2-deoxyglucose autoradiography of single motor units: labeling of individual acutely active muscle fibers

2-Deoxy-d-[l-¹⁴]glucose (2DG) was given intravenously during repetitive stimulation of single motor units in adult cats and autoradiographs were made of frozen sections of the target muscles in order to evaluate methods designed to improve the spatial resolution of [¹⁴C]2DG autoradiography. With the modifications used, acutely active muscle fibers, independently identified by depletion of intrafiber glycogen, were associated with highly localized accumulations of silver grains over the depleted fibers. The results indicate that [¹⁴C]2DG autoradiography can successfully identify individual active muscle fibers and might in principle be used to obtain quantitative data about rates of glucose metabolism in single muscle fibers of defined histochemical type. The modifications may be applicable also to other tissues to give improved spatial resolution with [¹⁴C]-labeled metabolic markers.     

Properties of medial gastrocnemius motor units and muscle fibers reinnervated by embryonic ventral spinal cord cells

Severe muscle atrophy occurs after complete denervation. Here, Embryonic Day 14-15 ventral spinal cord cells were transplanted into the distal tibial nerve stump of adult female Fischer rats to provide a source of neurons for muscle reinnervation. Our aim was to characterize the properties of the reinnervated motor units and muscle fibers. Some reinnervated motor units contracted spontaneously. Electrical stimulation of the transplants at increasing intensity produced an average (+/- SE) of 7 +/- 1 electromyographic and force steps. Each signal increment represented the excitation of another motor unit. These reinnervated units exerted an average force of 12.0 +/- 1.5 mN, strength similar to that of control fatigue-resistant units. Repeated transplant stimulation depleted 17% of the muscle fibers of glycogen, an indication of some functional reinnervation. Reinnervated (glycogen-depleted), denervated (no cells transplanted), and control fibers were of histochemical type I, IIA, or IIB. Fibers of the same type were grouped after reinnervation. The proportion of fiber types also changed. Reinnervated fibers were primarily type IIA, whereas most fibers in denervated and control muscles were type IIB. Reinnervated fibers of each type had significantly larger cross-sectional areas than the corresponding fiber types in denervated muscles. These data suggest that neurons with different properties can reside in the unusual environment of the adult rat peripheral nerve, make functional connections with muscle, specify muscle fiber type, and reduce the amount that each type atrophies.     

Electrophysiological characteristics of motor units and muscle fibers in trained and untrained young male subjects

We hypothesized that the amplitudes of compound muscle action potentials (CMAPs) and interference pattern analysis (IPA) would be larger in trained subjects compared with untrained subjects, possibly due to hypertrophy of muscle fibers and/or increased central drive. Moreover, we hypothesized that the untrained muscle is less excitable compared with the trained muscle. An electromyographic (EMG) needle electrode was used to record the IPA at maximal voluntary effort. The CMAP was obtained by stimulating the musculocutaneous nerve and recording the brachial biceps muscle using surface electrodes. CMAPs were obtained by direct muscle stimulation (DMS) with two stainless‐steel subdermal electrodes placed subcutaneously in the distal third of the muscle. Amplitudes of CMAP and IPA were significantly larger in trained subjects compared with untrained subjects. We found no differences between trained and untrained subjects in IPA power spectrum and turns per second or amplitude of the CMAPs obtained by DMS. Muscle fiber hypertrophy and/or altered central drive may account for our results, but there was no indication of changes in muscle fiber excitability. Muscle Nerve, 2010     

Inhibitory phenomena in individual motor units induced by transcranial magnetic stimulation

It is well known that a silent period (SP) can be observed in voluntary tonic EMG activity starting directly after the initial early response when magnetic stimuli are delivered through the skull over the contralateral primary motor cortex. It is, however, unknown as to how an individual motoneurone (MN) contributes to the SP observed in the surface EMG. The present investigation was conducted to investigate inhibitory phenomena at the level of individual motor units. It demonstrates that the duration of the SP in single motor units is inhomogeneously distributed within the pool of active MNs. At various stimulation strengths, SP durations in single motor units can be similar or longer when compared to that observable in surface EMG records. In some motor units, which show low thresholds for early excitation and appearance of the SP, durations of SP can exceed 1000 msec. The length of suppression of spontaneous MN firing is maximal at stimulus intensities a little higher than those required for an early excitatory response. Although in general thresholds for early excitation and appearance of SPs are similar, at threshold stimulation in a number of trials inhibitory effects on the firing of voluntarily activated motoneurones were present, even in the absence of early excitations. This proves the independent nature of inhibitory as opposed to excitatory effects induced by transcranial magnetic stimulation. An SP in the absence of early excitation underlines its cortical origin. Inhibition and excitation of single MNs were maximal over the same small scalp area. We suggest that cortical inhibitory control plays an important role in the organization of natural movements.     

Firing rate of individual motor units in voluntary contraction of abductor digiti minimi muscle in man

The frequency and variability of discharges of motor units in abductor digiti minimi muscle of 11 human subjects were investigated. In the first series, contraction was performed so as to raise the tension as linearly as possible from 0 to maximum in 2, 3.2, 5, 8, and 10 sec. There was a rise in discharge frequency when the tension was raised; this rise was steeper when the contraction was faster. The frequency at the start of discharge was higher, and the peak value of frequency was also higher at contractions with greater speeds. Irregular fluctuations in the discharge frequency was greater and appeared at earlier stages during the more rapid contractions. In the second series of experiments, the tension was held constant at various levels covering the entire range of contraction. At contractions with larger steady tension the discharge frequency of units was found to be higher and irregularity of discharge was enhanced. The units recruited at smaller tension vary over a greater range of discharge frequencies than those recruited at larger tension.     

Uniformity of metabolic enzymes within individual motor units

Individual muscle fibers of 10 motor units from the tibialis posterior muscle of cat were identified by glycogen depletion techniques, characterized for histochemical type, diameter, and intramuscular locations, and analyzed by quantitative biochemical methods. Four enzymes, representing different energy-yielding pathways, were quantitatively assayed in muscle fibers belonging to motor units selected from each of the three major physiological types. All four enzymes demonstrated identical activities among fibers within a motor unit, while showing up to 11-fold differences among fibers belonging to different motor units. Moreover, fibers within a single motor unit, but of substantially different diameters, were nevertheless homogeneous in specific enzyme activities.     

Superficial motor units are larger than deeper motor units in human vastus lateralis muscle

Previous studies have suggested that regionalization may occur for human motor units, whereby smaller motor units are located in deeper parts of the muscle and larger motor units are located in more superficial portions. We examined this possibility in the human vastus lateralis muscle using macro-EMG (electromyography) to estimate motor unit size. The sample consisted of nine individuals from whom 114 motor units were recorded at forces ranging between 5% and 60% MVC. Peak-to-peak macro-EMG amplitude was well correlated with macro area (Spearman rho = 0.96). There was a statistically significant inverse relationship between recording depth and macro peak-to-peak amplitude (rho = -0.402, p < 0.001). We conclude that there is a nonrandom distribution of motor units in human muscle, with larger motor units located in more superficial regions and smaller units located in deeper regions. Clinicians who monitor motor unit activity need to recognize that a representative sample of motor unit recordings should include motor units from both deeper and more superficial regions of muscle.     

Extraocular muscle motor units characterized by spike-triggered averaging in alert monkey

Single-unit recording in macaque monkeys has been widely used to study extraocular motoneuron behavior during eye movements. However, primate extraocular motor units have only been studied using electrical stimulation in anesthetized animals. To study motor units in alert, behaving macaques, we combined chronic muscle force transducer (MFT) and single-unit extracellular motoneuron recordings. During steady fixation with low motoneuron firing rates, we used motoneuron spike-triggered averaging of MFT signals (STA-MFT) to extract individual motor unit twitches, thereby characterizing each motor unit in terms of twitch force and dynamics. It is then possible, as in conventional studies, to determine motoneuron activity during eye movements, but now with knowledge of underlying motor unit characteristics.We demonstrate the STA-MFT technique for medial rectus motor units. Recordings from 33 medial rectus motoneurons in three animals identified 20 motor units, which had peak twitch tensions of 0.5-5.25 mg, initial twitch delays averaging 2.4 ms, and time to peak contraction averaging 9.3 ms. These twitch tensions are consistent with those reported in unanesthetized rabbits, and with estimates of the total number of medial rectus motoneurons and twitch tension generated by whole-nerve stimulation in monkey, but are substantially lower than those reported for lateral rectus motor units in anesthetized squirrel monkey. Motor units were recruited in order of twitch tension magnitude with stronger motor units reaching threshold further in the muscle's ON-direction, showing that, as in other skeletal muscles, medial rectus motor units are recruited according to the “size principle”.     

Synaptic effects induced in lamprey motor neurons by direct stimulation of individual presynaptic fibers]

Parallel intracellular recording from giant reticulospinal axons and target motoneurons of the lamprey (Lampetra fluviatilis) revealed unitary synaptic actions produced in the post-synaptic membrane by direct stimulation of individual presynaptic fibres. It is shown that monosynaptic reticulomotoneuronal EPSPs contain both electrical and chemical components which have different synaptic delay, time course, amplitude, sensitivity to Cadeficit and operating characterisitcs. A single motoneuron may receive direct electrotonic inputs from several giant axons. Individual giant axons may affect segmental motoneurons not only via monosynaptic connections but also via additional synaptic relays.     

The effect of activation history on tension production by individual muscle units.

Isometric responses of individual gastrocnemius muscle units of both fast (F) and slow (S) twitch type were studied during repetitive intracellular stimulation of the innervating motoneurons in anesthetized cats. Paired stimuli usually produced maximum responses (measured in terms of tension-time area) with interpulse intervals between 5 and 10 msec. However, longer pulse trains produced maximum tension-time area per pulse with relatively long interpulse intervals (referred to as the optimum interval). For each unit, stimulus intervals around the optimum produced maximum modulation of tension during unfused tetani. Post-activation potentiation produced shifts in the optimum toward longer intervals. The optimum intervals for type F units were shorter (mean 49.4 msec) than those for type S units (mean 86.9 msec), but there was no difference between the groups when the mean intervals were expressed in ratio with the twitch contraction time, Tc (F units: 1.42 X Tc; S units: 1.31 X Tc). The range of optimum intervals for all gastrocnemius muscle units corresponded to motoneuron firing frequencies of 7-40 pulses/sec, well within the frequency ranges observed for gastrocnemius motoneurons firing under natural synaptic drive. Insertion of a single short interval (5--10 msec) at the onset of a relatively low frequency stimulus train produced a prolonged 'catch-like' enhancement of tension output, particularly in type S units. The duration of the catch-like effect depended on the interval between pulses in the underlying basic train, and was maximum for intervals between 1 and 2 X Tc. The catch-like effect was also seen in type F units but its duration was curtailed by another process called the 'sag' property. The 'sag' property occurred in type F units over a wide range of stimulus intervals (usually 0.5-3 X Tc) but was present in type S units only with relatively long intervals (greater than 2 X Tc). The 'sag' property appears to represent a disenhancement process intrinsic to muscle unit fibers activated in unfused tetani.     

Competition for survival between motor units in mouse skeletal muscle

The soleus nerve on one side of neonatal mice was crushed and the soleus muscle on the same side was surgically reduced in size. Some animals also had their lumbar spinal nerve 5 (L5) cut and misdirected in the same operation. Three months later, the number of L5 and L6 soleus motor units in the operated muscles was counted electrophysiologically and the number of muscle fibers was counted histologically. The number of L6 motor units in reduced size muscles without L5 innervation was significantly greater than it was in muscles where both L5 and L6 motor units were present. This result supports the concept that motor neuron survival during development is dependent upon the number of muscle fibers available for innervation and the number of motor neurons competing to innervate them.     

Heterogeneity of type 1 skeletal muscle fibers revealed by monoclonal antibody to slow myosin

Monoclonal antibodies (McAbs) were generated against slow myosin from the chicken anterior latissimus dorsi (ALD) muscle and their reactivity was checked against fast (pectoralis), slow (ALD), cardiac (ventricular), and smooth (gizzard) myosins by radioimmunoassay (RIA), immunoautoradiography (immunoblots), and indirect immunofluorescence techniques. In RIAs, the McAb (ALD-58) described in this article reacted specifically with slow myosin, with only a weak cross-reactivity to cardiac myosin. In immunoblots against whole muscle homogenates and purified myosins, it bound selectively to the 200 Kd myosin heavy chain band. The ALD-58 antibody stained the fibers of the ALD muscle uniformly but gave three grades of reactions (strong, weak, and negative), with histochemically identified type 1 fibers of sartorius and gastrocnemius muscles demonstrating the immunological heterogeneity of myosins in type 1 skeletal muscle fibers.     

Force regulation and electrical properties of motor units in overloaded muscle

Introduction: The influence of long‐term muscle overload on force regulation and electrical properties of motor units (MUs) was investigated in rats. Methods: Compensatory overload of the medial gastrocnemius was induced by tenotomy of its synergists. Electrophysiological experiments were performed on functionally isolated MUs 3 months after the surgery. Results: Force–frequency curves for overloaded MUs were shifted rightward compared with control, thus MUs developed the same relative tetanic forces at higher frequencies. Higher force increase was achieved in response to an increase in stimulation frequency in overloaded fast MUs compared with control. The optimal tetanic contraction, characterized by the highest force–time area per pulse, was evoked at higher stimulation frequencies for all overloaded MUs except FF. Only minor adaptive changes in MU action potentials occurred. Conclusions: Compensatory muscle overload leads to substantial modifications in MU force development mechanisms, which are MU‐type‐specific and influence whole muscle force regulation. Muscle Nerve 53 : 96–106, 2016     

Functional differentiation of motor units in human opponens pollicis muscle.

Studies were made on the parameters of the single contraction and the size of the summated equivalent potential determined through location of motor units in human opponens pollicis muscles upon electrical stimulation and during voluntary activity. The investigation of the single contraction parameters upon electrical stimulation was performed before and after stimulation with a frequency of 30 impulses/sec for 3 min. Three types of motor units were differentiated in the opponens pollicis: fast contracting, resistant to fatigue (FR); slow contracting, resistant to fatigue (S); fast contracting, fatiguable (FF). The motor units of type FR were the smallest, those of type S were middle in size, those of type FF were the largest. During voluntary activity the lowest-threshold motor units were of types S and FR.     

Firing rates of human motor units in partially denervated muscle.

Single motor unit firing rates were measured from the first dorsal interosseous muscle (FDI) of normal subjects and patients with partial denervation of that muscle. Motor unit discharges were recorded at various levels of voluntary, stationary, isometric contraction of the FDI. The mean increase in firing rate associated with an increased muscle tension of 100 gm was significantly greater in severely weak muscle. However, when an "adjustment" was made for the degree of weakness, reduced firing rate responses were observed which correlated with the degree of muscle weakness.     

Changes in recruitment order of motor units in the human biceps muscle

Changes in recruitment threshold of individual motor units of the human biceps (caput longum), a multifunctional muscle, were investigated during different tasks, i.e., isometric flexion of the elbow, isometric supination of the forearm, and isometric exorotation of the humerus of the 110° flexed semiprone horizontal arm. The activity of 17 motor units was recorded by means of fine wire electrodes. Some units were found that could be recruited only by one force, e.g., flexion. In such cases recruitment did not depend on other forces. Most units, however, were recruited when a linear combination of exerted forces exceeded a certain threshold. The contribution of a force to this combination could be different for different motor units. Units with a high threshold for flexion tended to show a lower threshold while simultaneously exerting force in another direction. Units with a low threshold for flexion were more difficult to recruit under this condition. The findings support the view that movements are programmed “directionally”.     

The afferent influence on the voluntary firing range of individual motor units in man

The firing ranges of 50 normal anterior tibial or short toe extensor motor units in sustained isometric voluntary contraction were studied in electromyographic recordings. The afferent inflow was decreased by compression of the sciatic nerve, thereby blocking large afferents before alpha efferents. Motor units with low minimum and low maximum rates before blockade had higher minimum and maximum rates before blockade had a lower minimum but an unchanged maximum rate on blockade. The firing range recorded for both types of motor unit on blockade was also recorded for all tonically firing motor units during early reinnervation after traumatic lesion to the muscle nerve in 14 patients. It is concluded that both facilitating and inhibiting proprioceptive afferent activity is involved in the differentiation of motor unit firing ranges in sustained isometric voluntary contraction.