Exocytosis · Grey Matter

Exocytosis is how a neuron releases its transmitter: a small vesicle filled with neurotransmitter fuses with the membrane of the axon terminal and empties its contents into the synaptic cleft, triggered by the calcium that enters when an action potential arrives.

Exocytosis. Exocytosis is how a neuron releases its transmitter: a small vesicle filled with neurotransmitter fuses with the membrane of the axon terminal and empties its contents into the synaptic cleft, triggered by the calcium that enters when an action potential arrives.

The sequence takes less than a millisecond. The spike depolarises the terminal and opens voltage-gated calcium channels clustered at the release site; calcium flows in and, within nanometres of the channels, reaches concentrations high enough to bind synaptotagmin, the calcium sensor anchored in the vesicle membrane. The vesicle has already been docked and primed by the SNARE proteins: synaptobrevin (also called VAMP) on the vesicle, syntaxin and SNAP-25 on the terminal membrane. Their helices wind together into a tight bundle that pulls the two membranes close, and the calcium-bound synaptotagmin triggers the final step in which they fuse.

One vesicle is one quantum. A vesicle holds thousands of transmitter molecules (direct counts for glutamate range from about 2,000 to about 8,000), and release happens in these packets, never by the molecule.

Release is probabilistic. A spike reaching a terminal releases a vesicle only some of the time, and that probability depends steeply on how much calcium enters, which makes it a main lever for both plasticity and drugs.

The vesicle is recycled. After fusion its membrane is retrieved, refilled by transporters that pump transmitter in, and docked again; proteins of the vesicle such as SV2A take part in keeping this cycle ready for release.

Toxins show the parts. Botulinum and tetanus toxins cut specific SNARE proteins and block release, which is how the role of each protein was confirmed.

A zipper closing two pieces of cloth is the usual picture of the SNARE bundle.

The proteins zip from the far end toward the membranes, and the energy of the zipping pulls the vesicle and the terminal into one.

Questions: How does a spike arriving at a terminal turn into released transmitter? The spike depolarises the terminal and opens voltage-gated calcium channels clustered at the release sites. Calcium enters and binds synaptotagmin on vesicles that the SNARE proteins (synaptobrevin on the vesicle, syntaxin and SNAP-25 on the terminal) have already half-zipped to the membrane. The calcium-bound synaptotagmin lets the zipping finish, the two membranes fuse and the vesicle empties into the cleft, all in well under a millisecond. Blocking the calcium channels blocks release, which is how calcium was shown to be the link. How many vesicles does a single spike release? At a typical synapse in the cortex or hippocampus, usually none or one: each release site has a probability of release of roughly 0.1 to 0.3 at low firing rates, and only sometimes do two or more vesicles go together. Reliability comes from numbers, since a neuron receives input through many such synapses at once. A motor nerve terminal is built the other way, with hundreds of release sites, and one spike releases tens of vesicles (about 20 to 60 at human neuromuscular junctions, more in larger muscles of other animals), enough to fire the muscle fibre every time. A range such as one to two hundred vesicles per spike mixes these two very different kinds of synapse. How much glutamate does one synaptic vesicle hold? Thousands of molecules, with estimates that depend on the method. Biochemical analyses of isolated vesicles gave about 2,000 to 4,000, models fitted to the response of postsynaptic receptors gave about 5,000 to 9,000, and a direct electrochemical count in single vesicles found about 8,000. Release happens in these packets (quanta), so the smallest signal a synapse can send is one vesicle's worth. Where in the release machinery does SV2A sit, and what is it known to do? SV2A is a protein of the vesicle membrane itself, present on the vesicles of almost every synapse, beside synaptobrevin and synaptotagmin. Synapses without it release transmitter less reliably and mice lacking it have severe seizures, so it helps keep vesicles ready for calcium-triggered fusion. Its precise molecular job is still unsettled: it resembles a transporter, it interacts with synaptotagmin, and its substrate, if it has one, is unknown. Structures published in 2024 show where the antiseizure drugs and botulinum toxin A bind on it. Where does the time go between a spike reaching a terminal and the next cell responding? Most of it goes into opening the calcium channels and fusing the vesicle; crossing the 20 nm cleft takes only microseconds, and the receptors begin to open almost as soon as the transmitter lands. At fast synapses in the rat cerebellum at body temperature, fusion lags calcium entry by about 60 microseconds and the postsynaptic current starts about 150 microseconds after the presynaptic spike begins. Textbooks give 0.5 to 1 ms for the synaptic delay, a figure from older and cooler preparations, and the postsynaptic potential then takes another millisecond or more to rise and reach the soma. The delay is short but it adds up: a signal that crosses five synapses has spent several milliseconds in transit at the synapses alone. Why are calcium channels anchored within nanometres of the vesicles they trigger? Calcium entering through an open channel is buffered and pumped away so fast that its concentration is high only in a tiny cloud around the channel mouth, tens of nanometres across, for well under a millisecond. A vesicle docked inside that cloud sees enough calcium to bind its sensor and fuse almost at once; one docked farther away sees too little, too late. Tethering proteins at the active zone therefore hold the channels next to the docked vesicles, which is what lets fusion follow calcium entry by about 60 microseconds at body temperature. The arrangement also keeps release tied to the spike, since the cloud vanishes as soon as the channels close. Why are neuropeptides released only when a neuron fires hard? Peptides are stored in large dense-core vesicles that sit away from the active zone, outside the tight calcium clouds around the release-site channels. A single spike raises calcium enough to fuse the small vesicles docked next to the channels but not these distant ones; high-frequency firing or bursts let calcium build up throughout the terminal, and only then do dense-core vesicles fuse, often outside the synapse. Since peptides are made in the cell body and shipped down the axon, their supply at the terminal is also limited. The result is a two-level code: the small transmitter reports every spike, the peptide reports sustained, intense activity.