Fusing vesicle, open receptor, the spikeGlutamate (and the vesicles that hold it)Ca²⁺, as a GCaMP indicator sees itNa⁺One excitatory synapse from a rodent hippocampus (CA3 axon onto a CA1 spine), as a teaching model with its near half cut away (ruler: 1 tick = 100 nm). Bouton, spine, vesicles (40 nm) and the active zone (about 0.04 µm²) are at true scale; the cleft, really 20 nm, is drawn 8× wide and membranes about 2× thick. The bouton holds about 200 vesicles and about 10 docked on the active zone; the half shown has 31 and 5. When a spike arrives, voltage-gated Ca²⁺ channels at the edge of the active zone open; Ca²⁺ triggers docked vesicles to fuse, each spilling some 2,000–4,000 glutamate molecules (each pink dot stands for ~150). Glutamate crosses the cleft, binds AMPA receptors on the postsynaptic density, and their pores let Na⁺ in: the recording pipette sees a small depolarisation, the EPSP. Glial transporters clear the cleft and new vesicles dock. Release probability rises with the fourth power of external Ca²⁺; this model is tuned to a strong synapse, and at low Ca²⁺ most spikes fail. The whole event, about 5 ms, is shown roughly 1,000× slower; the cleft crossing, under a microsecond in life, is slowed far more so you can watch it.
20 nm突触间隙宽度（图中放大 8 倍）synaptic cleft (drawn 8× wide)~40 nm突触小泡直径synaptic vesicle diameter~10 / ~200活性区停靠小泡 / 每个突触小体的小泡（大鼠 CA1）docked per active zone / vesicles per bouton (rat CA1)2,000–4,000每个小泡中的谷氨酸分子glutamate molecules per vesicle~0.5 ms突触延搁：峰电位到突触后电流synaptic delay, spike to postsynaptic current∝ [Ca²⁺]⁴释放概率（Dodge & Rahamimoff 1967）release probability (Dodge & Rahamimoff 1967)