Synapse formation and elimination during growth of the pectoral muscle in Xenopus laevis.

Abstract

SUMMARY1. Synapse formation and synapse elimination were studied in the pectoral muscle of Xenopus laevis.2. Histology showed that fibres were not added during postmetamorphic growth. Most fibres were innervated at two widely separated junctions and this number did not change as frogs grew.3. Intracellular recording revealed that fibres with two junctions could be mononeuronally innervated, or innervated in one of three different polyneuronal patterns. A growth-related shift in innervation pattern was observed, with the polyneuronal patterns replaced by mononeuronal innervation.4. Endplate potentials (EPPs) evoked by low-frequency nerve stimulation were simultaneously measured at both junctions on individual fibres. For each fibre, the ratio of EPP amplitudes (smaller/larger) was calculated. When the two junctions were innervated by different motoneurones (A-B), the median EPP ratio was smaller than when the two junctions were innervated by the same motoneurone (A-A), although the difference was not significant. 5. The difference in the ratio of EPP amplitudes became significant, however, if junctions were conditioned by a train of fifty stimuli at 10 Hz. Immediately after such a train, EPP ratios for A-B fibres were significantly smaller than ratios for A-A fibres. This difference was due to greater synaptic depression at one of the junctions on A-B fibres.6. We concluded that enhanced depression of the EPP upon repetitive stimulation is a physiological correlate of the competition that underlies synapse elimination.

Specificity and development of innervation pattern ofXenopus pectoralis muscle

“…Although the overall proportion of fibers showing innervation of both end plates by the same axon with or without inputs from other axons ( a _ / a-) was very high (82.3%), the fraction showing pure DMI ( a / a , 57%) was significantly lower than reported from earlier experiments (66% in Nudell and Gnnnell, 1985; 92% in large adults in Dunia and Herrera, 1993 ). Many inputs in the present experiment were less than 1 mV in amplitude and would not have been detected had curare or other postsynaptic blocking agents been used to reduce EPPs below the threshold.…”

“…This suggests some fiber addition with growth, but we do not consider this a significant change because the fibers were much smaller in the smaller individuals and some fibers may have been missed. Dunia and Herrera ( 1993) found no addition of fibers, based on fiber counts in the procoracoid region of the muscle in small and large Xenopus. Haimann et al ( 1981a) reported that the anterior and posterior nerves contained approximately 19 and 35 motor axons, respectively, in nerves that had their spinal ganglia destroyed, removing the sensory component.…”

“…Although we are aware of no careful biomechanical study of the role of this muscle in movement of the forelimb, its shape and the fact that it appears to end in a broad tendon encircling a significant fraction of the proximoventral surface of the humerus suggests that, depending on what portion of the muscle is contracting, it pulls or rotates the humerus and the forelimb in different directions. This muscle has been used to study competition between inputs to the same and different end plates (Haimann et al, 1976(Haimann et al, , 1981bDunia and Herrera, 1993 ) and to derive some of the rules that appear to govern the number, size, and strength of synaptic inputs to the muscle Grinnell, 1982, 1983; Dunia and Herrera, 1993).…”

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

“…Cases of apparent synapse elimination and formation of new polyneuronal innervation have been directly observed in frogs (Herrera and Werle, 1990;Herrera et al, 1991). In addition, synapse replacement, wherein the innervation provided by one nerve terminal is substituted by innervation from another terminal, seems to be common in the pectoralis muscle (supracoracoideus) of Xenopus laevis, in which it can be strongly inferred from physiological measurements (Dunia and Herrera, 1993).…”

Neuromuscular Remodeling and Myofiber Turnover in <i>Rana pipiens’ </i>Jaw Muscles

“…The developmental progression of the trigeminal motor system is in stark contrast to other amphibian systems that have been presented as models of neuromuscular ontogenesis [Letinsky and Morrison-Graham, 1980;Morrison-Graham, 1983;Bennett et al, 1985;Jans et al, 1986;Herrera et al, 1990;Dunia and Herrera, 1993]. Most of these have focused on limb muscles, such as the sartorius and cutaneous pectoris, which show a more typical pattern of neuromuscular maturation.…”