A non-membrane bound, electron dense structure associated that extends perpendicular to the presynaptic membrane in ribbon synapses. The ribbon's surface is studded with small particles to which synaptic vesicles tether via fine filaments. The tethered vesicles function as a pool, several fold greater than the docked pool available for fast release, which supports sustained release of vesicles. Synaptic ribbons may be plate like or spherical.
synaptic ribbon