Some kinds, usually unmyelinated, axons are provided with long rows of rounded swellings: varicosities. The whole thing can be likened to a sparse string of pearls that wraps around in the nerve tissue, where the beads correspond to the varicosities and intermediate parts of conventional axon.
The varicosity contains swarms of membrane vesicles that resemble synaptic vesicles and contain transmitter substance (in this context, one often speaks of modulator substance instead). The varicosities are also provided with densified membrane sections similar to the presynaptic membrane parts.
When the varicous axon is active (transmitting nerve impulses), transmitter substance is released from the varicosities, and then usually not to some limited postsynaptic membrane section but directly into the volume of the extracellular space. Here, the transmitter substance can now spread over large areas and be active for a comparatively long time, from seconds to several minutes, and affect the groups of nerve cells (and glial cells) that are equipped with receptors sensitive to the transmitter (modulator) substance.
We are talking here about volume transmission and/or "global transmitter power".
Compare with synaptic transmission and "local transmitter power".
Transmitter substances common in these contexts are: