Influx of calcium ions in axon terminals question

[aychamo]

Hey guys

I've a quick question. As an action potential propogates down an axon, as it reaches the end of an axon it causes calcium influx into the terminal end of the axon. This calcium helps the vesicles move towards the pre-synaptic membrane so that they can release neurotransmitters. My question is how does the influx of calcium influence the vesicles to move towards the membrane?

Thank you :-)

 

[DocToxyn]

First off, there are already synaptic vesicles docked at the membrane, ready for release following the action potential and depolarization. This collective group is generally called the "readily-releasable pool" of neurotransmitter. Upon the entrance of calcium localized to the post-synaptic density the docked visecles begin the process of fusion of their membrane with the plasma membrane of the terminal to achieve release of their contents. This process is run by a fairly complicated and not entirely understood mechanism involving vesicle associated membrane proteins (VAMPs), soluble NSF attachment proteins (SNAPs), SNAP receptors (SNAREs) and a host of other proteins. Needless to say they all work together to open the vesicle to the synapse.
As far as getting vesicles to move towards the active zone, a process called recruitment, the following happens. The vesicles away from the active zone are tethered to the cytoskeleton by synapsins which are substrates for various calcium-dependent kinases (enzymes that phosphorylate other things). The addition of phosphate by the kinases is believed to release the hold that synapsin has on the cytoskeleton and thus free the vesicle to travel to the active zone and participate in neurotransmitter release. I haven't looked recently to check up on the progress of identification of all these mechanisms, but this is the going theory. Please, if any of this was confusing, or you want more, ask away!