Competition between motor nerves in the establishment of neuromuscular junctions in striated muscles of Xenopus laevis

Specificity and development of innervation pattern of Xenopus pectoralis muscle

Multielectrode recordings were used to identify and measure the axonal inputs to each end plate on contiguous surface fibers covering about 25% of the Xenopus pectoralis muscle in mature and developing animals. The mature innervation pattern was remarkably precise. Individual axons tended to innervate fibers of similar input resistance (R(in)) in compact motor units restricted to only a portion of the region studied. Motor units comprising fibers of similar R(in) overlapped mainly near their borders. Most fibers had two end plates. In more than 80% of these fibers, both end plates received input from the same axon. In 57%, this was the only input to both end plates. This implies a powerful mechanism for excluding or eliminating inputs from other axons. About 16% of the mature junctions showed focal polyneuronal innervation, with the weaker end plate potential component often less than 1 mV in noncurarized preparation. However, we have no evidence that the weaker inputs were being eliminated. During development, motor units became more compact, which was associated with synapse elimination; but from the earliest times studied, soon after metamorphosis when many fibers were adding second end plates, a majority of those that had two end plates were innervated at both sites by the same axon.

Synaptic efficacy at singly- and dually-innervated neuromuscular junctions in the frog, Rana pipiens

Electrophysiological investigation was performed on excised, curarized cutaneous pectoris and sartorius muscles of the frog. Sixteen percent of cutaneous pectoris and 7% of sartorius end-plates received dual innervation from two different axons. The resting membrane potential and input resistance of muscle fibers at single synapses were similar to those at dual junctions. The e.p.p.s. generated by individual axons in dual synapses were, on average, smaller in amplitude than e.p.p.s recorded from singly-innervated junctions. However, the total e.p.p. amplitude (sum of the two component e.p.p.s) at dually-innervated end-plates was similar to that of end-plates innervated by a single axon. Our results at polyneuronally-innervated end-plates are consistent with a model of simple competition between motor axons for a limited synaptic space.

Neuromuscular transmission at terminals of sprouted mammalian motor neurones

Motor nerve sprouting was induced in the tensor fasciae latae muscle of mice by partial denervation produced either by cutting (to prevent reinnervation) or crushing (to allow subsequent reinnervation) spinal nerve L4 unilaterally. The quantum content (m) of endplate potentials recorded intracellularly in vitro in the presence of high-Mg2+ and low-Ca2+ ion concentrations was determined up to 400 days later in non-reinnervated, reinnervated and contralateral control muscles. The muscles were then either fixed and stained with silver and cholinesterase for light microscopy, or fixed and examined in the electron microscope. The average value of m in control muscles increased by 4-5-fold as the animals matured in the 4 months following the operations. The average value of m at terminals of sprouted motor neurones in the absence of reinnervation also increased with time after partial denervation but was always less than the value in the corresponding control muscle. In electron micrographs of muscles following L4 section the nerve terminals closely apposed on average only two-thirds of the proportion of junctional folds apposed to terminals in control muscles. When muscles were reinnervated following L4 crush the average value of m at terminals of sprouted and reinnervating motor neurones equalled and sometimes exceeded m in contralateral control muscles. A proportion of muscle fibres had endplate potentials from reinnervating and sprouted axons, and the silver stain showed that these muscle fibres were innervated at the site of the original endplate. At these endplates the fraction of the total quantum content contributed by presumed sprout terminals fell significantly in the 4 months following L4 crush. It is concluded that: (i) in the absence of reinnervation, sprout terminals grow in size but a significant number never occupy all endplate site available to them; and (ii) in the presence of reinnervation axons terminals share some endplates with sprout terminals and grow at the expense of the sprout terminals which are eventually withdrawn from some shared endplates.

The fate of foreign endplates in cross-innervated rat soleus muscle

After transplantation of the superficial fibular and the medial plantar nerve to neighbouring sites in the proximal region of adult rat soleus muscles many muscle fibres were initially innervated by axons in both foreign nerves after resection of the original soleus nerve. The foreign endplates were formed at ectopic sites and were often separately located on individual muscle fibres. After 3-4 weeks many endplates had been eliminated and most muscle fibres were innervated by only a single foreign axon. Many muscle fibres still had multiple esterase-staining endplate sites in the region innervated by the foreign nerve. On examination by electronmicroscopy, some of these sites were seen to have lost their presynaptic terminal while the postsynaptic structure of the endplate remained intact. Other sites were only partially occupied by motor axon terminals. On each muscle fibre there was always at least one fully occupied endplate region. In some instances separate endplate sites on the same muscle fibre were innervated by branches of the same motor axon. We conclude that the elimination of endplates is due to a competitive interaction between motor axons innervating the same muscle fibre. Morphologically, the elimination of functional endplates is caused by a retraction of nerve terminals from the postsynaptic site.

Sprouting and regression of the nerve at the frog neuromuscular junction in normal conditions and after prolonged paralysis with curare

A light microscopical, histochemical and electron microscopical investigation of the frog neuromuscular junction has been performed on muscles from animals in different functional states of activity. The combined staining of axon terminals and cholinesterase (ChE) allows a precise description of the nerve terminal arborization and its synaptic contacts. Most terminal arborizations form long continuous contacts with the muscle cell. Distinquishable from these are nerve branches (usually of small diameter)d or distal endings of branches with one or several small and isolated contacts. It is assumed that these are sprouts with newly-formed synaptic sites. Other sprouts end without apparent synaptic contact. At the uttrastructural levet, nerve sprouts end without apparent synaptic contact. At the uttrastructural levet, nerve sprouts growing into empty, well-differentiated synaptic gutters or inducing the formation of new synaptic sites were observed. In other sites, ChE is apparently located at postsynaptic gutters with no nerve present. Similarly, in the electron microscope, well-differentiated synaptic gutters lacking any nerve or Schwann cell elements were observed. In addition, synaptic gutters only partially occupied by the nerve were frequently seen. These features have been interpreted as signs of regression of the nerve terminals. Nerve regression and sprouting were found in animals chronically paralysed with curare over several weeks as well as in untreated frogs (winter and summer frogs, laboratory frogs, fed and unfed). When quantitatively evaluating the occurence of presumed features of nerve sprouting and nerve regression, differences were found between different experimental groups. From this it is concluded that, in addition to developmental changes, the degree of nerve sprouting and regression is controlled by external factors such as muscle activity and seasonal variations. Signs of sprouting and nerve regression can be simultaneously present in a single synapse. It appears that the frog neuromuscular synapse is not a static structure, but is in a state of permenent remodelling.

Suppression of foreign innervation in axolotl muscle may not be dependent on juxtaposition of native and foreign nerve terminals

The posterior supracoracoideus nerve of the axolotl, Ambystoma mexicanum, was induced to make synapses outside its normal muscle territory. Muscle fibres with inputs from both native and foreign nerves were studied during the period of suppression of foreign transmission and in only 8% of fibres were foreign and native terminals found within 120 micrometer of each other. A combined cholinesterase/silver staining technique revealed non-innervated endplates in foreign-innervated fibres just prior to the return of the native nerve. These results suggest a mechanism enabling suppression of foreign synapses at some distance from native synapses probably beginning with the reinnervation of empty sites by the native nerve.

Systems-matching by degeneration. I. A quantitative electron microscopic study of the generation and degeneration of retinal ganglion cells in the chicken

The total number of optic nerve fibers of the chicken was determined at twenty sequential developmental stages from incubation day 5 to 104 days after hatching. It was found that the total number of optic nerve fibers increases from 4400 on incubation day 5 to about 4.0 million on incubation days 10 and 11. Thereafter, it decreases to a final value of about 2.4 million by incubation day 18 and remains constant from that time on until adulthood. Thus, 40% of optic fibers degenerate. Degenerating ganglion cells in the retina are first detectable by incubation day 9. Initially degenerating cells are located mainly in the central retina, but on subsequent days they can be found predominantly in peripheral zones. It is postulated that cell death occurs because of competition for adequate arborization space. If more retinal afferent fibers arrive than tectal termination sites are available, supernumerary fibers may degenerate. By degeneration the two systems retina and optic tectum, are matched in size.