| Summary: | During development of the nervous system an excess number of synapses are formed, most of which are subsequently pruned, resulting in functional neural networks. The precise mechanisms that determine which synapses are formed and which synapses are maintained are not thoroughly understood. The aim of this thesis is to investigate the intra-neuronal constraints and influences on synapse formation and elimination during development. In the first part of the thesis I investigated intra-neuronal influences on synapse elimination. Synapse elimination is known to occur at polyneuronally innervated neuromuscular junctions through competition, leading to mononeuronally innervated muscle fibres. However, whether synapse elimination ever occurs in the absence of competition, leading to muscle fibres becoming denervated, has not been resolved. The data presented in this thesis suggest that only large motor units undergo a reduction in motor unit size in the absence of competition. Using the Rasmussen and Willshaw (1993) version of the Dual Constraint Model I show that these data are consistent with the prediction that synapses will be eliminated from muscle fibres when a neuron's resources become stretched, for instance as a result of the normal growth of the animal. Larger motor units, which innervate more synapses, will thus be more vulnerable to the extra demand put upon them by the growth of each synapse. The model predicts that synapse elimination in the absence of competition should occur at least over the first 6 months of life and not only during the first two postnatal weeks, when most polyneuronal innervation is normally eliminated. In the second part of the thesis, I investigated intra-neuronal influences on synapse formation. Specifically I tested the hypothesis that each branch of a motor neuron forms synapses randomly and independently of other branches. If true there should be instances where two branches from the same neuron initially innervate the same endplate (sibling branch convergence). Sibling branch convergence was experimentally investigated in both regenerating and developing motor neurons. The evidence suggests that sibling branches can converge on the same muscle fibre and that they can competitively eliminate each other. However, it appears that convergence does not occur at the frequency that would be expected, suggesting that branches from the same motor neuron do not form synapses independently of each other. At present there are limitations in imaging immature networks due to the spatial resolution limit of light microscopy. The last part of this thesis explores thin serial sectioning and reconstruction as a possible technique for increasing resolution in the z-axis. This technique has potential to be developed but I show that at present it does not provide sufficient resolution to discriminate between developing motor axons in neonatal lumbrical muscles.
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