Channel pores, a calcium sensor and merging vesicles – explained for readers who never studied physiology, with the EU wording on calcium at the close of the section.
Open the ChronicleA nerve cell does not hand information on to the next cell as an electric current. At the very end of its fiber, in a swelling that specialists call the presynaptic terminal, the electrical event is turned into the release of a chemical substance. The sequence below shows at which point calcium ions are needed along the way.
When an action potential runs all the way into the terminal, the voltage across the cell membrane there shifts for a split second. For a brief moment the interior carries a less negative charge than it does at rest.
This change in voltage opens channel proteins that let calcium ions through and nothing else. At most synapses of the central nervous system, these are the Cav2.1 and Cav2.2 variants. Because roughly ten thousand times more free calcium sits outside the cell than inside, the ions flow in without any input of energy.
The wall of every vesicle that holds messenger substance contains the protein synaptotagmin-1. Two of its segments, the C2A and C2B domains, offer five binding sites for calcium between them. Only once several of those sites are occupied does the protein shift its position relative to the membrane.
Three proteins – synaptobrevin on the vesicle, syntaxin-1 and SNAP-25 on the cell membrane – are already twisted into a bundle. The signal from synaptotagmin sets off the final step: the two lipid layers merge, and the contents spill into the narrow cleft facing the neighboring cell.
Calcium contributes to normal neurotransmission.
EU-authorized wording · Regulation (EU) No 432/2012Physiologists already suspected toward the end of the 19th century that messenger release depends on calcium: with no calcium in the bathing solution, a nerve terminal gave no response. What remained unclear was where, and at what moment, the ion steps in.
In London, Bernard Katz and Ricardo Miledi worked on the motor end plate of the frog, an easy-to-reach point of contact between nerve and target cell. They took the calcium out of the surroundings and then put it back through a fine glass pipette, aimed at one spot and only for a brief moment. In 1967 they reported the outcome: messenger substance was released only if calcium was on hand shortly before or during the electrical excitation. If it came afterward, nothing was released.
That same year, the pair examined an unusually large synapse in the stellate ganglion of the squid. There they shut the sodium channels with tetrodotoxin and applied the voltage jump artificially instead. The terminal released messenger substance all the same. This showed that the process is set in motion not by the inflow of sodium but by the voltage and the calcium that enters in response.
Also in 1967, Frederick Dodge and Rami Rahamimoff altered the calcium content of the bathing solution one step at a time. The response of the target cell did not keep pace in a straight line but rose roughly with the fourth power. Their reading: several calcium ions have to bind together at a site nobody had yet identified. More than twenty years on, synaptotagmin was singled out as a candidate for precisely that site, and in 2013 James Rothman, Randy Schekman and Thomas Sudhof received the Nobel Prize for working out how vesicles fuse. The wording authorized today reads: Calcium contributes to normal neurotransmission – EU-authorized wording · Regulation (EU) No 432/2012.
The sequence sketched above, told at full length: measured values at the active zone, the experimental series of the 1960s, how the fusion machinery is put together, and an annotated reading list.
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