Physiological regulation of synaptic effectiveness at frog neuromuscular junctions.

Grinnell, Alan D.; Herrera, A A

doi:10.1113/jphysiol.1980.sp013436

Cited by 87 publications

(66 citation statements)

References 24 publications

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“…The muscle was rinsed with normal frog Ringer's solution containing (in mM): NaCl 116, KCl 2, CaCl 2 1.8, TES buffer 5, pH 7.2, for at least 20 min and a coverslip was gently placed on the muscle surface. We did not use ␣-bungarotoxin to visualize acetylcholine receptors (AChR) in vivo, because many fibers are innervated by neuromuscular junctions with low safety margins (Grinnell and Herrera, 1980). Inactivity is well known to stimulate junctional remodeling (Wernig and Herrera, 1986).…”

Section: Methodsmentioning

confidence: 99%

Precision of Reinnervation and Synaptic Remodeling Observed in Neuromuscular Junctions of Living Frogs

1996

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Repeated in vivo observations were used to study regenerated nerve terminals in neuromuscular junctions of the adult frog Rana pipiens. Sartorius junctions in living animals were stained with the fluorescent vital dye RH414 and viewed with video fluorescence microscopy. Each junction was observed in the intact muscle and then again 7, 10, and 13 weeks after nerve crush. At 13 weeks, junctions were determined to be mono-or polyneuronally innervated using intracellular recording. Between 7 and 13 weeks, most identified junctions were reinnervated less precisely and completely than described previously. Although some of the original synaptic gutters were reoccupied by regenerated terminal branches, other gutters were only partially occupied, and many appeared abandoned. Junctions showing precise recapitulation of original terminal arborizations comprised a small number of the total examined, as did those where reinnervation was very imprecise. Striking differences in the precision of reinnervation were found within the muscle such that distal terminals regenerated more precisely and completely than did proximal terminals. Terminals in reinnervated muscles were more dynamic than terminals in unoperated muscles over equivalent times. In singly innervated junctions, terminal growth was favored over regression. In doubly innervated junctions, regressive events were more common. Imprecise reinnervation is explained in terms of multisite innervation of muscle fibers and the activity dependence of synaptic stability. We hypothesize that when axons reinnervate the second or third junctions on a fiber, they do so less precisely, because the activity restored by reinnervation of the first junction renders later sites less attractive or less stable.

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Section: Methodsmentioning

confidence: 99%

Precision of Reinnervation and Synaptic Remodeling Observed in Neuromuscular Junctions of Living Frogs

1996

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“…A high safety margin can be inferred if the twitch tension was virtually unaffected when [Caz+] is far below physiological levels. If other factors are equal, a high safety margin implies a high quanta1 content of EPPs (Grinnell and Herrera, 1980;Grinnell and Trussell, 1983;Herrera and Grinnell, 1985). However, the measurement of safety margin during the period of synapse elimination is complicated by the fact that the number of inputs per fiber changes.…”

Section: Safety Margin Of La Synapsesmentioning

confidence: 99%

“…4,5). Several factors generally influence the safety margin of neuromuscular synpases, including the size of muscle fibers, the number of synapses per fiber, and EPP quanta1 content (Grinnell and Herrera, 1980;Grinnell and Trussell, 1983;Herrera and Grinnell, 1985). The diameter of LA muscle fibers increases significantly between P14 and P28 (Jordan et al, 1989a).…”

Section: Control Of Motor Unit Size During Developmentmentioning

confidence: 99%

Hormonal regulation of motor unit size and synaptic strength during synapse elimination in the rat levator ani muscle

et al. 1992

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Previous anatomical studies suggest that androgen regulates synapse elimination in the androgen-sensitive levator ani(LA) muscle of the rat. Androgen treatment beginning on postnatal day 7 (P7) prevents some of the normal loss of multiaxonal innervation in this muscle. The present study used physiological techniques to measure the number and size of LA motor units during the synapse elimination period in muscles from normals, and castrates treated with either testosterone propionate or oil. The number of increments in LA twitch tension as nerve stimulation intensity increased, a measure of the number of motor units, was the same at the end (P28) of synapse elimination as near the beginning (P7) of this process. This result indicates that motoneuronal cell death does not contribute to synapse elimination in the LA. Moreover, androgen during this period did not influence the number of LA motor units. In contrast, between P7 and P28, there was a dramatic decline in the size of LA motor units, as indicated by a decrease in the percentage of twitch or tetanus tension of individual motor units relative to the maximal twitch or tetanus tension of the whole muscle. In addition, androgen treatment of castrated males during this period prevented some of the normal decline in the size of LA motor units. Estimates of the number of inputs per LA muscle fiber derived from the number of LA motor units and their average size indicate that androgen maintains polyneuronal innervation in the LA muscle. This finding supports previous anatomical studies suggesting that androgen can prevent synapse elimination in this muscle. The strength of LA synapses was also examined by measuring the tetanus: twitch ratio of individual motor units and by measuring the safety margin of LA synapses. Both measurements indicated that the average strength of LA synapses increases during synapse elimination. Moreover, androgen appeared to spare synapses from elimination without increasing their strength, since androgen-treated muscles generally had larger motor units but the same mean tetanus:twitch ratio and safety margins as untreated LA muscles except at P28, when synapses in androgen-treated LA muscles had appreciably lower safety margins than normal. These results suggest that androgen regulates synapse elimination through a mechanism(s) independent of synaptic strength.

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“…It is of interest to know the extent to which this variability in synaptic efficacy reflects intrinsic differences between motoneurones and the extent to which it is a consequence of peripheral influences. Previous investigations have shown that the average quantal content of junctions in the cutaneous pectoris muscle is 3-fold higher than that of the sartorius (Grinnell & Herrera, 1980a). Since motoneurones of the cutaneous pectoris maintain one-half to one-third fewer terminals than do sartorius motoneurones, it might be postulated that synaptic efficacy falls with increasing motor unit size because of the dilution of a critical supportive substance.…”

Section: Introductionmentioning

confidence: 99%

Synaptic strength as a function of motor unit size in the normal frog sartorius.

Grinnell¹,

Trussell²

1983

The Journal of Physiology

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SUMMARY1. A wide range of motor unit sizes exists in each frog sartorius, with values of tetanus and twitch tensions extending over 62-and 400-fold ranges, respectively. These differences primarily represent differences in the number, rather than the size, of muscle fibres in each motor unit.2. Tetanus-to-twitch tension ratios varied markedly between different motor units, ranging from 1-5 to over 190. Because directly stimulated muscle fibres have tetanus/twitch ratios no larger than 3, it is concluded that high tetanus/twitch ratios arise from recruitment of muscle fibres during the tetanus which respond with only subthreshold depolarization during a twitch.3. Small motor units, as judged either by their twitch or tetanus tension, were associated with higher tetanus/twitch ratios, suggesting that the average safety margin of a motor unit increases with the motor unit's size. Indeed, when the tetanus/twitch ratio of a motor unit is used to determine the fraction of fibres in the unit with subthreshold neuromuscular junctions, it is observed that there is a direct linear relationship between the size of the motor unit and its over-all efficacy of synaptic transmission.

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Physiological regulation of synaptic effectiveness at frog neuromuscular junctions.

Cited by 87 publications

References 24 publications

Precision of Reinnervation and Synaptic Remodeling Observed in Neuromuscular Junctions of Living Frogs

Precision of Reinnervation and Synaptic Remodeling Observed in Neuromuscular Junctions of Living Frogs

Hormonal regulation of motor unit size and synaptic strength during synapse elimination in the rat levator ani muscle

Synaptic strength as a function of motor unit size in the normal frog sartorius.

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