May 06, 2006

The Means of Communication Between Nerves

Otto Loewi, On the Humoral Transmission of the Action of the Cardiac Nerve, 1921

Nerves were discovered during dissections performed by the ancient Greeks. The idea that nerves transmit electricity, and cause various quick responses in the body, was built up slowly through many small experiments. Luigi Galvani showed that electricity causes the contraction of frog muscles in the 1790s. Camillo Golgi figured out how to stain nerves cells, opening the way to the study of individual neurons starting in 1873. Santiago Ramón y Cajal showed that neurons transmit signals from their dendrites to their axons, and that there is a tiny gap between neurons, called the synapse, during the 1890s.

Most physiologists assumed that electric signals bridged the synapse, as well as travelling the length of the neurons. This is not to say that they didn't consider the other possibilities: at the turn of the century there were a number of scientists who proposed that chemical messengers were used to transmit nerve impulses across the synapse. But no strong evidence was produced, and by the end of World War I the electrical theory was generally accepted.

In 1921, Otto Loewi was 47 years old. He lectured, taught classes, and regularly published excellent work, but was not obviously a remarkable scientist. Nonetheless, he was about to disprove the idea that if you haven't done any groundbreaking work by the time you turn 30, then you're never going to.

The experiment described in his paper is very simple. With a little work, a frog heart can be kept alive and beating on a lab bench for several hours. During this time, it is immersed in something called Ringer solution, which physiologists have long used to keep living organs and animals healthy during experiments and surgery. Loewi collected some of the Ringer solution immersing his frog hearts, then stimulated the nerves that reduce the rate and strength of heart beats. While this was happening, he collected some more Ringer solution. After the reaction had died down, he removed the nerve that he had stimulated.

Loewi added some of the first batch of Ringer solution back into the mix. Nothing happened. Then, he added some of the second batch, and the heart reacted exactly as if he had stimulated it again with the now unattached nerve.

Conclusion: a chemical is either synthesized or released by the nerve, and this is what causes the reaction in the heart, not electrical stimulation.

Over the next decade, Otto Loewi performed many more experiments, eventually showing conclusively that neurotransmitters are pre-synthesized, then released by the nerves when an electrical signal arrives, and that the various neurotransmitters are detected on the other side of the synapse by receptors that react only to specific chemicals, many of which he identified.

Otto Loewi, then, basically opened up a whole new subfield of biology, one which has become incredibly important to drug research companies. Now that we understand what is going on in the synapse, we've begun to meddle with it, using drugs like Prozac and Ritalin. Some people are disturbed by the idea that we can manipulate emotions and behaviour with drugs, but I can see that it could come in handy in certain situations.


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