Neurotransmitters as the Chemical Messengers of Certain Circuits and Pathways

Abstract

The fact that the brain is composed of a myriad of individual cells (neurons), rather than being a fused reticulum, was established early in the twentieth century by Santiago Ramón y Cajal, regarded as the father of modern neuroanatomy. This finding posed questions concerning the nature of the communication between nerve cells, as to whether it is electrical or chemical. Otto Loewi first demonstrated chemical transmission in the peripheral nervous system using a vagus nerve/heart preparation. He placed a frog heart with attached vagus in a container perfused with saline. Electrical stimulation of the nerve elicited slowing of the heart. He then removed some of the fluid and introduced it into another container with the same preparation, causing slowing of the second heart. He called the substance “Vagusstoff,” which was assumed to be acetylcholine (ACh) as was established later. Subsequently, Loewi demonstrated that stimulation of the sympathetic nerves to the heart elicited cardiac acceleration, also chemically mediated. He called this substance “Acceleransstoff,” ultimately identified as noradrenaline. Sir Henry Dale, who showed that ACh is the transmitter at the neuromuscular junction, shared the 1936 Nobel Prize with his friend Loewy. Electrical synapses, found much later, have received much less attention than chemical synapses, but are of importance.