William P. Hurlbut

Effects of calcium and magnesium on the frequency of miniature end‐plate potentials during prolonged tetanization

1. End‐plate potentials (e.p.p.s) and miniature end‐plate potentials (min.e.p.p.s) were recorded intracellularly from the cutaneous pectoris nerve‐muscle preparation of the frog during prolonged stimulation at low frequencies (5/sec—50/sec).

New Evidence Supporting the Vesicle Hypothesis for Quantal Secretion at the Neuromuscular Junction

Acetylcholine (ACh) is stored in two compartments within vertebrate motor terminals: the cytoplasm and the synaptic vesicles (Whittaker et al., 1964; Dunant et al., 1972); it is released from terminals in two ways: continuously in a relatively steady molecular stream (Katz and Miledi, 1977; Vyskocil and Illes, 1979), and intermittantly in pulses, or quanta, (Fatt and Katz, 1952, del Castillo and Katz, 1954) which contain about 104 molecules (Kuffler and Yoshikami, 1975). The function of the continuous leak is not understood (Edwards et al., 1985), but the spontaneous or neurally evoked release of quanta generates the discrete, transient miniature endplate potentials (mepps) or endplate potentials (epps) that mediate neuromuscular transmission. Two hypotheses have been proposed for the origin of the quanta: a) the cytoplasmic hypothesis which postulates that quanta are comprised of cytoplasmic ACh which diffuses in pulses through channels in the axolemma which become intermittantly permeable to it (Israel and Manaranche, 1985), and b) the vesicle hypothesis which postulates that quanta are comprised of ACh which is released by exocytosis from the interiors of synaptic vesicles whose membranes have fused with the axolemma (del Castillo and Katz, 1956). In our opinion, the vast preponderance of evidence supports the vesicle hypothesis of quantal secretion, though its proof is not yet absolute and most of the mechanistic details of the fusion and recovery processes have yet to be worked out.