The nervous system can be diagrammatically presented as a telephone system, with a series of PBX and larger exchanges, through which messages are transmitted electrically. The only operators needed are the automatic synapses, which occur by millions of millions.
Until recently, understanding of this exchange advanced gradually. Two techniques have served to clarify and fortify knowledge acquired painfully slowly. One has been the electron microscope, which has taken the synapse out of the uncomfortable status of being a postulate, and has established its concrete anatomical basis. The second has been an engagingly simple device (simple in theory, perhaps not in practice), namely, the technique of iontophoresis on a minute scale.
Acetylcholine has long been known as the chemical mediator of the electric impulse from the motor nerve ending to the muscle cell. Under the electron microscope the presynaptic end bulb, which is the terminus of the axon of the excitor
CHEMISTRY OF THE NERVOUS IMPULSE. JAMA. 1966;197(1):51. doi:10.1001/jama.1966.03110010103029