Boutons originating from the same axon do not participate equally in synaptic transmission.

Morphological changes in the synaptic boutons located on the ciliary neurons were observed following stimulation of the Edinger-Westphal nucleus (EWn). A reduction in the number of clear and dense core vesicles (DCVs) per unit area and an increased surface density of vacuoles were observed in boutons located proximal to the afferent axon. Only a slight reduction in the number of DCVs per unit area was observed on boutons located distally to the afferent axon. Several mechanisms are proposed to explain why all boutons do not participate equally in synaptic transmission.