Passing the message on

Cmmunicating information between nerve cells occurs at breathtaking speed. To allow an electrical impulse to pass from one cell to another, chemical neurotransmitters are released at the nerve end, cross the narrow gap, or synapse, that separates the cells, pass on the message to the next cell – be it a nerve cell, muscle cell or gland cell – and disappear, all within a fraction of a second.

The story of how neurotransmitters are stored, released and inactivated within such a short space of time is told by the achievements rewarded with the 1970 Nobel Prize in Physiology or Medicine. Sir Bernard Katz used microscopic recording electrodes to measure the electrical changes that occur when the chemical transmitter acetylcholine is released by nerve cells. From a series of measurements, he deduced that acetylcholine is released in highly defined amounts, and that acetylcholine molecules are stored in small bubble-like compartments, or synaptic vesicles, in nerve endings. When an impulse arrives, millions of acetylcholine molecules are released together into the synapse almost immediately, and Katz showed that calcium plays a key role in triggering this so-called “quantal release” of acetylcholine.

Ulf von Euler discovered another neurotransmitter, noradrenaline, and he established how it works. He found that nerve cells rich in noradrenaline were linked to many mammalian tissues and organs, and he showed that noradrenaline release correlates directly with muscular work and activity, such as an immediate rise after standing. Von Euler also showed that this neurotransmitter is stored and synthesised within the nerve fibres themselves. Julius Axelrod’s work completed the cycle, revealing what happens to noradrenaline after it has finished transmitting a nerve impulse. By radioactively tracking noradrenaline’s movements in cat nerves, Axelrod discovered that, unlike acetylcholine, which is inactivated by enzymes, much of the noradrenaline is absorbed back into storage sites within the nerve ending that had just released it. Further studies made it evident that this recycling system is the rule rather than the exception for neurotransmitters. Axelrod also discovered drugs that inhibit the re-uptake process, and his discovery that antidepressants work in this manner stimulated the search for a much needed new generation of treatments for depression.

By Sophie Petit-Zeman, for

This Speed read is an element of the multimedia production “Nerve Signaling”. “Nerve Signaling” is a part of the AstraZeneca Nobel Medicine Initiative.

First published 16 September 2009

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