One of the most interesting things about
our brains is how neurons (nerve cells) talk to each other.
First, a little anatomy. Neurons have three basic parts, each of which
does
a different job:
 |
• The Axon: Relatively
thick and often very long 'cables', axons are the transmitter
towers of the neuron–they send out signals to other neurons.
• The Dendrite: Thin,
moderately long projections, dendrites act like antennas–they
pick up signals sent by other neuron's axons.
• Cell Body: The big
central part of a neuron is called the cell body. It contains
the cell's nucleus, DNA, and all the mechanisms cells need to
live and work. Like dendrites, the cell body can receive signals
from axons that are in contact with it. |
Although there are neurons everywhere
in the brain, the modulatory networks (such as the serotonin system)
have their own home areas: They live in clusters in the brain stem
called the raphe nuclei. From their home in the brain
stem, the serotonin neurons send long, highly branched axons out to
the rest of
the
brain:
In this picture, the size of the serotonin
neurons has been greatly exaggerated; there are several hundred thousand
serotonin neurons in the human brain, operating vast networks of fine
axons. The serotonin axons 'curl around' the brain instead of spreading
out strait because of how the brain develops. (The modulatory networks
are evolutionarily ancient, seen in the brains of all vertebrates...the
parts of the brain that can tell a dirty joke or order a pizza were
added on relatively recently.)
Call me!
When one neuron wants to talk to another,
it tells its axon to send out a signal. This signal is sent by releasing
a chemical called a neurotransmitter. Types of neurons are
named for the type of neurotransmitter they use. For instance, dopamine
neurons use the neurotransmitter dopamine to signal other neurons, serotonin
neurons use serotonin, and so forth.
The place where an axon meets up with
another neuron is called a synapse. In a synapse, a small open
space separates the two neurons. Technically this gap between neurons
is called the synaptic cleft, but people usually just call it
'the synapse.'
Let's take a closer look at the structure
of a synapse:
That looks like a mess, but we'll take
it one step at a time. The new players:
• Vesicles: These are temporary
storage tanks for neurotransmitters. You can see one vesicle being dumped
into the synapse to signal the second neuron.
• Receptor: These are the
actual 'sensors' that detect a neurotransmitter. Each type of receptor
has a different shape that fits a different neurotransmitter, like a
key in a lock. One of the receptors shown here has been 'turned on'
(activated) by the right neurotransmitter fitting into it. What the
neuron will do in response to one of its receptors being activated
varies...often it will cause the neuron to fire (release its own neurotransmitters
to signal even more neurons.)
• SERT: The Serotonin Transporter. Rather than just let the neurotransmitter
escape and be wasted, the axon uses these little pumps to suck it back
up and reuse it.
So, the whole cycle is:
1. An axon releases neurotransmitters into
the synapse.
2. The neurotransmitters fit into receptors,
activating them (delivering the signal.)
3. The transporter (SERT in the case of serotonin)
pumps the released neurotransmitter back into the axon for reuse.
This
process isn't perfect...some of the neurotransmitter will escape, but
not much under normal circumstances. To replace what does get away,
the body is continually making more.
How much/how often neurotransmitters
are released determines how strongly the neuron on the receiving end
will
be affected. If only a little serotonin is released, you may feel irritable,
anxious, or even depressed. On the other hand, if a lot of serotonin
is released you will probably feel calm, relaxed, and happy.
MDMA works primarily by causing the
serotonin axons to release a lot more serotonin than they normally
would. As the amount of serotonin in the synapse increases, the rate
of serotonin receptor
activation increases...and you start to feel different, and probably
very good.
The downside is that, by forcing a lot
of your serotonin to be released (and interfering with the SERT's
ability
to get it back) a great deal of serotonin escapes the synapse and is
broken down in your body. Given time (it may take days, even a week
or so) your body will replace the lost serotonin (see the 5-HTP
section in Preloading). In the meanwhile,
however, your brain may not be able to release as much serotonin
as
it wants to, causing irritability, anxiety, and other symptoms. Taking
5-HTP to quickly replace serotonin is a very useful tool to prevent
such side effects following the use of MDMA.
MDMA
affects dopamine and norepinephrine neurons in much the same way.
On to MDMA
at work...How it releases serotonin.
