摘要：

In freeze-fractured frog sartorius muscles, the long terminal branches of motor axons possess a series of narrow transverse ridges on their surface, bordered by rows of relatively large particles within the presynaptic membrane. By their exclusive location opposite muscle folds, it is apparent that these ridges represent an en face view of the electron-dense cytoplasmic bands around which synaptic vesicles cluster. In resting terminals there is no sign that vesicles underlie these ridges, except for an occasional bulge where a vesicle presses against the plasmalemma; but in terminals stimulated briefly in fixative, the ridges are surrounded by a number of small dimples where synaptic vesicles attach to the plasmalemma. Such 'vesicle sites' do not appear when Mg ++ is used to prevent the transmitter release that results from stimulation, so they presumably represent sites of transmitter discharge. However, more vesicle sites appear in terminals stimulated in more slowly acting fixatives, so they appear to accumulate for some time during fixation and do not indicate the instantaneous level of transmitter release. Vesicle sites occur in a variety of sizes and shapes that may represent different stages of vesicle discharge. Dimples which appear during stimulation under Schwann processes, where endocytosis of coated vesicles has been found to occur, are sometimes larger than vesicle sites but otherwise look much the same; so it is not possible to readily distinguish between the freeze fracture images of synaptic vesicle discharge and coated vesicle formation at this synapse. The muscle membrane beneath nerve terminals is paved with clusters of relatively large particles which mostly appear on the cytoplasmic half of the membrane after fracture. These clusters of particles occur in regions of the postsynaptic membrane that are coated by plaques of electron-dense cytoplasmic fuzz. Immediately around the clusters are a number of tightly-packed, orthogonal aggregates of slightly smaller particles.

展开