A Mossy Fiber Bouton (MF) is shown making multiple synapses with its postsynaptic partner, a branched dendritic spine. This complex spine is also referred to as a 'thorny excrescence'. The origin of the spine at its parent dendrite (DEN) is indicated by the black arrow. Scale Bar = 1 micrometer.
For more detailed information on the ultrastructure of these synapses see Chicurel and Harris (1992)