Motor units in a skeletal muscle of neonatal rat: mechanical properties and weak neuromuscular transmission.

Abstract

SUMMARY1. Isometric twitch and tetanic tensions were recorded from whole muscles and single motor units in isolated fourth deep lumbrical muscles from neonatal rats (most at 3-5 days old) and from older rats of various ages.2. Whole-muscle time to peak contraction reduced from about 120 ms at birth to about 20-25 ms at 20 days and older.3. The number of motor units in the muscle was constant with age (eleven on average) and there was no significant branching of motor axons below the common peroneal nerve branching point in the thigh.4. In the 3-5 days age range, mean twitch: tetanus ratio for whole muscles was 0-299 and for single units was 0-177. As a consequence, mean motor unit size (as a percentage of whole-muscle tension) was greater for tetani (29-7 00) than for twitches (19-9 %). This was not the case in muscles from animals 22 days or older. Evidence is given that the cause of this is low junctional efficacy in some neuromuscular junctions in neonatal muscle. Intracellular recordings supported this view.5. The relationships of motor-unit size to the contraction time, to the ratio of contraction time: half-relaxation time, and to fatigue index are given. There was no indication of clear segregation of motor units into more than one population, but it is concluded that small motor units are more likely to contain a higher proportion of slowly contracting, fatigue-resistant fibres than large units.6. The level of overlap by axons in the lateral plantar nerve onto muscle fibres in a single sural nerve motor unit was greater in tetani than in twitches. The results indicate that the distribution of weak and strong inputs was not random, but that there was a tendency for one strong input to accompany a number of weak inputs (on average about two) on each muscle fibre.7. Intracellular recording indicates that about 120 of fibres at 3-5 days may be electrically coupled.

Competitive mechanisms underlying synapse elimination in the lumbrical muscle of the rat

“…To address this question, Ridge and colleagues measured the composition of individual 4DL motor units at 3-5 days of age (Jones and Ridge, 1987;Jones et al, 1987a,b). Fibers belonging to single motor units were identified by glycogen de-pletion (intense stimulation of a single motor axon followed by periodic acid Schiffs staining of frozen midbelly sections for glycogen); fiber typing was performed using the slow myosin antibody on adjacent sections.…”

“…Could the results described above, showing the apparent mixture of S and F fibers in the 4 day units, be explained by electrical coupling? The estimate of 12% electrical coupling at 4 days by Jones & Ridge (1987) was based on quite a small sample (8 out of 68 fibers), and possibly intracellular recording biases the sample towards larger and more mature fibers as well. Gap junctions originate between primary and developing secondary myotubes so that activity originating in the primary is more likely to be conveyed to the secondary than in the reverse direction, because of the favorable impedance matching of a large to a small fiber.…”

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

“…All these units were INTRODUCTION Present knowledge of the development of motor units in the rat is largely based on studies of three hindlimb muscles: soleus (e.g. Brown, Jansen & Van Essen, 1976;Thompson, Sutton & Riley, 1984), extensor digitorum longus (EDL; Balice-Gordon & Thompson, 1988) and fourth deep lumbrical muscle (4DL; Betz, Caldwell & Ribehester, 1979;Jones & Ridge, 1987;Jones, Ridge & Rowlerson, 1987 a, b). The motor unit composition of soleus and EDL in normal adults is comparatively well known (Close, 1967;Kugelberg, 1973;Chamberlain & Lewis, 1989), but similar knowledge in 4DL is lacking.…”

Excitability of Skeletal Muscle during Development, Denervation, and Tissue Culture

“…39 This increase in transmission efficiency may result in the same level of input from the nerve having a greater effect upon the muscle fiber, lowering the threshold for transmission of an action potential and subsequent contraction of the muscle fiber, and thus increasing the excitability. Another mechanism that may also explain the increase in excitability with development is that the percentage of EDL muscle fibers that are innervated in rats increased from 15 days through *45 days.…”