You Think You've Got Chemistry... Part 1

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Welcome to the "Function" section of The Lab Rats Guide to the Brain.  The first post in this section is actually the post on Oxygen and Glucose utilization that I posted about a month ago (http://teddysratlab.blogspot.com/2011/03/give-me-air.html).  We will start with some recap some previous discussions about neurotransmitters and then look further into the chemistry of the brain.

Like all of the other cells of the body, the brain rests in a salt-water solution.  When popular scientific articles use this fact as evidence of evolution from sea-life, they often make a huge mis-statement about that salty solution - usually in claiming that the salinity of blood, lymph and cerebrospinal fluid is the same as seawater.

It isn't.

Seawater on average ranges from 2 to 4% salt, and is often higher in concentration in some of the "brackish" wetlands that are home to many transitional creatures.  The saline solution in the body is a pretty precise 0.9% salt.  Even salt-water fish do not have much higher than 1% salt in their body fluids, the gills only allow water through into the blood and intracellular fluids - salt is concentrated and excreted, much like what our kidneys do.

Yet the salt solution in our bodies has some very specific functions:  in the kidneys, regions with different concentrations of salt (ranging from 1-2%) serve the important purpose of helping to retain and conserve water and keep the body from being dehydrated (or losing too many chemicals in the urine).  In the brain, the compounds comprising the salts (mostly sodium-, potassium-, calcium-, and magnesium- as -chlorides, -sulfates and -carbonates) are separated, resulting in charged molecules (ions) that can be used to form electrical and chemical gradients (above, right).  This separation of ionic charge allows voltage to be built up, then discharge when channels that pass these ions are selectively opened (left).Any separation, then selective discharge of electrical charge can be harnessed to do *work* as the physicists term it.  In the case of the brain, the "work" is the transfer of information from neuron to neuron as a pattern of electrical pulses.

So, on the one hand, the chemistry of the brain is one of electrolytes and electrical currents.  Then there are the chemical neurotransmitters that act in place of a "spark" jumping from neuron to neuron. 

However, if all neurons had the exact same chemistry, there would be no way to make different neurons do different jobs.  After all, if all neurons were simple on-off switches, the brain would be a *Very* dumb switching station.  Even a digital computer relies on different voltages, and transistors which have some very exotic properties other than a simple on-off. 

The brain has a much more complex circuitry - there are excitatory neurons, inhibitory neurons, neurons which switch roles depending their targets.  To allow the *same* basic unit - a neuron - to perform wildly different functions, the brain - and nervous system in general - uses different neurotransmitters to connect neurons.  In fact, a single neuron will quite frequently have receptors for most of the common neurotransmitters (at right) so that inputs from different brain areas have different functions.  While most neurons will only *make and release* one of the neurotransmitters, they can respond to any neurotransmitter for which they have the appropriate "receptor".  The neurotransmitter and receptor act as a sort of "lock and key" system, where the receptor on a neuron is the lock, and the key is the neurotransmitter chemical.  Only if the appropriate neurotransmitter "key" is present, will the receptor "unlock" and perform the appropriate action on the neuron.  In addition, there can be different receptors that recognize different parts of the chemical, and have different actions on the neuron (such as excitation vs. inhibition). 

With all of that chemical activity, is it any wonder than the chemicals we ingest (eat, drink, sniff, smoke, inject or smear on the skin) can have such a profound effect on brain function?  In the next blog we will look at the different ways that externally applied drugs appear similar in structure to neurotransmitters in order to alter brain function.

So stay tuned for "You Think You've Got Chemistry? Part 2" the day after tomorrow. 

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