In order to understand how the signaling function of the brain can be
disrupted, it is first necessary to have a basic idea of how this signaling
works. The brain is essentially a mass of cells, or neurons, which
are capable of transmitting chemical signals to one another and propagating
electrical signals internally. This information can be transmitted
because neurons are each equipped with several special features that make
them different from other cells.
All neurons have a cell body and several extensions from this cell body
called dendrites and axons. A typical neuron usually has many dendrites
and only one very long axon. The dendrites contain receptors embedded
in the membrane, which are proteins that respond to chemical signals.
The axon has a terminal at its end that releases the chemical messengers,
or neurotransmitters, to the next neuron.
One of the most important ideas to consider about signal transmission in
neurons is that there is an actual physical space between one neuron's
axon and the next neuron's dendrites. This space is called the synapse
and is the site communication between cells. The neuron releasing
the neurotransmitter is called the pre-synaptic neuron and the one accepting
the neurotransmitter is called the post-synaptic neuron. The pre-synaptic
neuron will release its neurotransmitter into the synapse. This chemical
messenger will diffuse across the synapse and interact with specific receptors
on the post-synaptic membrane.
The interaction between the neurotransmitter and receptor will cause changes
in the post-synaptic neuron which will cause an electrical potential (or
signal) to be generated. This electrical signal will be driven down
the length of the axon until it reaches the terminal. When the signal
hits this part of the axon it will cause the terminal to release neurotransmitter
into the next synapse. These neurotransmitters will cross the synapse
and interact with the receptors on the next neuron, thus continuing the
All neurons in the brain generally function in this manner, but it is also
important to realize just how many neurons there are in the brain (about
100 billion) and how many different structures these neurons give rise
to. The real mystery of the brain is how these signals on a cellular
level give rise to our abilities as human beings. When a certain
structure in the brain is damaged by a tumor or stroke, for example, the
signals can't get through to their appropriate connections and the function
of that structure is impaired. Each structure is responsible for
controlling a different aspect of our behavior, so depending on where the
damage is, many different kinds of deficits can arise. This is why
imaging is so imperative to medical diagnosis. Locating the damage
precisely can make an enormous difference on how it is treated.