiring diagram of the brain help you understand it?

Every budding chemist sits through a statistical mechanics course, in which the insanity and inutility of knowing the position and velocity of each and every of the 10^23 molecules of a mole or so of gas in a container is brought home.  Instead we need to know the average energy of the molecules and the volume they are confined in, to get the pressure and the temperature.

However, people are taking the first approach in an attempt to understand the brain.  They want a ‘wiring diagram’ of the brain. e. g. a list of every neuron and for each neuron a list of the other neurons connected to it, and a third list for each neuron of the neurons it is connected to.  For the non-neuroscientist — the connections are called synapses, and they essentially communicate in one direction only (true to a first approximation but no further as there is strong evidence that communication goes both ways, with one of the ‘other way’ transmitters being endogenous marihuana).  This is why you need the second and third lists.

Clearly a monumental undertaking and one which grows more monumental with the passage of time.  Starting out in the 60s, it was estimated that we had about a billion neurons (no one could possibly count each of them).  This is where the neurological urban myth of the loss of 10,000 neurons each day came from.  For details see https://luysii.wordpress.com/2011/03/13/neurological-urban-legends/.

The latest estimate [ Science vol. 331 p. 708 ’11 ] is that we have 80 billion neurons connected to each other by 150 trillion synapses.  Well, that’s not a mole of synapses but it is a nanoMole of them. People are nonetheless trying to see which areas of the brain are connected to each other to at least get a schematic diagram.

Even if you had the complete wiring diagram, nobody’s brain is strong enough to comprehend it.  I strongly recommend looking at the pictures found in Nature vol. 471 pp. 177 – 182 ’11 to get a sense of the  complexity of the interconnection between neurons and just how many there are.  Figure 2 (p. 179) is particularly revealing showing a 3 dimensional reconstruction using the high resolutions obtainable by the electron microscope.  Stare at figure 2.f. a while and try to figure out what’s going on.  It’s both amazing and humbling.

But even assuming that someone or something could, you still wouldn’t have enough information to figure out how the brain is doing what it clearly is doing.  There are at least 3 reasons.

l. Synapses, to a first approximation, are excitatory (turn on the neuron to which they are attached, making it fire an impulse) or inhibitory (preventing the neuron to which they are attached from firing in response to impulses from other synapses).  A wiring diagram alone won’t tell you this.

2. When I was starting out, the following statement would have seemed impossible.  It is now possible to watch synapses in the living brain of awake animal for extended periods of time.  But we now know that synapses come and go in the brain.  The various papers don’t all agree on just what fraction of synapses last more than a few months, but it’s early times.  Here are a few references [ Neuron vol. 69 pp. 1039 – 1041 ’11, ibid vol. 49 pp. 780 – 783, 877 – 887 ’06 ].  So the wiring diagram would have to be updated constantly.

3. Not all communication between neurons occurs at synapses.  Certain neurotransmitters are generally released into the higher brain elements (cerebral cortex) where they bathe neurons and affecting their activity without any synapses for them (it’s called volume neurotransmission)  Their importance in psychiatry and drug addiction is unparalleled.  Examples of such volume transmitters include serotonin, dopamine and norepinephrine.  Drugs of abuse affecting their action include cocaine, amphetamine.  Drugs treating psychiatric disease affecting them include the antipsychotics, the antidepressants and probably the antimanics.

Statistical mechanics works because one molecule is pretty much like another. This certainly isn’t true for neurons. Have a look at http://faculties.sbu.ac.ir/~rajabi/Histo-labo-photos_files/kora-b-p-03-l.jpg.  This is of the cerebral cortex — neurons are fairly creepy looking things, and no two shown are carbon copies.

The mere existence of 80 billion neurons and their 150 trillion connections (if the numbers are in fact correct) poses a series of puzzles.  There is simply no way that the 3.2 billion nucleotides of out genome can code for each and every neuron, each and every synapse.  The construction of the brain from the fertilized egg must be in some sense statistical.  Remarkable that it happens at all.  Embryologists are intensively working on how this happens — thousands of papers on the subject appear each year.



Now is the winter of our discontent

One of the problems with being over 80 is that you watch your friends get sick.  In the past month, one classmate developed ALS and another has cardiac amyloidosis complete with implantable defibrillator.  The 40 year old daughter of a friend who we watched since infancy has serious breast cancer and is undergoing surgery radiation and chemo.  While I don’t have survivor’s guilt (yet), it isn’t fun.

Reading and thinking about molecular biology has been a form of psychotherapy for me (for why, see the reprint of an old post on this point at the end).

Consider ALS (Amyotrophic Lateral Sclerosis, Lou Gehrig disease).  What needs explaining is not why my classmate got it, but why we all don’t have it.  As you know human neurons don’t replace themselves (forget the work in animals — it doesn’t apply to us).  Just think what the neurons  which die in ALS have to do.  They have to send a single axon several feet (not nanoMeters, microMeters, milliMeters — but the better part of a meter) from their cell bodies in the spinal cord to the muscle the innervate (which could be in your foot).

Supplying the end of the axon with proteins and other molecules by simple diffusion would never work.  So molecular highways (called microtubules) inside the axon are constructed, along which trucks (molecular motors such as kinesin and dynein) drag cargos of proteins, and mRNAs to make more proteins.

We know a lot about microtubules, and Cell vol. 179 pp. 909 – 922 ’19 gives incredible detail about them (even better with lots of great pictures).  Start with the basic building block — the tubulin heterodimer — about 40 Angstroms wide and 80 Angstroms high.  The repeating unit of the microtubule is 960 Angstroms long, so 12 heterodimers are lined up end to end in each repeating unit — this is the protofilament of the microtubule, and our microtubules have 13 of them, so that’s 156 heterodimers per microtubule repeat length which is 960 Angstroms or 96 nanoMeters (96 billionths of a meter).  So a microtubule (or a bunch of microtubules extending a meter has 10^7 such repeats or about 1 billion heterodimers.  But the axon of a motor neuron has a bunch of microtubules in it (between 10 and 100), so the motor neuron firing to  the muscle moving my finger has probably made billions and billions of heterodimers.  Moreover it’s been doing this for 80 plus years.

This is why, what needs explaining is not ALS, but why we don’t all have it.

Here’s the old post

The Solace of Molecular Biology

Neurology is fascinating because it deals with illnesses affecting what makes us human. Unfortunately for nearly all of my medical career in neurology ’62 – ’00 neurologic therapy was lousy and death was no stranger. In a coverage group with 4 other neurologists taking weekend call (we covered our own practices during the week) about 1/4 of the patients seen on call weekend #1 had died by on call weekend #2 five weeks later.

Most of the deaths were in the elderly with strokes, tumors, cancer etc, but not all. I also ran a muscular dystrophy clinic and one of the hardest cases I saw was an infant with Werdnig Hoffman disease — similar to what Steven Hawking has, but much, much faster — she died at 1 year. Initially, I found the suffering of such patients and their families impossible to accept or understand, particularly when they affected the young, or even young adults in the graduate student age.

As noted earlier, I started med school in ’62, a time when the genetic code was first being cracked, and with the background then that many of you have presently understanding molecular biology as it was being unravelled wasn’t difficult. Usually when you know something you tend to regard it as simple or unimpressive. Not so the cell and life. The more you know, the more impressive it becomes.

Think of the 3.2 gigaBases of DNA in each cell. At 3 or so Angstroms aromatic ring thickness — this comes out to a meter or so stretched out — but it isn’t, rather compressed so it fits into a nucleus 5 – 10 millionths of a meter in diameter. Then since DNA is a helix with one complete turn every 10 bases, the genome in each cell contains 320,000,000 twists which must be unwound to copy it into RNA. The machinery which copies it into messenger RNA (RNA polymerase II) is huge — but the fun doesn’t stop there — in the eukaryotic cell to turn on a gene at the right time something called the mediator complex must bind to another site in the DNA and the RNA polymerase — the whole mess contains over 100 proteins and has a molecular mass of over 2 megaDaltons (with our friend carbon containing only 12 Daltons). This monster must somehow find and unwind just the right stretch of DNA in the extremely cramped confines of the nucleus. That’s just transcription of DNA into RNA. Translation of the messenger RNA (mRNA) into protein involves another monster — the ribosome. Most of our mRNA must be processed lopping out irrelevant pieces before it gets out to the cytoplasm — this calls for the spliceosome — a complex of over 100 proteins plus some RNAs — a completely different molecular machine with a mass in the megaDaltons. There’s tons more that we know now, equally complex.

So what.

Gradually I came to realize that what needs explaining is not the poor child dying of Werdnig Hoffman disease but that we exist at all and for fairly prolonged periods of time and in relatively good shape (like my father who was actively engaged in the law and a mortgage operation until 6 months before his death at age100). Such is the solace of molecular biology. It ain’t much, but it’s all I’ve got (the religious have a lot more). You guys have the chemical background and the intellectual horsepower to understand molecular biology — and even perhaps to extend it.


Cultural appropriation, neuroscience division

If Deng Xiaoping can have Socialism with Chinese Characteristics, I can have a Chinese saying with neuroscientific characteristics — “The axon and the dendrite are long and the nucleus is far away” mimicking “The mountains are high and the Emperor is far away”. The professionally offended will react to the latest offense du jour — cultural appropriation  — of course.  But I’m entitled and I spoke to my Chinese daughter in law, and people over there found it flattering and admiring of Chinese culture that the girl in Utah wore a Chinese cheongsam dress to her prom.

Back to the quote.  “The axon and the dendrite are long and the nucleus is far away”.  Well, neuronal ends are far away from the cell body — the best example are axons from the sacral spinal cord which in an NBA player can be a yard long.  But forget that, lets talk about the ends of dendrites which are much closer to the cell body than that.

Presumably neurons have different types of dendrites so they can respond to different types of inputs. Why should dendrites respond identically if their inputs are different? They don’t.    A dendrite responding to acetyl choline will express neurotransmitter receptors distinct from another dendrite on the same neuron distinct from a dendrite responding to dopamine.  The protein cohorts of axons and dendrites are different.  How does this come about?  Because the untranslated part of mRNA on the 3′ end (3’UTR) contains a sequence called a zipcode which binds to specific proteins which then move the mRNA to a specific location in the neuron (axon or dendrite).  Presumably all dendrites initially had the same complement of mRNA.

So depending on what’s happening at a particular dendrite on a neuron, more or less of a given protein is made.   This is way too abstract.  Suppose you want to strengthen a synapse.  You’d make more of a neurotransmitter receptor or an ion channel for whatever transmitter that dendrite is getting.

It is well established that axons and dendrites store mRNAs and make proteins from them far from the nucleus (aka the emperor).  If you think about it, just how a receptor for dopamine gets to a dendrite receiving dopamine and not to a dendrite (on the same neuron) getting glutamic acid as a transmitter, is far from clear.  There are zipcodes distinguishing axons from dendrites, but I’m unaware that there are zipcodes for dopamine dendrites distinct from other types of dendrites.

If that weren’t enough consider [ Neuron vol. 98 pp. 495 – 511 ’18 ].  Even for an mRNA coding for the same protein (presumably transcribed from just one gene), there can be more than one type of 3’UTR (and this in the same cell).  Note also that 3’UTRs are longer in neurons than in other tissues.

So the authors looked at the mRNAs in dendrites — they did this by choosing a tissue (the hippocampus) where rows of cell bodies are well separated from their dendrites.  They found that for a given dendritic mRNA there was more than one 3’UTR, and that the mRNAs with longer 3’UTRs had longer halflives.  Even more exquisitly neuronal activity altered the proportion of the different 3’UTR isoforms. The phenomenon is quite general — over 50% of all genes and over 70% of genes enriched in neurons showed multiple 3′ UTRs.

So there is a whole control system built into the dendritic system, and it varies with what is happening locally.

The emperor emits directives (mRNAs) but what happens locally is anyone’s guess