Model illustrating presynaptic short-term facilitation by Ca²⁺/CaM-mediated Munc18-1 release from mGluR4. Step  1: At the resting Ca²⁺  level, Munc18-1 is sequestered by mGluR4. Step  2: Owing to the low availability of Munc18-1 for SNARE, the initial synaptic vesicle release is small. Steps  2-3: Following the action potential, CaM sensing residual Ca²⁺ liberates Munc18-1 from mGkuR4. Step  4: The higher availability of Munc18-1 for SNARE enables a greater release of synaptic vesicles in response to the second action potential, which is derived in rapid succession. Adapted from Nakajima et al., 2009 [28].