A neuronal circuit is in principle a lot like an electrical circuit.  Two places
in the brain connected by a living "wire" of an axon, and communicating through a
living junction, called a synapse.  The sending cell ( or source neuron)
transmits a chemical message in the form of a   neurotransmitter(s); this signal 
changes  the activity of the receiving  cell (or target  neuron).  At this level
of study, we need to define the three basic patterns of neuronal circuitry.  In
the next section  "Transmitters/Actions" we will consider the transmitters and
their effects.