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Signals

      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 process.    

      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.