Tessa Gordon — Page 2 | PubMed Health

PubMedDate not supplied

Mechanisms controlling axonal sprouting at the neuromuscular junction.

This review considers the relative roles of sprouting stimuli, perisynaptic Schwann cells and neuromuscular activity in axonal sprouting at the neuromuscular junction in partially denervated muscles. A number of sprouting stimuli, including insulin-like growth factor II, which are generated from inactive muscle fibers in partially denervated and paralyzed skeletal muscles, has been considered. There is also evidence that perisynaptic Schwann cells induce and guide axonal sprouting in adult partially denervated muscles. Excessive neuromuscular activity significantly reduces bridging of perisynaptic Schwann cell processes between innervated and denervated endplates and thereby inhibits axonal sprouting in partially denervated adult muscles. Elimination of neuromuscular activity is also detrimental to sprouting in these muscles, suggesting that calcium influx into the nerve is crucial for axonal sprouting. The role of neuromuscular activity in axonal sprouting will be considered critically in the context of the roles of sprouting stimuli and perisynaptic Schwann cells in the process of axonal sprouting.

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PubMedDate not supplied

The resilience of the size principle in the organization of motor unit properties in normal and reinnervated adult skeletal muscles.

Henneman's size principle relates the input and output properties of motoneurons and their muscle fibers to size and is the basis for size-ordered activation or recruitment of motor units during movement. After nerve injury and surgical repair, the relationship between motoneuron size and the number and size of the muscle fibers that the motoneuron reinnervates is initially lost but returns with time, irrespective of whether the muscles are self- or cross-reinnervated by the regenerated axons. Although the return of the size relationships was initially attributed to the recovery of the cross-sectional area of the reinnervated muscle fibers and their force per fiber, direct enumeration of the innervation ratio and the number of muscle fibers per motoneuron demonstrated that a size-dependent branching of axons accounts for the size relationships in normal muscle, as suggested by Henneman and his colleagues. This same size-dependent branching accounts for the rematching of motoneuron size and muscle unit size in reinnervated muscles. Experiments were carried out to determine whether the daily amount of neuromuscular activation of motor units accounts for the size-dependent organization and reorganization of motor unit properties. The normal size-dependent matching of motoneurons and their muscle units with respect to the numbers of muscle fibers per motoneuron was unaltered by synchronous activation of all of the motor units with the same daily activity. Hence, the restored size relationships and rematching of motoneuron and muscle unit properties after nerve injuries and muscle reinnervation sustain the normal gradation of muscle force during movement by size-ordered recruitment of motor units and the process of rate coding of action potentials. Dynamic modulation of size of muscle fibers and their contractile speed and endurance by neuromuscular activity allows for neuromuscular adaptation in the context of the sustained organization of the neuromuscular system according to the size principle.

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