The size of motor units during post‐natal development of rat lumbrical muscle.

Betz, William J.; Caldwell, John H.; Ribchester, Richard R.

doi:10.1113/jphysiol.1979.sp013051

Cited by 148 publications

(143 citation statements)

References 16 publications

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“…However, it is also possible that some of this apparent overlap at P7 could be due to electrical coupling among developing muscle fibers (Dennis et al, 1981), which would cause our estimates of motor unit size to be spuriously large. Nonetheless, the finding that motor unit size but not number decreases during the period when multiaxonal innervation is eliminated in the LA muscle (Jordan et al, 1988) indicates that synapse elimination in the LA involves an elimination of axon terminals from d@rent motoneurons, as has been described in numerous other rat muscles (Redfern, 1970;Brown et al, 1976;Rosenthal and Taraskevich, 1977;Betz et al, 1979;Dennis et al, 198 1;Lavidis, 1984b, Balice-Gordon andThompson, 1988). Because synapse elimination in the LA involves the same cellular changes as synapse elimination in other developing muscles, there may also be common underlying molecular mechanisms.…”

Section: Control Of Motor Unit Size During Developmentmentioning

confidence: 82%

“…Consequently, developing mammalian muscle fibers are initially polyneuronally innervated (Redfem, 1970) rather than singly innervated as in adults. During synapse elimination, motoneurons retract a large portion of their axonal arbor (Bixby,198 1;Riley,198 l), thus reducing the size of motor units (Brown et al, 1976;Betz et al, 1979;Fladby, 1987; Balice-Gordon and Thompson, 1988) until the adult pattern of single innervation is established (reviewed in Van Essen, 1982;Thompson, 1986;Betz, 1987;Jansen and Fladby, 1990). Recent evidence in the androgen-sensitive levator ani (LA) suggests that steroid hormones may be important regulators of synapse elimination (Jordan et al, 1989a(Jordan et al, ,b, 1990.…”

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confidence: 99%

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

et al. 1992

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Previous anatomical studies suggest that androgen regulates synapse elimination in the androgen-sensitive levator ani(LA) muscle of the rat. Androgen treatment beginning on postnatal day 7 (P7) prevents some of the normal loss of multiaxonal innervation in this muscle. The present study used physiological techniques to measure the number and size of LA motor units during the synapse elimination period in muscles from normals, and castrates treated with either testosterone propionate or oil. The number of increments in LA twitch tension as nerve stimulation intensity increased, a measure of the number of motor units, was the same at the end (P28) of synapse elimination as near the beginning (P7) of this process. This result indicates that motoneuronal cell death does not contribute to synapse elimination in the LA. Moreover, androgen during this period did not influence the number of LA motor units. In contrast, between P7 and P28, there was a dramatic decline in the size of LA motor units, as indicated by a decrease in the percentage of twitch or tetanus tension of individual motor units relative to the maximal twitch or tetanus tension of the whole muscle. In addition, androgen treatment of castrated males during this period prevented some of the normal decline in the size of LA motor units. Estimates of the number of inputs per LA muscle fiber derived from the number of LA motor units and their average size indicate that androgen maintains polyneuronal innervation in the LA muscle. This finding supports previous anatomical studies suggesting that androgen can prevent synapse elimination in this muscle. The strength of LA synapses was also examined by measuring the tetanus: twitch ratio of individual motor units and by measuring the safety margin of LA synapses. Both measurements indicated that the average strength of LA synapses increases during synapse elimination. Moreover, androgen appeared to spare synapses from elimination without increasing their strength, since androgen-treated muscles generally had larger motor units but the same mean tetanus:twitch ratio and safety margins as untreated LA muscles except at P28, when synapses in androgen-treated LA muscles had appreciably lower safety margins than normal. These results suggest that androgen regulates synapse elimination through a mechanism(s) independent of synaptic strength.

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Section: Control Of Motor Unit Size During Developmentmentioning

confidence: 82%

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confidence: 99%

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

et al. 1992

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“…Interestingly, the amplitude distribution of miniature EPSCs in cultured neurons, which are believed to originate from single synapses, is also positively skewed (Bekkers and Stevens, 1995;van Rossum et al, 2000). We would like to note that the distribution of motor unit sizes is also reminiscent of a log-normal function (Betz et al, 1979;Taxt, 1983a,b;Lu et al, 2009), which raises the possibility that a log-normal distribution of synaptic strength observed in cortical neurons represents a more general principle of organization in the nervous system.…”

Section: Log-normal Distribution Of Spine Sizesmentioning

confidence: 88%

Multiplicative Dynamics Underlie the Emergence of the Log-Normal Distribution of Spine Sizes in the Neocortex In Vivo

Loewenstein¹,

Kuras²,

Rumpel³

2011

Journal of Neuroscience

220

265

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What fundamental properties of synaptic connectivity in the neocortex stem from the ongoing dynamics of synaptic changes? In this study, we seek to find the rules shaping the stationary distribution of synaptic efficacies in the cortex. To address this question, we combined chronic imaging of hundreds of spines in the auditory cortex of mice in vivo over weeks with modeling techniques to quantitatively study the dynamics of spines, the morphological correlates of excitatory synapses in the neocortex. We found that the stationary distribution of spine sizes of individual neurons can be exceptionally well described by a log-normal function. We furthermore show that spines exhibit substantial volatility in their sizes at timescales that range from days to months. Interestingly, the magnitude of changes in spine sizes is proportional to the size of the spine. Such multiplicative dynamics are in contrast with conventional models of synaptic plasticity, learning, and memory, which typically assume additive dynamics. Moreover, we show that the ongoing dynamics of spine sizes can be captured by a simple phenomenological model that operates at two timescales of days and months. This model converges to a log-normal distribution, bridging the gap between synaptic dynamics and the stationary distribution of synaptic efficacies.

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“…After birth, the peripherally located nuclei within the muscle fibers lose the capacity for mitotic activity and therefore, it is believed that fiber number is established at the time of birth (Van De Graaff 1984). Other investigators, however, have found an increase (Betz et al 1979) and decrease (Layman et al 1980) in fiber number in rat muscle tissue shortly after birth.…”

mentioning

confidence: 85%

Gender differences in strength and muscle fiber characteristics

Miller

MacDougall

Tarnopolsky

et al. 1993

Europ. J. Appl. Physiol.

828

562

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A gender difference in absolute muscle strength is well documented.The e'xtent to which quantitative (fiber area and number) and qualitative (specific tension) differences in muscle contribute to this is not well understood. The purpose of this study was to examine a variety of muscle characteristics in the biceps brachii and vastus lateralis in a sample of males (n-8) and females (n=8) The difference in type II fiber area in the biceps brachii was not statistically significant despite the fact that these fibers were almost twice as large in the males as in 2 the females (8207 vs. 4306 urn). No significant gender difference was found in biceps fiber number (180,620 vs.l56,872) or muscle area to fiber area ratio in the vastus lateralis (451,468 vs. 465,007).No significant gender differences were found in any of the motor unit characteristics.The results indicate that the primary determinant of the greater muscle strength of males is their larger mean fiber areas which results in greater muscle cross-sectional areas.iv ACKNOWLEDGEMENTS

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The size of motor units during post‐natal development of rat lumbrical muscle.

Cited by 148 publications

References 16 publications

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

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

Multiplicative Dynamics Underlie the Emergence of the Log-Normal Distribution of Spine Sizes in the Neocortex In Vivo

Gender differences in strength and muscle fiber characteristics

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