High Quality Content by WIKIPEDIA articles! Synapsin II is the collective name for Synapsin IIa and Synapsin IIb, two nearly identical phosphoproteins in the synapsin family that in humans are encoded by the SYN2 gene. Synapsins associate as endogenous substrates to the surface of synaptic vesicles and act as key modulators in neurotransmitter release across the pre-synaptic membrane of axonal neurons in the nervous system.