Organized variability in the neuromuscular system: a survey of task-related adaptations

This survey concerns the physiology of the neuromuscular system, as studied at the level of the single mammalian limb muscle and its motoneurones (MNs). Particular attention is devoted to the ways in which the properties and the organization of spinal MNs are adapted for the control of muscle (unit) force. These questions are discussed in relation to: a) The general and basic task of the system: providing a smooth and finely gradeable force by the mechanisms of rate- and recruitment-modulation of MN activity. b) Gradation problems in relation to specific peripheral requirements in connection with: (i) tasks of different duty-time (i.e. problems related to fatigue and endurance); (ii) tasks of different speed; (iii) task using different muscle lengths; (iv) tasks requiring different adjustment-gains. c) The adaptational properties of the neuromuscular system as it is subjected to long-term changes in its motor tasks. These matters are largely discussed in relation to experiments for studying the responses of the neuromuscular system to different patterns of chronic electrical stimulation. d) The manner in which the neuromuscular system, at the level of a single unidirectional muscle, is used for different motor programs. Evidence is summarized which shows the presence of task-related variations in MN recruitment patterns, and it is pointed out that such variations may be related to the intraspinal topography of the respective MNs. It is suggested that these task-related variations in MN recruitment behaviour might largely reflect topographic (and other) differences in the organization of spinal interneuronal systems responsible for the execution of different motor programs.