Science

A Brain Primer

We know mental illness has to do with the brain, but do we know why?

The brain is a three-pound mass containing some 100 billion nerve cells - neurons - thousands of different kinds, each forming more than a thousand synaptic connections with other neurons. In all there may be anywhere between 100 trillion and a quadrillion synapses organized into elaborate networks that account for the brain's vast complexity.

Just to give you an idea of how truly complicated the brain is, of 35,000 genes in the human genome, some half to three-quarters of these go into the brain's makeup.

So what happens when someone - say a little man inside - decides to throw a switch? To start, signals from the neuron travel out a single extension called the axon that may end in several terminals. These signals are picked up by branches - dendrites - extending from other neurons.

Communication occurs at specialized structures called synapses, organized into two parts for sending and receiving. The presynaptic structure is located on a terminal portion of the sending neuron that contains packets of sending chemicals, or neurotransmitters. The postsynaptic structure on the receiving neuron has receptors for these molecules.

There are two main types of molecules that function as neurotransmitters: small molecules - including dopamine, serotonin, and norepinephrine - and larger molecules that are essentially protein chains. All told, there are some 100 types of neurotransmitters in the brain. According to the Report:

"A neurotransmitter can elicit a biological effect in the postsynaptic neuron by binding to a protein called a neurotransmitter receptor. Its job is to pass the information contained in the neurotransmitter message from the synapse to the inside of the receiving cell."

The neurotransmitter dopamine has five known receptors, and serotonin has at least 14.

One class of neurotransmitter receptors allows either a positive or negative charge to enter the cell, either exciting or inhibiting that cell's firing action. Glutamate is an example of an excitatory neurotransmitter and GABA an example of an inhibitory one.

The neurotransmitters we are most familiar with - dopamine, serotonin, and norepinephrine - are neither. They interact with signaling proteins - G proteins - found inside the cell membrane in a way which allows the receiving neurons to process signals from glutamate and GABA. According to the Report:

"To use a metaphor of a musical instrument, if glutamate, the excitatory neurotransmitter, is puffing wind into a flute or clarinet, it is the modulatory neurotransmitters such as dopamine or serotonin that might be seen as playing the keys."

For every 200,000 cells in the brain, only one makes dopamine. Even fewer make norepinephrine. Dopamine-producing neurons are clustered in but a few brain regions, deep in the midbrain. Norepinephrine is made even further down, in the brain stem, and serotonin-producing neurons in somewhat larger numbers are spread along the brain stem.

Thanks to the neurons' axons and their ability to extend throughout the brain, their communicating neurotransmitters have the capacity to light up the entire nervous system. According to the Report:

"These neurotransmitters are responsible for brain states such as degree of arousal, ability to pay attention, and for putting emotional color or significance on top of cold cognitive information ... It is no wonder that these modulatory neurotransmitters and their receptors are critical targets of medications used to treat mental disorders - for example, the antidepressant and antipsychotic drugs - and also are the targets of drugs of abuse."

But we are not just a bag of cells and chemicals. The crowning complexity of the brain, according to the Report, is that it is not static. Every time a person learns something new, that experience alters the structure of the brain. In the words of the Surgeon General:

"Experience that is salient enough to cause memory creates new synaptic connections, prunes away old ones, and strengthens or weakens existing ones ... The end result is that information is now routed over an altered circuit. Many of these changes are long-lived, even permanent. It is in this way that a person can look back 10 or 20 or 50 years and remember family, a home or school room, or friends."

And this is one reason healing is possible. We may see ourselves as prisoners of the hardwiring inside our heads, but the brain's seemingly limitless capacity to remap and transform itself ultimately gives us a fighting chance. Even as our mind may be telling us that life is hopeless, unnoticed inside the head a hundred billion neurons are working overtime laying the foundations for a new tomorrow.

Thanks to the neurons' axons and their ability to extend throughout the brain, their communicating neurotransmitters have the capacity to light up the entire nervous system. According to the Report:

Updated Feb 11, 2008