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Abstract

The orderly arrays of intramembranous particles (IMPs) found in the p-face of freeze-fracture replicas of the frog neuromuscular junction ('active zones') are believed to be involved in transmitter release. Some or all of the particles represent voltage-dependent Ca2+ channels. Since there is a great heterogeneity in the amount of transmitter released by different frog motor nerve terminals we sought to determine whether active zone (AZ) structure displayed a similar heterogeneity by using a novel freeze-fracture procedure providing large, intact replicas containing significant portions of motor nerve terminals from the cutaneous pectoris muscle of the frog, Rana pipiens. Using only junctions in which more than 50 AZs or more than 50 microm of nerve terminal were included in the fractures, we measured AZ length, AZ intramembranous particle density, terminal width at each AZ, space between AZs, the angle of AZ orientation with respect to the longitudinal axis of the nerve terminal, exposed pre-synaptic nerve terminal surface area and a calculated value for mean AZ length per unit terminal length. The analysis led to the following conclusions. There is an approximate 5-fold range in mean AZ length/micrometre terminal length. Terminal width is a good predictor of AZ length. Particle density does not vary significantly within a given AZ, nor between AZs from the same or different junctions. The distance between AZs is not related to AZ length, i.e. shorter AZs are no more or less likely to be closer to the adjacent AZ. The probability of release from any AZ on action potential invasion is small. If most of the IMPs are Ca2+ channels, either the probability of channel opening or the efficacy of triggering release is very low or both. That the variability in release efficacy in different terminals is much greater than ultrastructural variability in terminals suggests that regulation of release is dominated by physiological processes that do not have obvious ultrastructural correlates. On the other hand, the apparent excess of AZ relative to the number of vesicles released indicates that the amount and variability in amount of AZ is important in ways that need to be elucidated.

Macromolecular connections of active zone material to docked synaptic vesicles and presynaptic membrane at neuromuscular junctions of mouse

“…The paired double rows of macromolecules are situated directly opposite infoldings (junctional folds) in the postsynaptic plasma membrane of the muscle fiber, and their long axis is orthogonal to the long axis of the mouths of the folds. Paired double rows of membrane macromolecules are also found at the active zones of NMJs in frog and on phasic muscle fibers in the lizard (Heuser and Reese, 1974; Pawson et al, 1998; Walrond and Reese 1985). In mouse the rows are about 80 nm long (Fukuoka et al, 1987), but both double rows of a pair are not always equal in length and, even when equal in length, they are not always in register [Fukuoka et al (1987)].…”

Estimation of presynaptic calcium currents and endogenous calcium buffers at the frog neuromuscular junction with two different calcium fluorescent dyes

“…The terminal was modeled as a cylinder with height of 2 μm and a diameter of 2 μm, with the calcium channel as a point source in its center. The height of frog's neuromuscular terminal (the height of the cylinder in the model—Figure S1 in the Supplementary Materials) was taken from Birks et al ( 1960 ) and Pawson et al ( 1998 ). In addition to fluorescent dye molecules in the cytoplasm, the presence of mobile and fixed buffers was also assumed (Klingauf and Neher, 1997 ).…”

Synaptic Vesicle Endocytosis in Different Model Systems

“…Sartorius and cutaneous pectoris motor neurons are anatomically similar (Grinnell and Herrera, 1980 ). Cholinergic motor neuron axons extend elongated unmyelinated branch terminals (total length 100–300 μm; Pawson et al, 1998 ) running parallel to the muscle fiber (Birks et al, 1960 ). Fusion sites, or active zones (Couteaux and Pécot-Dechavassine, 1970 ), are organized in periodic 1.0 μm-long “stripes” running transverse to the terminal, with roughly 0.8–1.1 μm spacing in between (Propst et al, 1986 ; Pawson et al, 1998 ).…”