High-frequency stimulation causes an increase in release called post-tetanic potentiation (Kandel 2003).
[citation needed] This presynaptic event is caused by calcium influx.
Calcium-protein interactions then produce a change in vesicle exocytosis.
The result of these changes is to make the postsynaptic cell more likely to fire an action potential.
[1] Lower elicitations of tetanic stimulation in aged muscles were shown to be caused by lower levels of anaerobic energy provision in skeletal muscles.