SYNAPTIC SPINULES ATTENDANT ON POSTTETANIC POTENTIATION IN CAT SYMPATHETIC-GANGLION

A synaptic spinule formed on the postsynaptic membrane in the cat supe rior cervical ganglion (SCG) in vivo was electron microscopically stud ied, When the preganglionic nerve fiber was tetanically stimulated at 10 Hz for more than 10 seconds, a post-tetanic potentiation (PTP) was electrophysiologically detected and maintained up to 30 min. Concurren tly with PTP, a small process, termed as synaptic spinule, was generat ed as a small projection standing out from the postsynaptic dendritic membrane with a straightly extended contour. The spinule produced a br eak at about a middle point of the postsynaptic density (PSD) and divi ded it into two parts forming a perforated synapse. Then, the spinule increased in size, invaginated into the cytoplasm of the nerve termina l, and subdivided the presynaptic terminal bag into two compartments. Each of the compartments retained a morphology of an active synaptic s ite with a cluster of synaptic vesicles associated with a presynaptic active zone standing opposite to the one part of the subdivided PSD. T he longer the tetanic stimulation in period, the more the synapses wit h the spinule increased in number. In addition to this, a few spinules (multiple type of spinule) were often formed after the stimulation of 1 min and more than two compartments were generated in a single nerve ending. The present observations have suggested that the synaptic spi nule is a morphological representation of the ''synaptic plasticity'' which is electrophysiologically displayed as the PTP in cat SCG in viv o.