Competitive interaction between foreign nerves innervating frog skeletal muscle.

Abstract

SUMMARY1. Competition between two foreign nerves innervating frog skeletal muscle has been studied by using pairs of somatic motor nerves (s.m.n.s) or one s.m.n. and the preganglionic splanchnic nerve (s.p.n.) implanted into a denervated sartorius muscle that has been transplanted to the lymph sac of the back.2. A single s.m.n. implanted into the muscle succeeded in innervating essentially every fibre within 2-3 months; tetanic stimulation of the nerve elicited 90-100 % of the maximal direct tetanus tension. Most of the e.p.p.s were suprathreshold, since a single indirect stimulus evoked a twitch 60-100% as large as that to a direct stimulus. 3. If two s.m.n.s were implanted simultaneously, tetanic stimulation of either elicited 80-100% of the maximal tension to direct stimulation. If one nerve was implanted 2-3 months before the other, the second, although usually less effective than the first, normally innervated 50-100% of the fibres, with approximately the same time course of innervation as a single s.m.n.4. Mutual synaptic repression was seen on examination of twitch tensions. With either simultaneous or staggered innervation, stimulation of each s.m.n. resulted in a twitch of 30-50 % of the total direct twitch tension, with little overlap between the fields driven by the two nerves. Intracellular recordings showed that the distribution of subthreshold and spike-producing e.p.p.s reflected the existence of separate twitch fields. Even if one s.m.n. was implanted several months before the other and had time to establish suprathreshold junctions on most muscle fibres, an s.m.n.

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

“…These findings are consistent with the view that there is ongoing competition between terminals at doubly innervated junctions, even in the absence of synapse elimination. Our results also are consistent with earlier findings that junctions on dually innervated fibers in frogs are substantially weaker and smaller than junctions on singly innervated fibers, both in intact muscles (Haimann et al, 1981a;Nudell and Grinnell, 1983) and reinnervated muscles (Grinnell et al, 1979;Haimann et al, 1981b).…”

The physiology, pharmacology, and trophic effectiveness of synapses formed by autonomic preganglionic nerves on frog skeletal muscle.

“…evoked twitches were stronger and did not show such severe synaptic depression during repetitive stimulation. In the case of the s.p.n., however, the ability of the nerve to evoke contraction was greatest at about 3-6 months, followed by an apparent decline to a lower plateau level of tension development in older preparations (Grinnell et al 1979). Fig.…”

“…Subsequent papers will describe the distribution and the morphology of the synapses formed by the splanchnic nerve (A. D. Grinnell & M. S. Letinsky, in preparation) and the competitive interaction between foreign nerves reinnervating the same muscle (Grinnell, Letinsky & Rheuben, 1979).…”

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

“…The selective elimination of multiple inputs may involve some form ofcompetition that is mediated via the muscle fiber (Brown et al, 1976;Thompson and Jansen, 1977;Betz et al, 1980Betz et al, , 1990Fladby and Jansen, 1987;Lichtman and Balice-Gordon, 1990). Moreover, synaptic effectiveness may be an important factor in determining the competitive strength of synapses (e.g., Grinnell et al, 1979;Wigston, 1980;Bennett and Lavidis, 1984b;Herrera, 1987, 1988;Herrera and Werle, 1990;Lichtman and Balice-Gordon, 1990). Since androgen may maintain polyneuronal innervation in the LA by preventing or altering synaptic competition, we wished to evaluate the effect of androgen on synaptic strength in the LA.…”