selective elimination of motor nerve terminals in the rat soleus muscle during development.

Abstract

SUMMARY1. The pattern of innervation and either frequency of miniature end-plate potentials (m.e.p.p.s) or acetylcholine (ACh) sensitivity was examined in individual fibres of the soleus muscle in new-born rats to elucidate the mechanism of elimination of motor nerve terminals.2. The number of motor nerve terminals innervating a muscle fibre was judged by the difference in latencies of end-plate potentials evoked by stimulation of the ventral roots L4 and L5.3. At early developmental stages, about 50 % of soleus muscle fibres were usually singly innervated by the L5 ventral root and this nerve to fibre relation did not change throughout the developmental days. In contrast to the L5 root, the L4 root innervated all the muscle fibres polyneuronally at day 6. In day 16 or older animals, the remaining half of the fibres were singly innervated by the L4 ventral nerves. Therefore, it is concluded that the elimination of motor nerve terminals in the soleus muscle occurs selectively only for the L4 ventral nerves.4. The number of polyneuronally innervated fibres was increased, to some extent, within 2 days after transaction of the spinal cord at day 12. It is suggested that the increased polyneuronal innervation following the inactivity produced by cordotomy may reflect the recovery of terminals which had lost function only a short time previously during elimination.5. Though frequency of m.e.p.p.s increased from day 6 to adult age as previously reported, there was no significant difference in the frequencies between singly and polyneuronally innervated fibres at any age. Cord transaction at day 12 caused no change in the frequency.6. ACh sensitivity observed in singly innervated fibres was similar to that in polyneuronally innervated fibres. Furthermore, cordotomy on day 12 did not induce an increase in ACh sensitivity around the end-plate. It is concluded that the elimination of terminals is not causally related to changes in ACh sensitivity.7. The possible mechanisms of selective elimination of terminals and the process of elimination are discussed.

Altered developmental changes of neuromuscular junction in hypo‐ and hyperthyroid rats

“…1 A). These two features have often been adopted as an indication of multiple innervation (Redfern, 1970;Brown et al 1976;Miyata & Yoshioka, 1980). Multiple innervation could be recognized fairly readily by these methods in younger animals, but discrimination became difficult in older animals where motor axon thresholds were similar among innervating nerves (Rosenthal & Taraskevich, 1977).…”

“…In this study, we have focused on the following developmental changes in neuromuscular junctions: (a) the elimination of excess axons in polyneuronal innervation (Redfern, 1970;Bennett & Pettigrew, 1974;Brown, Jansen & Van Essen, 1976;Rosenthal & Taraskevich, 1977); (b) the increase in frequency of miniature end-plate potentials (m.e.p.p.s) (Diamond & Miledi, 1962;Kelly, 1978;Miyata & Yoshioka, 1980); (c) the reduction of extrasynaptic acetylcholine (ACh) sensitivity (Diamond & Miledi, 1962;Bevan & Steinbach, 1977;Miyata & Yoshioka, 1980); and (d) the shift from facilitation to depression of end-plate potentials (e.p.p.s) (Bennett & Pettigrew, 1974). These properties were investigated in normal, hypo-and hyperthyroid new-born rats and it was demonstrated that thyroid hormone is important for the normal maturation of neuromuscular synapses; the lack or excess of thyroid hormone retarded or accelerated the developmental changes in these properties, respectively.…”

Histochemical profiles of fibers in the rat tibialis anterior muscle during early postnatal development.

“…Whether the histochemical properties of muscle fibers under multiple innervation are regulated by neuronal factors of the predetermined spinal motoneurons, the functional activity, and/or neurotrophic factors has been discussed in previous studies (THOMPSON et al, 1984;GORDON and VAN ESSEN, 1985;JONES et at., 1987a, b;FLADBY and JANSEN, 1988;REDENBACH et al, 1988). Although the mechanism underlying the type shift of muscle fibers under multiple innervation is not clear, our findings suggest that collateral sprouting from type-different motoneurons which differentiate earlier causes the type shift of muscle fibers during early postnatal development, as observed by MIYATA and YOsHIOKA (1980) All muscle fibers 1 day after birth showed a uniform reaction for the enzyme, but fast-twitch (dark) and slow-twitch (light) fibers were found at 9 days. Thereafter, the type shift of fibers from slow-twitch to fast-twitch was observed during postnatal development while multiply innervated (13 days).…”

Communications

“…Measurements of contractions show that the proportion of soleus muscle fibres innervated by axons from both L4 and L5 roots is as much as 80 % up to 8 days of age, and is reduced to less than 10 % by 14 days. This reduction is due almost exclusively to the loss of innervation by L4axons (see also Miyata & Yoshioka, 1980); in the present experiments this change in the pattern of innervation has been exploited to distinguish between potentially surviving (L5) and non-surviving (L4) nerve terminals.…”

“…of mean, n = 19 fibres) for L4 and 6-7 + 0 99 (n = 12) for L5. These values are the sums of the quantal release from all the activated nerve terminals at these immature end-plates; since there may be fewer axon terminals from L5 than from L4 (Miyata & Yoshioka, 1980), the observed difference in quantum content is probably an underestimate of the real difference between individual L4 and L5 terminals. Quantum size in magnesium-poisoned muscles was estimated from histograms of the e.p.p.…”