Is there any research as to the average number of synaptic vesicles per axon terminal in a human neuron's axon? On top of that, what percentage of those vesicles are used in a single action potential cycle i.e. membrane depolarization, hyperpolarization and return to resting potential?
Several to thousands of vesicles per synapse. In the cortex, 0-1 are released per typical synapse. At neuromuscular junctions or specialized CNS synapses there can be more.
It doesn't make much sense to talk about an "average", since there are different types of synapses with very different properties, but we can talk about some examples from synapses used as models. In Rosenmund & Stevens, 1996 they estimate about 10 readily releasable vesicles at one synapse in hippocampus. Rizzoli & Betz, 2004 count hundreds at a frog neuromuscular junction.
In hippocampus and neocortex, for a typical synapse there are from 0-1 vesicles released per spike. Higher release probabilities to activation of single cells are achieved by making multiple synaptic contacts; in a typical cortical synapse, there's really just one active site so a max of one vesicle released (Silver et al 2003; Auger & Marty 2000 and Abbott & Regehr, 2004 also discuss this a bit).
Short-term plasticity can strongly influence release probability (Abbott and Regehr 2004 is a good reference to thinking about synaptic computations like this). Multiple action potentials occurring quickly can cause a sustained increase in calcium concentration in the presynaptic cell, increasing release probability at otherwise low-probability synapses (short-term facilitation). Alternatively, many spikes over a longer time period or at higher probability synapses can exhaust the pool (short-term depression). Rosenmund & Stevens used this to estimate the readily releasable pool size of about 10: they triggered a ton of rapid-fire spikes and counted release events, finding only 10. Rizzoli & Betz mention that 10 seconds of 30 Hz stimulation (300 pulses) depleted the available vesicle pool.
Abbott, L. F., & Regehr, W. G. (2004). Synaptic computation. Nature, 431(7010), 796-803.
Auger, C., & Marty, A. (2000). Quantal currents at single‐site central synapses. The Journal of physiology, 526(1), 3-11.
Reyes, A., Lujan, R., Rozov, A., Burnashev, N., Somogyi, P., & Sakmann, B. (1998). Target-cell-specific facilitation and depression in neocortical circuits. Nature neuroscience, 1(4), 279-285.
Rizzoli, S. O., & Betz, W. J. (2004). The structural organization of the readily releasable pool of synaptic vesicles. Science, 303(5666), 2037-2039.
Rosenmund, C., & Stevens, C. F. (1996). Definition of the readily releasable pool of vesicles at hippocampal synapses. Neuron, 16(6), 1197-1207.
Silver, R. A., Lübke, J., Sakmann, B., & Feldmeyer, D. (2003). High-probability uniquantal transmission at excitatory synapses in barrel cortex. Science, 302(5652), 1981-1984.