Neurons and Basic Chemistry

Post #2 of this series on the Brain (#1). As I mentioned last time, these posts are from a series of lectures by Prof. Jan Schnupp, and I want to make sure he is properly quoted and credited. Many parts are his lectures verbatim. However, for any errors, mistakes or inaccuracies in anything I write in here, I take all the credit. I probably misunderstood him or made it up. His course material is available here.

Neurons come in lots of different shapes and sizes.

This image here is a so-called Purkinje cell, which lives in the cerebellum. People always show that because it's a particularly beautiful neuron.

It really looks almost like a coral. In documentaries you may have seen these sort of corals that extend out all these sort of branches, which just feed from the current. And the Purkinje cells are arranged in the cerebellar cortex, one after the other in a stack, and you get lots and lots and lots of axons, sort of basically cables coming from other neurons that will run through this, and you can almost imagine that they are basically filtering out information that they want.

And the things that are growing out of there are called the dendrites of the neuron. Dendrites from Greek dendron meaning tree. They've got basically tree-like branches growing out of them, very beautiful in the Purkinje cell.

By The original uploader was Cahass at English Wikipedia. - Transferred from en.wikipedia to Commons., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=651365

Perhaps less beautiful but still very well developed is the so-called pyramidal cell from the cerebral cortex. This is called pyramidal because it's got a little bit of a pyramid shape in its cell body. It is the most common cell type in the cerebral cortex. And so a typical neuron, in as far as there is such a thing, will have one little body called its soma. The plural of soma is somata.

It may have one or several dendrites growing out of it and it will typically have one axon growing out of it, only one. But this one axon that grows out can branch very heavily. And neurons, as we will see later, are really devices for collecting information and for processing information. And they collect their information almost exclusively through their dendrites and a little bit through their soma.

And they then output their information to the axon.

We basically collect information and then we think about it and integrate it and then we basically make a decision about that information and we send our decision out to the axon. We'll look at how this particular process works in more detail.

Neurons have dendrites and axons. Under the microscope they look a bit like this.

‘Celula del lobulo cerebral electrico del torpedo. Coloracion por el liquido de boberi.’ by Santiago Ramon y Cajal. Wellcome Collection. CC BY.

These are sort of classic drawings by a famous neuroanatomist called Ramon y Cajal, who sort of drew them in the 1900s, early 1900s. And, well, they look like biological cells. They have little membranes around them. They have got a nucleus inside them. But, what are they made of and what would we see if we zoomed in even further? Well, ultimately there comes a point where if we zoom in further, we'll see that this is all made out of molecules and atoms.

So, atoms are made out of a positively charged nucleus and then they have got a shell that is made out of negatively charged electrons.

Usually you have a certain number of positive charges which come from so-called protons that sit in the nucleus. And the more positive charges you have in there, the more negative charges, the more electrons, you have in your shell, effectively. So, the more positive charges you have in the centre, the more negative charges you have in the outside.

There are very complicated reasons that have to do with quantum physics on why atoms actually become particularly stable if they can have a certain numbers of electrons in their shells. But sometimes, these numbers aren't necessarily the same number as the number of positive charges. If you are an atom with just one positive charge, it turns out you would be stable if you have two electrons. So, if we have a hydrogen atom which just has one positive charge in the middle and one electron in the shell, then the most stable thing is to find another atom with which it can share electrons, so that they can have a pair.

So, if you then have two hydrogen atoms linked together, then you end up with two hydrogen atoms making a hydrogen molecule, which is linked together by a so-called covalent bond. Covalent bond simply means that you have atoms that are linked together into a molecule by sharing electrons. So, we now have atoms making molecules, and we have molecules making everything else. So, if we were to zoom into this, we would ultimately see atoms and molecules, but there are certain atoms and molecules that we would see a lot and substances that are going to be of particular importance.

These are water, salt, fat, and protein. These are the ingredients, if we are going to make a neuron. Firstly take water, about 60% or so is going to be water. Salt, much less, but it is going to play a hugely important role. Fat and protein are the other main ingredients. Water, our main thing, has an oxygen atom and two hydrogen atoms.

They will link together in covalent bonds just in order to be able to produce sort of an outer shell where the electrons can whiz about which is nice and stable. So, this is by far the most abundant molecule in your brain, and it plays an important role.

                         Source: Climate Science Investigations (CSI), NASA

Something important about the shell of the water molecule is that the electrons do not whiz about in a way so that they are everywhere the same amount of time.

They will actually spend more time around the big fat oxygen atom than they'll spend around the two hydrogen atoms. That means that the electrons which are negatively charged will be more around the oxygen atom, so therefore it will be a little bit more electrically negative on one side.

Why is that important? Well, negative charges are attracted, a bit like little magnets, to positive charges. This is the electrostatic force. This electrostatic force, of course, means that if we have a water molecule that has a negative side and a positive side, the positive side of one water molecule will be attracted to the negative side of the other. You can imagine they will stick together. So, that makes water molecules sticky relative to other water molecules and this is one of the reasons why water likes to form drops.

It also helps water dissolve salt. Salt is another really important ingredient for brains. Brains cannot work without salt. It doesn't mean you should eat too much salt, but certainly if you do not have enough salt in your diet, there would come a point where your brain would stop functioning and your muscle cells would also stop functioning. You need salts in order to generate the electricity that your body needs.

How does that work? You may have heard that table salt is sodium chloride. That means you have sodium atoms and you have chlorine atoms that make it up. But your sodium atom will normally have 11 protons and will therefore be entitled to 11 electrons in its shell and the chlorine has 17 protons and is entitled to 17 electrons. But it turns out that actually, rather than sharing electrons, the sodium atom is relatively happy to just give up one electron and just go around with 10 electrons in its shell, meaning that it becomes an ion where it is basically a free-floating atom that has one extra positive charge, so it is like a big positive charge.

Whereas the chlorine atom will quite happily steal this electron from the sodium atom, so that it becomes a chloride ion which has an extra electron and is therefore negatively charged. So, if you then have a salt crystal, you have these chlorine ions and the sodium ions, which are negatively and positively charged little particles and they will be mutually attracted to each other and they will pack themselves nice and densely together in a sort of nice crystal lattice. We have a little salt crystal.

But as soon as you get water coming into contact with these crystals, what is going to happen is that the positively charged hydrogen atoms are going to become attracted to the negatively charged chlorine.

And that means that the water will basically start to group itself around the chlorine ion and that will allow the chlorine ion to basically then leave this crystal lattice and float off. Similarly, the positively charged sodium ion will be attracted to the negatively charged oxygen atoms and make the sodium float away. So you end up with chlorine ions and sodium ions being able to just float off and disperse.

These are then freely floating ions which have water molecules that are electrically attracted to them, densely packed around them.

This is known as a hydration shell. Basically, if you are an ion and you are in water, all the water molecules, because they themselves are polar, will become attracted to you and will flock around you. This is why the salts quite happily dissolve in water.

Now, as soon as you have charges, electrically charged particles being able to flow about and move about, you have what people call electrical currents.

If you switch on the electric light and there is a current flowing through your switch, that current will normally be carried by electrons that in the metal can whiz from one electron shell to another. So all the electrical items that we use in our domestic lives in the modern world will have electrical currents carried by electrons.

In your body, in your brain, in your muscle cells, in your heart cell, there are lots of electrical currents going on. They are all carried by ions that are in solution.

The fact that the fluids in your cells and between the cells in your body have salts dissolved in them makes the water a conductor of electricity.

Next post: Diffusion and Membranes