Motor units of the fourth deep lumbrical muscle of the adult rat: isometric contractions and fibre type compositions.

Abstract

SUMMARY1. Isometric twitch and tetanic tensions were recorded from whole muscles and single motor units in fourth deep lumbrical muscles isolated from young adult (60 days) rats. Muscles were superfused with oxygenated Ringer solution at 25°C except where stated otherwise.2. It was confirmed that the muscle is supplied most commonly by eleven motor axons, nine via the lateral plantar nerve (LPN), and two via the sural nerve (SN). Motor units whose axons were isolated from either LPN or SN were studied. There was no difference in mean motor unit size.3. In their unfused tetani most units showed 'sag' and some 'no sag', with no segregation between LPN and SN. 'No sag' units were always small (unit tetanic tension < 8% whole-muscle tetanic tension), tended to be relatively slowly contracting and relaxing during an isometric twitch, and tended to have relatively low twitch: tetanus ratios. Units showing sag ranged from large to small. 4. In some motor units muscle fibres were depleted of their glycogen by repetitive stimulation at 30°C in glucose-free Ringer solution, and the muscle and its unstimulated control frozen and sectioned. Neighbouring sections were stained for glycogen and for binding of two myosin-specific antibodies, one specific for slow myosin and the other for type IIA myosin. Myosin ATPase and succinic dehydrogenase histochemistry were also carried out in some muscles.5. Serial reconstructions showed that all or virtually all extrafusal fibres in the muscle were present in a midbelly section, and that the myosin type of individual fibres did not change significantly along their length. Spindle profiles were seen frequently and in two muscles eight and twelve spindles were identified.6. Of twenty-six motor units examined twenty contained almost exclusively muscle fibres of the recently described type IIX. All these units showed sag in their isometric tetani.7. Six units each contained 50% or more of slow myosin-containing fibres (IIC and a few type I). The remaining fibres in these units were IIA. All these units were H. -J. GATES, R. M. A. P. RIDGE AND A. ROWLERSON therefore of mixed fibre composition, and are discussed as IIC/IIA units. In whole muscles slow-myosin-containing fibres were generally distributed evenly (nonrandomly) throughout the muscle cross-section.8. Whole muscles contained on average 970 fibres (S.D. + 70) of which 82 (± 9) were slow-myosin-containing. A few muscles from older rats (3-24 months) contained very few such fibres.9. We conclude that of an average eleven motor units in this muscle two or three are of mixed fibre composition (JIC/JIA), the two constituent kinds of fibre differing in myosin type and cross-sectional area. They probably also differ in developmental origin, and this is discussed.

Mechanomyograms recorded during evoked contractions of single motor units in the rat medial gastrocnemius muscle

“…Table 4 Mean (SD) and range (in square brackets) of ratios of the properties characterising the twitch and the fused tetanus. The results of comparisons of dierences between mean values for the three types of motor units are presented at the bottom part of the Table. (T Tw á (T Tet ) )1 Ratio of tensions of the twitch to the tetanus, v Tw á (m Tet ) )1 ratio of velocities of increase in tension of the twitch to the tetanic contraction measured from the baseline to 50% of the peak value for both kinds of contraction, Ampl MMG Tw á (Ampl MMG Tet ) )1 ratio of amplitudes of MMG signals during the twitch to the tetanus) It is well known that the value of the T Tw :T Tet ratio is the highest for FF and the lowest for the S type of motor unit (Bagust et al 1973;Stephens and Stuart 1975;Gates et al 1991;Celichowski and Grottel 1993). In addition, it was observed that ratios of MMG amplitudes and velocities if changes in tension are also the highest for FF and the lowest for S motor units.…”

The lumbrical muscle: a novel in situ system to evaluate adult skeletal muscle proteolysis and anticatabolic drugs for therapeutic purposes

“…Adult rat lumbrical muscles have contraction properties similar to those of EDL muscles (20), but their major advantage is that they are thin enough to allow a suitable measurement of tyrosine release (Fig. 2), which is not possible with EDL muscles because of the inevitable necrosis of inner muscle fibers (3).…”

Differential protection of neuromuscular sensory and motor axons and their endings in Wld mutant mice

“…Innervated muscle spindles viewed after microdissec-tion from unoperated mouse DL muscles contained be-tween two and five intrafusal muscle fibres, each less than 10 1-m in diameter, as described in other mouse muscles (Tourtellotte et al, 2001;Desaki et al, 2010) or rat lum-brical muscles (Porayko and Smith, 1968;Ovalle, 1972;Gates et al, 1991;Kucera et al, 1991). Each spindle was supplied by one large primary afferent axon, penetrating the capsule and terminating in annulospiral endings that wrapped the equatorial region of the intrafusal muscle fibres (Fig.…”

“…We therefore tested for functional com-petence of the annulospiral endings themselves, using FM1-43 as a fluorescent reporter. Muscle stretch, which depolarises annulospiral sensory endings, facilitates up-take of amphipathic styryl dyes such as FM1-43 and RH795 (Gates et al, 1991;Bewick et al, 2005). We dis-sected lumbrical muscles 3-5 days after axotomy and pinned preparations of these at extended lengths in phys-iological saline containing 8-1-M FM1-43.…”