Calcium ions play a crucial role in regulating neurotransmitter release and short-term plasticity in neurons. Two distinct roles of intracellular calcium concentration ([Ca²⁺]) are identified: one that accelerates "molecular priming" of vesicles (docking and release machinery assembly), and another that promotes tight coupling between releasable vesicles and calcium channels. This coupling is essential for vesicles to respond to short calcium transients during action potentials. The intracellular calcium signal has multiple regulatory functions, and its role in neurotransmitter release is achieved through a low-affinity calcium sensor. High calcium concentrations, which persist briefly during action potentials, are spatially localized near open calcium channels. In contrast, other functions are mediated by slow, smaller amplitude calcium variations that build up during high synaptic activity and decay during rest. These slow increases in calcium accelerate vesicle recruitment, influence release probability during action potentials, and may trigger asynchronous release. The calyx of Held synapse is a key preparation for studying these processes, providing quantitative dose-response curves and accurate kinetic information. It allows for simultaneous pre- and postsynaptic voltage clamp, combined with fluorimetric calcium measurement and caged-calcium stimulation. The study highlights the complexity of calcium's role, with changes in synaptic properties rarely being a direct one-to-one relationship with molecular changes. Calcium-dependent vesicle recruitment is enhanced by globally increased intraterminal calcium. This leads to a dynamic equilibrium between a reserve pool of vesicles and a primed pool, with recruitment rates determined by calcium levels. In phasic synapses, such as those in the hippocampus and cortex, short-term depression (STD) occurs due to the partial exhaustion of a release-ready vesicle pool. In tonic synapses, however, synaptic strength increases due to enhanced recruitment, leading to synaptic enhancement. Facilitation, synchrony, and spontaneous release are influenced by global calcium elevation. The "residual-calcium" hypothesis suggests that elevated calcium after a first stimulus increases release probability during a second pulse. However, this view is debated, as small increments in basal calcium may not significantly affect microdomain calcium levels. Recent studies suggest that global calcium dynamics are closely linked to paired pulse facilitation (PPF), with changes in calcium transients affecting PPF. The calyx of Held synapse is a model system for studying vesicle pools and release properties. It allows for the distinction between changes in vesicle pools and intrinsic release properties. The synapse has a heterogeneous vesicle pool, with vesicles differing in release probability. Fast and slow vesicles are recruited differently, with fast vesicles being released rapidly and slow vesicles recovering more slowly. The recruitment of fast-releasing vesicles is influenced by global calcium levels, with a linear relationship between recruitment rate and calcium concentration. At higher calcium concentrations, recruitment rates exceed release rates, leading to a dynamic equilibrium. The calyx of Held synapse is a key model for understanding the molecularCalcium ions play a crucial role in regulating neurotransmitter release and short-term plasticity in neurons. Two distinct roles of intracellular calcium concentration ([Ca²⁺]) are identified: one that accelerates "molecular priming" of vesicles (docking and release machinery assembly), and another that promotes tight coupling between releasable vesicles and calcium channels. This coupling is essential for vesicles to respond to short calcium transients during action potentials. The intracellular calcium signal has multiple regulatory functions, and its role in neurotransmitter release is achieved through a low-affinity calcium sensor. High calcium concentrations, which persist briefly during action potentials, are spatially localized near open calcium channels. In contrast, other functions are mediated by slow, smaller amplitude calcium variations that build up during high synaptic activity and decay during rest. These slow increases in calcium accelerate vesicle recruitment, influence release probability during action potentials, and may trigger asynchronous release. The calyx of Held synapse is a key preparation for studying these processes, providing quantitative dose-response curves and accurate kinetic information. It allows for simultaneous pre- and postsynaptic voltage clamp, combined with fluorimetric calcium measurement and caged-calcium stimulation. The study highlights the complexity of calcium's role, with changes in synaptic properties rarely being a direct one-to-one relationship with molecular changes. Calcium-dependent vesicle recruitment is enhanced by globally increased intraterminal calcium. This leads to a dynamic equilibrium between a reserve pool of vesicles and a primed pool, with recruitment rates determined by calcium levels. In phasic synapses, such as those in the hippocampus and cortex, short-term depression (STD) occurs due to the partial exhaustion of a release-ready vesicle pool. In tonic synapses, however, synaptic strength increases due to enhanced recruitment, leading to synaptic enhancement. Facilitation, synchrony, and spontaneous release are influenced by global calcium elevation. The "residual-calcium" hypothesis suggests that elevated calcium after a first stimulus increases release probability during a second pulse. However, this view is debated, as small increments in basal calcium may not significantly affect microdomain calcium levels. Recent studies suggest that global calcium dynamics are closely linked to paired pulse facilitation (PPF), with changes in calcium transients affecting PPF. The calyx of Held synapse is a model system for studying vesicle pools and release properties. It allows for the distinction between changes in vesicle pools and intrinsic release properties. The synapse has a heterogeneous vesicle pool, with vesicles differing in release probability. Fast and slow vesicles are recruited differently, with fast vesicles being released rapidly and slow vesicles recovering more slowly. The recruitment of fast-releasing vesicles is influenced by global calcium levels, with a linear relationship between recruitment rate and calcium concentration. At higher calcium concentrations, recruitment rates exceed release rates, leading to a dynamic equilibrium. The calyx of Held synapse is a key model for understanding the molecular