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.