Patterns of motor innervation in the pectoral muscle of adult Xenopus laevis: evidence for possible synaptic remodelling.

Abstract

SUMMARY1. An anatomical and electrophysiological study was performed on the pectoral muscle of Xenopus laevis.2. Silver-impregnated preparations revealed immature endings, collateral and terminal sprouting and signs of synaptic regression.3. Twenty percent of the fibres received a dual innervation from two different nerves. The synapses of 250 of these fibres are formed in close vicinity.4. Some of the singly innervated and most of the dually innervated end-plates generated only subthreshold electrical activity. Synaptic efficacy in dually innervated muscle fibres with closely spaced or distant endings was, on the average, one third and two thirds, respectively, of that obtained in singly innervated fibres.5. Fibres with subthreshold electrical activity displayed normal ACh sensitivity.6. The existence of non-transmitting synapses, of dually innervated end-plate sites and of morphological signs of the sprouting of new endings and the degeneration of old ones suggests that synaptic remodelling may occur in normal adult muscles.

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).…”

Specificity and development of innervation pattern ofXenopus pectoralis muscle

“…Although the name pectoralis has been used for this muscle in all of the recent physiological literature, it might more accurately be termed the supracoracoideus muscle (Grobbelaar, 1924;Tyson, 1987). Haimann et al (1981a) reported that there are approximately 900 fibers in the muscle. We have done comparable fiber counts on transverse sections in muscles of three adults (nose-vent body length 5.2-6 cm) and four much smaller animals ( 1.5-2.1 cm body length), finding means of 814 k 100 (S.D.)…”

“…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. This corresponds to a mean motor unit size of 17 muscle fibers (assuming no overlap of innervation).…”

“…The common pectoralis nerve was exposed and freed and the anterior branch was sectioned just distal to the branch point, allowing independent stimulation of the posterior and anterior nerves. These contain separate populations of axons, because antidromic stimulation of one branch does not lead to activ-ity in fibers innervated by the other (Haimann et al, 1981a;Nudell and Grinnell, 1985).…”

Neuronal Lysosome Accumulation in Degrading Synapses of Sensory-Motor and Limbic Subsystems in the Duck <i>Anas platyrhynchos</i><i>:</i> Indication of Rearrangements during Avian Brain Development?

“…Principles underlying productive synaptic development essentially demand consideration of regressive events which refer to both the irreversible elimination corre lated with selective stabilization of labile synapses [3,4] and the remodelling comprising structural modifications of persisting synapses [5][6][7][8].…”