[Results of surgical treatment of radio-humeral epicondylopathy]

We examined extent and affection of an assumed neuromuscular transmission disorder by performing a prospective clinical study on 75 patients with therapy-resistant radiohumeral epicondylopathy. Before operation, we electromyographically diagnosed an increased rate of polyphasic potentials of the long wrist extensors as well as a prolonged motor latency of the respective muscles. Corresponding to a hereby implied damage to the distal part of the motor neuron, disordered neuromuscular recruitment combined with a reduced maximum strength and -elasticity could be proven. Both effects were significantly reversible (p < 0.001) through operative intervention. We found a significant correlation (corr < 0.90) between the normalization of the motor latency and increased strength. Subgroups were formed depending on different pre-operative diagnostic efforts and differing redicality regarding the performed soft-tissue operation, thus the clinical validity of the findings diagnosed in the anatomic and neurophysiologic part of the study was additionally examined. It was proven that the failure rate varies between 10% and 30%, depending on the radicality of tenotomy, which could be interpreted as a general indication for complete extensor carpi radialis brevis tendon release. In this connection it is remarkable that the clinical result of an electromyographically localized damage in the area between epicondyle and arcade of Frohse could not be improved through open neurolysis. Dealing with strictures located on the proximal side of the epicondyle on the other hand, this technique seems to play an important role for recurrence prophylaxis.

Recruitment characteristics of nerve fascicles stimulated by a multigroove electrode

The recruitment characteristics of fascicle-selective nerve stimulation by a multigroove electrode have been investigated both theoretically and in acute experiments. A three-dimensional (3-D) volume conductor model of fascicles in a multigroove device and a model of myelinated nerve fiber stimulation were used to calculate threshold stimuli of nerve fibers in these fascicles. After their exposition, fascicles from rat sciatic nerve were positioned in different grooves of appropriate sizes and stimulated separately. The device appeared to be suitable for fascicle-selective stimulation, because both computer simulations and acute animal experiments showed that crosstalk between neighboring fascicles is not a problem, even when monopolar stimulation was used. The threshold stimulus was lower for a small fascicle than for a large one. When the amount of (conducting) medium between contact and perineurium or its conductivity was reduced, threshold stimuli were lower. Moreover, simulations predict that the slopes of recruitment curves are smaller and inverse recruitment order is less pronounced. Simulations also showed that a small contact is preferable to a large on, because a small contact gives a slightly smaller slope of the recruitment curve. Both experimentally and theoretically a significantly smaller slope of recruitment curves was obtained by stimulation with a cathode and an anode at opposite sides of the fascicle, driven by two current sources giving simultaneous pulses with different, but linearly dependent amplitudes.

Neuroethological and morphological (Neo-Timm staining) correlates of limbic recruitment during the development of audiogenic kindling in seizure susceptible Wistar rats

Acute audiogenic seizures are a model of generalized tonic-clonic seizures, induced by high intensity acoustic stimulation in genetically susceptible rodents. The neural substrate are sensory motor brainstem nuclei. Recruitment of forebrain structures takes places upon repetition of acoustically evoked seizures. The term audiogenic kindling means forebrain kindling evoked by repeated brainstem seizures and has been described in several strains of genetically epilepsy-prone rats. Thus, the present work was conducted in order to test the hypothesis that audiogenic kindling recruits the forebrain, which may be behaviorally evaluated and associated with morphological changes as well. The behavioral sequences observed during the development of audiogenic kindling were assessed by neuroethological methods (cluster analysis), with the ETHOMATIC program. Seizure severity indexes (brainstem and limbic seizures) and latencies of wild running and tonic-clonic seizures were measured to quantify seizure evolution. Densitometric analysis of Neo-Timm staining was used for assessing morphological changes associated with audiogenic kindling. In group I, II resistant (R) and 16 susceptible (S) animals were stimulated (120 dB) 21 times, and allowed a 10 day recovery period prior to retesting. In group II, 22 R and 20 S were stimulated 60 times, and allowed a 2 month recovery period prior to retesting. Repetition of the acoustic stimulation in group I and group II susceptible animals led to a progressive and statistically significant attenuation of the behaviors associated with brainstem seizures and a concomitant increased expression of the behaviors associated with limbic seizures. After either a 10 day (group I) or 2 month (group II) recovery period, acoustic stimulation preferentially evoked brainstem-associated behaviors and seizures rather than limbic ones in the audiogenic susceptible animals, although in some animals overlapped brainstem and limbic seizures were detected. Latencies for the wild running and tonic seizures after acoustic stimulation significantly increased during audiogenic kindling for both group I and group II susceptible animals. The quantitative ethological evaluation in both group I and group II, illustrated by flowcharts, showed the evolution of the kindling installation by the presence of limbic seizure clusters, competing in time with the original tonic-clonic clusters. Expression of limbic seizures by group I animals, after acoustic stimulation, was not associated with changes in the mossy fiber Neo-Timm staining pattern of these animals. In group II however, Neo-Timm staining revealed mossy fiber sprouting in the ventral hippocampus (but not in the dorsal), and a significant change in the optical density of amygdaloid nuclei and perirhinal cortex in susceptible animals as compared to resistant ones. In conclusion, audiogenic kindling effectively recruits forebrain structures, responsible for the appearance of limbic seizures. It is possible that the paradigm used in group I was subthreshold for the development of clear-cut synaptic reorganization in the hippocampal mossy fiber system, since the behavioral patterns reverted ten days after the last seizure induction. In group II, however, an increased number of evoked seizures and a more prolonged time after the last chronic seizure showed structural re-arrangements in amygdala, perirhinal cortex and hippocampus, associated with permanence in terms of behavioral data (lack of regression of limbic seizures to control values).

Comparison of force and peak EMG during a maximal voluntary isometric contraction at selected angles in the range of motion for knee extension

For 5 adults maximal isometric strength at 110 degrees, 90 degrees, 70 degrees, 50 degrees, and 30 degrees of knee flexion for the vastus medialis, rectus femoris, and vastus lateralis varied across angles but EMG responses were similar. That motor recruitment patterns were similar throughout the range of joint motion requires replication on a larger sample.

Contraction patterns of intrinsic laryngeal muscles induced by orderly recruitment in the canine

The specific performance of intrinsic laryngeal muscles has been difficult to evaluate, especially in living subjects. To reproduce natural contractions, we artificially induced orderly recruitment of their innervating axons according to the size principle. In 5 dogs, both recurrent laryngeal nerves (RLNs) were stimulated with 50 through 10 Hz, 300 to 1000 microA currents while 600 Hz, 1000 to 0 microA decreasing blocking currents were administered. Surface electromyography electrodes placed on the thyroarytenoid, posterior cricoarytenoid, and lateral cricoarytenoid muscles were used to determine the amplitudes (in mA) of compound muscle action potentials. There was a highly statistically significant difference (P<.004) between the thyroarytenoideus which had the fastest rate of recruitment (8.38%), and posterior cricoarytenoideus, which had the slowest (4.81%). There was an intermediate recruitment rate (6.72%) of the lateral cricoarytenoideus, a divergence attributed to a more equal distribution in fast and slow types of myofibers and a smaller sample. We submit that RLN axons can be recruited in an orderly manner according to their sizes and that the rates are unique to the muscle classes they innervate. The parameters defining these contraction patterns may offer key information for laryngeal pacing.

Motor unit control properties in constant-force isometric contractions

1. The purpose of this study was 1) to characterize the decrease observed in mean firing rates of motor units in the first 8-15 s of isometric constant-force contractions and 2) to investigate possible mechanisms that could account for the ability to maintain force output in the presence of decreasing motor unit firing rates. 2. The decrease in mean firing rates was characterized by investigating myoelectric signals detected with a specialized quadrifilar needle electrode from the first dorsal interosseus (FDI) and the tibialis anterior (TA) muscles of 19 healthy subjects during a total of 85 constant-force isometric contractions at 30, 50, or 80% of maximal effort. The firing times of motor units were obtained from the myoelectric signals with the use of computer algorithms to decompose the signal into the constituent motor unit action potentials. Time-varying mean firing rates and recruitment thresholds were also calculated. 3. Motor units detected from the TA muscle were found to have a continual decrease in their mean firing rates in 36 of 44 trials performed during isometric ankle dorsiflexion at force values ranging from 30 to 80% of maximal effort and a duration of 8-15 s. Likewise, motor units detected in the FDI muscle displayed a decrease in firing rate in 32 of 41 trials performed during constant-force isometric index finger abduction for contractions ranging from 30 to 80% of maximal effort. In 14 contractions (16% of total), firing rates were essentially constant, whereas in 3 contractions (4%), firing rates appeared to increase. 4. Motor units with the higher recruitment thresholds and lower firing rates tended to display the greater decreases in firing rate over the constant-force interval, whereas motor units with lower recruitment thresholds and higher firing rates had lesser rates of decrease. Furthermore, increasing contraction levels tended to intensify the decrease in the motor unit firing rates. 5. Three possible mechanisms were considered as factors responsible for the maintaining of force output while motor units decreased their firing rates: motor unit recruitment, agonist/antagonist interaction, and twitch potentiation. Of these, motor unit recruitment was discarded first because none was observed during the 8-15 s duration of any of the 85 contractions. Furthermore, contractions outside the physiological range of motor unit recruitment (at 80% of maximal effort) revealed the same decreasing trend in firing rates, ruling out recruitment as the means of sustaining force output. 6. The role of agonist or antagonist muscle interaction was investigated with the use of the muscles controlling the wrist joint. Myoelectric signals were recorded with quadrifilar needle electrodes from the wrist extensor muscles while myoelectric activity in the wrist flexor muscles was concurrently monitored with surface electrodes during constant-force isometric wrist extension at 50% of maximal effort. Firing rates of the motor units in the wrist extensor muscles simultaneously decreased while the flexor muscles were determined to be inactive. 7. All the findings of this study regarding the behavior of the firing rates could be well explained by the reported characteristics of twitch potentiation that have been previously documented in animals and humans. 8. The results of this study, combined with the results of other investigators, provide the following scenario to explain how a constant-force isometric contraction is sustained. As the contraction progresses, the twitch force of the muscle fibers undergoes a potentiation followed by a decrease. Simultaneously, the "late adaptation" property of the motoneuron decreases the firing rate of the motor unit. Findings of this study suggest that voluntary reduction in firing rates also cannot be ruled out as a means to augment the adaptation in motoneurons. (ABSTRACT TRUNCATED)

Motor unit activity during human single joint movements

1. To explore the neural control of single joint movements in humans, the activity of 47 motor units in triceps brachii muscle was recorded during elbow flexion and extension movements. Movements were performed with different but changing deceleration magnitudes, while the acceleration magnitude was kept constant, to determine the relationship between motor unit activity and the acceleration and deceleration characteristics of single joint movements. 2. The number of motor unit action potentials was found to vary with the magnitude of the movement deceleration. In addition the duration of the discharge of a motor unit was found to parallel the duration of the acceleration phase of the movement, when the acceleration duration was varied while acceleration magnitude was kept constant. 3. Approximately half of the recorded motor units in triceps brachii were active both in the initiation and in the termination of the extension movements. However, motor units were identified that participated in specific phases of the movement (i.e., either during the 1st agonist or 2nd agonist burst of muscle activity) depending on the magnitude of the acceleration or deceleration. 4. During flexion movements, when the triceps muscle served as an antagonist, approximately half of the motor units were recruited only when the magnitude of the flexion deceleration was large. Further, this deceleration magnitude was larger than that evident during the extension movements in which the motor unit discharged. 5. The findings of this study demonstrate that the nervous system activates the same motor units whether the muscle is functioning as an agonist or antagonist so as to control the characteristics of acceleration and deceleration of single joint movements.

Analysis of the electromyographic interference pattern

The electromyographic interference pattern (EMG-IP) contains information about the number, firing rate, and recruitment characteristics of motor units, and information regarding the waveforms of the recruited motor units. Muscle and nerve diseases produce characteristic changes in the IP that can be distinguished by IP analysis. This analysis complements analysis of the motor unit potentials. The electromyographer usually assesses the IP signals subjectively by their appearance on the oscilloscope screen and by their sound on the audio monitor. Techniques have been developed to automate IP analysis with and without force monitoring. These techniques give objective information, quantitate the degree of abnormality, and permit electromyographers-in-training to compare their subjective analysis of the IP with more objective findings.

Ionic mechanisms underlying synchronized oscillations and propagating waves in a model of ferret thalamic slices

1. A network model of thalamocortical (TC) and thalamic reticular (RE) neurons was developed based on electrophysiological measurements in ferret thalamic slices. Single-compartment TC and RE cells included voltage- and calcium-sensitive currents described by Hodgkin-Huxley type of kinetics. Synaptic currents were modeled by kinetic models of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), gamma-aminobutyric acid-A (GABAA) and GABAB receptors. 2. The model reproduced successfully the characteristics of spindle and slow bicuculline-induced oscillations observed in vitro. The characteristics of these two types of oscillations depended on both the intrinsic properties of TC and RE cells and their pattern of interconnectivity. 3. The oscillations were organized by the reciprocal recruitment between TC and RE cells, due to their manual connectivity and bursting properties. TC cells elicited AMPA-mediated excitatory postsynaptic potentials (EPSPs) in RE cells, whereas RE cells elicited a mixture of GABAA and GABAB inhibitory postsynaptic potentials (IPSPs) in TC cells. Because of the presence of a T current, sufficiently strong EPSPs could elicit a burst in RE cells, and TC cells could generate a rebound burst following GABAergic IPSPs. Under these conditions, interaction between the TC and RE cells produced sustained oscillations. 4. In the absence of spontaneous oscillation in any cell, the TC-RE network remained quiescent. Spindle oscillations with a frequency of 9-11 Hz could be initiated by stimulation of either TC or RE neurons. A few spontaneously oscillating TC neurons recruited the entire network model into a "waxing-and waning" oscillation. These "initiator" cells could be an extremely small proportion of TC cells. 5. In intracellular recordings, TC cells display a reduced ability for burst firing after a sequence of bursts. The "waning" phase of spindles was reproduced in the network model by assuming an activity-dependent upregulation of Ih operating via a calcium-binding protein in TC cells, as shown previously in a two-cell model. 6. Following the global suppression of GABAA inhibition, the disinhibited RE cells produced prolonged burst discharges that elicited strong GABAB-mediated currents in TC cells. The enhancement of slow IPSPs in TC cells was also due to cooperativity in the activation of GABAB-mediated current. These slow IPSPs recruited TC and RE cells into slower waxing-and-waning oscillations (3-4 HZ) that were even more highly synchronized. 7. Local axonal arborization of the TC to RE and RE to TC projections allowed oscillations to propagate through the network. An oscillation starting at a single focus induced a propagating wavefront as more cells were recruited progressively. The waning of the oscillation also propagated due to upregulation of Ih in TC cells, leading to waves of spindle activity as observed in experiments. 8. The spatiotemporal properties of propagating waves in the model were highly dependent on the intrinsic properties of TC cells. The spatial pattern of spiking activity was markedly different for spindles compared with bicuculline-induced oscillations and depended on the rebound burst behavior of TC cells. The upregulation of Ih produced a refractory period so that colliding spindle waves merged into a single oscillation and extinguished. Finally, reducing the Ih conductance led to sustained oscillations. 9. Two key properties of cells in the thalamic network may account for the initiation, propagation, and termination of spindle oscillations, the activity-dependent upregulation of Ih in TC cells, and the localized axonal projections between TC and RE cells. In addition, the model predicts that a nonlinear stimulus dependency of GABAB responses accounts for the genesis of prolonged synchronized discharges following block of GABAA receptors.

Motor unit recruitment in a distributed model of extraocular muscle

1. Eye position commands frequently are treated as a lumped, single-valued variable that is related linearly to eye position. As a step toward investigating how system-level linearity might be achieved despite nonlinear components, a distributed model of motor units in the abducens nucleus and lateral rectus muscle was constructed. 2. Parameters in the model were estimated using data from three main sources: measurement of length-tension curves for eye muscle in people, electrophysiological recording of ocular motoneuron properties in monkeys, and investigations of oculomotor unit properties in cat. Units (n = 100) in the distributed model were assigned equal strengths and for a given fixation command, the force developed by each unit was calculated, and the sum of unit forces compared with the active force in the entire muscle as measured experimentally. 3. The properties of the active units then were adjusted to reduce the size of any resultant error in a manner related to gradient descent methods for neural-net training. Distributed models were "trained" in this fashion for a series of eye positions drawn at random from the oculomotor range until performance stabilized. The goal of the training procedure was to obtain a good match between the output of the model and the experimental data on muscle force as a function of eye position. 4. Plots of trained motor-unit strength against ocular motoneuron threshold revealed a U-shaped pattern with the strongest units being recruited at both extremes of the oculomotor range and the weakest units recruited in the middle. The pattern remained unaltered qualitatively over a range of assumptions about the distribution of ocular motoneuron parameters and the relation between motoneuron firing rate and unit force. 5. The right-hand limb of the U-shaped pattern is similar to that observed in spinal motoneurons, where stronger units tend to have higher recruitment thresholds. The left-hand limb may reflect the two specializations of eye muscle: the functional need for very precise control of eye position in the middle of the oculomotor range and the use of multiply innervated muscle fibers to provide ripple-free control of eye position at low firing frequencies.

In vivo IIX and IIB fiber recruitment in gastrocnemius muscle of the rat is compartment related

The purpose of the present study was to investigate to what extent fast-twitch IIX and IIB fiber recruitment was related to the natural existing muscle compartments (subvolumes of muscle innervated by different primary nerve branches) in rat medial gastrocnemius. Three groups (n = 6) of rats trotted on a motor-driven treadmill (20 degrees incline) at different speeds. A fourth group served as controls, and a fifth group received in situ electrical stimulation of all medial gastrocnemius muscle fibers. Postexercise glycogen levels (periodic acid-Schiff staining intensities) were made. Running caused more and in situ stimulation caused less glycogen breakdown in the proximal IIX and IIB fibers compared with the fibers of the same type in the most distal compartment. Furthermore, the boundaries of the most distal compartment could often be recognized in the periodic acid-Schiff-stained cross sections. It was concluded that during running the proximal IIX and IIB fibers were recruited to a greater extent (and at lower treadmill speeds) compared with the distal IIX and IIB fibers, respectively.

Patterns of muscle activity during different behaviors in chicks: implications for neural control

The large behavioral repertoire that spans the embryonic and postembryonic stages of development make chicks an ideal system for identifying patterns of muscle activity that are common to different behaviors and those that are behavior-specific. The main goal of this work was to identify the similar and dissimilar aspects of the recruitment patterns and the regulation of muscle activity during three distinct postembryonic behaviors: walking, swimming and airstepping. We identified two synergies that were common to each of these behaviors. The synergies were not disrupted by the absence of FT1 activity in airstepping. Within each synergy the recruitment time, recruitment order and duration of activity were not rigid, but varied according to the context-specific resistance that the leg encountered. Unlike the other muscles, FT2 activity was not recruited as part of the same synergy in each behavior. When weight-bearing contact with the substrate did not occur, as in swimming and airstepping, as well as in walking in chicks with deafferented legs, FT2 activity was not recruited as part of either synergy, but was recruited during the time between them. Although not identical, embryonic motility and hatching motor pattern both show the two synergies described for the postembryonic behaviors. Like the latter behaviors, the synergies tolerated the absence of activity from specific muscles. Thus, we suggest that the CNS produces different behaviors using many of the same muscles by organizing the patterned activity around two common synergies while permitting the different muscles that participate in a synergy to be modified in tandem or on an individual basis. Furthermore, the common synergies are established early during prenatal development in chicks.

Motor unit recruitment strategy changes with skill acquisition

The modifications of motor unit recruitment strategy due to skill acquisition was determined in the elbow flexor-extensor muscles of normal human subjects. The median frequency of the power density spectra of the electromyograms recorded from the biceps and triceps muscles during a 3-s linear increase in flexion force in the range of 0-100% maximal voluntary contraction (MVC) was calculated for each subject, every 2 weeks over a total 6-week period during which subjects practiced linear flexion force increase three times a week. Electromyograms were recorded with two pairs of electrodes of different size and electrode spacing. It was shown that skill acquisition due to the 360 practice trials over the 6-week period caused an increase in the initial motor unit recruitment phase of the agonist's force generation cycle from about 0-65% MVC to about 0-85% MVC. The increase in the recruitment range was gradual and statistically significant for the measurements made every 2 weeks. The recruitment range of the antagonist triceps demonstrated a minor, but statistically insignificant, decrease over the same training period. There was a minor, but statistically insignificant, advantage of using small electrodes and inter-electrode spacing. It was concluded that skill acquisition, due to repeated functional use of a muscle in the same contraction mode, results in a slower, prolonged recruitment of motor units in the initial segment of the force generation cycle, thereby allowing a more precise and accurate control of the increments of force increase. Such conclusions reinforce the concept advocating the plasticity of motor unit control according to the functional demands imposed on the muscle. The results have significant implications in the design of various athletic, occupational and rehabilitation training modalities for optimal performance of various movement functions.

Recovery from habituation in Caenorhabditis elegans is dependent on interstimulus interval and not habituation kinetics

The habituation of the tap withdrawal reflex of Caenorhabditis elegans was assessed to determine whether the kinetics of recovery from habituation were dependent on the interstimulus interval (ISI) used during habituation training, or alternately, on the rate and asymptotic level of habituation produced at a given ISI. Two groups of intact animals were trained at either a 10-s (CON10) or a 60-s (CON60) ISI. Laser ablation was used to alter the habituation kinetics in one further group of animals (PLM10), independent of ISI. Although the PLM10 animals trained at a 10-s ISI habituated like CON60 worms, the recovery from habituation of the PLM10 animals very closely resembled the recovery of the CON10 worms. Thus recovery kinetics are dictated by consequences of a given ISI, which do not impact upon habituation rate and asymptote. This suggests the recruitment of multiple ISI-dependent processes during habituation in C. elegans.

Position-selective activation of peripheral nerve fibers with a cuff electrode

The degree of spatial selectivity which can be obtained with longitudinal dot tripoles in an insulating cuff was quantified in terms of the overlap between fiber populations activated by different tripoles. Previous studies have failed to take into account the relative influences of transverse current and longitudinal current on position-selective activation, and furthermore have not controlled for the differing sensitivities of large and small nerve fibers to electrical stimuli. In this study, these factors were taken into account. Transverse current from an anode positioned opposite the stimulating cathode was found to improve spatial selectivity, and selectivity was enhanced when the ratio of transverse current to longitudinal current was increased. Large fibers were excited before small fibers, irrespective of fiber position, indicating a combination of position and size selectivity.

Muscle relaxation in Parkinson's disease: a reaction time study

We tested the hypothesis that the relaxation reaction time in Parkinson's disease (PD) is delayed, as a sign of disorder in the control of voluntary motoneuron derecruitment. We compared, in the triceps brachii muscle, the reaction times (RTs) of the onset (O-RT) of electromyographic (EMG) activity during initiation of a contraction with the RTs of the termination of EMG tonic activity during full relaxation (R-RTs). Fourteen patients with idiopathic PD and 10 normal controls were examined. Mean R-RTs for all controls were 30 ms shorter than mean O-RTs. Mean R-RTs for all patients were approximately 70 ms longer than mean O-RTs. In two untreated patients levodopa therapy improved both O-RT and R-RT, but the difference between the two was unchanged. There was no correlation between EMG level and R-RT or between peak force and O-RT in either controls or patients. O-RT and R-RT were correlated with the bradykinesia score. In some patients, bursts of late activity were recorded after the R-RT; the duration of this activity was correlated with the duration and staging of the disease and with bradykinesia and rigidity scores. The reversed latency of onset and termination of muscle contraction in PD suggests an abnormality in the inhibitory spinal mechanisms, possibly stemming from a defect in the pathways descending to the spinal cord.

Electromyographic activity of the hyoepiglotticus muscle in dogs

We examined the respiratory-related electromyographic (EMG) activity of the hyoepiglotticus muscle using fine wire bipolar electrodes, inserted perorally in five anaesthetised (IV chloralose) tracheostomised dogs studied in the prone, mouth open position. The integrated HE EMG was measured in arbitrary units (a.u.) during resting breathing via the upper airway, and on a breath-by-breath basis during progressive increases in respiratory drive induced by infusion of CO2 into the inspired gas. The HE demonstrated inspiratory activity which increased linearly in relation to ventilation (r = 0.85 +/- 0.06, p < 0.001) due to an increase in both phasic (8.8 +/- 1.8 to 32.4 +/- 9.2 a.u.) and tonic (0.2 +/- 0.1 to 26.3 +/- 13.3 a.u.) activity (both p < 0.05). In addition, HE EMG developed substantial phasic expiratory activity (1.3 +/- 1.1 to 13.8 +/- 4.4 a.u., p < 0.05). We conclude that the canine HE exhibits inspiratory and expiratory related activity which is augmented during increased respiratory drive. These findings imply active control of epiglottic position during breathing in dogs.

Effects of operantly conditioning the amplitude of the P200 peak of the SEP on pain sensitivity and the spinal nociceptive withdrawal reflex in humans

This study attempted to replicate and extend earlier work that reported that the amplitude of the P200 peak of the human somatosensory evoked potential (SEP) can be increased and decreased when reward is made contingent upon change and that these changes are accompanied by alterations in pain sensitivity. Twenty-one subjects were able to make the amplitude of the P200 peak evoked by sural nerve stimulation larger during increased training (up-training) than during decreased training (down-training). There were no differences in the sural nerve compound action potential between up-training and down-training. This finding demonstrates that the change in P200 amplitude was not due to a change in stimulus efficacy, but rather to a change within the central nervous system. Subjective pain ratings and a nociceptive spinal reflex were the same in up-training as in down-training. Thus, conditioned changes in P200 amplitude do not alter pain sensitivity.

Recruitment of triceps surae motor units in the decerebrate cat. II. Heterogeneity among soleus motor units

1. On the basis of the orderly activation of motoneurons in a pool, one would predict that motor unit activity and whole muscle force will change at least roughly in parallel: active motor units should continue to fire as net muscle force increases and quiescent motor units should remain inactive as muscle force decreases. We have consistently observed this relationship in our studies of the medial gastrocnemius (MG) muscle, but here we report an uncoupling of the soleus muscle and some of its motor units. 2. Physiological properties and firing behaviors of 20 soleus motor units were characterized in five decerebrate cats with the use of intra-axonal stimulation and recording. Motor unit firing was elicited in reflexes initiated by muscle stretch, nerve stimulation, and mechanical stimulation of the heel. Particular emphasis was placed on the heterogenic reflexes produced in soleus by ramphold-release stretches of the MG muscle. In agreement with previous reports, either net heterogenic excitation or inhibition of the soleus muscle was produced in separate trials of MG stretch. 3. During excitation of soleus in autogenic stretch reflexes and in crossed-extension reflexes, all 20 units were recruited or increased firing, i.e., unit firing was coupled with soleus force. In the other reflexes, however, unit firing and muscle force were uncoupled for 10 of these units. Six tonically active motor units were inhibited during an increase in soleus force produced by MG stretch or by mechanical stimulation of the heel. Four motor units were activated during a decrease in soleus force produced by the same stimuli. 4. Six motor units were studied during both soleus inhibition and excitation evoked by MG stretches. One motor unit was consistently coupled to the soleus muscle response; firing increased during soleus excitation and decreased during inhibition. However, four soleus motor units were inhibited under both conditions, and one unit was excited under both conditions. Thus the firing behavior of five of these six motor units was the same in response to MG stretch, irrespective of the soleus response. 5. The uncoupling was most clearly recognized when tonically active units ceased firing during net excitation of the soleus muscle and when silent units began firing during net inhibition of the soleus muscle. Unit responses were not as striking in all trials of MG stretch (spike number increased or decreased relative to prestretch values by 1-4 spikes), but the responses were consistent across trials; in multiple stretches, spike number commonly either increased or decreased. Intertrial regularity was also observed in units for which firing was coupled with the net reflex response of the soleus muscle. 6. Divergence in the firing of soleus motor units was also observed in three cases in which records were taken simultaneously from two motor units. In one pair, one unit increased and the other decreased firing during MG stretch-evoked inhibition of soleus. In the other two pairs, one unit increased and the other decreased firing when soleus was excited by heel stimulation. In all pairs, the unit that decreased firing under these conditions had the lowest recruitment threshold in response to the soleus stretch. 7. Although all soleus motor units were classified as slow-twitch (type S), variation in their physiological properties bore some relation to firing behavior. Those units recruited during periods of soleus inhibition exhibited among the fastest conduction velocities and contraction times in our sample. In all three unit pairs sampled, the unit expressing decreases in firing had the slower conduction velocity and contraction time. 8. These findings demonstrate that soleus motor units are differentially activated and deactivated by peripheral afferents. (ABSTRACT TRUNCATED)

Human motor unit activity during post-vibratory and imitative voluntary muscle contractions

Applying mechanical vibration for short periods to a muscle tendon induces long-lasting involuntary contractions which develop soon after the vibration offset in the previously vibrated muscle. In the present study, the question was raised as to whether these post-vibratory motor responses are mediated by the activity of supraspinal neural population or whether they may involve in addition some peripheral facilitatory influences operating at the motoneuronal level. To investigate this question, we analysed the unitary activity of 48 motor units belonging to the wrist extensor radialis muscles of the human hand, after attempting to classify them as slow or fast, during both post-vibratory and voluntary contractions having almost the same amplitudes and time-courses. The motor units were found to be activated in much the same way with both types of contraction. Similarities were observed as regards: the nature of the motor units activated, the order of recruitment of the motor unit population, the motor units' force recruitment thresholds, the mean interspike interval and the standard deviation. These analogies suggest that post-vibratory contraction may mainly involve a supraspinal tonic drive, but the possibility that these involuntary contractions may have a spinal origin cannot be completely ruled out.

On-line control of a speeded motor response

Evidence for a point of no return in a motor act has been reported to occur very late in its preparation and even to the point at which the response is executed. We report a qualitative example from electromyographic (EMG) data of an elbow-extension movement from 1 of 12 adults which suggests that the response can be stopped at any time up to its production, from which we conclude that the response is subject to on-line control at all times.

Recruitment order and dendritic morphology of rat phrenic motoneurons

The detailed morphology of rat phrenic motoneurons (PMs) was studied in 40 electrophysiologically identified cells with intracellular injection of Neurobiotin. In 15 cells, the dendritic trees were fully analyzed by using path-distance analysis, and total surface area and volume were estimated. Based on their relative onset times (ROT; i.e., the time of firing onset relative to the onset of whole phrenic activity), PMs were classified into three types; early recruited (type E; ROT < 10%), late recruited (type L; ROT > 12.5%), and quiescent (type Q; not recruited under normal conditions). Dendrites constituted 93.3% of the surface area of cells and 38.9% of the cell volumes. The number of primary dendrites (nPD) averaged 10.1, and the mean number of terminations was 38.8. The combined diameters of primary dendrites of PMs correlated well with the total dendritic surface area and the number of dendritic terminations. Comparisons among cell types revealed that type Q cells had greater dendritic surface areas and volumes than type E or type L cells. With path-distance analysis, this difference was found to be due to differences between the cell types in the numbers of their dendrites, their combined dendritic lengths, and the number of their branches. The differences between these data and those available for cat motoneurons are discussed. The input resistance of PMs correlated with their total surface area but did not correlate with their somal surface area, indicating that, in rat, PM input resistance is a function of the entire neuronal membrane rather than of the somal surface alone.

Non-invasive measurement of the input-output properties of peripheral nerve stimulating electrodes

A non-invasive method was developed to determine the input-output (I/O) properties of peripheral nerve stimulating electrodes. An apparatus was fabricated to measure the 3-dimensional (3-D) isometric torque generated at the cat ankle joint by electrical activation of the sciatic nerve. The performance of the apparatus was quantified, and the utility of the method was demonstrated by measuring the recruitment properties of multiple contact nerve cuff electrodes. Torque-twitch waveforms, recruitment curves of peak torque as a function of stimulus current amplitude, and 2-D joint torque vectors were used to analyze the recruitment properties of the cuff. The peak of the twitch torque was an accurate measure of excitation even for muscles having fibers with varying speeds of contraction. The evoked twitch waveforms and torque vectors generated by selective stimulation of individual nerve branches with a hook electrode were compared to those produced by stimulation of the nerve trunk with the cuff electrode. These data allowed determination of the regions of the nerve trunk that were activated by different electrode geometries and stimulus parameters. The positional stability of electrode recruitment properties could be quantified by measuring I/O characteristics at different limb positions. The methods described are useful for characterization of neural stimulating electrodes and for studies of motor system physiology.

Pure stimulus-sensitive truncal myoclonus of propriospinal origin

The clinical and electrophysiological features of stimulus-sensitive truncal myoclonus are described in a 49-year-old woman. Touching the skin of the back and abdomen would evoke jerks in both ipsilateral and contralateral axial muscles; there was no spontaneous jerking. Multichannel EMG recordings showed bilateral short-latency muscle bursts at truncal recording sites both rostral and caudal to stimulus sites. The short latencies of muscle bursts in adjacent segments give evidence of a spinal origin of myoclonus; the pattern of recruitment and the velocity of spread suggest the involvement of propriospinal pathways. The presence of intrathecal IgG synthesis and of oligoclonal bands in the CSF point towards an inflammatory process which may underly the unusual type of myoclonus in this patient.

Synaptic recruitment during long-term potentiation at synapses of the medial perforant pathway in the dentate gyrus of the rat brain

Long-term potentiation (LTP) in synapses of the medial perforant pathway of the rat dentate gyrus has been studied using the whole-cell voltage clamp technique and a standard hippocampal slice preparation. The rate of LTP induction by 2-4 brief trains of stimuli at 100 Hz, paired with postsynaptic depolarization to -20 mV, in individual granule neurons was only 42% but the average magnitude was large. In a representative series of nine experiments the average potentiation was 339% (s.d. 255%). The variable magnitude of LTP appeared to be related to the relative size of the NMDA receptor dependent current in individual neurons. LTP was further characterized by the selective enhancement of the AMPA (but not the NMDA) component in the excitatory synaptic responses. This selective enhancement of the AMPA component and a graphical variance analysis suggest that the large magnitude of LTP in dentate gyrus can be best explained by recruitment of previously silent synapses by a combination of pre- and post-synaptic mechanisms.

The kindling-activated neuronal network: recruitment of somatostatin-synthesizing neurons

The present study demonstrates the anatomical extent of the kindling-activated neuronal network in general, and specifically the recruitment of extrahippocampal somatostatin (SST)-synthesizing neurons into this network. It has been known that SST neurons of the hippocampal formation are activated during episodes of seizure, however, it was not known if this activation was a local event or extended to other areas in the brain. We were therefore interested in determining if and which SST neurons outside the hippocampal formation might be recruited into this seizure-activated neuronal network. Using the kindling model of seizure elicitation, expression of the Fos protein in activated, depolarized neurons was utilized to identify seizure-activated neurons. Subsequently, the mRNA for SST was identified through in situ hybridization in the same tissue section, allowing the identification of seizure-activated, SST-synthesizing neurons. The results show that: (a) the majority of SST-synthesizing neurons in the forebrain and diencephalon became activated during the kindling development; (b) their recruitment into the kindling-activated neuronal network occurred progressively; and, (c) these SST-synthesizing neurons represented a component of the kindling-activated neuronal network throughout the development of kindling-induced seizures.

Activity of single motor units in attention-demanding tasks: firing pattern in the human trapezius muscle

Activity of single motor units in relation to surface electromyography (EMG) was studied in 11 subjects in attention-demanding work tasks with minimal requirement of movement. In 53 verified firing periods, single motor units fired continuously from 30 s to 10 min (duration of the experiment work task) with a stable median firing rate in the range of 8-13 Hz. When the integrated surface EMG were stable, the motor units identified as a rule were continuously active with only small modulations of firing rate corresponding to low-amplitude fluctuations in surface EMG. Marked changes in the surface EMG, either sudden or gradual, were caused by recruitment or derecruitment of motor units, and not by modulations of the motor unit firing rate. Motor unit firing periods (duration 10 s-35 s) in low-level voluntary contractions (approximately 1%-5% EMGmax) performed by the same subjects showed median firing rates (7-12 Hz) similar to the observations in attention-related activation.

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. I. Motor-unit recruitment

1. The recruitment order of 64 pairs of motor units, comprising 21 type-identified units, was studied during centrally evoked muscle contractions of the cat medial gastrocnemius (MG) muscle in an unanesthetized, high decerebrate preparation. Motor units were functionally isolated within the MG nerve by intra-axonal (or intramyelin) penetration with conventional glass microelectrodes. 2. Graded stimulation of the mesencephalic locomotor region (MLR) was used to evoke smoothly graded contractions, which under favorable conditions was estimated to reach 40% of maximum tetanic tension of the MG muscle. With this method of activation, 100% of slow twitch (type S) units, 95% of fast twitch, fatigue-resistant (type FR) units, 86% of fast twitch, fatigue-intermediate (type FI) units, and 49% of fast twitch, fatigable (type FF) units studied were recruited. 3. Motoneuron size as estimated by axonal conduction velocity (CV) was correlated with muscle-unit size as estimated by maximum tetanic tension (Po). Although the correlation between these properties was significant among type S and FR units, no significant correlation was found for these properties among type FI and FF units. 4. Motor-unit recruitment was ordered by physiological type (S > F, 100% of pairs; S > FR > FI > FF, 93% of pairs). Although none of the motor-unit properties studied predicted recruitment order perfectly, motor-unit recruitment was found to proceed by increasing Po (89% of pairs), decreasing contraction time (79% of pairs), decreasing fatigue index (80% of pairs), and increasing CV (76% of pairs). These percentages were significantly different from random (i.e., 50%). Statistically, all four motor-unit properties were equivalent in predicting recruitment order. These results are similar to those reported by other investigators for motor-unit recruitment order evoked from other supraspinal centers, as well as from peripheral sites. 5. When, however, motor-unit recruitment within pairs of motor units containing two fast-twitch (type F) units was examined, Po was a significantly better predictor of recruitment order than CV (85% vs. 52% of pairs). One explanation for this observation is that the correlation between Po and CV is high among type S, type FR units, and possibly among the lower-tension type FF units, but not among the remaining higher-tension type FF units. 6. The reproducibility of recruitment order in multiple contractions was investigated in 16 motor-unit pairs. Recruitment order was found to be variable in only three motor-unit pairs, all of which contained units of similar physiological type and recruitment threshold. 7. Analysis of recruitment order by pair-wise testing confirms the general conclusion reached in human studies that the muscle force level at recruitment for a motor unit is highly correlated with its strength. As an additional confirmation, the whole-muscle force level at recruitment for 41 units was measured in a series of contractions in which the rate of rise of muscle tension was limited to rates < 1,000 g/s. For these contractions, a significant correlation was found between muscle tension at recruitment and motor-unit Po.

One-trial adaptation of movement to changes in load

1. We analyzed the rapid adaptation of elbow movement to unexpected changes in external load conditions at the elbow joint. The experimental approach was based on the lambda model, which defines control variables (CVs) setting the positional frames of reference for recruitment of flexor and extensor motoneurons. CVs may be specified by the nervous system independently of the current values of output variable such as electromyographic (EMG) activity, muscle torques, and kinematics. The CV R specifies the referent joint angle (R) at which the transition of flexor to extensor activity or vice versa can be observed during changes in the actual joint angle, theta, elicited by an external force. The other CV, the coactivation (C) command, instead of a single transition angle, defines an angular range in which flexor and extensor muscles may be simultaneously active (if C > 0) or silent (if C < 0). Changes in the R command result in shifts in the equilibrium state of the system, a dynamic process leading to EMG modifications resulting in movement or isometric force production if movement is obstructed. Fast movements are likely produced by combining the R command with a positive C command, which provides movement stability and effective energy dissipation, diminishing oscillations at the end of movement. 2. According to the model, changes in the load characteristic (e.g., from a 0 to a springlike load) influence the system's equilibrium state, leading to a positional error. This error may be corrected by a secondary movement produced by additional changes in R and C commands. In subsequent trials, the system may reproduce the CVs specified after correction in the previous trial. This behavior is called the recurrent strategy. It allows the system to adapt to the new load condition in the subsequent trials without corrections (1-trial adaptation). Alternatively, the system may reproduce the CVs specified before correction (invariant strategy). If the movement was perturbed only in a single trial, the invariant strategy allows the system to reach the target in the subsequent trials without corrections. 3. To test the assumption on the dominant role of the recurrent strategy in rapid adaptation of movement to new load conditions, we performed experiments in which subjects (n = 6) used a pivoting manipulandum and made fast 60 degrees movements to a target. After a random number of trials (5-10) with no load, we introduced opposing (experiment 1), assisting (experiment 2), or randomly varied opposing or assisting loads (experiment 3) for 5-10 trials before unexpectedly switching loads again (14-18 switches in total). The opposing or assisting torque was created by position feedback to a torque motor and was a linear function of the displacement of the manipulandum form the initial position (springlike load). Subjects were instructed to correct positional errors as soon as possible to reach the target. The EMG activity of two elbow flexors (biceps brachii and brachioradialis) and two elbow extensors (triceps brachii and anconeus), elbow position, velocity, and torque were recorded. Kinematic and EMG patterns were compared with those obtained in similar experiments in which subjects were instructed not to correct errors. 4. In 94% of the trials in which a change in the load occurred, the primary movement was in error and was followed by a corrective secondary movement. In primary movements, both the phasic and tonic levels of EMG activity as well as the kinematics were load dependent, implicating reflex and intramuscular mechanisms in the adaptation of muscle forces counteracting external loads. These mechanisms, however, were not sufficient to eliminate positional errors. 5. An undershoot error occurred in trials with an opposing load after those with no load or in trials with no load after those with an assisting load. After adaptation to a new load condition, a sudden return to the previous load condition resulted in an error of the oppo

Activation of type-identified motor units during centrally evoked contractions in the cat medial gastrocnemius muscle. III. Muscle-unit force modulation

1. The aim of this study was to examine the extent of muscle-unit force modulation due to motoneuron firing-rate variation in type-identified motor units of the cat medial gastrocnemius (MG) muscle, and to investigate the contribution of muscle-unit force modulation to whole-muscle force regulation. The motoneuron discharge patterns recorded from 8 pairs of motor units during 12 smoothly graded muscle contractions evoked by stimulation of the mesencephalic locomotor region (MLR) were used to reactivate those units in isolation to estimate what their force profiles would have been like during the evoked whole-muscle contractions. 2. For most motor units, muscle-unit force modulation was similar to motoneuron firing-rate modulation, in that muscle-unit force increased over a limited range (120-600 g) of increasing whole-muscle tension and was then maintained at a near maximal (> 70%) output level as muscle force continued to rise. Most muscle units also decreased their force outputs over a slightly larger range of declining whole-muscle force before relaxing. This second finding was best explained by the counterclockwise hysteresis recorded in the motor units' frequency-tension (f-t) relationships. 3. In those instances when whole-muscle force fluctuated just above the recruitment threshold of a motor unit, a substantial percentage (10-25%) of the change in whole-muscle force could be accounted for by force modulation in that motor unit alone. This finding suggested that few motor units in the pool were simultaneously simultaneously undergoing force modulation. To evaluate this possibility, the extent of parallel muscle-unit force modulation within the 8 pairs of simultaneously active motor units was evaluated. As with parallel motoneuron firing-rate modulation, the extent of parallel muscle-unit force modulation was limited to unit pairs of the same physiological type and recruitment threshold. In several instances, pairs of motor units displayed parallel motoneuron firing-rate modulation but did not show parallel muscle-unit force modulation because of the nature of the motor units' f-t relationships. 4. The limited extent of parallel muscle-unit force modulation seen in these experiments implies that the major strategy for force modulation in the cat MG muscle, involving contractions estimated to reach 30-40% of maximum, may be motor-unit recruitment rather than motor-unit firing-rate variation with resulting force modulation. Given, however, that the majority of motor units are already recruited at these output levels (< 40%), it is proposed that motor-unit firing-rate variation with resulting force modulation may take over as the major muscle force modulating strategy at higher output levels.

H-reflexes in masseter and temporalis muscles in man

In contrast with limb muscles, studies on H-reflexes in the trigeminal system are scarce. The present report aimed at reevaluating the responses obtained in the masseter and temporalis muscles after electrical stimulation of their nerves. Twenty-four subjects participated in the experiments. The reflexes were elicited in the masseter and temporal muscles by monopolar stimulation and recorded using surface electrodes. Stimulation of the masseteric nerve evoked an M-response in the masseter and an H-reflex in both the masseter and the temporal muscles. In contrast with the masseter muscle, where the homonymous H-reflex disappeared at higher stimulation intensities, the heteronymous temporal H-reflex remained and reached a plateau. Simultaneous stimulation of the masseteric and deep temporal nerves resulted in an M-response and an H-reflex in both the masseter and temporal muscles. Increasing stimulus intensitites led to disappearance of the H-reflex in both muscles. The results were compared with those obtained by others on limb muscles. As in these muscles, the presence of heteronymous H-reflexes in the jaw muscles can be used in future studies of motoneuronal excitability.

Changes in the hemiparetic limb with training. II. EMG signal

Following a cerebro-vascular accident, motor deficits are usually associated with a selective atrophy of fast fatiguable muscle fibers. The reduction of output torque in spastic patients is considered to be caused by unfused twitches resulting from the reduction in the motor unit firing rate. To regain control of a paretic limb, these patients have to learn, through training, how to recruit an adequate number of intact motor units to generate the required functional movement. In hemiplegia, intra-subject variability makes difficult the assessment of the effects of any therapy treatments. Our objective here was thus to address the problem of the data variability and to analyze the electromyographic signal obtained from hemiplegic patients under biofeedback training. The experimental situation chosen was a very isolated but controlled motor activity (an isometric shoulder forward flexion). The results show that after training, 5 out of 11 patients could produce EMG signal from their affected deltoid whose mean value was 50% greater than before the training, while no appreciable change was observed for their unaffected deltoid. While these EMG changes on the HEMI side are correlated with greater torque output, the coefficient of correlation is slightly smaller after than before the training (r = 0.87 vs 0.89). It can be hypothesized that while the training was directed toward the deltoid, the patients in some ways developed new strategies for contracting their shoulder.

Recruitment of GABAA inhibition in rat neocortex is limited and not NMDA dependent

1. The recruitment of evoked fast inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs) was examined using whole cell voltage-clamp recordings from layer V pyramidal neurons in slices of rat somatosensory cortex. Synaptic currents were evoked with graded electrical stimulation to assess the relative activation of IPSCs and EPSCs. Fast GABAA ergic IPSCs were selectively recorded by holding cells at potentials equal to EPSC reversal (approximately 0 mV). EPSCs were likewise isolated by holding cells at IPSC reversal potential (about -75 mV). 2. As stimulus intensities were increased, the magnitude of the postsynaptic currents also increased. Over the range of stimuli applied (2-10 V), EPSCs did not exhibit an upper limit. However, fast gamma-aminobutyric acid-A (GABAA-mediated IPSCs reached a maximum at intensities approximately 2 times threshold. 3. The limit on fast inhibition was unresponsive to alterations in N-methyl-D-aspartate (NMDA)-mediated excitation. Exposure to nominally magnesium-free solutions or to the NMDA antagonist 3-[(RS)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid did not affect the fast IPSC maximum. Shifts in the input-output curves for submaximal activation of IPSCs were seen, which were attributed to polysynaptic excitation. 4. Blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (non-NMDA) receptors with 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) completely abolished synaptically driven, fast GABAA-mediated inhibition. These findings suggested that neocortical inhibitory cells could be driven exclusively through non-NMDA transmission. 5. By comparison, in hippocampal CA1 pyramidal neurons maximal fast inhibition was sensitive to both NMDA and non-NMDA receptor blockade. 6. The results in neocortex were corroborated by direct intracellular recordings from layer V-VI interneurons. Non-NMDA receptor blockade with CNQX prevented synaptic activation of action potentials in these cells, even during cotreatment with magnesium-free solution. 7. Together, these results suggest that recruitment of GABA(A) ergic IPSCs in neocortex is ultimately driven via glutamatergic afferents arriving at non-NMDA receptors on interneurons. Properties limiting fast inhibition would favor the propagation of enhanced excitatory activity through the neuronal network.

Some effects of vagal blockade on abdominal muscle activation and shortening in awake dogs

1. The mechanisms of abdominal muscle activation are thought to be different during expiratory threshold loading (ETL) compared with hypercapnia. Our objectives in the present study were to determine the effects of removing excitatory vagal feedback on abdominal muscle activation, shortening and pattern of recruitment during ETL and hypercapnia. Six tracheotomized dogs were chronically implanted with sonomicrometer transducers and fine wire EMG electrodes in each of the four abdominal muscles. Muscle length changes and EMG activity were studied in the awake dog during ETL (6 dogs) and CO2 rebreathing (3 dogs), before and after vagal blockade. 2. Following vagal blockade, the change in volume (increase in functional residual capacity, FRC) during ETL was greater and active phasic shortening of all the abdominal muscles was reduced, when shortening was compared with a similar change in lung volume. Similarly, at comparable minute ventilation, abdominal muscle active shortening was also reduced during hypercapnia. The internal muscle layer was recruited preferentially in both control and vagally blocked dogs during both ETL and hypercapnia. 3. The degree of recruitment of the abdominal muscles during ETL and hypercapnia in awake dogs is influenced by vagal feedback, but less so than in anaesthetized dogs. These results illustrate the importance of the vagi and abdominal muscle activation in load compensation. However, vagal reflexes are apparently not contributing to the preferential recruitment of the internal muscle layer. In awake dogs during vagal blockade abdominal muscle recruitment still occurs by extravagal mechanisms.

Characteristics of motor unit discharge in subjects with hemiparesis

We studied the discharge rates and recruitment characteristics of single motor units in paretic and contralateral arm muscles of 6 hemiparetic subjects. Motor unit activity in biceps brachii was recorded at different elbow torques, and the activity related both to the mean level of surface electromyographic activity, and to the degree of weakness. In 3 of the 6 subjects, there were significant reductions in mean discharge rate of motor units in the paretic muscle. All 6 subjects showed compression of the range of motoneuron recruitment forces, and a failure to increase motor unit discharge rate during voluntary force increases in paretic muscles. These rate reductions could potentially alter the precise match of motoneuron properties to the mechanical properties of the innervated muscle fibers, and reduce the efficiency of muscle contraction. This reduction could lead, in turn to increased effort, to fatigue, and ultimately to a sense of weakness for voluntary force generation.

Functional differentiation within latissimus dorsi

"Functional differentiation" within skeletal muscle refers to the ability of the Central Nervous System (CNS) to control, with a degree of independence, individual subunits of a muscle during a particular muscle contraction. Essentially, the concept of functional differentiation within skeletal muscle suggests an ability of the CNS to selectively activate those segments of a muscle which have the most appropriate line of action for the task as a means of ensuring the muscles efficient utilisation. The present study was undertaken to determine whether functional differentiation was present within the radiate muscle, Latissimus Dorsi, during a series of isometric muscle contractions. Six surface electrode pairs were placed along the origin of the muscle and electromyographic (EMG) potentials were recorded during isometric contractions in various planes and levels of contraction intensity. The EMG waveforms were integrated (IEMG) and then normalised. The normalised EMG waveforms (nIEMG) were then statistically compared to determine if muscle fibres within the detection area of each electrode pair had varied their contribution to the total activity of the muscle, from one isometric contraction to another. Alterations in the contribution of each site were taken to be indicative of functional differentiation. The results of this study indicated that functional differentiation did occur within Latissimus Dorsi. In essence, the most caudal fibres were utilised preferentially when subjects performed adduction from an abducted position of the shoulder joint. Conversely, activity was distributed over the entire muscle when subjects attempted adduction from the anatomical position. A similar situation was observed when subjects performed horizontal extension from either a flexed or abducted position.(ABSTRACT TRUNCATED AT 250 WORDS)

The cricothyroid muscle does not influence vocal fold position in laryngeal paralysis

The status of the cricothyroid muscle, which is innervated by the superior laryngeal nerve, is believed to influence the vocal fold position in laryngeal paralysis. It is believed that isolated lesions of the recurrent laryngeal nerve generally result in the paralyzed vocal fold assuming a paramedian position but that with lesions of both the superior and recurrent laryngeal nerves, a more lateral (intermediate or cadaveric) vocal fold position can be expected. Twenty-six consecutive patients with unilateral vocal fold paralysis underwent transnasal fiberoptic laryngoscopy (TFL) and laryngeal electromyography (LEMG). By TFL, the vocal fold positions were paramedian in 8 patients, intermediate in 7, and lateral in 11. By LEMG, 13 patients had isolated recurrent laryngeal nerve lesions and 13 patients had combined (superior and recurrent laryngeal nerve) lesions. There was no correlation between the vocal fold position and the status of the cricothyroid muscle, i.e., the status of the cricothyroid muscle by LEMG did not predict the vocal fold position nor did the vocal fold position by TFL predict the site of lesion. In addition, we investigated the possibility that the degree of thyroarytenoid muscle recruitment (tone) might correlate with vocal fold position, but no relation was found. We conclude that 1. the cricothyroid muscle does not predictably influence the position of the vocal fold in unilateral paralysis; 2. thyroarytenoid muscle recruitment (tone) does not appear to influence vocal fold position; and 3. still unidentified and unknown factors may be responsible for determining vocal fold position in laryngeal paralysis.

The influence of premotor interneuron populations on the frequency of the spinal pattern generator for swimming in Xenopus embryos: a simulation study

Our aim was to test the hypothesis that the frequency of neuronal rhythm-generating networks is partly controlled by the size of the active premotor interneuron population. We have tested possible mechanisms for frequency changes in a population model of the Xenopus laevis embryo spinal rhythm-generating networks for swimming. After initiation by a brief sensory excitation, the frequency of swimming activity decreases to a steady level determined by the properties of the 24 interneurons and their connections. The initial frequency decrease was dependent on the time-course of initiating sensory synaptic excitation. When some premotor excitatory interneurons were given weaker synaptic connections to reflect the variability in the spinal cord, they could drop out and stop firing during the initial frequency decrease while swimming activity continued. If the synaptic input of such weak excitatory interneurons was graded finely, they could drop out consecutively. This led to further decreases in the level of tonic excitation and in network frequency which depended on the number, type and distribution of excitatory interneurons that stopped firing. Silent weak excitatory interneurons could be recruited by a second sensory excitation and cause an increase in tonic depolarization and frequency which outlasted the sensory input. Such recruitment could occur on both sides after local sensory stimulation to only one region or one side of the model. We conclude that these computer simulations support the hypothesis that premotor interneuron drop-out and recruitment is one mechanism which can control frequency in a locomotor central pattern generator.

Sensitivity of nucleus accumbens neurons in vivo to intoxicating doses of ethanol

The nucleus accumbens septi (NAcc) is considered an important component of the final common pathway involved in the reinforcing properties of ethanol. We studied the effects of intraperitoneal administration of ethanol on spontaneous, glutamate-activated, and fimbria-activated NAcc neurons in acute anesthetized and freely moving unanesthetized rats. Ethanol significantly reduced the firing rate of spontaneous and glutamate-activated NAcc neurons in both electrophysiological preparations. Stimulation of the ipsilateral fimbria evoked single-unit activity in NAcc neurons with two characteristic latencies (early, 7.21 +/- 0.74 msec; late, 18.24 +/- 0.66 msec). Intoxicating doses of ethanol inhibited the recruitment of late, but not of early, fimbria-activated NAcc neurons. These data demonstrate electrophysiological evidence for the existence of neurons in the core region of the NAcc that are sensitive and insensitive to acute systemic ethanol administration.

Nonlinear behaviour of human motoneurons

When ramp-and-hold currents are injected into a motoneuron of an anesthetized cat, the motoneuron responds with a high initial firing rate (dynamic phase), which then adapts to a lower steady-state firing rate. The firing rates during the dynamic and the steady-state phases are linearly related to the rate of change and the magnitude of the injected current, respectively. In human subjects, where inputs to the motoneurons are not accessible, force parameters are used to describe motoneuron behaviour. Population responses of human motoneurons, measured in terms of gross electromyographic (EMG) activity, increase linearly with the magnitude and the rate of change of force. No study has attempted to examine the question of linearity of single motor units during the dynamic as well as the steady-state phases. The following study recorded single motor unit and EMG activities simultaneously from the flexor carpi radialis muscle in human subjects completing ramp-and-hold force trajectories. Although the results confirmed the linear relationship between EMG activities and the rate and magnitude of the force, a nonlinear activity pattern was observed between the single motor unit firing and the force parameters, suggesting that recruitment must be responsible for the linear behaviour of EMG activity. Comparisons of different background activity levels on the firing patterns of a given motor unit, as well as comparisons of two simultaneously recorded units, further supported nonlinear response patterns of single motor units.

Laryngeal function in postpolio patients

Of the 250,000 survivors of the polio epidemics, approximately 25% experience progressive muscle weakness known as postpolio syndrome (PPS). Laryngeal function in postpolio patients previously has not been studied. This paper presents data detailing laryngeal function in a group of postpolio patients who had been evaluated for swallowing complaints. Nine patients underwent comprehensive history and physical exam, acoustical voice analysis, and laryngeal videostroboscopic endoscopy. Three patients underwent laryngeal electromyography (EMG) evaluation. Results indicated some degree of phonatory or laryngeal deficit in all subjects. Subjects with dysphagia also demonstrated vocal fold paralysis. EMG revealed decreased recruitment and increased amplitude, findings consistent with EMG studies in skeletal muscle in postpolio patients. Results suggest that postpolio patients who complain of swallowing difficulties are at risk for laryngeal pathology.

Head movement trajectory in three-dimensional space during orienting behavior toward visual targets in rhesus monkeys

Head movement trajectories in three-dimensional space were studied in two monkeys with their heads free during natural and spontaneous orienting behavior toward objects of interest displayed in a horizontal plane. The main interest of this study lies in understanding the process responsible for behavioral variability during the execution of head movements, with special reference to "units of movement." The head movements were recorded by an optoelectronic movement analyzer working with passive markers. Algorithms have been designed to reconstruct the three-dimensional trajectories of the center of gravity of the head. Simultaneously, electromyographic activity in the four pairs of suboccipital muscles was studied. A quantitative evaluation of the involvement of the head in orienting behavior toward visual targets shows that the gaze shift is always produced by eye movements in combination with head movements, even with target eccentricities of less than 10 degrees. On the basis of 80 trials performed by the two monkeys, head trajectories and recruitment patterns of the four pairs of suboccipital muscles have been analyzed. We have been able to identify four elementary kinematic units which can be described as a rightward or leftward turning associated with a contralateral or ipsilateral bending. Each of these four elementary units are underlain by a precise fixed recruitment pattern in the four pairs of suboccipital muscles. These four sets of motor strategies can be combined in order to offer a certain amount of plasticity from which the animal builds its own head trajectory.

Excitability of the soleus motoneuron pool revealed by the developmental slope of the H-reflex as reflex gain

The excitability of a motoneuron (MN) pool was evaluated by the developmental slope of H-reflexes (Hslp) evoked at a range of a stimulus intensity less than the threshold of an M-response. The Hslp has been regarded as the "reflex gain", which is the changing rate in MN excitability as a function of the increase of Ia input to an MN pool. In a comparison of two parameters used in the H-reflex technique, such as the ratio of the maximal H-reflex to the maximal M-response and the ratio of the threshold of an H-reflex to that of an M-response, the Hslp was predicted to be a reasonable parameter to evaluate motoneuronal excitability, because the Hslp is free from the effect of any collision between the H-reflex discharge and the antidromic volley drived from the occurrence of an M-response within the alpha-efferents, and the Hslp can estimate the recruitment properties of a whole MN pool. The Hslp was alleviated during dorsi-flexion and steepened during plantar-flexion, according to the inhibitory or facilitatory synaptic modifications onto soleus MNs. The developmental slope of an M-response (Mslp), which shows the recruitment property of axons of soleus MNs, was alleviated especially in plantar-flexion. In order to exclude the peripheral neuromuscular factors in evaluating substantial MN excitability, the Hslp/Mslp is proposed as a more effective parameter than the Hslp.

Needle electromyographic evaluation of the diaphragm

Needle electromyography of the diaphragm is a useful tool in diagnosis and prognosis of patients with diaphragmatic dysfunction. Spontaneous activity, polyphasic motor units and decreased recruitment can be found in phrenic nerve and spinal cord injury. We describe a safe technique for studying the diaphragm using needle electrodes.

Alterations in synaptic input to motoneurons during partial spinal cord injury

An acute animal model (dorsal hemisection of the spinal cord in the decerebrate cat preparation) has been developed that closely mimics the spasticity in humans that occurs subsequent to partial spinal cord injury and hemiparetic stroke. In this animal model, there are severe disruptions in the pattern of recruitment and rate modulation of motoneurons. The cellular mechanisms of these deficits are being studied with a combined experimental/computer simulation approach. The initial studies indicate that changes in the intrinsic properties of motoneurons are not important, which means the mechanism for changes in recruitment and rate patterns must reside in an alteration in the organization of the synaptic input to motoneurons. Computer simulation studies of the effects of different synaptic inputs on motoneuron outputs show that inhibitory inputs can, under certain conditions, generate substantial disruptions in recruitment and rate modulation. Recent data indicate that the monoamines noradrenalin and serotonin, which are released by fiber tracts originating in the brainstem, may play an important role in maintaining normal levels of inhibition in spinal circuits. Pharmacological interventions based on the monoamines may provide effective means of reducing the deficits in recruitment and rate modulation.

Independence of soleus H-reflex tests in control and spastic subjects shown by principal components analysis

Different soleus H-reflex tests are used in the study of neurophysiological mechanisms of motor control. We studied the interdependence pattern of a number of soleus H-reflex tests, i.e., vibratory inhibition, the ratio of the reflex response to direct muscle potential (H/M ratio) and the homonymous recovery curve with a principal components analysis in 48 healthy controls and 38 patients with signs of the upper motoneuron syndrome. In controls, the analysis showed 3 independent principal components (PCs). Vibratory inhibition and H/M ratio loaded on separate components. Late facilitation and late inhibition variables of the recovery curve loaded on the third component due to the positive correlation (P < 0.001) between these variables. In spastic patients the analysis identified 4 independent PCs corresponding with vibratory inhibition, H/M ratio, late facilitation and late inhibition variables, respectively. The findings suggest that the mutual independence of the different soleus H-reflex tests in patients with the upper motoneuron syndrome has retained the control situation to a large extent.

Effect of neck posture on the activation of feline neck muscles during voluntary head turns

1. To determine whether neck posture affects the usage of neck muscles during a specific motor task, we recorded the electromyographic (EMG) patterns of neck muscles in four cats, which made horizontal, head-turning movements to fixate eccentrically placed targets. In some trials, the cervical column was oriented vertically whereas in other trials, the cervical column was oriented more horizontally. 2. During horizontal head movements, five muscles (obliquus capitis inferior, splenius, levator scapulae, complexus, and biventer cervicis) displayed activation patterns that were consistent from cat to cat and did not change when the cats adopted a different neck posture. Most of these muscles are large, superficial muscles that attach to the skull and span many cervical joints. 3. Posturally dependent patterns of activation were observed in five other neck muscles (semispinalis cervicis, longissimus capitis, levator scapulae ventralis, scalenus anterior, and obliquus capitis superior). Most of these muscles lie deeper and more laterally within the neck musculature and generally span fewer cervical joints than the muscles that displayed invariant patterns of activation. 4. These results suggest that the set of invariantly activated muscles may compose part of a basic motor program that is triggered during head movements in the horizontal plane. This motor program appears to be modified by the selective activation of ancillary muscles, which are recruited in a manner related to the neck posture. The deep positioning of the ancillary muscles may permit them to regulate the mobility of the cervical column and to adjust the net muscular force applied across the neck to the skull. Organizing the motor output in this manner might simplify the task of computing the appropriate patterns of neck-muscle activation.

Recruitment of a projection neuron determines gastric mill motor pattern selection in the stomatogastric nervous system of the crab, Cancer borealis

1. In the isolated stomatogastric nervous system of the crab Cancer borealis (Fig. 1), the muscarinic agonist oxotremorine elicits several distinct gastric mill motor patterns from neurons in the stomatogastric ganglion (STG; Fig. 2). Selection of a particular gastric mill rhythm is determined by activation of distinct projection neurons that influence gastric mill neurons within the STG. In this paper we identify one such neuron, called commissural projection neuron 2 (CPN2), whose rhythmic activity is integral in producing one form of the gastric mill rhythm. 2. There is a CPN2 soma and neuropilar arborization in each commissural ganglion (CoG). The CPN2 axon projects through the superior esophageal nerve (son) and the stomatogastric nerve (stn) to influence neurons in the STG (Figs. 3 and 4A). 3. CPN2 activity influences most of the gastric mill neurons in the STG. Specifically, CPN2 excites gastric mill neurons GM and LG (gastric mill and lateral gastric, respectively) and inhibits the dorsal gastric (DG), anterior median (AM), medial gastric (MG), and inferior cardiac (IC) neurons (Figs. 5 and 6). CPN2 also indirectly inhibits gastric mill neurons Int1 and VD (interneuron 1 and ventricular dilator neuron, respectively) through its activation of LG. The CPN2 excitatory effects are mediated at least partly via discrete excitatory postsynaptic potentials (EPSPs; Fig. 4B), whereas its inhibitory effects are produced via smooth hyperpolarizations. 4. Within the CoG, CPN2 receives excitatory synaptic input from the anterior gastric receptor neuron (AGR), a gastric mill proprioceptive sensory neuron (Fig. 7) and inhibitory synaptic input from the gastric mill interneuron, Int1 (Fig. 8). 5. During one form of the gastric mill rhythm, CPN2 fires rhythmically in time with the gastric mill motor pattern, whereas it is silent or fires weakly during other gastric mill rhythms (Fig. 9). 6. When CPN2 rhythmic activity is suppressed during a CPN2-influenced gastric mill rhythm, the gastric mill rhythm continues, but the pattern is altered (Fig. 10). Moreover, transiently stimulating CPN2 during any ongoing gastric mill motor pattern can reset the timing of that rhythm (Fig. 11). 7. Tonic activity in CPN2 is insufficient to elicit a gastric mill rhythm (Fig. 12). Phasic activity in CPN2 can elicit a gastric mill rhythm only in preparations in which gastric mill neurons are already in an excited state (Figs. 12 and 13). 8. CPN2 recruitment plays a pivotal role in determining the final form of the gastric mill rhythm.(ABSTRACT TRUNCATED AT 400 WORDS)

Motor mechanisms in schizophrenic patients with tardive dyskinesia

A specific aspect of the motor output of patients with tardive dyskinesia was investigated with an experimental procedure referenced in force control function (i.e., the maintenance of sustained force output). We hypothesized that response output (button press) at a given level would be significantly altered when preceded by an output of a higher force than when preceded by an output of a lower force. The difference could be interpreted as a behavioral measure of fatigability and/or threshold change, and its quantitative aspects could serve as a marker of motor impairment. Three groups of subjects participated in the study: schizophrenic patients with tardive dyskinesia (TD), schizophrenic patients without tardive dyskinesia, and normal control subjects. The test used was a target-matching task with force output ranging from 5 to 560 cN. Three dependent measures were used: output force, steadiness of force maintenance, and latency of target capture. Results indicated that precedent output at the previous trial significantly altered force, but even better group separation was attained with the measure of steadiness. The TD schizophrenic group's performance was least efficient, the control group's was most efficient, and the non-TD schizophrenic group's performance was intermediate. The principal conclusions reached were that the target-matching response reflects mechanisms related to both fatigability and recruitment of motor units involved in output control.

Macro-EMG and motor unit recruitment threshold: differences between the young and the aged

The relationship between macro-EMG (electromyography) and motor unit recruitment threshold was studied in the first dorsal interosseous (FDI) muscle of normal young and aged subjects. During voluntary isometric contraction, motor unit action potentials (MUAP) were collected by a special quadrifilar electrode and decomposed to each MUAP train (MUAPT) using an EMG signal decomposition technique. Macro-EMG was obtained from the electrode shaft, then triggered and averaged for each MUAPT. A positive linear correlation was observed in both the young and aged subjects. However, the correlation coefficients were significantly lower in the aged individuals than in the young individuals.