Skeletal motor neurones innervate the specialized 'types' of fibres comprising most mammalian muscles in a characteristic fashion: each motor neurone forms a 'motor unit' by innervating a set of fibres all of the same type. Because the type expression of adult muscle fibres is plastic and apparently controlled by their innervation, each motor neurone is thought to impose a common type differentiation on all the fibres in its motor unit. However, the situation in developing muscles cannot be this simple. Muscle fibres in neonates receive synaptic input from several motor neurones and achieve the adult, single innervation only after a period of 'synapse elimination. Despite this polyneuronal innervation, differentiated fibre types are present in neonatal muscles. This means either that the motor neurones polyneuronally innervate fibres in a random fashion and type expression is not determined by innervation or that the polyneuronal innervation is ordered in such a way that each fibre could receive unambiguous instructions for type differentiation. We have investigated these possibilities here by determining the fibre type composition of motor units in neonatal rat soleus muscle. We find that even during the time of polyneuronal innervation each motor neurone confines its innervation to largely one of two fibre types present in the muscle. Therefore, some mechanism during early development segregates the synapses of two groups of soleus motor neurones onto two separate populations of soleus muscle fibres.

No abstract

The intermediate filament nestin is localized postsynaptically at rodent neuromuscular junctions. The protein forms a filamentous network beneath and between the synaptic gutters, surrounds myofiber nuclei, and is associated with Z-discs adjacent to the junction. In situ hybridization shows that nestin mRNA is synthesized selectively by synaptic myonuclei. Although weak immunoreactivity is present in myelinating Schwann cells that wrap the preterminal axon, nestin is not detected in the terminal Schwann cells (tSCs) that cover the nerve terminal branches. However, after denervation of muscle, nestin is upregulated in tSCs and in SCs within the nerve distal to the lesion site. In contrast, immunoreactivity is strongly downregulated in the muscle fiber. Transgenic mice in which the nestin neural enhancer drives expression of a green fluorescent protein (GFP) reporter show that the regulation in SCs is transcriptional. However, the postsynaptic expression occurs through enhancer elements distinct from those responsible for regulation in SCs. Application of botulinum toxin shows that the upregulation in tSCs and the loss of immunoreactivity in muscle fibers occurs with blockade of transmitter release. Extrinsic stimulation of denervated muscle maintains the postsynaptic expression of nestin but does not affect the upregulation in SCs. Thus, a nestin-containing cytoskeleton is promoted in the postsynaptic muscle fiber by nerve-evoked muscle activity but suppressed in tSCs by transmitter release. Nestin antibodies and GFP driven by nestin promoter elements serve as excellent markers for the reactive state of SCs. Vital imaging of GFP shows that SCs grow a dynamic set of processes after denervation.