Cat hindlimb motoneurons during locomotion. III. Functional segregation in sartorius

Cat sartorius has two distinct anatomical portions, anterior (SA-a) and medial (SA-m). SA-a acts to extend the knee and also to flex the hip. SA-m acts to flex both the knee and the hip. The objective of this study was to investigate how a "single motoneuron pool" is used to control at least three separate functions mediated by the two anatomical portions of one muscle. Discharge patterns of single motoneurons projecting to the sartorius muscle were recorded using floating microelectrodes implanted in the L5 ventral root of cats. The electromyographic activity generated by the anterior and medial portions of sartorius was recorded with chronically implanted electrodes. The muscle portion innervated by each motoneuron was determined by spike-triggered averaging of the EMGs during walking on a motorized treadmill. During normal locomotion, SA-a exhibited two bursts of EMG activity per step cycle, one during the stance phase and one during the late swing phase. In contrast, every recorded motoneuron projecting to SA-a discharged a single burst of action potentials per step cycle. Some SA-a motoneurons discharged only during the stance phase, whereas other motoneurons discharged only during the late swing phase. In all cases, the instantaneous frequencygram of the motoneuron was well fit by the rectified smoothed EMG envelope generated by SA-a during the appropriate phase of the step cycle. During normal locomotion, SA-m exhibited a single burst of EMG activity per step cycle, during the swing phase. The temporal characteristics of the EMG bursts recorded from SA-m differed from the swing-phase EMG bursts generated by SA-a.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cat hindlimb motoneurons during locomotion. I. Destination, axonal conduction velocity, and recruitment threshold

Fine flexible wire microelectrodes chronically implanted in the fifth lumbar ventral root (L5 VR) of 17 cats rendered stable records of the natural discharge patterns of 164 individual axons during locomotion on a treadmill. Fifty-one out of 164 axons were identified as motoneurons projecting to the anterior thigh muscle group. For these axons, the centrifugal propagation of action potentials was demonstrated by the technique of spike-triggered averaging using signals recorded from cuff electrodes implanted around the femoral nerve. The axonal conduction velocity was measured from the femoral nerve cuff records. For 43/51 motoneurons, the corresponding target muscle was identified by spike-triggered averaging of signals recorded from bipolar EMG electrodes implanted in each of the anterior thigh muscles: vastus intermedius, medialis and lateralis, sartorius anterior and medialis, and rectus femoris. For 32/51 motoneurons, the recruitment threshold during locomotion was determined from the mean value of the rectified digitally smoothed EMG of the target muscle measured at the time when the motoneuron fired its first spike for each step. The recruitment threshold of every motoneuron was relatively constant for a given speed of walking, but for some units there were small systematic variations as a function of treadmill speed (range: 0.1-1.3 m/s). Recruitment thresholds were standardized with respect to the mean value of peak EMG activity of the target muscle during 16 s of walking at 0.5 m/s. For 28/51 motoneurons recorded in nine cats, recruitment thresholds (range: 3-93% of peak target muscle EMG) were linearly correlated (r = 0.51, P less than 0.02) to axonal conduction velocities (range: 57-117 m/s). In addition, for seven recorded pairs of motoneurons that projected to the same muscle in the same cat, the recruitment thresholds were ordered by relative conduction velocities. Taken together, these results are consistent with the notion that, in normal cat locomotion up to a medium trot, anterior thigh motoneurons are progressively recruited in an orderly fashion.     

Cat hindlimb motoneurons during locomotion. IV. Participation in cutaneous reflexes

The responses of 11 individual motoneurons, the muscle to which each projected, plus all other muscles in the anterior thigh of the cat, were recorded following single non-noxious electrical stimuli to cutaneous nerves while the intact animal walked on a treadmill. The various excitatory and/or inhibitory responses were qualitatively similar for stimuli within the range 1.1-10 times threshold for group I fibers in the stimulated nerve (usually saphenous). Monarticular knee extensor muscles in the vastus group and their motoneurons were usually inhibited in the period 10- to 25-ms poststimulus. The faster contracting vastus medialis and lateralis muscles tended to have an excitatory rebound at approximately 25- to 40-ms poststimulus that was confined to the stance phase of the step cycle when these muscles were normally active. Biarticular hip flexor muscles rectus femoris and both the anterior and medial parts of sartorius and their motoneurons all had similar bimodal excitatory responses, including an early period 3- to 18-ms poststimulus and a later period 20- to 35-ms poststimulus. The short-latency excitatory responses appeared to be proportional to the normal recruitment of the muscles in the step cycle, whereas the long-latency responses tended to be phase advanced with respect to normal recruitment. Motoneurons projecting to muscles with two excitatory peaks tended to have similar excitatory responses at both latencies and occasionally responded at both latencies to a single stimulus.     

Effects of acute dorsal spinal hemisection on motoneuron discharge in the medial gastrocnemius of the decerebrate cat

1. The discharge of single alpha-motoneuron axons was recorded from small cut filaments of the medial gastrocnemius (MG) muscle nerve in the decerebrated cat preparation before and after a dorsal hemisection of the thoracic spinal cord. The remainder of the MG muscle nerve was left intact, and muscle force and multiunit electromyographic (EMG) activity were recorded along with alpha-motoneuron discharge, while motor output was varied by manual stimulation of the contralateral hindlimb. 2. We recorded activity in 32 motoneurons before and after the spinal lesion, and pre- and postlesion recruitment forces and minimum firing rates were determined for 30 of these. Postlesion decreases in minimum firing rates were observed in 25/30 motoneurons, and decreases in recruitment force were seen in 21/30 motoneurons. The remaining motoneurons, which generally had low presection recruitment forces and minimum rates, exhibited postlesion increases in both parameters (see below). 3. The effects of the spinal lesion on the recruitment force and minimum firing rate of a motoneuron were related to the prelesion values of these parameters; the largest postlesion decreases were seen in motoneurons with the highest prelesion rates and recruitment forces. Spinal lesions thus acted to shift and compress the range of recruitment forces and minimum firing rates, so that after the lesion all motoneurons tended to exhibit discharge behavior typical of that seen only in the lowest threshold motoneurons before the lesion. In addition, motoneurons with low prelesion recruitment forces (less than 1.0 N of active force) generally showed an increase in recruitment force after the lesion, indicating that the lesion may have led to changes in the prelesion recruitment order. Direct evidence of recruitment reversals was obtained in 4/14 experiments where two or more motoneurons were followed pre- and postlesion. 4. The lesion-induced changes in motoneuron discharge characteristics were associated with changes in the relations between muscle force, rectified EMG, and motoneuron rate. Postlesion discharge rates were always significantly lower than the prelesion rates when compared over the same range of EMG levels. This postlesion drop in discharge rates was generally associated with inefficient force production, as evidenced by a significant drop in muscle force for matched EMG levels. 5. The degree of discharge synchrony in MG motoneurons was assessed by calculating a spike-triggered average (STA) between axonal discharge and multiunit rectified EMG. Significant STA peaks were rare before the lesion (4/32 motoneurons) but were quite common after the lesion (29/32 motoneurons).(ABSTRACT TRUNCATED AT 400 WORDS)     

Adaptive control for backward quadrupedal walking. II. Hindlimb muscle synergies

1. To compare the basic hindlimb synergies for backward (BWD) and forward (FWD) walking, electromyograms (EMG) were recorded from selected flexor and extensor muscles of the hip, knee, and ankle joints from four cats trained to perform both forms of walking at a moderate walking speed (0.6 m/s). For each muscle, EMG measurements included burst duration, burst latencies referenced to the time of paw contact or paw off, and integrated burst amplitudes. To relate patterns of muscle activity to various phases of the step cycle, EMG records were synchronized with kinematic data obtained by digitizing high-speed ciné film. 2. Hindlimb EMG data indicate that BWD walking in the cat was characterized by reciprocal flexor and extensor synergies similar to those for FWD walking, with flexors active during swing and extensors active during stance. Although the underlying synergies were similar, temporal parameters (burst latencies and durations) and amplitude levels for specific muscles were different for BWD and FWD walking. 3. For both directions, iliopsoas (IP) and semitendinosus (ST) were active as the hip and knee joints flexed at the onset of swing. For BWD walking, IP activity decreased early, and ST activity continued as the hip extended and the knee flexed. For FWD walking, in contrast, ST activity ceased early, and IP activity continued as the hip flexed and the knee extended. For both directions, tibialis anterior (TA) was active throughout swing as the ankle flexed and then extended. A second ST burst occurred at the end of swing for FWD walking as hip flexion and knee extension slowed for paw contact. 4. For both directions, knee extensor (vastus lateralis, VL) activity began at paw contact. Ankle extensor (lateral gastrocnemius, LG) activity began during midswing for BWD walking but just before paw contact for FWD walking. At the ankle joint, flexion during the E2 phase (yield) of stance was minimal or absent for BWD walking, and ankle extension during BWD stance was accompanied by a ramp increase in LG-EMG activity. At the knee joint, the yield was also small (or absent) for BWD walking, and increased VL-EMG amplitudes were associated with the increased range of knee extension for BWD stance. 5. Although the uniarticular hip extensor (anterior biceps femoris, ABF) was active during stance for both directions, the hip flexed during BWD stance and extended during FWD stance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Sartorius muscle afferents influence the amplitude and timing of flexor activity in walking decerebrate cats

Recent investigations have demonstrated that afferent signals from hindlimb flexor muscles can strongly influence flexor burst activity during walking and during fictive locomotion in decerebrate cats. We have reported previously that modifying afferent feedback from the sartorius (Sart) muscles by assisting or resisting hip flexion has a marked effect on the magnitude and duration of activity in iliopsoas (IP) as well as the sartorius muscles. The objective of the present investigation was to identify the afferents responsible for these effects by examining, in walking decerebrate cats, the influence of electrically stimulating sartorius afferents on burst activity in the IP and tibialis anterior (TA) muscles. Stimulation of the sartorius nerve at group I strength resulted in an increase in the duration of IP and TA bursts and an increase in the magnitude of IP bursts. The effect on burst durations was only observed at stimulus strengths of 1.6 T and higher. At lower stimulus strengths, there was a strong excitatory effect on IP bursts but no effect on TA bursts. Stimulation of the sartorius nerve at group II strength yielded variable results. When group II stimulation was delivered repeatedly during a walking sequence, the initial response was usually a strong inhibition of burst activity in IP and TA followed by a progressive reduction in inhibition and the emergence in IP of an excitatory response. This observation, together with findings of previous studies, suggests the existence of parallel excitatory and inhibitory pathways from sartorius group II afferents to flexor motoneurons. Taken together, these results support an earlier speculation that feedback from large afferents from the sartorius muscles has a strong influence on the generation of flexor burst activity in walking cats. Electronic Publication     

Properties of human motor units after prolonged activity at a constant firing rate

The primary purpose of this study was to examine if there are changes in the intrinsic properties of spinal motoneurons after prolonged submaximal contractions. To do this, we assessed whether or not the synaptic drive to motoneurons needs to increase in order to maintain a constant firing rate of a motor unit. Recruitment of new units and an increase in total electromyographic (EMG) activity of the muscle of interest were taken as estimates of an increase in synaptic drive. Subjects were asked to maintain a constant firing rate of a clearly identifiable (targeted) motor unit from the first dorsal interosseous muscle for approximately 10 min, while surface EMG and force were recorded simultaneously. For the 60 units studied, the duration of the constant-firing-rate period ranged from 73 to 1,140 s (448 ± 227 s; mean ± SD). There was a significant increase (t-test, p<0.001) in the magnitude of mean surface EMG, and DC force while the targeted motoneuron maintained a constant rate suggesting an increase in the net excitatory input to the motoneuron pool. Changes occurring simultaneously in other parameters, namely, variability in interspike interval, magnitude of force fluctuations, the duration of motor unit action potentials, and the median power frequency of surface EMG were also computed. The firing rates of 16 concurrently firing motoneurons, not controlled by the subject, remained constant. The key finding of this study is that after prolonged activity, a motoneuron requires a stronger excitatory input to maintain its firing rate. Additional results are indicative of significant changes in the characteristics of the synaptic inputs, changes at the neuromuscular junction (both pre- and postsynaptic regions) and the sarcolemma.     

Synaptic efficacy of inhibitory synapses and repetitive firing in the reinnervating trigeminal and hypoglossal motoneurons

The synaptic efficacy and repetitive firing in masseteric motoneurons after the self-union operation and in tongue protruder motoneurons after their cut axons were reunited to tongue retractor muscles, the styloglossus muscle, were studied in cats. To ensure the correct identification of reinnervating motoneurons, the muscle response produce by an induced spike in a motoneuron by intracellularly injected depolarizing current was recorded. In both masseteric and tongue protruder motoneurons there were no differences on the patterns of postsynaptic potentials produced in reinnervating and non-reinnervating motoneurons by peripheral nerve stimulation, suggesting that the recovery of the synaptic efficacy of inhibitory synapses is time-dependent rather than muscle reinnervation. However, the present study demonstrated that the recovery of processes that control rhythmical firing of motoneurons is probably dependent on muscle reinnervation.     

Single-unit activity in bulbar reticular formation during food ingestion in chronic cats

1. Single-unit activity was recorded from 215 neurons in the medial bulbar reticular formation during the masticatory sequence, from intake to deglutition, of 3 kinds of food (cat food pellets, canned fish, and milk) in 8 chronically prepared, unanesthetized, spontaneously respiring cats with their head fixed to a stereotaxic apparatus without pain or pressure. The firing patterns were compared to the simultaneously recorded EMGs of the jaw-closing and -opening muscles and to the jaw movement. 2. Fifty neurons changed their firing patterns during mastication. Nine neurons increased and one neuron decreased or stopped firing coincident with the masticatory sequence without an apparent rhythmical modulation of frequency corresponding with the masticatory rhythm (nonphasic group). The firing pattern of the remaining 40 neurons was modulated in phase with jaw movement (phasic group); 34 neurons either showed a spike burst or attained the highest firing frequency during the jaw-opening phase (opening type), while 6 neurons did so during the jaw-closing phase (closing type). The firing patterns of each neuron were essentially the same regardless of the kind of food ingested, except for 2 opening-type neurons that showed a rhythmical burst during mastication of solid food and tonic activity during lapping milk. 3. For 16 phasic neurons, there were significant correlations between some aspects of the firing pattern and a parameter of the movement during ingestion of solid food and/or milk. With one exception, these relationships did not appear to be due to sensory feedback. 4. We detected a monosynaptic excitatory projection from 3 opening-type neurons to the anterior digastric motoneurons, and monosynaptic inhibitory projections to the temporal or masseter motoneurons from 3 other opening-type neurons, by spike-triggered averaging of the full-wave rectified EMG of the jaw-closing and -opening muscles. No monosynaptic projections from the closing-type neurons or nonphasic group neurons to either jaw-opener or -closer motoneurons were detected. 5. The instantaneous firing frequency of all 3 inhibitor premotor neurons was positively correlated with the opening velocity, and the firing of 2 was also related to the jaw displacement.(ABSTRACT TRUNCATED AT 400 WORDS)     

Increases in reflex excitability of monkey masseter motoneurons before a jaw-bite reaction-time response.

Reflex excitability of the motoneurons innervating the masseter muscle of monkeys was tested before a phasic voluntary activation of the jaw-closing muscles (a RT bite response). Single test shocks were delivered to the Mes V which supplies a monosynaptic excitatory input to the jaw-muscle motoneurons. Changes in reflex excitability were assessed by measuring the amplitude of the synchronous muscle potential evoked by the test shock. Amplitudes of the muscle potentials evoked by shocks which occurred just before the beginning of the voluntary EMG response, as judged by the onset of EMG activity of the masseter muscle contralateral to the test shock were many times larger than potentials evoked immediately following the visual RT stimulus. Curves relating the average amplitude of the evoked response to its time before the beginning of the voluntary response suggest that the reflex excitability of the motoneuron pool begins to increase 25-45 ms before the first detectable EMG activity occurs. These results suggest that inputs arrive at the motoneurons of agonist muscles used in rapid RT tasks substantially before changes in the EMG of the muscle are noted. These results, in part, would account for the time interval noted between the beginning of neural activity in suprasegmental structures which presumably excites spinal motoneurons, and the first EMG activity of muscles which are innervated by these motoneurons.     

A central pattern-generating network contributes to "reflex-reversal"-like leg motoneuron activity in the locust.

We introduce a new rhythmic preparation of the locust mesothoracic segment that exhibits long-lasting rhythmicity without pharmacological treatment. In most experiments, isolation of the locust mesothoracic ganglion from the anterior and posterior ganglia causes episodes of patterned activity to be generated in leg motoneurons that supply the femur-tibia (FT) joint. Flexor and extensor tibiae motoneuron pools exhibit alternating bursts of activity mostly composed of doublets and triplets of bursts. Motor activity during these episodes appears to be centrally generated because it persisted after complete deafferentation in 37% of the preparations; however, proprioceptive signals from the middle leg strongly influenced the patterning of motoneuron activity. Mimicking FT joint flexion by elongating the femoral chordotonal organ during an extensor burst terminates extensor activity and initiates flexor activity. The reverse is true for a mimicked extension during a flexor burst. This motor activity represents a reflex reversal that is typically observed in the locomotor state of the stick insect walking system. Our results provide evidence that this "reversed" reflex is caused by the action of central pattern-generating networks.     

Ectopic expression of Hoxd10 in thoracic spinal segments induces motoneurons with a lumbosacral molecular profile and axon projections to the limb.

Hox genes encode anterior-posterior identity during central nervous system development. Few studies have examined Hox gene function at lumbosacral (LS) levels of the spinal cord, where there is extensive information on normal development. Hoxd10 is expressed at high levels in the embryonic LS cord but not the thoracic cord. To test the hypothesis that restricted expression of Hoxd10 contributes to the attainment of an LS identity, and specifically an LS motoneuron identity, Hoxd10 was ectopically expressed in thoracic segments in chick embryos by means of in ovo electroporation. Regional motoneuron identity was assessed after the normal period of motoneuron differentiation. Subsets of motoneurons in transfected thoracic segments developed a molecular profile normally shown by LS motoneurons, including Lim 1 and RALDH2 expression. In addition, motoneurons in posterior thoracic segments showed novel axon projections to two muscles in the anterodorsal limb, the sartorius and anterior iliotibialis muscles. At thoracic levels, we also found a decrease in motoneuron numbers and a reduction in gonad size. These last findings suggest that early and high levels of Hox expression impeded motoneuron development and neural-mesodermal interactions. Despite these adverse effects, our data indicate that Hoxd10 expression is sufficient to induce LS motoneuron identity and axon trajectories characteristic of motoneurons in the LS region.     

Adaptation of the walking pattern to uphill walking in normal and spinal-cord injured subjects

 Lower-limb movements and muscle-activity patterns were assessed from seven normal and seven ambulatory subjects with incomplete spinal-cord injury (SCI) during level and uphill treadmill walking (5, 10 and 15°). Increasing the treadmill grade from 0° to 15° induced an increasingly flexed posture of the hip, knee and ankle during initial contact in all normal subjects, resulting in a larger excursion throughout stance. This adaptation process actually began in mid-swing with a graded increase in hip flexion and ankle dorsiflexion as well as a gradual decrease in knee extension. In SCI subjects, a similar trend was found at the hip joint for both swing and stance phases, whereas the knee angle showed very limited changes and the ankle angle showed large variations with grade throughout the walking cycle. A distinct coordination pattern between the hip and knee was observed in normal subjects, but not in SCI subjects during level walking. The same coordination pattern was preserved in all normal subjects and in five of seven SCI subjects during uphill walking. The duration of electromyographic (EMG) activity of thigh muscles was progressively increased during uphill walking, whereas no significant changes occurred in leg muscles. In SCI subjects, EMG durations of both thigh and leg muscles, which were already active throughout stance during level walking, were not significantly affected by uphill walking. The peak amplitude of EMG activity of the vastus lateralis, medial hamstrings, soleus, medial gastrocnemius and tibialis anterior was progressively increased during uphill walking in normal subjects. In SCI subjects, the peak amplitude of EMG activity of the medial hamstrings was adapted in a similar fashion, whereas the vastus lateralis, soleus and medial gastrocnemius showed very limited adaptation during uphill walking. We conclude that SCI subjects can adapt to uphill treadmill walking within certain limits, but they use different strategies to adapt to the changing locomotor demands. Received: 10 March 1998 / Accepted: 29 December 1998     

Is there a ‘normal’ profile of EMG activity in gait?

The aim of the present study was to validate the notion that there is a normal profile for electromyographic (EMG) signals in gait. For eight subjects, surface electrodes were used to record signals over ten strides from the soleus, rectus femoris, biceps femoris, vastus medialis and tibialis anterior muscles. Analyses of EMG linear envelopes, normalised with respect to both time and amplitude, demonstrated that statistically significant differences exist in amplitude of activity across subjects for all muscles, and that within subjects the data were highly repeatable. More importantly, differences in profiles of activity were found especially for the rectus femoris muscle. It was also found that a given muscle from a given subject can present specific profiles, but usually these specificities are averaged out in the pooled between-subject data. Thus limits exist with the use or the notion of a normal profile of EMG for gait.     

Doublet discharges in motoneurons of young and older adults

The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 +/- 3.56 yr) and eight older (74.1 +/- 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.     

Activation patterns of hindlimb motor units in the awake rat and their relation to motoneuron intrinsic properties.

The activity of hindlimb motor units from the lateral gastrocnemius and tibialis anterior muscles in the awake rat was compared during locomotion and during slow, sinusoidal muscle stretch. The majority of units were activated with high initial frequencies and often commenced firing with an initial doublet or triplet, even when activated by slow muscle stretch. The high firing rates at recruitment occurred without jumps in the firing rates of other concurrently activated units, the firing rate profiles of which were used as a measure of the net synaptic drive onto the motoneuronal pool. This suggested that the sharp recruitment jumps were not due to an abrupt increase in synaptic drive but rather due to intrinsic properties of the motoneuron. In addition, motor units that were activated phasically by the muscle stretch fired more action potentials during muscle shortening than during muscle lengthening, resulting in rightwardly skewed, asymmetrical firing profiles. In contrast, when the same units fired tonically during the imposed muscle stretch, the frequency profiles were modulated symmetrically and no nonlinearities were observed. Tonically firing units were modulated symmetrically throughout a wide range of firing frequencies, and discrete jumps in rate (i.e., bistable firing) were not observed. The sharp recruitment jumps during locomotion and muscle stretch are proposed to have resulted from the additional depolarization produced by the activation of plateau potentials at recruitment. Likewise, the sustained activation of plateaus subsequent to recruitment may have produced the prolonged firing of the motor units during sinusoidal muscle stretch.     

Discharge patterns of levator palpebrae superioris motoneurons during vertical lid and eye movements in the monkey.

1. We recorded single-unit activity in the caudal central nucleus (CCN) of the oculomotor complex in monkeys trained to make vertical saccadic, smooth-pursuit, and fixation eye movements. We confirmed that our recordings were from motoneurons innervating the upper lid, because small lesions placed at the sites of responsive units were recovered among neurons labeled by horseradish peroxidase (HRP) injections into the levator palpebrae superioris muscle. 2. For fixations above a threshold lid position, levator motoneurons discharged at a steady rate, which increased linearly with upward lid position. The average position sensitivity during fixation was 2.9 spikes/s per deg, and the average lid motoneuron was recruited into steady firing when the eye was looking 10 degrees down. 3. During upward saccades, levator motoneurons discharged a burst of spikes that began, on average, 7.3 ms before the lid movement if the saccade started from a straight-ahead position; the lead time decreased considerably as the initial eye and lid positions shifted downward. The firing rate usually reached its peak (130-280 spikes/s) at the very onset of the burst and declined gradually during the course of the saccade. The steady rate associated with the new fixation position was reached about halfway during the saccade. All units exhibited a pause in firing during the initial half of large downward saccades; during small saccades, the pause was inconspicuous or absent. 4. During vertical sinusoidal smooth pursuit, levator motoneurons exhibited a sinusoidal modulation in firing rate, which led eye position by an average of 23 degrees at 0.3 Hz. The average velocity sensitivity calculated from such data was 0.63 spikes/s per deg/s. 5. Although they exhibit a number of qualitative similarities, the discharge patterns of levator motoneurons and superior rectus motoneurons differ in several respects. First, during a blink, when the lid undergoes a large depression but the eye exhibits only a brief transient displacement, levator motoneurons cease firing completely, whereas superior rectus motoneurons continue to discharge. Second, for all types of coordinated lid and eye movements, levator motoneurons discharge at lower firing rates than do superior rectus motoneurons. Third, during saccades, levator motoneurons have less conspicuous and shorter-lasting bursts and pauses than do motoneurons involved in rotating the eye. 6. During upward gaze, the qualitative similarity of their burst-tonic discharge patterns suggests that levator and superior rectus motoneurons receive input signals that originate from a common source, but that the signals are processed differently to deal with the different loads facing these muscles.(ABSTRACT TRUNCATED AT 400 WORDS)     

Functional coupling of motor units is modulated during walking in human subjects

Time- and frequency-domain analysis of the coupling between pairs of electromyograms (EMG) recorded from leg muscles was investigated during walking in healthy human subjects. For two independent surface EMG signals from the tibialis anterior (TA) muscle, coupling estimated from coherence measurements was observed at frequencies 相似文献   

The role of Renshaw cells in locomotion: antagonism of their excitation from motor axon collaterals with intravenous mecamylamine

Summary The contribution of Renshaw cell (RC) activity to the production of fictive locomotion in the mesencephalic preparation was examined using the nicotinic antagonist mecamylamine (MEC). After the i.v. administration of 3 doses of MEC (1.0 mg/kg) the following observations were made: 1) ventral root (VR) evoked discharge of RCs was decreased by up to 87.7%, 2) recurrent inhibitory postsynaptic potentials recorded in alpha motoneurons were greatly reduced or abolished, and 3) the rhythmic firing of RCs during the fictive step cycle was abolished in 83% of the cells examined. Locomotor drive potentials (LDPs) in motoneurons persisted during the fictive step cycle after MEC administration. Bursts of motoneuron firing during each fictive step cycle were characterized by increased frequency and number of spikes after MEC, although the burst duration was unaltered for similar step cycle lengths. A greater number and frequency of spikes per burst was also observed in Ia inhibitory interneurons (IaINs), which remained rhythmically active after MEC administration. It is concluded that Renshaw cells are not an integral part of the spinal central pattern generator for locomotion, nor do they control the timing of the motoneuron or IaIN bursts of firing during fictive locomotion. The data are consistent with a role for RCs in limiting the firing rates of motoneurons and IaINs during each burst.