Chemical Communication

Experiments were increasingly showing that signals are carried across synapses by chemical messengers called 'neurotransmitters', which are released at the axon tip. Katz showed that when molecules of acetylcholine, the first known transmitter, are released from a motor neuron, they spread rapidly - in fact almost instantly - across the synapse to muscle cells. There they bind to receptors, like a key slotting into the right lock, and as they slide home, they open gates in the cell membrane, letting charged ions flood in, and setting the synaptic potential on its way. These chemical molecule or 'ligand-triggered' ion gates are different from the 'voltage-triggered' ion gates that set off action potentials, allowing both sodium and potassium ions to flood through each way.

Soon a whole range of neurotransmitters had been discovered, such as serotonin. Some are typically excitors, such as glutamate, while others are mainly inhibitory, such as GABA. But it is the receptor they bind to that excites or inhibits, not the transmitter itself. So while glutamate excites most nerves, there are some it inhibits.

There was a twist to the soupers' victory tale, when in the late 1950s, two students of Katz, Edwin Furshpan and David Potter, found a few examples of electric signalling across synapses. But in the brain at least, synaptic signalling is mainly chemical. Katz, as well as Ulf von Euler and Julius Axelrod, went on to discover just how this process worked, for which they shared the 1970 Physiology or Medicine Prize.

Katz found that transmitters are not released molecule by molecule but spill out in a flood from tiny packets of 5,000 or so molecules contained in vesicles, which fuse with the nerve cell wall and open into the synapse. Discovering another neurotransmitter, noradrenaline, von Euler established that this chemical is created in the nerve fibres themselves and stored in swellings concentrated at nerve endings. Axelrod revealed how unused noradrenaline is reabsorbed by nerve cells after the chemical message has been delivered. While this answered many fundamental questions about neurotransmitters, a key piece of information missing was what tasks these chemicals actually carry out when they reach their target nerve cells.

How neurotransmitters work, part 1.



Chemicals called neurotransmitters are responsible for transmitting nerve impulses across the synapse from one nerve cell to another, or from one nerve cell to a target organ or gland. When a nerve impulse reaches the synapse, neurotransmitters spill out from tiny packets of around 5,000 molecules contained in vesicles within the nerve terminal. The released neurotransmitters spread rapidly across the synapse and bind to chemical receptors on the surface membrane of a receiving nerve cell. In most cases unused neurotransmitter is reabsorbed by nerve cells after the chemical message has been passed on.
Copyright: The Nobel Committee for Physiology or Medicine at Karolinska Institutet.


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