The reorganization of motor units in motor neuron disease

We studied 78 patients with motor neuron disease (MND) using concentric needle electromyography. Analysis on weak and maximal effort was performed using our own, fully automated, computer method, EMG-LAB. In addition to the conventional parameters of single motor unit action potentials (MUAPs) and interference pattern, new criteria were applied: the range of the acting motor units and the functional recruitment order. A total of 375 muscles of MND patients and 120 control muscles were investigated. The electromyographic data were analyzed separately in five groups of muscles, classified A, B, C, D, and E according to their clinical condition. Those results allowed us to discern six neurophysiological stages (N(0,1,2,3,4,5)) from the early to the most advanced phase. It has been confirmed that reinnervation in MND is adequate to compensate for the loss of over 50% of motor neurons but it is only a transitory phase in the morbid course. At stages N(O-5), the electrophysiological data reflect structural and functional integrity of the functioning motor units. Evaluation of not only single MUAPs but also of the full range of acting motor units and their recruitment order allowed a deeper look into the underlying pathophysiological mechanisms.     

Directional tuning of single motor units

The directional activity of whole muscles has been shown to be broadly and often multimodally tuned, raising the question of how this tuning is subserved at the level of single motor units (SMUs). Previously defined rules of SMU activation would predict that units of the same muscle (or at least of the same neuromuscular compartment) are activated homogeneously with activity peaks in the same "best" direction(s). In the present study, the best directions of SMUs in human biceps (both heads) and deltoid (anterior, medial, and posterior portions) were determined by measuring the firing rate and threshold force of units for recruitment during isometric force ramps in many different directions. For all muscles studied, neighboring motor units could have significantly different best directions, suggesting that each muscle receives multiple directional commands. Furthermore, 17% of the units sampled clearly had a second-best direction, consistent with a convergence of different directional commands onto the same motoneuron. The best directions of the units changed gradually with location in the muscle. Best directions did not cluster into separate groups, thus, not supporting the existence of clearly distinguished neuromuscular compartments. Instead, the results reveal a more gradually distributed activation of the biceps and deltoid motoneuron pools. A model is proposed in which the central control mechanism optimizes the fulfillment of the continuously changing directional force requirements of a movement by gradually recruiting and derecruiting those units ideally suited for the production of the required force vector at any given time.     

Loss of motor units in Alzheimer's disease

Little attention has been paid in the literature to the state of the peripheral nervous system (PNS) in Alzheimer's disease (AD). We conducted a comprehensive electrophysiological study in 15 AD patients looking for functional abnormalities within their PNSs. A reduction of the number of functioning motor units (MU) was found in the thenar and soleus muscles of most of these patients without enlargement of the remaining MUs territories, while the motor and sensory conduction velocities of the peripheral nerves were preserved. These results suggest dysfunction of the spinal motoneurones in patients afflicted with this condition.     

Heteronymous H-reflex in temporal muscle motor units

An electric stimulation of the masseteric nerve elicits a heteronymous H-reflex in the temporal muscle. The characteristics of this reflex response were investigated by analysis of the firing probability changes of single motor units. Eleven healthy subjects participated in the experiments. The heteronymous H-reflex of the temporal muscle was electrically elicited by stimulation of the masseteric nerve at 120% of the intensity needed for the maximal masseteric M-wave. From 8 to 24 motor units were sampled from the temporal muscle of each subject. Peri-stimulus time histograms of motor unit recordings were built with a 0.5-ms bin width. The mean firing probability was calculated for the 20 ms preceding the stimulus. The firing probability was considered increased when it exceeded the mean by 3 standard deviations. Of 104 sampled motor units, 40 motor units showed a significant increase of the firing probability, which lasted 1 ms or less in 29 of them. In 12 out of 16 motor units, a significant increase of firing probability also persisted at a lower stimulation intensity (120% of the threshold needed to elicit a masseteric M wave). These data indicate that: (1) some temporal muscle motor units are modulated by afferents from the masseter muscle, (2) the heteronymous H-reflex has a monosynaptic component, and (3) there might be a more complex than just monosynaptic organization serving the heteronymous temporal H-reflex. For the latter conclusion regarding synaptic wiring, however, PSTH studies like the present one can offer only indirect evidence, and this question could be better studied in animals.     

Organization of single motor units in feline sartorius

1. We depleted single motor units in feline sartorius muscles of glycogen by stimulating their motoneurons intracellularly. We mapped the intramuscular distribution of depleted fibers by inspecting histological cross-sections throughout the length of sartorius. 2. We selected ten depleted motor units for detailed study and quantitative analysis. Nine motor units were located in the anterior head of sartorius. One was located in a muscle whose distal half appeared to have been damaged some time before the acute experiment. A single motor unit was located in the medial head of sartorius. 3. Five motor units were composed of fast-twitch glycolytic (FG) muscle fibers, two of fast-twitch oxidative glycolytic (FOG) muscle fibers, and three of slow-twitch oxidative (SO) muscle fibers. Estimates of the numbers of depleted fibers in motor units of anterior sartorius indicated that FG motor units were larger (mean 566 fibers) than FOG and SO motor units (SO mean 190, FOG mean 156 fibers). The SO motor unit in the damaged muscle had 550 fibers. One motor unit depleted in the medial head of sartorius had 270 fibers with FG profiles. 4. Muscle fibers belonging to each anterior motor unit were never distributed throughout the whole cross-section of anterior sartorius at any proximodistal level. Furthermore, fibers were distributed nonuniformly along the proximodistal axis of the muscle. In most muscles at least a few depleted fibers were found at all proximodistal levels. However, in one normal muscle and the damaged muscle, depleted fibers were confined to the proximal end. 5. The fibers in the medial motor unit were confined to a strip that did not extend across the whole cross-section of the muscle head. Fibers within this strip were scattered quite evenly from origin to insertion. This medial FG motor unit occupied a smaller territory and contained fewer fibers than anterior motor units of the same histochemical type. 6. These results show that sartorius motor units are not distributed uniformly in the mediolateral plane; those in anterior sartorius were distributed asymmetrically in the proximodistal axis as well. This finding has important functional implications for the way in which we model force development and transmission in sartorius and other long muscles.     

Properties of motor units of the frog sartorius muscle

1. The mechanical properties of single motor units in the sartorius muscle of the frog Litoria aurea were examined during single shock and repetitive stimulation of motor axons. 2. The tetanic tension developed by motor units lay in the range 1-40% of whole muscle tension with two peaks in the distribution, in the range 5-10% and 25-30%. The large units had briefer times-to-peak for the twitch than the small units and were more readily fatigued during prolonged repetitive stimulation. 3. Histological examination of the muscle gave a count of 620 muscle fibres with a diameter range of 28-128 mum. Cholinesterase stained preparations showed that the majority of muscle fibres had several nerve terminals (mean 3, range 1-5). 4. Muscle fibres received their multiple innervation from different axons (polyneuronal) or branches of the same axon (multiterminal). The presence of polyneuronal innervation of muscle fibres was confirmed by a comparison of the tensions when each of a pair of motor units was stimulated alone and when they were stimulated together. The tension excess, or overlap, was up to 60% when expressed in terms of the tension developed by either unit alone. Motor units developing similar amounts of tension tended to show more overlap in their innervation than units with very different tensions. 5. An estimate of the amount of multiterminal innervation gave variable results but could account for up to 60% of a motor unit's tension. No correlation could be detected between the values for multiterminal innervation and any other measured parameter. However, it is argued that because of the limitations of the measurements the existence of a relationship between the extent of multiterminal or polyneuronal innervation and the mechanical properties of the motor unit cannot be excluded.     

Responses of human single motor units to transcranial magnetic stimulation

The authors analyse the results of 51 unicompartmental knee prostheses with 1 to 12 years follow-up (mean follow-up: 5 years). The results were evaluated using the scoring system of the "Knee Group" of the SO.B.C.O.T. (Société Belge de Chirurgie Orthopédique et de Traumatologie). This analysis demonstrates that the quality of the results depends on implant positioning. The authors suggest positioning the tibial implant parallel with the healthy plateau and slightly distal, i.e. to position the tibial implant perpendicular to the epiphyseal axis and not to the mechanical axis, as is systematically done with the usual tibial cutting guides. When this ideal positioning was respected, 77.5% of the patients had a score above 90 points (out of a possible maximum of 100 points) and 12.5% had a score between 75 and 89 points. When this condition was not respected, none of the knees obtained more than 75 points. The difference was statistically significant (p = 0.0001).     

Properties of motor units after immobilization of cat peroneus longus muscle

Changes in contractile properties of cat peroneus longus motor units were studied 2, 5, and 8 wk after selective immobilization of this muscle, which was achieved by fixing the distal tendon of the peroneus longus to the fibula either at the muscle minimal physiological length ("short" length) or at the length for a 90 degree ankle joint ("neutral" length). In each muscle, 75-90% of the units [slow (S), fast resistant to fatigue (FR), fast intermediate (FI), and fast fatigable (FF)] were studied. Immobilization elicited a permanent decrease in tetanic force developed by single motor units, which was larger for resistant-to-fatigue units (S, FR). In most instances this decrease was not related to the immobilization length. In all units, twitch contraction and half-relaxation times underwent a transient increase, the extent and time course of which were influenced by immobilization length. The relationship between the frequency of motor units activation and the ratio of unfused to maximal tetanic force was studied. For fast units, there was a transient shift of the relation toward low frequencies after 2 and 5 wk of immobilization at neutral and short length, respectively.     

Recruitment stability in masseter motor units during isometric voluntary contractions

Recruitment of single motor units (SMUs) of the masseter muscle was studied using macro representation (MacroRep) as the indicator of motor unit size. When subjects followed a slow isometric force ramp, units were usually recruited in order of MacroRep size. However, pooling the data from repeated ramps in the same subject resulted in a weak relationship between MacroRep size and force recruitment threshold, probably due to marked variations in the relative contributions of the jaw muscles, and varying levels of cocontraction, in the development of total bite force in each ramp. The force recruitment thresholds of individual SMUs showed marked variability, but recruitment threshold stability was improved when expressed as a percentage of maximum surface electromyographic (SEMG) activity in the ipsilateral masseter. Therefore the SEMG recruitment threshold was concluded to be a more stable and accurate indicator of the SMU's position in the recruitment hierarchy in a given muscle. It was concluded that SMUs in masseter are recruited according to the size principle, and that when investigating recruitment in jaw muscles, SEMG recruitment threshold should be used in preference to force recruitment threshold.     

Motor unit size estimation of enlarged motor units with surface electromyography

Surface EMG is hardly used to estimate motor unit (MU) characteristics, while its non-invasiveness is less stressful for patients and allows multi-electrode recordings to investigate different sites of the muscle and MU. The present study compares motor unit potentials (MUPs) obtained with surface EMG and macro EMG during voluntary contraction of the biceps brachii muscle of patients with enlarged MUs caused by prior poliomyelitis. Averaged surface MUPs were obtained by means of needle EMG (SMUP1) and surface EMG (SMUP2) triggering. The MUPs area and peak amplitudes correlated well when comparing the macro MUP and SMUP1 of the same MUs. When MU populations of different patients were compared, the SMUP1s and SMUP2s were equally sensitive to pathology as macro MUPs. In this, the late non-propagating positive wave (only present in unipolar recordings) is more robust than the triphasic propagating wave. Therefore, surface EMG can be used for detecting enlarged MUs.     

Composition of newly forming motor units in prenatal rat intercostal muscle

We have examined the composition of rat intercostal motor units during the period of late gestation, when most muscle fibres are formed, in order to see the pattern of the contacts initially made between single motoneurons and myotubes. At this early stage, the muscle contains two types of myotubes, primary and secondary myotubes, and a major aim was to see whether individual motoneurons preferentially made contact with a particular myotube type. The technique used to define myotubes contacted by a single motoneuron was anterograde labelling of the neuron, followed by electron microscopic detection of labelled terminals and their postsynaptic targets. We find that prenatal motor units are inhomogeneous with respect to their primary/secondary myotube composition. Most individual motoneurons show many permutations of contact with primary myotubes, secondary myotubes, and undifferentiated cells, including single nerve terminals which contact both primary and secondary myotubes. Our results are interpreted in terms of changes to the composition of both the muscle and of the motor units during the final 5 days of gestation. We demonstrate that motoneurons necessarily make their initial contacts on primary myotubes, but that these are surprisingly sparse. As secondary myotubes appear and become innervated, motor units are at first all similar and all heterogeneous. However, primary myotubes are represented more often in motor units than in the muscle as a whole. This probably reflects the relative densities of polyinnervation of primary vs. secondary myotubes. By embryonic day 20, motor units have become divergent in composition, with some dominated by primary myotubes and others by secondaries. We propose that motoneurons initially establish contacts at random on either myotube type, but then begin to express preference for one type or the other and reorganise their periphery. Refining of motor unit composition towards homogeneity in the postnatal period probably involves other elements, such as mutability of muscle fibre and/or motoneuron characteristics as a function of usage and muscle position, perhaps influenced by sensory feedback mechanisms.     

Adaptations of rat lateral gastrocnemius motor units in response to voluntary running

This study investigated the effects of 12 wk of voluntary wheel running on motor units from rat lateral gastrocnemius. Motor units were isolated via ventral root splitting (L5) from active or sedentary rats and were classified into slow, fast-fatigue-resistant, and fast-fatigable (FF) units. An overall increase in mean motor unit tetanic tension (35%) was accompanied by a decrease in mean motor unit fatigue resistance (-10%). These adaptations were localized in the fast unit population but varied among fast motor unit subtypes. The overall increase in tetanic force was due primarily to increases in fast-fatigue-resistant units (300%), whereas changes in fatigue resistance (-43%) were confined to FF units. However, the changes seen with activity may have been partly obscured by classifying fast motor units based on fatigability, since a significant increase in tetanic force accompanied by a decreased twitch one-half relaxation time was apparent in units falling in the midrange of the tetanic force continuum and included a number of FF units. These data provide direct demonstration of nonuniform motor unit adaptations subsequent to increases in normal functional activity.     

The relationship between surface potentials and the number of active motor units

One of the assumptions inherent in a technique recently devised for enumerating motor units in human muscles is that the surface potentials from active motor units summate in a linear fashion. We present an electrical model of a muscle which predicts that a linear relationship between the number of active units and the electrical response recorded at the surface overlying the muscle would not be expected. The extent of the non-linearity, and hence the error in the calculation of the number of motor units in a given muscle, depends upon the ratio between the mean conductance of the motor units themselves and that of the external conduction pathway through which the electrical signal is fed (Gu/Ge). The extent of non-linearity is assessed experimentally in human hypothenar muscles using a "collision" technique. The average underestimate introduced into the calculation of the number of motor units in this particular case was concluded to be 26%. The value of Gu/Ge derived from these experiments, in 2 subjects, was checked by simulating an intramuscular action potential and determining the attenuation at the surface. The 2 independently obtained values were sufficiently close to suggest that the model may be a valid one. We conclude that caution should be employed in the interpretation of experiments which purport to determine the number of motor units in a muscle by means of surface recordings.     

Phosphotransferases associated with the regulation of kinesin motor activity

Kinesin, a plus-end-directed microtubule motor protein, functions in concert with accessory factors that have been shown to regulate enzyme activity and may also provide cargo specificity. This report identifies teh 79-kDa kinesin-associated phosphoprotein as a phosphoisoform of kinesin light chain. Increased phosphorylation of this light chain isoform is sufficient to account for the increase in kinesin-mediated microtubule-gliding activity. Additionally, it was found that the degree of phosphorylation of this isoform is regulated by a 100-kDa kinase and 150-kDa type 1 phosphatase. Both the kinesin light chain kinase and phosphatase co-purify with the kinesin heavy chain, suggesting that kinesin exists in a large complex capable of self-regulation.     

Longitudinal study of the contractile and electrical properties of single human thenar motor units

Serial motor unit number estimates have shed important light on the extent and rates of motoneuron losses in aging and amyotrophic lateral sclerosis. However, the estimates alone provide few clues to the health and functional status of surviving motor units. A reliable means for assessing the functional status of the surviving motor units would therefore by a welcome addition to our present tools for studying motor units. Examining the physiological properties of samples of motor units drawn at intervals during the course of a motoneuronal disease suffers from the important limitation that the samples may not be representative of one another. The latter problem could be circumvented by serially studying the same motor units. This study describes a noninvasive technique capable of longitudinally tracking the contractile and electrical properties of specific single thenar motor units in healthy subjects, in some instances over several years. The technique proved to be reasonably reliable and provided information on a wide range of contractile and electrical properties of motor units. Such an approach could serve as a potentially powerful and sensitive means of studying the life histories of single motor units in aging, diseases of the motoneuron, and in the latter instances, the responses of the motoneurons to treatment.     

Neuromuscular contacts of expanded motor units in rat soleus muscles are rescued by leupeptin

In the soleus muscle of the rat following section of the L5 ventral ramus (partial denervation) the remaining motor axons increase their territory by sprouting. Nerve sprouts are first seen two to three days after the operation, their number peaks at 10-14 days and subsequently remains at this level. The time course of the initial sprouting in partially denervated muscles is not altered by paralysing the muscles with alpha-bungarotoxin, and the initial extent of the sprouting is, if anything, greater in the paralysed muscles. However, unlike in controls, this level of sprouting is not maintained and neuromuscular contacts are lost when muscles recover from the paralysis. The loss of these contacts can be prevented by treatment of these partially denervated paralysed muscles with leupeptin, an inhibitor of calcium-activated neutral protease. Interestingly, more contacts are rescued when leupeptin is applied 10 days after alpha-bungarotoxin treatment, when sprouting has reached high levels, than at three days, when sprouting has just begun. The neuromuscular connections rescued by leupeptin are functional. Maximum tetanic tension produced by untreated soleus muscles two to five months after partial denervation is 66 +/- 9% of contralateral control muscles, but only 39 +/- 8% when the muscles were paralysed with alpha-bungarotoxin for 12-14 days after partial denervation. However, when partially denervated paralysed muscles were treated with leupeptin three and 10 days after alpha-bungarotoxin treatment their tension output is 74 +/- 3% and 81 +/- 8%, respectively. After partial denervation alone, motor units are twice their normal size. Short-term paralysis with alpha-bungarotoxin prevents this increase in motor unit territory. However, the application of leupeptin to the paralysed muscles rescues neuromuscular contacts, allowing motor unit size to remain expanded, at around 2-2.5-fold. Thus, following recovery from temporary paralysis with alpha-bungarotoxin, there is a sudden withdrawal of neuromuscular contacts and these can be rescued by treatment with leupeptin.     

Mechanical properties and behaviour of motor units in the tibialis anterior during voluntary contractions

The present work was carried out to analyse the properties and behaviour of Tibialis anterior motor units (MUs) during voluntary contractions in humans. A total of 528 single MU mechanical properties was recorded in 10 subjects by means of the spike-triggered averaging (STA) technique. MU recruitment thresholds and discharge frequencies were recorded during linearly increasing maximal voluntary contraction (MVC). The results indicate a mean (+/- SD) MU torque of 25.5 +/- 21.5 mN.m. and a mean time-to-peak of 45.6 +/- 13.6 ms. A comparison of the average MU twitch torque with that of the muscle allowed an estimate of about 300 MUs in the Tibialis anterior. A positive linear relationship was recorded between the MU twitch torque and the recruitment threshold. The mean minimal and maximal discharge frequencies of MUs were 8.4 +/- 3.0 Hz and 33.2 +/- 14.7 Hz, respectively. The results of the present work indicate that MU behaviour during voluntary contractions is different in the tibialis anterior and in the adductor pollicis.