||"Synapses are very small. This narrow gap of extracellular space is approximately 20-40 nanometers (nm) wide. For an idea of scale, one inch is about 25.4 million nm long. The thickness of a single sheet of paper is about 100,000 nm." - See more at: link See Ahmari 2002 PMID 11988164 p.333 right column 4th paragraph [organism is rat]:"Mature CNS synapses share an established set of ultrastructural characteristics. The canonical excitatory synapse comprises presynaptic and postsynaptic active zones separated by a uniform 20 nm cleft (Figure 1A)."