The chemical signals that neurons use to transmit information are referred to by
the general term neurotransmitter" (abbreviated here as Xtr).  As there are many
different chemical forms of transmitter, and many features that distinguish them,
and how they act, we need some additional schemes to organize our thoughts.  We
will consider the three major chemical varieties of neurotransmitters first     
(the amino acids, the amines, and the peptides) and then quickly consider their
different types of actions.