Different classes of volume transmission signals exist in the central nervous system and are affected by metabolic signals, temperature gradients and pressure waves.

Volume transmission (VT) is the mode of intercellular communication involving the diffusion of transmitters, via extracellular fluid (ECF) pathways, from nerve cells selectively capable of producing the signal (signal source) to nerve and glial cells selectively capable of recognizing it (signal target). The proposal is now put forward that at least two classes of VT signals can be distinguished: (a) the private-code signals, exemplified by neurotransmitters, which are released by a limited group of nerve cells and recognized, via high affinity G-protein coupled receptors or by cytosolic enzymes such as guanylate cyclase in the case of nitric oxide, by a specific group of cells; and (b) the accessible-code signals, such as the electrical signals that are released by all neuronal cells and decoded by almost every CNS cell. In the present paper, it will be underlined that carbon dioxide, hydrogen ions, temperature gradients and pressure waves may be regulators of wiring transmission and VT.