PTP and the RRP of synaptic vesicles at the calyx of Held nerve terminal. The calyx of Held presynaptic nerve terminal has two types of RRPs of vesicles: a fast-releasing pool that recovers slowly from pool depletion and a more slowly releasing pool of vesicles that recovers quickly from vesicle pool depletion. The fast-releasing pool of vesicles probably constitutes those vesicles docked at the plasma membrane that are primed for exocytosis and also tightly coupled to Ca²⁺ channels, whereas the slowly releasing pool of vesicles may constitute docked vesicles that are located at larger distances from Ca²⁺ channels (loosely coupled vesicles). Under low-frequency stimulation conditions, the free Ca²⁺ concentration level in the nerve terminal may not reach sufficiently high levels for the activation of significant amounts of CaM and MLCK. However, after a prolonged (4–5-s) and high-frequency (100 Hz) action potential tetanic stimulation of the calyx nerve terminal, high free Ca²⁺ concentration levels apparently activate significant amounts of CaM, which then activates MLCK. Now MLCK can activate myosin II, a molecular motor that may drive the translocation of vesicles in the slowly releasing pool toward locations where they are more tightly coupled to Ca²⁺ channels. The size of the fast-releasing pool of vesicles is thus increased at the expense of the slowly releasing pool size. The amplitude of the excitatory postsynaptic current is thereby potentiated.