The fine structural characteristics of motor endplate (paradigm of a cholinergic synapse) and central synapses are briefly summarized giving major emphasis on recent observations in freeze fractured material. Special attention is paid to the concepts of "Active Zones" (presynaptic membrane complex) and "Specific Sites" (postsynaptic membrane complex). Comparison and functional interpretations are made specifically with respect to secretion and receptor mechanisms that are so characteristic of chemically transmitting junctions.

Animals↗

PubMedDate not supplied

[Freeze fracture study of vesicle attachment sites during mediator liberation in spinal motoneuron synapses].

Freeze-fractured appearances of vesicle attachement sites (VAS) in presynaptic active zone were studied in the spinal cord of 4-aminopyridine (4-AP) treated and nembutalized rats. The VAS were classified into: type 1 containing no intramembranous particles, type 2 with particles around the edge of openings, type 3 containing several particles within the VAS. The application of 4-AP, which is known to enhance transmitter release, affected strikingly the total number of VAS (p less than 0.001) but not the proportion of the three types. The predominance of type 3 outside the active zone and its similarity to particle-loaded indentations possibly corresponding to coated vesicle formation, suggested that type 3 may represent endocytosis. It is likely that the remaining two types represent exocytosis.

Aminopyridines↗