THE FINAL COMMON PATHWAY IN POSTURAL CONTROL - DEVELOPMENTAL PERSPECTIVE

A brief review is given concerning postural specialisations among mamm alian muscle fibres and motor units. Most skeletal muscles contain a m ixture of fibres with different characteristics, and their slow-twitch (S) units are well-known to possess properties suitable for postural tasks: they are highly fatigue-resistant, well equipped for oxidative metabolism, and their slowness makes them energetically cheap in (semi -)isometric contractions. These features are adequately employed in mo tor behaviour owing to characteristics of the associated motoneurones. In adult mammals, the way in which a muscle is used can influence its proportion of S units. This adjustment occurs within a restricted 'ad aptive range' which differs between muscles and animal species, presum ably being preset at an early age. In the course of early foetal devel opment, part of the slow vs. fast differentiation of muscle fibre prop erties can take place independently of innervation. Once innervation h as taken place, how ever, motoneurones influence the differentiation i n various ways. On the whole, a well coordinated timing seems to exist between the early differentiation of central motor mechanisms and of the peripheral machinery, largely causing the neuromuscular system to be/become ready for use when the brain needs it. (C) 1998 Elsevier Sci ence Ltd. All rights reserved.