We present a stochastic model of the presynaptic terminal in the calyx of Held synapse. This model exploits process calculi as a representation language and has a direct computational implementation that supports quantitative simulation trials of the behaviour of the synapse. To our knowledge, it represents the first model of synaptic activity based on process calculi. The model builds upon available data, the fitting of some parameters and developed working hypotheses. Experiments about plasticity have been carried out regarding synaptic facilitation and potentiation. Also, synaptic depression has been considered in a model exhibiting dynamical equilibrium. Overall, the simulation results are coherent with the experimental findings appearing in the literature about the modeled reality. These results represent a quite detailed description of the presynaptic activity. This multidisciplinary work validates some aspects of the approach based on process calculi with respect to the new application domain, such as abstraction, expressiveness and compositionality.