Different quantal responses within single frog neuromuscular junctions.

Abstract

SUMMARY1. At frog neuromuscular junctions spontaneous miniature end-plate potentials (m.e.p.p.s) were recorded from several isolated spots within single synapses. This was done by consecutively placing an extracellular glass micro-electrode (focal electrode) at different recording sites, while the intracellular electrode remained in one place.2. After each set of recordings, muscles were stained to reveal both axon terminals and cholinesterase (ChE) such that the exact position of each recording site could be determined.3. In many nerve terminal branches a similar quantum size was found at several different spots. In other instances, however, mean quantum amplitudes varied by 10-60 % at different spots along the same terminal branches.4. As a control, individual spots were recorded from repeatedly after repositioning the focal electrode. In these recordings mean m.e.p.p. amplitude varied by only 5-10 %.5. It is concluded that quantum size within a single junction is similar at many spots, but deviates markedly at others. Correlation of this variation with the stained preparations suggested that spots where quanta significantly larger or smaller than normal were recorded were either at ChE rings or at the distal ends of nerve branches, respectively; at different nerve terminal branches within the same junction, quantum amplitudes were similar in many cases but deviated in others.6. The results are consistent with ultrastructural evidence that frog neuromuscular junctions are non-homogeneous structures which undergo continual remodelling.

The probability of quantal secretion along visualized terminal branches at amphibian (Bufo marinus) neuromuscular synapses.

“…2 shows results for three such series of measurements. The largest amplitude e.p.c.s (about 2-3 mV), presumably those originating from the release sites closest to the micro-electrode (see Bieser et al 1984), decay approximately exponentially with distance of the micro-electrode from the sites giving an average length constant for the decay of 6 8 +0 8 jam (+ S.E. of mean, n = 6).…”

“…The end-plate current density decays in the extracellular space such that an external electrode records quantal release originating only over a distance of about 10-20 ,um along the length of a terminal branch to either side of the electrode tip (del Castillo & Katz, 1956;Katz & Miledi, 1965a; 9-2 259 M. R. BENNETT, P. JONES AND N. A. LA VIDIS Bieser, Wernig & Zucker, 1984). However, if the cholinesterase reaction product is not continuous along a terminal branch, changes occur in the extracellular resistivity in the vicinity of the cholinesterase discontinuity (Bieser et al 1984).…”

“…LA VIDIS Bieser, Wernig & Zucker, 1984). However, if the cholinesterase reaction product is not continuous along a terminal branch, changes occur in the extracellular resistivity in the vicinity of the cholinesterase discontinuity (Bieser et al 1984). As a consequence, terminal branches were only analysed if the cholinesterase was continuous (except in the case of Fig.…”

“…These were not included in the terminals analysed above. It was of interest to see if me declined along these branches in which interruptions may occur in the formation of release sites apposed to post-synaptic membrane (Bieser et al 1984). There was an over-all decline in me along these terminal branches (Fig.…”

Size-related differences in the branching pattern of the motor nerve terminals in triangularis sterni muscle of the mouse

“…These observations are not surprising if a variety of situations in some ex-perime~atal models are taken into account in which the more proximal part of the motor nerve endings behaves as the more significant portion of the terminal arborization in relation to neurotransmission, probably indicating the metabolic supremacy of the initial portion of the nerve endings [24]. For instance, the frequency of the spontaneous quantal release measured as miniature endplate potentials at the neuromuscular junction of the frog is higher in the proximal portion than in the distal regions [6,23,29,30]. Also, evoked transmitter release seems to be higher in the proximal region than in the distally placed tips of the terminals [3,4,11,34].…”