Friday, November 26, 2010

Acetylcholine and Loewi’s Dream

In 1936, Otto Loewi was awarded the Nobel Prize in Physiology or Medicine for the discovery of the role of acetylcholine and other chemicals in nerve and muscle transmission. Russ Hobbie and I don’t mention Loewi in the 4th edition of Intermediate Physics for Medicine and Biology, but we do discuss acetylcholine. In Chapter 6, we write
At the end of a nerve cell the signal passes to another nerve cell or to a muscle cell across a synapse or junction. A few synapses in mammals are electrical; most are chemical…In electrical synapses, channels connect the interior of one cell with the next. In the chemical case a neurotransmitter chemical is secreted by the first cell. It crosses the synaptic cleft (about 50 nm) and enters the next cell.

At the neuromuscular junction the transmitter is acetylcholine (ACh). ACh increases the permeability of nearby muscle to sodium, which then enters and depolarizes the muscle membrane. The process is quantized. Packets of acetylcholine of definite size are liberated.
In Homework Problem 20 in Chapter 4, we ask the student to calculate the time required for acetylcholine to diffuse across the synaptic cleft. The release of acetylcholine at the nerve-muscle junction in discrete quanta provides a nice example of Poisson Statistics described in Appendix J. In Chapter 7, when discussing the heart, we mention how acetylcholine, released by parasympathetic nerves, decreases the heart rate.

The Left Hand of the Electron, by Isaac Asimov, superimposed on Intermediate Physics for Medicine and Biology.
The Left Hand of the Electron,
by Isaac Asimov.
I can’t tell you about Otto Loewi and acetylcholine without mentioning the fascinating tale of Loewi’s dream. Since Isaac Asimov is a much better storyteller than I am, I will simply quote from his essay “The Eureka Phenomenon” published in The Left Hand of the Electron.
The German physiologist Otto Loewi was working on the mechanism of nerve action, in particular, on the chemicals produced by nerve endings. He woke at 3 A.M. one night in 1921 with a perfectly clear notion of the type of experiment he would have to run to settle a key point that was puzzling him. He wrote it down and went back to sleep. When he woke in the morning, he found he couldn't remember what his inspiration had been. He remembered he had written it down, but he couldn't read his writing.

The next night, he woke again at 3 A.M. with the clear thought once more in mind. This time, he didn't fool around. He got up, dressed himself, went straight to the laboratory and began work. By 5 A.M. he had proved his point and the consequences of his findings became important enough in later years so that in 1936 he received a share in the Nobel prize in medicine and physiology.

1 comment:

  1. Otto Loewi began a journey that still continues today. Specie dependent, region dependent,.. it has different effects /roles throughout the heart (and body). For a physics student interested in applying their skills to medicine, I would encourage further reading within the field of molecular biology.