Exp Brain Res (1987) 66:597-606 Experimental BrainResearch Springer-Verlag 1987 Force and fatiguability of sprouting motor units in partially denervated rat plantaris P. Gardiner, R. Michel, A. Olha, and F. Pettigrew* Sciences de l'Activit6 Physique, D6partement d'Education Physique, Universit6 de Montr6al, C.P. 6128, Succ. A, Montr6al, Qudbec H3C 3J7, Canada Summary. The contractile properties of single motor units of rat plantaris were measured in situ 7 days following muscle partial denervation, achieved by section of radicular nerve L4. Partially denervated muscles weighed less, generated weaker twitch and tetanic forces, and contained denervated fibers, as evidenced by indirect/direct stimulation force ratios less than 1. Fast motor units (over 90% of unit pool) showed elevated twitch and tetanic responses (222% and 171% of controls, respectively) and elevated twitch-to-tetanic force ratios. Although partial denervation did not alter the mean fatiguability of fast motor units, fewer proportions of units remained in the extreme categories of fatigue resistance, with a clustering of units in the intermediate ranges. Slow units, while showing elevated twitch and tetanic responses, did not change in fatiguability. Glycogen depletion of the fibers of two fast motor units in partially denervated muscles revealed the presence of fibers varying in size, and in staining intensities for succinate dehydrogenase and ATPase, within the same motor unit, as a result of motoneurone sprouting. Key words: Sprouting - Motor units - Partial dener- vation Introduction Sprouting of motoneurones in response to partial denervation of its target muscle has been demon- strated in previous literature using histological and physiological criteria. In rodents, increased muscle * Present address: Faculty of Physical Education, University of Western Ontario, London, Ontario, Canada Offprint requests to: P. Gardiner (address see above) force evoked by electrical stimulation of spared roots can be detected as soon as 3 to 5 days following the lesion (Brown and Ironton 1978), and sprouting can result in increases in motor unit tension of 4-5 fold (Thompson and Jansen 1977; Brown and Ironton 1978) and perhaps more (Gorio et al. 1983) within a relatively short period of time. This robust response, usually measured using indirectly-evoked twitch or tetanic contractions, is in contrast to the rather delayed response on the part of the nerve cell body metabolism to adjust to its expanded peripheral field. For instance, terminals of a sprouted motoneurone become less complex (Slack and Hopkins 1982), and contain reduced quantal content of end-plate poten- tials (Slack and Hopkins 1982; Lowrie et al. 1985), during the period when contractile evidence of sprouting is present. Although the functional corre- lates of these motoneurone changes are not known, evidence has been presented previously that sprouted motor units demonstrate enhanced "sag" in response to indirect stimulation at 50 Hz, whereas direct stimulation produces no such response, implying a presynaptic failure such as transmitter depletion or intermittent conduction block in the sprouts (Brown and Ironton 1978). In our previous studies (Gardiner et al. 1984a; Gardiner and Faltus 1986) we have used twitch and tetanic contractile responses of the rat plantaris to indirect stimulation to obtain an estimation of the extent of functional motoneurone sprouting occur- ring during the first two weeks after partial denerva- tion, the latter accomplished by transection of radicu- lar nerve L4. These studies involved stimulation of the radicular nerve left intact (L5) following the partial denervation, a procedure which includes an intrinsic degree of variability due to the inter-animal variation in the proportions of motoneurones origi- nally innervating the muscle by L4 and L5 axons. In the present study, we describe the contractile charac-