No abstract

1. The present study uses chronic low frequency stimulation of cat medial gastrocnemius (MG) muscle to investigate the relative contribution of innervation ratio to the wide range of motor unit force in large mammalian muscles by reducing the normal variation in muscle fibre cross-sectional area and specific force. 2. Isometric force recordings from isolated and physiologically characterized motor units were made 42-240 days after stimulation. Innervation ratio, fibre area and fibre type (I, II A, II B) were determined in one glycogen-depleted motor unit per muscle. 3. After 42 days of stimulation, all motor units were non-fatigable and were classified as either slow (S) or fast-fatigue resistant (FR). Despite the absence of fast-fatigable (FF) motor units, all three muscle fibre types were present, as identified according to their myofibrillar ATPase reactivity. After 143 days, all motor units and muscle fibres were classified as type S and type I, respectively. 4. A rapid decline in muscle and motor unit force to 30% of normal values after 42 days of chronic stimulation was accounted for by a reduction in muscle fibre area. Fibre areas did not change further with longer periods of stimulation but type II fibres were converted to type I. All stimulated muscle fibres were the size of normal type I fibres; the size of the fibres within single motor units covered the full range of the muscle fibre population. 5. In long-term stimulated muscles (> 100 days) when all muscle fibres were type I and all motor units type S, only differences in innervation ratio could account for the remaining range in motor unit force. Estimates of this range from the minimum and maximum values recorded and from values of tetanic force between the 5th and 95th percentiles indicate that the range in innervation ratio in the MG muscles is at least 15-fold and may be as large as 38-fold. Enumerations of glycogen-depleted muscle fibres from single motor units were consistent with this explanation. 6. The findings provide evidence that there is a wide range of innervation ratios in large muscles, which can account for the large range in motor unit forces in the muscles. Since motor unit force and innervation ratio vary with motoneurone size, these studies provide further support that the size of the peripheral field of innervation of motoneurones is related to their size.

SUMMARYThe imposition of long-term lengthened immobilization on rabbit tibialis anterior (TA) muscles resulted in rapid increases in slow oxidative (SO) fibre number. After 2 weeks, SO number had increased 2-fold and was 5 times greater after 6 weeks immobilization. There were also fibre-type-specific effects on SO, fast oxidative glycolytic (FOG) and fast glycolytic (FG) fibre areas. Twitch strength was unchanged throughout immobilization whilst production of tetanic tension was impaired during the initial period but had returned to control levels by 6 weeks. Twitch contraction times and isometrically determined rates of rise and relaxation were largely unaffected by immobilization despite the marked increase in expression of slow myosin. The change in phenotypic expression of the lengthened TA was not attributable to chronically increased levels of activation since integrated EMG activity was unchanged from control values throughout the immobilization period. Thus it is suggested that a chronic increase in tension consequent on the lengthening procedure is a potent stimulus for fast-to-slow myosin transformation.

SUMMARYUnilateral immobilization of rabbit hindlimbs was carried out for 2 weeks with an ankle angle of 90 deg. This was followed either by natural recovery for a further 2 or 4 weeks, or by chronic electrical stimulation of the soleus nerve during 2 weeks recovery using a low-frequency pattern of activation. Immobilization caused gross degeneration and functional disruption of soleus. Mean fibre area was reduced by 60 % compared with unoperated controls, twitch and tetanic tensions by 90 % and there was speeding of the rising phase of contraction. Natural recovery for 2 weeks had little effect; some regeneration occurred but muscle fibres remained atrophied and immature as indicated by the histochemical expression of both fast and slow myosin. Function continued to be grossly impaired, twitch and tetanic tensions were 66 and 79 % reduced and the time-to-peak twitch tension 31 % faster than controls. Prolonging the natural recovery period to 4 weeks increased muscle fibre area to 60 % of normal, and increased the acquisition of mature staining properties but most functional characteristics remained impaired. Stimulated muscles had normal fibre areas, mature appearance and functional improvements which matched and in most cases exceeded those seen after natural recovery for 4 weeks. Thus chronic low-frequency stimulation of soleus muscles accelerates recovery of structure and function following degenerative immobilization atrophy and may represent an important therapeutic aid in patient rehabilitation.

SUMMARYImmobilization of rabbit tibialis anterior (_fast) -ausele if -a lengthened pusitioru Aor o weeks led to a fivefold increase in the number of slow oxidative fibres. Unlike the situation in other models of phenotypic change in adult muscle, the increased expression of slow phenotype was not associated with an increase in the level of electrical activation, as monitored in the stretched muscles by chronic telemetric electromyography .