Ultrastructural diversity in motor units of crustacean stomach muscles

The physiological and ultrastructural properties of muscle fiber.s comprising three motor units in the gastric mill of blue crabs are described. In their contractile properties muscle fibers in all motor units are similar and resemble the slow type fibers in crustacean limb muscles. The majority of fibers generate large excitatory post-synaptic potentials which do not facilitate strongly. Structurally two types of fibers are found. The one type has long sarcomeres (greater than 6 mum), thin to thick myofilament ratios of 5-6:1 and diads located near the ends of the A-band. The other type has shorter sarcomeres (less than 6 mum), thin to thick myofilament ratios of 3:1 and diads located at mid sarcomere level. Both types of fibers occur within a single motor unit and this differs from the vertebrate situation. Furthermore, the finding of fibers with a low thin to thick myofilament ratio of 3:1 demonstrates that they are not exclusive to fast type crustacean muscle but also occur in slow stomach muscles.     

Discharge frequencies of single motor units in human diaphragm and parasternal muscles in lying and standing

Single motor unitdischarge was measured directly in diaphragm and parasternalintercostal muscles to determine whether neural drive to humaninspiratory muscles changes between lying and standing. The finaldischarge frequency of diaphragmatic motor units increased slightly, by1 Hz (12%; P < 0.01), when subjects were standing [182 units, median 9.1 Hz (interquartile range 7.6-11.3 Hz)]compared with lying supine [159 units, 8.1 Hz (6.6-10.3Hz)]. However, this increase with standing occurred in onlytwo of six subjects, in one of whom tidal volume increasedsignificantly during standing. Parasternal intercostal motor unitfinal discharge frequencies did not differ between standing [116units, 8.0 Hz (6.6-9.6 Hz)] and lying [124 units, 8.4 Hz(7.0-10.3 Hz)]. The discharge frequencies at the onset ofinspiration did not differ between lying and standing for eithermuscle. A larger proportion of motor units in both inspiratory muscleshad postinspiratory or tonic expiratory activity for lying comparedwith standing (15 vs. 4%; P < 0.05). We conclude thatthere is no major difference in the phasic inspiratory drive to thediaphragm with the change in posture.

     

Fatigue of single motor units in human masseter

The spike-triggered averaging technique was used to determine the time course and extent of fatigue of single motor unit twitches in the human masseter. This is the first report of a fatigue test having been applied to masseter motor units in either animals or humans. The human masseter was found to be comprised predominantly of fast-twitch motor units with a broad spectrum of fatigability. Very few physiological type S units were found, despite histochemical evidence for a substantial population of type I fibers in the masseter. In addition, there was no significant correlation between fatigability and either twitch amplitude or contractile speed in the motor units studied. The latter observations are consistent with the unusual histological features of the masseter. Comparison with other human fatigue data suggests that the extent of fatigue in the present population of masseter motor units after approximately 3,000 activations is similar to that reported for populations of units in first dorsal interosseous and medial gastrocnemius.     

Digit flexor muscles in the cat: their action and motor units

The relation between muscle action and the mechanical properties of motor units has been explored in the main digit flexors of the cat hind limb: plantaris (PL); flexor digitorum brevis (FDB); flexor hallucis longus (FHL); and, flexor digitorum longus (FDL). General observations on muscle action revealed that PL is an ankle extensor as well as a digit flexor. PL and FHL were shown to be the major force contributors to digit flexion with FDL playing a lesser but still significant role. The mechanical properties of PL, FHL and FDB motor units were studied by noting twitch and tetanic tensions produced by electrical stimulation of single alpha axons, functionally isolated from the ventral root filaments. These data were compared to similar data reported by Olson and Swett (1966) for flexor digitorum longus (FDL). Our sample (114 PL, 60 FDB and 124 FHL units) disclosed that PL, FDB and FHL have units of uniformly fast contraction times (means 22, 27 and 27 msec respectively). PL units developed the most tetanic tension (3 to 160, mean 62 gm-wt) followed by FHL (2 to 87, mean 31 gm-wt) with FDB units producing very little tension (1 to 20, mean 6 gm-wt). Swett and Olson's FDL sample (108 units) showed tensions ranging from 0.3 to 100 gm-wt (mean 10 gm-wt). A division of labor among the four muscles is proposed. The large PL units are advantageous for forceful phasic inputs to the digits during the locomotion and in keeping with PL's additional role as an ankle exstensor. The low output forces of FDB units are optimal for discrete input to the digits during subtle adjustments of posture. We propose that the larger fast contracting units of FHL are used primarily for forceful digit flexions required in locomotion and for phasic protrusion of the claws while the predominately small and slow contracting units of FDL are used for sustained claw protrusion.     

Short term bed-rest reduces conduction velocity of individual motor units in leg muscles

Space permanence simulations such as prolonged bed-rest can mimic some of the physiological modifications in the human body and provide study conditions that are more accessible than during space flight. A short term bed-rest experiment was organized to simulate the effects of weightlessness for studying the adaptation to this condition. Eight healthy young volunteers were studied before and immediately after the 14 day periods of strict bed-rest.Surface EMG signals were detected with linear electrode arrays from vastus medialis, vastus lateralis and tibialis anterior muscle during isometric voluntary contractions at 20% MVC. Motor unit action potentials (MUAPs) of individual motor units were extracted from the interference EMG signals with a partial decomposition algorithm and averaged.MUAP templates generated by the same motor unit could be retrieved before and after bed-rest period. Muscle fiber conduction velocity (CV) was estimated from each averaged MUAP template and from the global EMG signal. Both global and single MU conduction velocity was observed to decrease by about 10% after the bed-rest period (p < 0.05). Amplitude and power spectral parameters did not significantly change after the bed-rest period.It is concluded that a short term bed-rest reduces the CV of individual motor units without a significant effect on muscle force or on other electrophysiological parameters.     

Reciprocal inhibition studied in discharging human motor units

The effect of excitation of group Ia afferents, evoked by stimulation of a mixed nerve, on the firing pattern of voluntarily activated single motor units of an antagonist muscle (biceps femoris, triceps surae, and tibialis anterior muscles) was studied. Poststimulus histograms were constructed for rhythmic sequences of motor unit potentials recorded by needle electrodes and the duration of interspike intervals was analyzed. Reciprocal inhibition and other effects accompanying nerve stimulation were discovered in the motoneurons of all three muscles. Distinguishing features of the manifestation of reciprocal inhibition in a discharging motoneuron were investigated; the effect was shown to depend on the time of occurrence of the inhibitory action in the interspike interval.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 6, pp. 626–636, November–December, 1978.     

Impairment of human soleus motor units during ischemia

It is generally accepted that ischemia produced by limb compression affects rapidly conducting large-diameter Ia afferents in the early stage and that the motor nerve-muscle complex is blocked later. This notion, however, seems to be controversial for several reasons, so an attempt to reveal the amount of motor unit (MU) impairment during ischemia was made. Observation of human soleus muscle electromyographic (EMG) signal recorded either by bipolar needle electrode or by surface electrodes at various levels of voluntary contraction during the course of ischemia showed that low-threshold small MUs were affected first while high-threshold large MUs survived longer. The changes in EMG patterns were temporally correlated with T-reflex deterioration. It is suggested that the early loss of low-threshold MUs may play a definite role in alterations of reflexes during ischemia.     

Training adaptations in the behavior of human motor units.

The purpose of this brief review is to examine the neural adaptations associated with training, by focusing on the behavior of single motor units. The review synthesizes current understanding on motor unit recruitment and rate coding during voluntary contractions, briefly describes the techniques used to record motor unit activity, and then evaluates the adaptations that have been observed in motor unit activity during maximal and submaximal contractions. Relatively few studies have directly compared motor unit behavior before and after training. Although some studies suggest that the voluntary activation of muscle can increase slightly with strength training, it is not known how the discharge of motor units changes to produce this increase in activation. The evidence indicates that the increase is not attributable to changes in motor unit synchronization. It has been demonstrated, however, that training can increase both the rate of torque development and the discharge rate of motor units. Furthermore, both strength training and practice of a force-matching task can evoke adaptations in the discharge characteristics of motor units. Because the variability in discharge rate has a significant influence on the fluctuations in force during submaximal contractions, the changes produced with training can influence motor performance during activities of daily living. Little is known, however, about the relative contributions of the descending drive, afferent feedback, spinal circuitry, and motor neuron properties to the observed adaptations in motor unit activity.     

Properties of twitch motor units of the ankle extensor muscles in the bullfrog Rana catesbeiana

A study of the mechanical properties of the twitch motor units in the ankle extensor muscles of bullfrogs was undertaken to expand our view of the diversity of motor unit properties among vertebrates. Two muscles were chosen that represent a wide range of extensor function: the plantaris longus (PL) is a large muscle providing most of the force for ankle extension in hopping and swimming, and the tibialis posticus (TP) is relatively small and may act as an ankle stabilizer or be primarily postural in function. Both muscles have highly fatigable motor units, but also some (especially in TP) low or non-fatigable ones. Mean tetanic tensions of motor units in both muscles are relatively large as compared with those of mammals but are especially large in PL, No clear correlations were found between contraction times and either motor unit tetanic tensions or fatigability, nor did the motor units fall into clearly defined types based on any functional parameters. Overall contraction and relaxation times are slow compared with those of mammals and are somewhat slow compared to those of other frogs; unlike results from earlier studies, the large units of PL are slower than the small units of TP. This results in PL units reaching fused tetani at lower stimulus frequencies. The twitch/tetanus and force/frequency ratios in PL motor units are much larger than those of TP, giving PL units greater relative forces at lower stimulus rates. These results are discussed in the context of motor unit function. © 1994 Wiley-Liss, Inc.     

Discharge properties of motor units during steady isometric contractions performed with the dorsiflexor muscles

The purpose of this study was to record the discharge characteristics of tibialis anterior motor units over a range of target forces and to import these data, along with previously reported observations, into a computational model to compare experimental and simulated measures of torque variability during isometric contractions with the dorsiflexor muscles. The discharge characteristics of 44 motor units were quantified during brief isometric contractions at torques that ranged from recruitment threshold to an average of 22 ± 14.4% maximal voluntary contraction (MVC) torque above recruitment threshold. The minimal [range: 5.8-19.8 pulses per second (pps)] and peak (range: 8.6-37.5 pps) discharge rates of motor units were positively related to the recruitment threshold torque (R(2) ≥ 0.266; P < 0.001). The coefficient of variation for interspike interval at recruitment was positively associated with recruitment threshold torque (R(2) = 0.443; P < 0.001) and either decreased exponentially or remained constant as target torque increased above recruitment threshold torque. The variability in the simulated torque did not differ from the experimental values once the recruitment range was set to ～85% MVC torque, and the association between motor twitch contraction times and peak twitch torque was defined as a weak linear association (R(2) = 0.096; P < 0.001). These results indicate that the steadiness of isometric contractions performed with the dorsiflexor muscle depended more on the distributions of mechanical properties than discharge properties across the population of motor units in the tibialis anterior.     

Discharge patterns in human motor units during fatiguing arm movements

The purpose of this study was to determinewhether short interspike intervals (ISIs of <20 ms) would occurnaturally during voluntary movement and would increase in number withfatigue. Thirty-four triceps brachii motor units from ninesubjects were assessed during a fatigue task consisting of fiftyextension and fifty flexion elbow movements against a constant-loadopposing extension. Nineteen motor units were recorded from thebeginning of the fatigue task; the number of short ISIs was 7.1 ± 4.1% of the total number of ISIs in the first one-third of the task(unfatigued state). This value increased to 11.8 ± 5.9% for thelast one-third of the task (fatigued state). Fifteen motor units wererecruited during the fatigue task and discharged, with 16.4 ± 6.0%of short ISIs in the fatigued state. For all motor units, the number of short ISIs was positively correlated(r² = 0.85) withthe recruitment threshold torque. Short ISIs occurred most frequentlyat movement initiation but also occurred throughout the movement. Theseresults document the presence of short ISIs during voluntary movementand their increase in number during fatigue.