Physiological responses of rat plantaris motor units to overload induced by surgical removal of its synergists

Abstract

1. Rat plantaris muscles were subjected to chronic overload by the surgical removal of the soleus and most of the gastrocnemius muscles. Twelve to 16 wk later whole muscle and motor unit (ventral root dissection technique) contractile properties as well as histochemistry were determined. 2. Motor units were categorized as fast, fatigable (FF), fast, intermediate fatigue-resistant (FI), fast, fatigue-resistant (FR), and slow (S) based on contractile characteristics. Muscle fibers were identified as type I and type II according to myofibrillar ATPase staining. 3. Whole muscles demonstrated increases in wet weight, tetanic force, proportion of type I fibers, and mean cross-sectional areas of both type I and II fibers, as a result of chronic overload. 4. Tetanic tension increased by the same relative magnitude in all motor units whereas twitch tension remained unchanged. A significant change in the proportions of the motor unit types occurred in overloaded muscles, such that the latter contained higher proportions of FF and S units, and lower proportions of FI and FR units, than normal muscles. 5. The fatigue profile of a composite constructed from a summation of motor unit responses revealed that the overloaded plantaris displayed fatigue resistance similar to that of the normal plantaris for a given absolute force output. 6. Glycogen-depleted fibers of hypertrophied single motor units demonstrated uniform myofibrillar ATPase and SDH staining characteristics suggesting that metabolic adaptations among fibers of the same unit were similar after 12-16 wk of overload. 7. The finding that overload caused a uniform increase in the tetanic strength of all motor units, whereas alterations in fatigue resistance varied in degree and direction among unit types, demonstrate that these two properties are not controlled in parallel in this model. The smallest units maintain or even increase their fatigue resistance during the hypertrophic process, whereas high threshold units actually decrease in fatigue resistance.

Adaptations of motoneuron properties to chronic compensatory muscle overload

“…The mean values of this ratio were 0.26 Ϯ 0.02% in the control group, 0.29 Ϯ 0.01% (P Ͻ 0.05) in the 5wkOL group, and 0.31 Ϯ 0.04% (P Ͻ 0.01) in the 12wkOL group. The observed results are in line with several previous studies on overload effects on muscle tissue (Huey et al 2010;Kryściak et al In press;Noble and Pettigrew 1989;Olha et al 1988).…”

“…However, in the present study fast MUs became more excitable (see Table 1), whereas the parallel study has indicated greater forces produced by fast MUs due to 3 mo overload (Kryściak et al In press). The latter observation is in accordance with several previous reports on overload effects on properties of muscle fibers or MUs (Walsh et al 1978;Olha et al 1988;Noble and Pettigrew 1989). Kryściak et al (in press) have also shown a decrease of the twitch-totetanus ratios, which indicates more efficient force development during tetanic contractions.…”

“…This indicates that the response to an increased level of chronic activation of MNs is relatively quick and stable. membrane properties; rat MUSCLE OVERLOAD CAN BE ACHIEVED by workload training (Fry 2004;McDonagh and Davies 1984), ablation or tenotomy of synergists (Gardiner et al 1991;Noble and Pettigrew 1989;Olha et al 1988), peripheral nerve damage, or denervation of synergistic muscles (Degens et al 1995). Overload induces measurable adaptive effects in the whole muscle, its motor units (MUs), and individual muscle fibers.…”

“…Several studies have reported changes in contractile properties of MUs in response to chronic muscle overload. They point on increased tetanic forces of all MU types, longer contraction times of fast resistant and slow MUs, decreased fatigue resistance of fast fatigable units, and altered proportions of MUs within an overloaded muscle (Noble and Pettigrew 1989;Olha et al 1988).It is well documented that motoneurons are highly susceptible to altered levels of physical activity . Hyperpolarized membrane potentials and voltage thresholds, lower spike rise times, and higher cell capacitances of fast-type motoneurons have been established in rat in response to exercise training (Beaumont and Gardiner 2003).…”

“…Several studies have reported changes in contractile properties of MUs in response to chronic muscle overload. They point on increased tetanic forces of all MU types, longer contraction times of fast resistant and slow MUs, decreased fatigue resistance of fast fatigable units, and altered proportions of MUs within an overloaded muscle (Noble and Pettigrew 1989;Olha et al 1988).…”