text: Neurotransmitter systems are systems of neurons in the brain expressing certain types of neurotransmitters, and thus form distinct systems. Activation of the system causes effects in large volumes of the brain, called volume transmission. Volume transmission is the diffusion of neurotransmitters through the brain extracellular fluid released at points that may be remote from the target cells with the resulting activation of extrasynaptic receptors, and with a longer time course than for transmission at a single synapse. Such prolonged transmitter action is called tonic transmission, in contrast to the phasic transmission that occurs rapidly at single synapses.