I believe varicosity in the context of your question refers to the enlarged and tortuous vein, artery, or lymphatic vessel in the area of the synapse. Therefore in the aplysia synaptic junctions between the sensory and motor neurons experience long term facilition in response to 5-HT and long term depression in response to RMRFamide. Research has shown macromolecules synthesis is needed for expression of the long lasting structural changes in the sensory cells and that this synthesis is porportional with long term functional modulation of sensorimotor synapses.