Effect of pain on the modulation in discharge rate of sternocleidomastoid motor units with force direction

ObjectiveTo compare the behavior of sternocleidomastoid motor units of patients with chronic neck pain and healthy controls.MethodsNine women (age, 40.4 ± 3.5 yr) with chronic neck pain and nine age- and gender-matched healthy controls participated. Surface and intramuscular EMG were recorded from the sternocleidomastoid muscle bilaterally as subjects performed isometric contractions of 10-s duration in the horizontal plane at a force of 15 N in eight directions (0–360°; 45° intervals) and isometric contractions at 15 and 30 N force with continuous change in force direction in the range 0–360°. Motor unit behavior was monitored during the 10-s contractions and the subsequent resting periods.ResultsThe mean motor unit discharge rate depended on the direction of force in the control subjects (P < 0.05) but not in the patients. Moreover, in three of the nine patients, but in none of the controls, single motor unit activity continued for 8.1 ± 6.1 s upon completion of the contraction. The surface EMG amplitude during the circular contraction at 15 N was greater for the patients (43.5 ± 54.2 μV) compared to controls (16.9 ± 14.9 μV; P < 0.05).ConclusionsThe modulation in discharge rate of individual motor units with force direction is reduced in the sternocleidomastoid muscle in patients with neck pain, with some patients showing prolonged motor unit activity when they were instructed to rest.SignificanceThese observations suggest that chronic neck pain affects the change in neural drive to muscles with force direction.     

Neuromuscular transmission in human single motor units.

A method, multiple point stimulation, has been reported to isolate 5-20 single hypothenar or thenar motor units for investigation. This method is attractive for testing neuromuscular transmission because the required stimulus intensities are much less than for supramaximal nerve stimulation, and movement artefact is less of a problem. In this investigation of controls, the changes in hypothenar and thenar motor unit surface voltage and latency of single muscle fibre action potentials belonging to the motor unit have been measured in response to trains of stimuli delivered to the motor nerve. In healthy motor units, increases in the surface peak-to-peak voltage and corresponding reductions in the peak-to-peak duration occurred, the changes being maximum at the shorter stimulus intervals and accompanied by increased synchronisation of motor unit muscle fibre action potential discharges. The investigations of the responses for healthy single motor units to repetitive stimulation provide the basis for investigations of neuromuscular transmission at the level of the motor unit in diseases of neuromuscular transmission.     

Time dependent selective recurrent discharge of motor units in F-response

The interaction among the recurrent discharge of motor units is studied in surface recorded composite F-response. In the first experiment, 200 serially elicited polyphasic F-responses from foot muscles of patients with different lower motor neurone (LMN) disorders were rearranged to understand the behaviour of individual negative and positive peaks. These peaks were considered on the basis of simultaneous recording with a single fibre EMG needle, to be the partial expression of either a single motor unit potential (MUP) or more than one MUP generated simultaneously. In another experiment, two serially averaged F-responses (50 each) were superimposed over each other to look for their similarities at 15 min intervals 6 times. Result analysis indicated interaction of MUPs by in-phase summation and out-of-phase subtraction. Less affected MUPs recurred as negative or positive peaks in morphologically different F-responses. Certain peaks were observed more frequently than the others. Two serially averaged F-responses were nearly identical on superimposition over each other but their morphologies differed at different time intervals. The study suggests more frequent generation of F-response in a specific group of alpha motoneurones at one point of time, which is replaced by another group at another point of time. The characteristic variation in F parameters is, however, mainly due to the interaction of these frequently generated F-response MUPs with other sporadically generated ones. This interaction can be appreciated on visual screening of F-responses in patients with lower motor neurone disorders, perhaps because of a reduction in the number of participating MUPs.     

The firing rate of motor units in neuromuscular disorders

Summary The firing frequency and interval variability of motor units was studied in healthy controls and in patients suffering from peripheral neuromuscular disorders. In patients with myopathic disorders, the motor unit reached a higher firing frequency than in the controls during a slight muscle contraction although interval variability did not differ significantly from controls. In neurogenic lesions the firing rate also reached a higher level than in controls; in addition the interval variability was significantly increased. The firing characteristics of motor units may prove to be a valuable aid in the differential diagnosis of neuromuscular diseases.

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Zusammenfassung Die Entladungsfrequenz und die Variationen zwischen den Intervallen motorischer Einheiten wurden sowohl bei Patienten, die an Störungen peripherer Neuromuskulatur leiden, als auch bei Normalfällen im Vergleich untersucht. Während einer leichten Muskelkontraktion wiesen die an myopathischen Störungen leidenden Patienten, verglichen mit den Normalfällen, eine höhere Entladungsfrequenz motorischer Einheiten auf, trotz der unbedeutenden Unterschiede in Intervallvariationen zwischen den Untersuchungsgruppen. Eine größere Steigerung der Entladungsrate wurde auch bei neurogenen Läsionen festgestellt, in diesem Zusammenhang hatten auch die Abweichungen zwischen den Intervallen beträchtlich zugenommen. Die Entladungscharakteristika motorischer Einheiten können einen wertvollen Beitrag zur Differentialdiagnostik bei neuromuskulären Störungen liefern.

     

Divergent synaptic influences affecting discharge patterning of genioglossus motor units

Discharge patterns of single motor units were recorded from 5 inframandibular muscles in acutely prepared adult rhesus monkeys and cats. The alterations in interspike interval patterns were studied in genioglossus motor units during respiration, swallowing, laryngeal, and precentral cortex stimulation in animals anesthetized with urethane. Genioglossus motor units were active in both inspiration and swallowing with 87% of the units demonstrating a significant difference in distributions of their first order interspike interval histograms in swallowing as compared to their respiratory pattern. Interaction between several swallows and the prolonged inhibition of respiration (1–7 sec) altered the discharge pattern of a genioglossus unit from its activity during normal respiration. This significant change in the distribution of the interspike intervals of the motor unit in both first order and autocorrelation histograms occurred during the marked increase in intensity and duration of the gross EMG of the genioglossus muscle during the return of respiration. Stimulation of the precentral cortex of the rhesus monkey indicated a third central synaptic influence on the genioglossus motoneurons with a given motor unit discharging in masticatory movements and swallowing. A first order latency histogram demonstrated a fourth synaptic influence with a correlation between the stimulus pulse to the superior laryngeal nerve innervating the laryngeal region and the latency to when a genioglossus motor unit discharged. The latency analysis suggested a vago-hypoglossal reflex from sensory input of the laryngeal region synaptically affecting genioglossus motoneurons by a polysynaptic pathway.     

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.     

Incidence of F waves in single human thenar motor units.

F-wave generation, axon conduction velocities, and contractile properties were compared in 44 healthy individual human thenar motor units. Force and muscle action potentials were recorded when single motor axons were stimulated intraneurally about 10 cm proximal to the elbow. Each stimulus usually evoked only one electromyographic (EMG) potential. However, in seven units (16%), a single stimulus elicited an F wave in response to 1.7 +/- 1.6% (mean +/- SD) of the stimuli applied. Axon conduction velocity proximal to the site of stimulation was faster than distal conduction velocity (72.7 +/- 8.0 m/s versus 64.2 +/- 10.5 m/s). Distal conduction velocities, twitch forces, and contraction times were similar for units that did and did not generate F waves. Thus, no obvious subset of thenar motor units generated F waves. These results provide valuable baseline information on F waves that can be used to assess changes in axon conduction, motor unit contractile properties, and motoneuron excitability in disease.     

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.     

Intraneural microstimulation of motor axons in the study of human single motor units

Single motor unit activity has been studied in depth since the first intramuscular electrodes were developed more than 70 years ago. Many techniques have been combined or used in isolation since then. Intraneural motor axon microstimulation allows the detailed study of single motor units in awake human subjects in a manner most analogous to that used in reduced animal preparations. A microelectrode, inserted percutaneously into a peripheral nerve, stimulates the axon of a single alpha-motoneuron at a site remote from the contracting muscle, allowing detailed analyses of the contractile properties of a single motor unit in an otherwise quiescent muscle, that is, without interference of simultaneously active motor units or the presence of an electrode within the muscle. The methods and results obtained using this technique are described and compared to those of other studies of single motor units in human subjects. Differences have been found between human and animal motor units and between motor units of various muscles. Studying human and animal motor units using an analogous technique provides insight into the interpretation of human data when results differ from animal data, and when human motor units cannot be examined in the same way, or at a similar level of detail, as animal motor units.     

Number of motor units in human abductor hallucis

Motor unit number was estimated for the human abductor hallucis (AH) muscle in 11 subjects by counting the number of increments in surface electromyographic responses to progressive increases in current-pulse amplitude applied to the muscle-nerve. The average motor unit count for AH (43) was substantially smaller than that estimated for other human muscles. Consequently, motor unit activity should be readily recordable up to high forces in AH, making it well suited for studies of recruitment and rate coding.     

Comparison of the recruitment and discharge properties of motor units in human brachial biceps and adductor pollicis during isometric contractions

Experiments were designed to assess the relative contribution of rate coding and motor unit recruitment to force production in two muscles of different fiber composition and function. Single motor unit action potentials were recorded during steady isometric contraction in biceps brachii, a large proximal limb muscle of mixed fiber composition, and adductor pollicis, a small hand muscle comprised mainly of type I muscle fibers. Action potential spike trains were obtained over the entire force range in each muscle. The results suggest that these two muscles are controlled in different ways. In biceps brachii, recruitment was observed from 0 to 88% maximum voluntary contraction (MVC). In adductor pollicis, no motor unit was observed to be recruited at forces greater than 50% MVC, with the majority of recruitment occuring below 30% MVC. On the average, motor units in adductor pollicis discharged at higher rates, with less regularity, and with a greater frequency of occurrence of short interspike intervals (intervals ≤ 20 msec) than those in biceps brachii. Such findings suggest that rate coding plays a more prominent role in force modulation in adductor pollicis, while recruitment plays a more important role throughout the contractile force range in biceps brachii.     

Three-dimensional structure of cat tibialis anterior motor units

The motor unit is the basic unit for force production in a muscle. However, the position and shape of the territory of a motor unit within the muscle have not been defined precisely. The territories of five motor units in the cat tibialis anterior muscle were reconstructed three-dimensionally (3-D) from tracings of the glycogen-depleted fibers belonging to each unit. The motor unit territories did not span the entire length of the muscle and their cross-sectional areas tapered along the proximodistal axis producing a conical shape. In addition, the position of the territory of each unit shifted in an anterior–posterior plane along the longitudinal axis of the muscle, presumably as a consequence of the pinnation of the fibers. The area of the motor unit territory at any given level along the proximodistal axis was highly correlated with the number of fibers within the territory at that level. Connective tissue boundaries (outlining fascicles) appeared to have a strong influence on the shape of the territory, territories showed abrupt changes at connective tissue boundaries as groups of motor unit fibers within a fascicle often terminated together while motor unit fibers in neighboring fascicles did not terminate. It is likely that the mechanical impact of the recruitment of a motor unit is affected by the location and shape of motor units within the same muscle area. Since there is a close relationship between the area of the territory of a motor unit and the number of fibers in the motor unit, while the density of unit fibers remains the same, then the same factor(s) which regulate the number of fibers innervated by a motoneuron may also determine the territory area.© 1995 John Wiley & Sons, Inc.     

Non-invasive imaging of single human motor units

ObjectiveTo determine the size, shape and distribution of single human motor units in-vivo in healthy controls of different ages.MethodsA novel diffusion-weighted magnetic resonance imaging (MRI) technique was used in combination with in-scanner electrical stimulation to quantify the shape, cross-sectional area, and dimensions of individual motor units in 10 healthy subjects.ResultsThirty-one discrete motor units were studied. The majority were elliptical or crescent shaped, but occasional split motor units were observed. The mean motor unit cross sectional area was 26.7 ± 11.2 mm², the mean maximum dimension was 10.7 ± 3.3 mm, and the mean minimum dimension was 4.5 ± 1.2 mm. Subjects aged over 40 had significantly larger maximum dimensions than those below this age (p < 0.05).ConclusionsMotor unit MRI (MUMRI) is a novel technique capable of revealing the size, shape and position of multiple motor units in human muscles. It is reproducible, non-invasive, and sufficiently sensitive to detect physiologically relevant changes in motor unit morphology with age.SignificanceTo our knowledge, these results provide the first imaging assessment of human motor unit morphology. The technique shows promise both as a diagnostic tool and as a biomarker in longitudinal studies of disease progression.     

Pattern of recruiting human motor units in neuropathies and motor neurone disease

The pattern of recruiting human motor units in the first dorsal interosseous muscle of the hand has been studied in 31 patients with ulnar neuropathies and motor neurone disease. Two years after surgical repair of an unilateral complete severance of the ulnar nerve, the twitch tensions increased to normal size. However, the normal orderly pattern of recruiting motor units of increasing size during increasing voluntary contractions was irretrievably lost. Among patients with pressure or entrapment neuropathies, the normal orderly pattern of recruiting motor units was always retained. Similarly, in patients with motor neurone disease (amyotrophic lateral sclerosis), the orderly pattern of recruitment was not disrupted.     

Magnetic fields produced by single motor units in human skeletal muscles.

OBJECTIVE: To develop new non-invasive ways of analyzing human skeletal muscle function, biomagnetic measurement was applied to the vastus lateralis and vastus medialis of 3 healthy adult males using a 64 channel superconducting quantum interference device system. METHODS: Discharges from single motor units were detected by simultaneously recorded surface electromyography. Magnetic signals were averaged 64-158 times at zero-crossing timing in the surface electromyographic signal. RESULTS: Six motor units detected in the 3 subjects produced large magnetic fields with peak-to-peak amplitudes of 1-2 pT. Magnetomyographic isofield maps showed current sources arising from motor endplate regions and propagating in directions opposite to fiber ends. The absolute intensity of current moments in muscle fibers within motor units was estimated based on dipole fitting. The estimated moment was 23.9-114.3 nAm for repolarization dipole. Dividing these moment values by the typical dipole moment of 0.286 nAm in a single muscle fiber, the number of muscle fibers in motor unit was estimated to be 84-400. CONCLUSIONS: Magnetic recording may provide a new non-invasive way of analyzing and diagnosing human muscle function.     

Methods for estimating the number of motor units in human muscles

The number of motor units in the thenar muscle group was calculated by dividing the surface electromyogram and twitch force, in maximal stimulation of the median nerve, by estimates of the average electromyogram and twitch force from single units. The following three techniques were used to estimate the average electromyogram from single units: spike-triggered averaging from units recorded with a needle electrode, intramuscular microstimulation of motor nerve branches, and graded whole nerve stimulation at the wrist. The first two techniques also provided independent estimates of motor unit numbers based on the average force generated by single units. The five estimates (three based on the electromyogram and two on force) ranged from 116 to 170 motor units in the thenar group. Correcting for cancellation when unit responses sum to form the compound action potential or twitch increased the estimated number of units, which ranged from 130 to 179. The estimates were not statistically different from one another but were substantially lower than some previous electrophysiological estimates based on graded whole nerve stimulation. The recruitment pattern of single units during whole nerve stimulation was recorded and simulated mathematically. The most likely reason for the higher estimates in previous studies using graded whole nerve stimulation is shown to be alternation of motor units. Potential errors in all the techniques are discussed and compared.     

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”.