Organization of the reticulospinal axon

At the ultrastructural level the majority of the release sites (65-70%) appear as a single cluster of synaptic vesicles accumulated at a single active zone (Fig. ID), which may contain specializations of pure chemical or mixed synapses (Pfenninger and Rovainen, 1974 Ringham, 1975 Christensen, 1976 Rovainen, 1979). The latter type thus contains gap junctions as well as active zones (Fig. ID). Most of the synapses are established on dendritic shafts. A distinct class of intemeuron, however, receives axo-somatic synapses (OS and LB unpublished). The majority of the postsynaptic cells receive multiple chemical or mixed synaptic contacts from a single axon. Therefore most of the EPSPs recorded in postsynaptic neurons are composed of two components, an electrotonic and a chemical component. 

Single active zones are surrounded by glial cell processes which surround the synaptic cleft like a collar and thereby provide a barrier to the extracellular space (Figs. ID and 3). 

---