Frog nerve-muscle preparations were quick-frozen at various times after a single electrical stimulus in the presence of 4-aminopyridine (4-AP), after which motor nerve terminals were visualized by freeze-fracture . Previous studies have shown that such stimulation causes prompt discharge of 3,000-6,000 synaptic vesicles from each nerve terminal and, as a result, adds a large amount of synaptic vesicle membrane to its plasmalemma . In the current experiments, we sought to visualize the endocytic retrieval of this vesicle membrane back into the terminal, during the interval between 1 s and 2 min after stimulation . Two distinct types of endocytosis were observed. The first appeared to be rapid and nonselective . Within the first few seconds after stimulation, relatively large vacuoles (-0.1 um) pinched off from the plasma membrane, both near to and far away from the active zones . Previous thin-section studies have shown that such vacuoles are not coated with clathrin at any stage during their formation . The second endocytic process was slower and appeared to be selective, because it internalized large intramembrane particles . This process was manifest first by the formation of relatively small (-0.05 wm) indentations in the plasma membrane, which occurred everywhere except at the active zones . These indentations first appeared at 1 s, reached a peak abundance of 5.5/,m2 by 30 s after the stimulus, and disappeared almost completely by 90 s . Previous thin-section studies indicate that these indentations correspond to clathrin-coated pits. Their total abundance is comparable with the number of vesicles that were discharged initially . These endocytic structures could be classified into four intermediate forms, whose relative abundance over time suggests that, at this type of nerve terminal, endocytosis of coated vesicles has the following characteristics : (a) the single endocytotic event is short lived relative to the time scale of two minutes ; (b) earlier forms last longer than later forms; and (c) a single event spends a smaller portion of its lifetime in the flat configuration soon after the stimulus than it does later on .The discovery of synaptic vesicles (27,30,33,34,38) and the evidence for the quantal release of transmitter (5) led to the hypothesis that transmitter release is accomplished by the exocytosis of synaptic vesicles (6). Electron microscopy of chemically-fixed nerve terminals revealed pockets in the presynaptic membranes suggestive of exocytosis (9, 22, 31), but it was not possible to make temporal and quantitative comparisons between these structures and the number of transmitter quanta released .A quick-freezing machine, based on earlier designs by Van Harreveld (40-42) was developed specifically so that these

Our study supports the idea that "hands-on" use of diagnostic DSSs can influence diagnostic reasoning of clinicians. The larger effect for students suggests a possible educational role for these systems.

Background: Chronic non-cancer pain is a common problem that is often accompanied by psychiatric comorbidity and disability. The effectiveness of a multi-disciplinary pain management program was tested in a 3 month before and after trial.

We describe a program for planning treatments of the eye with charged particle beams. Many of its features apply to treatment planning programs in general. These include the three-dimensional definition of the tumor volume and of normal structures, the possibility of delivering the treatment beam from any direction in space, the provision of arbitrary viewpoints including a "beam's eye" point of view, the design and fabrication of the field shaping aperture, the specification of the necessary beam quality (range and modulation), the calculation and display of isodose contours on any plane in space and on relevant surfaces, the identification of the extent to which normal structures are included or excluded from the beam, and the stimulation of the desired alignment film with which patient alignment can be verified.