Signal to Charge

The nervous system behaves like a series of microscopic generators, with electrical pulses repeatedly created and fired along nerve cells in response to a stimulus, such as touch or heat. By showing how these impulses are generated and transmitted, the three scientists who received an equal share of the 1963 Nobel Prize in Physiology or Medicine revealed the key triggers that spark the nervous system’s in-built electrical system into life.

Seeking ways of measuring electrical currents inside nerves, Alan Hodgkin and his student Andrew Huxley turned to giant nerve fibres in the squid, which are almost a thousand times thicker than their human counterparts. Using tiny electrodes to record the electrical difference between the inside and outside of these nerves, they were surprised to find that the polarity did not drop from negative to zero during the transmission of an impulse as predicted, but in fact reversed, becoming electrically positive. By carrying out a series of measurements and using complex mathematical models to interpret the findings, Hodgkin and Huxley formulated a theory to propose how impulses are formed. Changes in the permeability of the cell membrane allow charged atoms to flow in and out of a nerve fibre, creating waves of electric charge that constitute the nerve impulse. During the rising phase of an impulse, positive sodium ions are allowed to flood in from the outside and in the falling phase potassium ions are allowed to migrate outwards from the inside.

Sir John Eccles showed how Hodgkin and Huxley’s findings also relate to the events that occur when an impulse is transmitted from one nerve cell from another. Eccles recorded minute but noticeable variations in electrical charge at the junctions, or synapses, between nerve cells shortly after an impulse arrives. This charge deviated in opposite directions depending on the synapse studied, which corresponded to the release of chemicals from the synapse that either excite or inhibit the neighbouring cell. Releasing these chemicals causes microscopic channels to open across the cell membrane, creating a sieve that allows specific ions to flood through the membrane in a particular direction. Each nerve cell is confronted by an enormous number of these excitor and inhibitor signals coming from different synapses, and the decision as to whether to transmit or inhibit a impulse ultimately comes down to which type of signal outweighs the other.

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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