There has always been a bit of a debate as to whether the vesicles in the presynaptic nerve terminal that contain transmitter are just near the presynaptic membrane or are in fact hemifused with it.

At the presynapse, vesicles containing neurotransmitter are prepared and aligned by the presynaptic membrane -- the process of synaptic release needs to be very rapid. When an action potential travels down the axon, calcium flows into the presynaptic terminal. This calcium activates SNARE proteins that are involved both in docking the vesicles near the membrane and fusing them with the membrane. The vesicle fuses with the membrane, releasing its contents into the synapse.

This is incredibly fast, but scientists have long wondered whether it is fast because the vesicles are already hemifused. What does it mean to be hemifused? The membranes of both vesicles and cells are composed of lipid bilayers. The lipids are a lot like soap. They have a charged water-loving end and uncharged oil-like tail. The oil-like tails like to hug together in order for them to have as little contact with water as possible. The best arrangement for them to do this is a bilayer with the charged ends on the outsides and the tails oriented in towards the middle.

It would be possible for the vesicle to just be floating around intact and tethered to the membrane. However, it is also possible for part of the outer bilayer of the vesicle membrane to have already blended into the inner bilayer of the cell membrane. This would mean that the vesicle could release its contents even more quickly.

Unfortunately it is really difficult to check this proposition. Vesicles and synapses are incredibly small, requiring electron microscopy to even see. However, Zampighi et al, publishing in the Biophysical Journal, used conical electron tomography to show that the majority of docked vesicles are hemifused.

This image shows a small number of vesicles. The blue vesicles are not docked. The red vesicles are hemifused. The white vesicle has fully fused with the membrane and is releasing its contents. The bottom panels show a reconstruction of what it would look like to see a vesicle fusing from outside the cell -- before and after fusion.

This image shows a larger view. The top panel is an electron micrograph showing the presynaptic and the postsynaptic terminal. The bottom panel color codes the constituents. The red vesicles are hemifused. The blue vesicles are not docked. The orange blobs are the presynaptic web. The white is the synaptic membranes in apposition. The brown is the postsynaptic density.

When the researchers did counts of the vesicles, they found that the vast majority that were docked. Of the docked vesicles about 75% were hemifused. This means that this hemifusion may be one of the reasons the synapse is so fast. For repeated activity you have other vesicles floating around, but for immediate use you have these vesicles ready to go.