Monday, 1 July 2013

The brain, like Gaul, is divided into parts

For a few centuries people have cut brains up and named the various lumps, bumps, holes, and sheets that they found. This could be likened to early astronomy. Let us call this desire to name bits of the brain cerebonomy, just to have the joy of coining a new word! Cerebonomy is simply naming the brain parts, like the naming of stars, without any reference to what a star is or why it shines. More recently, with the aid of microscopes, researchers have been able to discern a multitude of layers, regions and divisions within each lump or bump that had previously been given a name. Each layer and region is then also given a name.  This makes the simple profusion of names one of the chief obstacles to reading brain-related literature, and there is no way I can think of to simplify things.

Here are some bits you should know in a simple glossary.
  • The brain stem is the bit of your brain that is obviously an extension of your spinal cord. It is not divided into 'left' and 'right' sections like the rest of the brain. On a side view of a human brain you can see just a little bit of the brain stem sticking out the bottom like the stem of a cauliflower, but it carries on up inside almost to the centre of the brain, and indeed incorporates some (or all) of what is usually called the mid-brain or mesencephalon.
  • The cerebral cortex (confusingly often just called cortex) is the folded 'cauliflower' bit that is the most visible feature of a human brain. It is a sheet that isn't very thick (cortex is from the latin for 'bark' of trees) and in humans it is around 4mm thick. It is divided into the left and right hemispheres although these are joined in various places by large, fast, bundles of connections. It is the 'greyest' part of the brain indicating that its connections are dense and mostly very short-range.
  • The next bit of interest, the diencephelon, refers to most of the bits that you cannot see from the outside (because they are covered by the cortex) and which are joined to the top of the brain stem and the mid-brain. If you turn the brain to look directly up the brain stem, you can catch a glimpse of part of the diencephalon.
  • An obvious feature of the human brain is the cerebellum.  It is almost like a separate little brain that sticks prominently out of the back of a human brain and is joined to the brain stem. Its surface is folded - more narrowly folded than the cerebral cortex - and consists of a single sheet of tissue in a folded arrangement like an accordion.
These are the major divisions of the brain - not really like the divisions of Gaul, more like the continents of the globe. Like the continents there aren't too many of them!

Part of: Martin’s Vastly Oversimplified and Woefully Incomplete Guide to Everything in the Brain as featured on the Brainsex website.

Saturday, 29 June 2013

50 Shades of Brain

People often talk about grey matter and white matter without making it clear what they mean; even a fictional character like Hercule Poirot, for example, makes references to his 'little grey cells'. The brain can be usefully divided into areas where the connections are very dense and so short that speed isn't much of an issue (grey matter); and other areas where the connections are less dense, but faster, and often cover longer distances, called the white matter.

A dead brain does look sort of grey and white in different areas. A living brain is mostly browny-pink, because it is well supplied with blood vessels, while the grey matter is slightly darker (browner).

The white matter looks white (or paler) because it is high in fat. The brain uses fat to surround and insulate the connections, to stop spikes from 'leaking away' as they travel to the synapses. So areas of the brain responsible for the long-range connections have very fatty axons, and these regions appear paler. And in contrast, areas containing only very short-range connections, which need very little fat, appear darker or grey.

To complete the picture there is a small amount of 'black matter' which is a distinct area of the brain, deep inside, which appears darker than the grey matter because the cells have pigment in them. This area is always referred to by its Latin designation substantia nigra - in contrast to grey and white matter which are hardly ever called substantia grisea and substantia alba.

Oh yes, and there is a blue bit. The locus coeruleus. I have never seen one but apparently it looks a bit blue.

Part of: Martin’s Vastly Oversimplified and Woefully Incomplete Guide to Everything in the Brain as featured on the Brainsex website.

Saturday, 22 June 2013

The chemical truth!

In other posts we have established that the brain is a network of cells, which almost (but not quite) touch each other, which send out pulses of electrochemical activity if there is enough of the right kind of activity around them, and that they have special structures called synapses to manage the communication between them.

Synapses are where the action truly is, and any discussion of how the brain works has to have them at its heart. It is useful to get a primitive idea of what they do split up into three time scales:

On the very short time scale (thousandths of a second), activity in the synapses is dominated by neurotransmitters. These these are chemicals that are produced inside the neurons and are responsible for carrying the wave of activity over the the synaptic cleft to the next neuron. These chemicals are not very 'famous' so we don't really need to know what they are called at this stage. They are either excitatory - that is they increase the likelihood that the post-synaptic neuron will fire - or they are inhibitory. They are released only when the spike reaches the synapse.

On a longer time scale (seconds to hours or even days), the behaviour of the synapses is changed by another, much more famous, group of chemicals called neuromodulators. These are not produced in the synapses they affect, but are usually made in other parts of the brain.

Many neuromodulators are well known because they are linked in the popular imagination to particular behaviours: adrenaline (fight and flight), dopamine (reward and pleasure), histamine (allergic reactions), oxytocin (love and bonding), serotonin (happiness!), melatonin (sleep cycles), and many, many others.

In reality, things are much more complicated than this picture (one modulator - one behaviour) suggests! Things are further complicated by the fact that many neuromodulators are also neurotransmitters (although not necessarily in the brain) and many of them are hormones with wide-ranging effects apart from their effects on synapses. A chemical like oestrogen for example (slightly controversial to include this as a neuromodulator - but justified I think) has hundreds of well-documented effects on pretty much every part of the body.

On the longest time scale (hours, and days, and months), synapses actually appear and disappear, are strengthened and weakened, grow and shrink. (And there may be hundreds of other behaviours yet to be documented.) This is controlled by a vast array of factors including the neuromodulators, genetics and epigenetic control. This is only just beginning to be documented, and is certainly not well understood. The longer ("developmental" some might say) time scale is more or less undiscovered country for neuroscientists at a cellular level, and patterns that emerge on this time scale are still something of a mystery.

Part of: Martin’s Vastly Oversimplified and Woefully Incomplete Guide to Everything in the Brain as featured on the Brainsex website.

Tuesday, 4 June 2013

The Neural Hypothesis

The brain is an organ made up of cells. This is just like any other tissue or organ in the body is made up of specialized cells. The heart is composed of heart cells that are found nowhere else in the body, the liver of liver cells, and so it is with the brain.

The most famous type of brain cell is the neuron. Actually there are hundreds of different types of neuron but we will ignore that here. Neurons differ from all other cells in the body because they send out projections to meet each other to form a complicated network capable of fast and flexible communication. All cells are capable of communicating with each other at some level, but we are talking about something much faster and more flexible found only in cells of the nervous system.

For a long time people thought that the cells of the brain (and the rest of the nervous system) were all joined up in a huge net; this was called the 'reticular hypothesis'. But towards the end of the 19th century it became clear that they don't actually join up, and that each neuron was separate and a "fully autonomous physiological canton" (Cajal, 1888). Thus was born the neuron hypothesis.

(We will ignore, for the moment, all brain cells that are not neurons - although it turns out that this is probably a big mistake.)

So, the neuron sends out information to other neurons down a thick-ish fibre called an 'axon', and gathers information from its surroundings, through thin thread-like projections called 'dendrites'. Dendrites and axons are usually, but not always, on opposite sides of the cell body which, if you look at pictures, is the 'blob' in the middle.

The most obvious sort of activity that is carried away from the neuron by the axon is the action potential, which is a spike of electrochemical activity. These spikes, or clicks, are the usual form in which information is encoded and moved around the brain. In the places where an outgoing axon gets close to, but never quite touches, a receiving dendrite, there is a special structure that controls the passage of information across the gap, or cleft, called the synapse.

You won't find this in many textbooks, but my money, and all the smart money is on the synapse (which is not yet that well understood) being the key to much of the really clever stuff that happens in the brain.

From: Martin’s Vastly Oversimplified and Woefully Incomplete Guide to Everything in the Brain as featured on the Brainsex website.

Thursday, 16 May 2013

Detecting activity in the brain

So, as has been mentioned elsewhere, neurons pick up the pulses or spikes of activity that travel away down the axons of its neighbouring neurons. This activity jumps the gap at the synapse on to the dendrites of the receiving neuron. The activity the neuron picks up can either be excitatory (making it more likely to generate its own spike) or inhibitory, but if there is enough excitatory activity the neuron responds by generating a spike of its own that travels away down its own axon.

The spikes are like Mexican waves of chemicals moving in and out across the walls of the axon which is hollow. The atoms that move in the wave all carry an electrical charge, so the propagating wave is, in some ways, like an electrical current in a wire. However this analogy can be pushed too far. The brain is not an electrical device in the usual sense (which would involve electrons moving through a solid or a gas) the brain is a liquid phase machine and the spikes are movements of charged atoms - or ions as they are known.

The only way to detect an individual spike in an individual neuron, is stick a glass needle into the brain and get it as close to the neuron as possible. This is fraught with difficulty and interpretation of the results can be very tricky.

However, groups of neurons tend to fire more or less at the same time, and the axons of the group are, more or less, all pointing in the same direction in many cases. The total resulting current is large enough to be detected by placing coils on the scalp. This technique is known as EEG (electroencephalography). The EEG is incredibly useful because it records exactly when the activity happens but, frustratingly, it is very difficult to pin down exactly where the signal came from - only a very rough answer is usually possible. And in any case this technique is limited to activity that is near the skull (mostly the outside 4mm or so of the brain known as the cortex).

If something causes the brain to generate more spiking activity than usual, then, between 2 and 6 seconds later, there is a corresponding increase in blood flow in the area of activity. Although this increase in blood flow is generated way after the event, doesn't directly measure the activity of the neuron, and contains no fine timing detail, it does have the the huge advantage of being easy to locate using a technique known as MRI (magnetic resonance imaging).

From: Martin’s Vastly Oversimplified and Woefully Incomplete Guide to Everything in the Brain as featured on the Brainsex website.