Section of fibular nerve affects activity pattern and contractile properties of soleus motor units in adult rats

"Fast" and "slow" muscle fibres in hindlimb muscles of adult rats regenerate from intrinsically different satellite cells

Myosin heavy chain (MyHC) expression was examined in regenerating fast extensor digitorum longus (EDL) and slow soleus (SOL) muscles of adult rats. Myotoxic bupivacaine was injected into SOL and EDL and the muscles were either denervated or neuromuscularly blocked by tetrodotoxin (TTX) on the sciatic nerve. Three to 10 or 30 days later, denervated SOL or EDL, or innervated but neuromuscularly blocked EDL received a slow 20 Hz stimulus pattern through electrodes implanted on the muscles or along the fibular nerve to EDL below the TTX block. In addition, denervated SOL and EDL received a fast 100 Hz stimulus pattern. Denervated EDL and SOL stimulated with the same slow stimulus pattern expressed different amounts of type 1 MyHC protein (8% versus 35% at 10 days, 13% versus 87% at 30 days). Stimulated denervated and stimulated innervated (TTX blocked) EDL expressed the same amounts of type 1, 2A, 2X and 2B MyHC proteins. Cross-sections treated for in situ hybridization and immunocytochemistry showed expression of type 1 MyHC in all SOL fibres but only in some scattered single or smaller groups of fibres in EDL. The results suggest that muscle fibres regenerate from intrinsically different satellite cells in EDL and SOL and within EDL. However, induction by different extrinsic factors arising in extracellular matrix or from muscle position and usage in the limb has not been excluded. No evidence for nerve-derived trophic influences was obtained.

Muscle shortening induced by tenotomy does not reduce activity levels in rat soleus

1. A slow postural muscle was tenotomized to determine the role of muscle stretch on chronic recruitment patterns in freely moving animals. 2. Different amounts of muscle shortening were induced in the soleus muscles of ten rats by severing the tendon of insertion (n = 3), the whole Achilles' tendon (n = 4) or the origins and insertions (n = 3). 3. Bipolar wire electrodes were implanted on each muscle to record the electromyographic activity (EMG) under control and tenotomized conditions. The complex interference pattern was continuously analysed to determine the number and amplitude of peak potentials (called turns). The numbers of these 'turns' and their amplitudes were determined during 4 control and at least 5 experimental days. Sham-operated controls and groups matched according to the type of tenotomy were analysed for length changes and pathological changes 5 and 10 days post-tenotomy. 4. The total activity levels in all three tenotomy conditions were not significantly changed when compared with their own control levels. No differences in total activity level were found between the three tenotomized conditions. 5. The normal diurnal patterns of muscle recruitment were preserved during the tenotomized conditions, with the highest levels consistently occurring during the first 3 h of the dark cycle. 6. Tenotomy of the soleus, whether induced by distal (ST), distal and proximal (DT) or Achilles' tenotomy (AT) resulted in muscle shortening (9-26 %). No muscle pathology was found in the ST or AT groups. Degeneration was found in the DT group after 5 days, with further increases at 10 days. 7. These data suggest that the absence of stretch had no discernible influence on the aggregate activity levels in the slow postural soleus muscle. Whether tenotomy caused changes in recruitment within individual step cycles was not evaluated.

Spontaneous electromyographic activity in adult rat soleus muscle

Single-motor-unit and gross electromyograms (EMG) were recorded from the soleus muscle in six unrestrained rats. The median firing frequencies of nine motor units were in the 16-25 Hz range, in agreement with previous studies. One additional motor unit had a median firing frequency of 47 Hz. This unit and one of the lower-frequency units regularly fired doublets. Motor-unit firing frequency was well correlated to whole-muscle EMG during locomotion. Integrated rectified gross EMG revealed periods of continuous modulation, phasic high-amplitude events, and tonic low-amplitude segments. The tonic segments typically were caused by a small number of motor units firing at stable high frequencies (20-30 Hz) for extended periods of time without detectable activity in other units. This long-lasting firing in single motor units typically was initiated by transient mass activity, which recruited many units. However, only one or a few units continued firing at a stable high frequency. The tonic firing terminated spontaneously or in conjunction with an episode of mass activity. Different units were active in different tonic segments. Thus there was an apparent dissociation between activity in different single motor units and consequently between single-motor-unit activity and whole-muscle EMG. It is proposed that the maintained tonic motor-unit activity is caused by intrinsic motoneuron properties in the form of depolarizing plateau potentials.

The incidence of initial doublets in the discharges of motoneurones of two different inspiratory muscles in the cat

1. Trains of action potentials in motoneurones frequently commence with an initial doublet; i.e. a uniquely short interspike interval. Previous authors have speculated on the functional importance of initial doublets. Here we test the hypotheses that these doublets are associated with particular classes of motoneurones or particular physiological conditions. 2. Discharges of inspiratory motoneurones were recorded extracellularly in the thoracic ventral horn of anaesthetized, paralysed cats. Seventy units (35 each with axons in the internal and external intercostal nerves) were classified on the basis of their maximum firing rates, start times in the respiratory cycle and axonal destination. 3. Initial doublets were defined by an interspike interval < 14 ms. Of seventeen units firing initial doublets, fifteen had axons in the external intercostal nerve and two had axons in the internal intercostal nerve. Neither maximum firing rate nor start time during the respiratory cycle predicted the occurrence of doublets. 4. The chemical drive to breathe was manipulated by altering the CO2 content of the inspired gas or by briefly stopping the respiratory pump. Varying the chemical drive to breathe had no consistent effect on the occurrence of initial doublets. 5. These results support the view that initial doublets are part of the normal pattern of discharge of motoneurones. However, because the incidence of doublets does not consistently support previous functional hypotheses, we argue that the occurrence of doublets may not necessarily be dictated by the CNS, but in some circumstances it is an epiphenomenon dependent on the state of the motoneurone, in particular on the statistical properties of its synaptic inputs.

Bistable firing properties of soleus motor units in unrestrained rats

EMG recordings from single motor units in the soleus muscle were performed in alert unrestrained rats. A cuff electrode around the tibial nerve and subcutaneously placed electrodes in the foot permitted stimulation of afferent fibres. The movements of the rat and the simultaneous EMG activity were displayed together on a video monitor. Most motor units were tonically active for extended periods during quiet standing. During tonic discharge, maintained shifts between stable low (8-12 Hz; quartiles) and high (16-22.8 Hz) frequency ranges were initiated by short-lasting synaptic excitation of the motoneuron pool by stimulation of Ia afferents, or inhibition by stimulation of skin afferents. The shifts were not related to gross limb movements. This phenomenon is referred to as a bistable firing pattern. Bistable firing also occurred spontaneously during quiet standing. Typically the firing frequency shifted between a low (9-12.5 Hz) and a high (20-24.5 Hz) mode. During recordings of simultaneous activity in two units, spontaneous and stimulus-induced frequency jumps causing maintained changes in firing frequency were regularly seen to occur in one unit, while the frequency in the second unit was unchanged or only phasically influenced. These results demonstrate for the first time a bistable firing pattern during postural activity in the intact animal. The firing pattern closely resembles the bistable behaviour described in spinal motoneurons in reduced preparations, where it is due to the presence of a plateau potential. This suggests that the bistable firing is unexplained by plateau potentials also in the intact animal.

Gradation of force output in normal fast and slow muscles of the rat

The discharge patterns of 16 motor units in extensor digitorum longus (EDL) and soleus (SOL) muscles of freely moving adult rats, described by Hennig & Lømo (1985), were further analysed with respect to their role in grading muscle force output. The units fell into three distinct classes, termed EDL-1, EDL-2 and SOL-1, probably corresponding to type FF, FR and S units. The EDL-1 units generated only single impulses or impulse trains of short duration (phasic firing) which had high frequency and usually started with a short interspike interval (initial doublet). The EDL-2 and SOL-1 units generated single impulses and impulse trains of both short and long durations (phasic and tonic firing) without initial doublets. The frequency was high in EDL-2 and low in SOL-1 units. In EDL-2 and SOL-1 units, the mean durations of the first interspike intervals in a train decreased as the number of impulses per train increased. In EDL-1 units they did not change. Both SOL and EDL muscles were simulated through the nerve at different regular frequencies and tension-frequency (T-F) curves constructed. The EDL-2 units fired naturally most often at frequencies corresponding to the steepest part of the EDL T-F curve. The EDL-1 and SOL-1 units fired naturally most often at frequencies where the T-F curves of their respective muscles began to flatten before maximum tetanic tension was reached. Stimulus trains starting with an initial doublet produced maximum rate of tension development (optimum impulse pattern). At optimum intervals the force increased from about 20 to 85% of maximum tetanic tension when the number of stimuli was increased from 1 to 7. It is concluded that the natural firing pattern of EDL-1 units and the contractile properties of EDL muscle fibres are normally matched so that the force can develop at maximum rate to maximum levels at the start of contractions. Tension output is apparently regulated primarily through varying number of impulses per train in EDL-1 units; in SOL-1 and EDL-2 units both rate and number of impulses are important.