The carpal tunnel syndrome. Relationship between median distal motor latency and graded results of needle electromyography

Needle Electromyography (NEMG) was performed in 228 patients who had been diagnosed by means of motor and sensory median nerve conductions of having Carpal Tunnel Syndrome (CTS). Evaluating abnormal spontaneous muscle activity (fibrillations, positive sharp waves, fasciculation potentials and myokymic discharges) and recruitment pattern, 51.7% of them showed abnormal NEMG. According to the amount of abnormality found, the studies were classified into: normal, grade 1, grade 2 and grade 3. Median Distal Motor Latency (DML) to APB was prolonged beyond the upper confidence limit in 155 (68%) hands, showing a mean value of 5.69 +/- 2.79 msec. Statistically significant difference (p < 0.001) of DML mean value among patients with a NEMG "normal", "grade 1" or "grade 2" have been found. We revise previous reports about NEMG on the CTS and discuss practical consequences of our findings.

Abdominal muscle activity during hypercapnia in awake dogs

We previously found the internal abdominal muscle layer to be preferentially recruited during expiratory threshold loading in anesthetized and awake dogs. Expiratory threshold loading increases end-expiratory lung volume and hence can activate reflex pathways such as tonic vagal reflexes, which could influence abdominal muscle recruitment. Our objectives in the present study were to determine the effects of hypercapnia on abdominal muscle activation and the pattern of recruitment in awake dogs. Five tracheotomized dogs were chronically implanted with sonomicrometer transducers and fine-wire electromyogram (EMG) electrodes in each of the four abdominal muscles: transversus abdominis, internal oblique, external oblique, and rectus abdominis. Muscle length changes and EMG activity were studied in the awake dog at rest and during CO2 rebreathing. CO2 rebreathing produced a tripling of tidal volume and activation of the abdominal muscles. Despite the increase in tidal volume, there was no significant change in abdominal muscle end-inspiratory length. Both tonic and phasic expiratory shortening were greater in the internal muscle layer (transversus abdominis and internal oblique) than in the external muscle layer (external oblique and rectus abdominis). We conclude that the internal abdominal muscles are preferentially recruited by hypercapnia and vagal reflexes probably do not contribute to this differential recruitment but that segmental reflexes may be involved. The mechanical consequences of this recruitment are discussed.

Fluctuations of excitability in the monosynaptic reflex pathway to lumbar motoneurons in the cat

1. It is well known that the amplitude of successive monosynaptic reflexes (MSR), elicited by afferent stimuli of constant strength, fluctuate from trial to trial. Previous evidence suggests that such excitability fluctuations within the motor pool can be introduced either pre- and/or postsynaptically. Using unanesthetized decerebrate or decerebrate/spinal cats, we attempted to evaluate the relative importance of pre- and postsynaptic mechanisms to MSR variability and the potential contribution of changes in the identities of responding motoneurons to such variability. 2. Comparisons between the MSR amplitude, measured in a severed ventral root, and the probability of firing of up to three individual motoneurons in fine filaments teased from the same root, confirmed that both correlated and uncorrelated fluctuations of motoneuron excitability are involved in MSR variability. Linear regression analysis from concurrent intracellular recordings from homonymous motoneurons showed that the MSR fluctuations were correlated with the variations in membrane potential baseline, as well as with the fluctuations in the monosynaptic excitatory postsynaptic potential peak amplitude. In all 11 cases tested, the former correlation was stronger than the latter. 3. Stimulation of the caudal cutaneous sural nerve (CCS) was used to alter the postsynaptic potential background on which triceps surae (GS) MSRs were generated. The interval chosen between CCS conditioning and the GS stimulation excluded the involvement of presynaptic inhibition. When conditioned by preceding CCS stimulation, GS population MSRs generally (8/9 cases tested) increased in amplitude without much change in their overall variance. However, the individual motoneurons that contributed to the population responses did show changes in both relative excitability and in the uncorrelated component of their response variance. About half of the concurrently recorded motoneurons (6/13) showed a decrease in relative excitability after CCS conditioning, 5/13 showed an increase, and 2/13 were unchanged. Comparison of unit and population responses indicated that the identities of the motoneurons that responded at any given level of population response were quite different with and without CCS conditioning. 4. High-frequency stimulation of Ia fibers was used to alter the state of presynaptic Group Ia-afferents that produced population MSRs. Post tetanic potentiation following high-frequency stimulation did not greatly alter the variance of population MSRs or ratio of correlated and uncorrelated fluctuations in MSR responses among individual motoneurons within the responding population. However, intratetanic depression and posttetanic potentiation of population MSRs were accompanied by marked shifts in individual motoneuron excitability relative to the population response, again indicated that changes in the identities of responding motoneurons contributes to population response fluctuations.(ABSTRACT TRUNCATED AT 400 WORDS)

Task-dependent differences between mono- and bi-articular heads of the triceps brachii muscle

We studied motor-unit recruitment and decruitment thresholds in the three heads of the human elbow extensor, the triceps brachii muscle (caput mediale and laterale, both mono-articular heads, and caput longum, the bi-articular head) by means of intramuscular electromyographic-recordings. Two experiments were performed: an 'isometric' and a 'movement' experiment. In the isometric experiment, subjects were asked to increase the elbow extension torque isometrically to a specific level, keep the torque at the level for 10 s, and then decrease the torque again to zero. In the movement experimental subjects moved their forearm from 90 degrees to 110 degrees extension against an increasing flexion torque, kept the latter position for 10 s and then moved their forearm back while the torque decreased. Results for caput longum showed that recruitment thresholds were higher than decruitment thresholds, whereas in caput mediale and laterale no difference in thresholds was found. In caput longum recruitment thresholds were found to be lower in movement conditions than in isometric conditions. The reverse effect was observed in caput mediale, whereas no difference in recruitment thresholds was found in caput laterale. Our results point to a transfer of force from mono-articular muscles in isometric conditions to bi-articular muscles in movement conditions. A similar transfer is found when recruitment and decruitment are compared. This means that the transfer is not only a property of the elbow-flexor muscles, but is a more common trait. A qualitative analysis of firing frequencies at recruitment and at decruitment in both conditions supports our findings.

Normalization of soleus H-reflex recruitment curves in controls and a population of spastic patients

We examined soleus H-reflex recruitment in 30 controls and 33 patients with spinal cord lesions and spastic spinal paresis. H-reflex gain and threshold were determined from recruitment curves after normalization of stimulus intensity as a multiple of the current for a threshold M-response. Reflex gain was expressed as the mean slope of the H-reflex recruitment curve up to the half-maximal response size. Up to this point the curve follows an almost linear trajectory and will mainly reflect Ia afferent stimulation. This slope had a large variability but was clearly correlated with the H/M ratio. The mean gain was equal in controls and patients. The relation between H- and M-thresholds was expressed as a ratio which had a lower mean value in the patients. Though both H- and M-thresholds may be influenced by peripheral factors, this lower ratio suggests an increase in spinal motoneuron excitability in patients.

Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction

The activity of 50 single motor units was recorded in the biceps brachii muscle of human subjects while they performed submaximal isometric elbow flexion contractions that were sustained to induce fatigue. The purposes of this study were to examine the influence of fatigue on motor unit threshold force and to determine the relationship between the threshold force of recruitment and the initial interimpulse interval on the discharge rates of single motor units during a fatiguing contraction. The discharge rate of most motor units that were active from the beginning of the contraction declined during the fatiguing contraction, whereas the discharge rates of most newly recruited units were either constant or increased slightly. The absolute threshold forces of recruitment and derecruitment decreased, and the variability of interimpulse intervals increased after the fatigue task. The change in motor unit discharge rate during the fatigue task was related to the initial rate, but the direction of the change in discharge rate could not be predicted from the threshold force of recruitment or the variability in the interimpulse intervals. The discharge rate of most motor units declined despite an increase in the excitatory drive to the motoneuron pool during the fatigue task.

Recruitment of long-lasting and protein kinase A-dependent long-term potentiation in the CA1 region of hippocampus requires repeated tetanization

To study how the late phase of long-term potentiation (LTP) in hippocampus arises, we examined the resulting LTP for its time course and its dependence on protein synthesis and different second-messenger kinases by applying various conditioning tetani. We find that one high-frequency train (100 Hz) produces a form of LTP that lasts longer than 1 hr but less than 3 hr (the early phase of LTP, or E-LTP). It is blocked by inhibitors of calcium/calmodulin kinase II (Cam kinase II) but is not affected by an inhibitor of cAMP-dependent protein kinase [protein kinase A (PKA) and the protein synthesis inhibitor anisomycin] nor is it occluded by the cAMP activator forskolin. In contrast, when three high-frequency trains are used, the resulting potentiation persists for at least 6-10 hr. The L-LTP induced by three trains differs from the E-LTP in that it requires new protein synthesis, is blocked by an inhibitor of cAMP-dependent protein kinase, and is occluded by forskolin. These results indicate that the two mechanistically distinctive forms of LTP, a transient, early component (E-LTP) and a more enduring form (L-LTP), can be recruited selectively by changing the number of conditioning tetanic trains. Repeated tetani induce a PKA and protein synthesis-dependent late component that adds to the amplitude and duration of the potentiation induced by a single tetanus.

Computer simulations of the effects of different synaptic input systems on the steady-state input-output structure of the motoneuron pool

1. The effects of different types of synaptic input on the steady-state input-output relations of the mammalian motoneuron pool were investigated by the use of computer simulations. The properties of the simulated motor units and their synaptic inputs were based as closely as possible on the experimental data from studies in the cat hindlimb. 2. Three basic types of synaptic input systems were simulated: postsynaptic, presynaptic, and neuromodulatory. The effects of these inputs on three aspects of the system input-output structure were studied: gain, precision, and motor-unit type utilization. 3. The gain analyses were based on a simulation of the steady-state homonymous Ia input. The gain of this steady-state Ia "reflex" was found to be determined largely by the slope of the pool input-output function. Precision was evaluated in two ways, from the amplitudes of the quantal steps due to motor-unit recruitment and from the sensitivity of the input-output function to noise. The pattern of motor-unit type utilization allowed indirect assessment of fatigue resistance: the larger the percentage of force generated by FF units, the lower the fatigue resistance. 4. A uniformly distributed input (i.e., one that generates equal input in all motoneurons) generates outputs that are solely determined by the intrinsic properties of the motor units. Thus the gain, precision, and motor-unit type patterns generated by a uniform input were used as the basis with which the effects of all other input systems were compared. 5. Postsynaptic excitatory inputs with nonuniform distributions within the pool did influence gain. The greatest effect was the increase mediated by the rubrospinal excitatory input (27% increase at 30% of maximal force). However, this input also greatly decreased both fatigue resistance and precision, due to increased activation of FF units at low force levels. In contrast, the Ia input slightly decreased gain (12% decrease at 30% of maximum force) while slightly increasing fatigue resistance and precision. 6. The simulated neuromodulatory input was based on the monoaminergic reticulospinal effect on motoneurons. Gain was generally increased by the monoaminergic input. However, the magnitude of the increase strongly depended on whether the monoaminergic effects were largest on S units (giving a 20% increase at 30% of maximum force), equal on all types (52%), or largest on FF units (102%). Presynaptic inhibition reduced gain with no effect whatsoever on fatigue resistance or precision. 7. Therefore Ia reflex gain was modifiable by all three types of input: postsynaptic, presynaptic, and neuromodulatory.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of chronic spinalization on ankle extensor motoneurons. II. Motoneuron electrical properties

1. Intracellular recording and stimulation techniques were used in a comparison of electrical properties of triceps surae and plantaris motoneurons between unlesioned and 6-wk chronic spinal (L1-L2) cats. The primary analysis was restricted to 195 motoneurons with action potential heights > or = 80 mV. 2. Voltage transients resulting from short-duration current pulses (0.5 ms) were used to estimate membrane time constant (tau m) and equivalent cylinder electrotonic length (L). Although L was unchanged, tau m and the equalization phase time constant were significantly lower (17%) in motoneurons from chronic spinal preparations. Estimated total cell surface area was also reduced by 11%. The incidence of sag conductances, as judged from observations of the decay of voltage transients, increased from 3% to 29% after chronic spinalization. 3. Input resistance, as measured from either the amplitude of voltage responses to long-duration (50 ms) hyperpolarizing pulses (RinL) or from the area of the short-duration current pulse-induced voltage transients, was unchanged in the chronic spinal preparation. 4. Rheobase current was unchanged but threshold voltage (V Th) was increased in chronic spinal motoneurons. Increased V Th was not a result of membrane hyperpolarization because both mean action potential height (88 mV) and resting membrane potential (70 mV) were identical in both preparations. 5. The threshold current for action potential activation by short-duration (0.5 ms) current pulses increased 28% in chronic spinal preparations. This is consistent with the increase in V Th in the same motoneurons. 6. Measured V Th was identical to that calculated from the product of RinL and rheobase in the unlesioned preparation but was significantly larger than calculated V Th in chronic spinal preparations. This may indicate an increased incidence or magnitude of subthreshold rectification processes in motoneurons from chronic spinal preparations. These results in barbiturate-anesthetized preparations suggest that ankle extensor motoneurons are less excitable in the chronic spinal state. 7. Mean afterhyperpolarization duration was 10% shorter in motoneurons from chronic spinal preparations, whereas amplitude was unchanged. 8. Electrical properties were also compared in chronic spinal and unlesioned preparations using motoneurons with action potential heights of 60-79 mV. In these motoneurons with presumably poorer impalements there were no significant differences between unlesioned and chronic spinal preparations. 9. Ia monosynaptic excitatory postsynaptic potentials (EPSPs) recorded in the same motoneurons have decreased half-widths and rise times and increased amplitudes.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of chronic spinalization on ankle extensor motoneurons. I. Composite monosynaptic Ia EPSPs in four motoneuron pools

1. We examined the effects of 6-wk chronic spinalization at the L1-L2 level on composite monosynaptic Ia excitatory postsynaptic potentials (EPSPs) recorded in medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus (SOL), and plantaris (PL) motoneurons. Amplitudes, rise times, and half-widths of composite monosynaptic Ia EPSPs evoked by low-strength electrical stimulation of peripheral nerves were measured in barbiturate-anesthetized cats and compared between unlesioned and chronic spinal preparations. 2. The mean amplitude of homonymous composite Ia EPSPs evoked by 1.2 times threshold (1.2T) stimulation and recorded in all four ankle extensor motoneuron pools increased 26% in chronic spinal animals compared with unlesioned controls. There was also an increased incidence of large-amplitude, short-rise time EPSPs. When the same data were separated according to individual motoneuron species, homonymous EPSP amplitudes in MG motoneurons were found to be unchanged. EPSPs recorded in LG motoneurons and evoked by stimulation of the combined LG and SOL nerve were increased by 46%. Mean EPSP amplitudes recorded in both SOL and PL motoneurons were larger after spinalization but statistical significance was only achieved when values from SOL and PL were combined to produce a larger sample size. 3. In LG motoneurons from chronic spinal animals, all EPSPs evoked by 1.2T stimulation of the LGS nerve were > or = 0.5 mV in amplitude. In unlesioned preparations, one fourth of the LG cells had EPSPs that were < or = 0.2 mV. 4. The mean amplitude of heteronymous EPSPs evoked by 2T stimulation of LGS and MG nerves and recorded in MG and LG motoneurons, respectively, doubled in size after chronic spinalization. Because homonymous EPSP amplitudes were unchanged in MG motoneurons, synaptic mechanisms and not passive membrane properties are likely responsible for increased heteronymous EPSP amplitudes in MG. 5. The mean 10-90% rise time of homonymous composite Ia EPSPs in pooled data from all motoneurons decreased 21% in 6-wk chronic spinal animals. Unlike EPSP amplitude, significant rise time decreases were found in all four motoneuron pools. Compared with the other motoneuron species, the mean homonymous rise time recorded in MG motoneurons was shortest and decreased the least in chronic spinal animals. Rise times of heteronymous Ia EPSPs in MG and LG motoneurons also decreased. The maximum rate of rise of homonymous EPSPs increased in all four motoneuron species. 6. The mean half-widths of Ia composite EPSPs decreased in 6-wk spinalized preparations in all motoneuron species.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of jaw orientation and position in mastication and speech

1. The kinematics of sagittal-plane jaw motion were assessed in mastication and speech. The movement paths were described in joint coordinates, in terms of the component rotations and translations. The analysis focused on the relationship between rotation and horizontal translation. Evidence was presented that these can be separately controlled. 2. In speech, jaw movements were studied during consonant-vowel utterances produced at different rates and volumes. In mastication, bolus placement, compliance, and size as well as chewing rate were manipulated. Jaw movements were recorded using the University of Wisconsin X-ray microbeam system. Jaw rotation and translation were calculated on the basis of the motion of X-ray tracking pellets on the jaw. 3. The average magnitudes of jaw rotation and translation were greater in mastication than in speech. In addition, in speech, it was shown that the average rotation magnitude may vary independent of the horizontal translation magnitude. In mastication, the average magnitude of vertical jaw translation was not dependent on the magnitudes of jaw rotation or horizontal jaw translation. 4. The magnitude of rotation and horizontal jaw translation tended to be correlated when examined on a trial by trial basis. Some subjects also showed a correlation between jaw rotation and vertical jaw translation. However, the proportion of variance accounted for was greater for all subjects in the case of rotation and horizontal translation. 5. Joint space paths in both mastication and speech were found to be straight. The pattern was observed at normal and fast rates of speech and mastication and for loud speech as well. Straight line paths were also observed when subjects produced utterances that had both the syllabic structure and the intonation pattern of speech. The findings suggest that control may be organized in terms of an equilibrium jaw orientation and an equilibrium jaw position. 6. Departures from linearity were also observed. These were typically associated with differences during jaw closing in the end time of rotation and translation. Start time differences were not observed in jaw closing and the movement paths were typically linear within this region.

Effects of chronic spinalization on ankle extensor motoneurons. III. Composite Ia EPSPs in motoneurons separated into motor unit types

1. In this paper we continue an examination of changes in composite Ia excitatory postsynaptic potentials (EPSPs) in ankle extensor motoneurons after 6-wk (L1-L2) spinal cordotomy. The ratio of rheobase to input resistance was used to divide motoneurons into three groups approximating fast-fatigable (FF), fast fatigue-resistant (FR), and slow (S) motor units in barbiturate-anesthetized cats. Homonymous monosynaptic Ia EPSPs evoked by low-strength [1.2 times threshold (T)] electrical stimulation and heteronymous EPSPs evoked by 2T stimulation were compared between groups of motoneurons in unlesioned and chronic spinal preparations. 2. The distribution of motor unit types of triceps surae and plantaris (PL) motoneurons according to the present classification scheme agrees well with that obtained elsewhere using mechanical typing. Chronic spinalization resulted in an increased proportion of type FF motoneurons in PL and type FR motoneurons in lateral gastrocnemius (LG) motoneurons. There was a numeric but insignificant increase in the proportion of fast medial gastrocnemius motor units. 3. Membrane time constant (tau m) and estimated total cell capacitance were significantly reduced in FF and S motoneurons in chronic spinal preparations. FF motoneurons from chronic spinal animals also had a reduced afterhyperpolarization duration. Mean values of membrane electrical properties in FR motoneurons were unaltered after spinalization. 4. Homonymous Ia EPSP changes after chronic spinalization occurred preferentially in type FR and S motor units. Amplitudes increased 69% in type FR and 38% type S motor units but were unchanged in type FF units. Furthermore, the amplitudes of heteronymous Ia EPSPs in type FF and S units in the chronic spinal preparation were almost double those in unlesioned preparations. 5. Homonymous EPSP 10-90% rise times decreased 25% in type FR motor units and 15% in type S motor units and were unchanged in type FF motor units. Homonymous EPSP half-width decreased in all three motoneuron groups. Normalization of EPSP rise time and half-width to tau m reduced the difference between EPSP shape indexes in unlesioned and chronic spinal preparations in type FF and S motoneurons but less so in type FR motoneurons. Normalized EPSP shape indexes in some type FR units were shorter after chronic spinalization than any in unlesioned preparations. 6. The increased amplitude and decreased rise time of Ia EPSPs in type FR motoneurons after spinalization occurred without changes in the electrical properties of type FR motor units.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustically induced cortical arousal increases phasic pharyngeal muscle and diaphragmatic EMG in NREM sleep

Six healthy subjects (3 males, 3 females) were studied to assess phasic inspiratory responses of upper airway (UA) and diaphragm muscles to electrocortical arousal independent of other potential respiratory stimulation. Transient electroencephalographic (EEG) arousal (abrupt EEG frequency shift > or = 3 s without awakening) was induced during supine stage 2 non-rapid-eye-movement (NREM) sleep with binaural tone bursts (0.5 s, 4 kHz, 25-95 dB). Electromyograms (EMG) of levator veli palatini (EMGlvp) and genioglossus (EMGgg) were obtained with intramuscular electrodes, and EMG of diaphragm (EMGdi) was obtained with esophageal electrodes. EMG signals were processed as moving time-averaged inspiratory activity over 100-ms windows. For each arousal, each of five consecutive postarousal breaths (R1-R5) was scored for peak inspiratory phasic EMG and normalized as percent averaged EMG of the three prearousal breaths for all muscles. After arousal, EMGlvp was increased for R1-R5 and EMGgg and EMGdi were increased for R1-R4. The increase in EMGlvp was greater than those of EMGgg and EMGdi for all response breaths. There was a significant increase in EMGlvp in all subjects, and EMGgg and EMGdi were significantly increased in three and two subjects, respectively. These data indicate that isolated transient electrocortical arousal is generally associated with phasic inspiratory recruitment of UA and diaphragm muscles in normal humans during NREM sleep; velopharyngeal muscle recruitment appears to be more consistent and of greater magnitude and duration than that of oropharyngeal muscle or diaphragm. We speculate that transient arousal from sleep may contribute to UA patency independent of chemical and mechanical respiratory stimuli.

Single fibre EMG studies in chronic fatigue syndrome: a reappraisal

Single fibre EMG studies were carried out on the right extensor digitorum communis muscle in 30 subjects with chronic fatigue syndrome and in 30 age and sex matched controls. Abnormal jitter was seen in five patients with chronic fatigue syndrome. Slight but significant differences between the mean consecutive differences in the remainder of the chronic fatigue subjects and the control subjects were recorded. Overall the differences were so minor that it seems unlikely that a disturbance of neuromuscular function as reflected by jitter measurement has a pathogenetic role. It is suggested that the increased jitter seen may be explained by the effects of the variability of motor unit firing rates on the myogenic component of the jitter.

Effects of scaphognathite nerve stimulation on the acutely deafferented crab ventilatory central pattern generator

1. Sensory axons from crab (Carcinus maenas) scaphognathites enter the thoracic ganglion primarily via the LNb branch of the levator nerve. The LNa branch of the levator nerve and the depressor nerve each contain relatively few sensory axons. 2. Acutely deafferented ventilatory central pattern generators show a free running burst rate which is lower than that observed in intact crabs. Electrical stimulation of the levator nerve, or of its LNb branch, increases the burst rate in a frequency dependent manner. Stimulation at high enough intensity to recruit afferents will restart a paused motor rhythm. Stimulation of the levator nerve with short pulse trains phase resets and can entrain the rhythm. 3. In addition to increasing the burst rate, LNb stimulation also causes a progressive elimination of motor neurons from the bursts as the stimulating frequency increases, probably due to depolarization of the 3 oval organ 'giant' afferent axons in this branch. Intracellular depolarization of single oval organ afferents will also inhibit some motor neurons as well as slow or stop the rhythm. 4. Continuous stimulation of the depressor nerve does not affect the ganglionic burst rate and this nerve contains only a few small diameter afferent axons; however, brief trains of stimuli can reset the rhythm in a phase-dependent manner.

Contractile activation and force generation in skinned rabbit muscle fibres: effects of hydrostatic pressure

1. Effects of hydrostatic pressure (range 0.1-10 MPa) on the isometric tension of skinned (rabbit psoas) muscle fibres were examined at 12 degrees C and at different levels of Ca2+ activation (pCa range 4-7); the effects on both the steady tension and the tension transients induced by rapid pressure release (< 1 ms) are described. 2. The steady tension was depressed by increased pressure (approximately 1% MPa-1) at a high level of Ca2+ activation (pCa approximately 4) whereas it was potentiated at lower Ca2+ levels (pCa > 6); the effects were reversible. 3. At maximal Ca2+ activation, the tension recovery following pressure release (10 MPa to atmospheric) consisted of a fast (approximately 30 s-1) and a slow (2-3 s-1) phase; the rate and the normalized amplitude (normalized to the steady tension at atmospheric pressure for a particular pCa) of the fast phase were invariant with changes in Ca2+ level. 4. The effects of changing Ca2+ level on the slow phase were complex; its positive amplitude at high Ca2+ levels changed to negative and the rate decreased to approximately 1 s-1 at low Ca2+ levels (pCa > 6.0). 5. Results are discussed in relation to previous studies on the effect of pressure on intact muscle fibres and the actin-myosin interaction. This work supports calcium regulation of cross-bridge recruitment rather than calcium regulation of the rate of a specific step in the cross-bridge cycle.

Activation-induced reduction of SDH activity in diaphragm muscle fibers

In this study, we examined whether exhaustive activation reduces succinate dehydrogenase (SDH) activity in diaphragm muscle fibers. In adult male rats (approximately 300 g), the costal diaphragm was excised and positioned in a chamber perfused with mammalian Ringer solution kept at 26 degrees C and oxygenated with 95% O2-5% CO2. The muscle was stimulated directly at 10 or 75 Hz in trains of 500 ms duration (1/s) for 8 min. An adjacent unstimulated segment of muscle served as control. The two muscle segments were frozen, and serial sections were stained for myofibrillar adenosinetriphosphatase activity after alkaline and acid preincubation to classify type I, IIa, and IIb fibers. The extent of glycogen utilization was also examined histochemically to confirm exhaustive activation of muscle fibers. SDH activity was quantified using a microdensitometric procedure implemented on an image-processing system. Exhaustive activation at both 10 and 75 Hz caused a significant decrease in SDH activity of all fiber types, with the decrease after 10-Hz stimulation being greater than that after 75-Hz stimulation. At both stimulation frequencies, type IIb fibers demonstrated the greatest decrease in SDH activity (36% after 10-Hz and 27% after 75-Hz stimulation), whereas type I and IIa fibers both displayed reductions of approximately 27 and approximately 19% after 10- and 75-Hz stimulation, respectively. The greater reduction of SDH activity in type IIb fibers indicates an inverse relationship between activation-induced reductions in SDH activity and fiber oxidative capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor unit recruitment and the gradation of muscle force

The capabilities of the different types of motor units are reviewed, and their properties in a variety of muscles are discussed. Because the tension-generating capacities of motor units are so different, the order in which they are recruited will have a strong influence on the way force output of the whole muscle is graded. Activation of motor units in a random order produces a roughly linear force increase with progressive recruitment, whereas recruitment of motor units in order of increasing force produces an approximately exponential force increase as the number of active motor units increases. The latter scheme allows fine control of weak movements and rapid production of powerful movements. Motor units are shown to be well adapted to the tasks they must perform, and a "compromise" motor unit will not fulfill all the tasks demanded of it. Finally, changes in motor unit properties produced by different activity patterns and by muscle reinnervation are reviewed, and the implications for rehabilitation are discussed.

Short-term synchronization of motor units in human extensor digitorum communis muscle: relation to contractile properties and voluntary control

Synchronous activity was studied in relation to the contractile properties of pairs of motor units (MUs) recorded with independent microelectrodes in the right extensor digitorum communis muscle (EDC) of human subjects during isometric finger extension. MU contractile properties were characterized in terms of the rise time and amplitude of twitch tensions extracted by spike-triggered averages of the extension force. Synchronization of MU discharges appeared in the form of narrow central peaks in the cross-correlograms of 35 of 50 pairs of MUs, suggesting the contribution of common last-order neurons. Synchronization peaks tended to be briefer and higher among pairs of MUs with slower and smaller twitches than among pairs of MUs with faster and larger twitches. The higher peaks of slow-contracting MUs suggest a greater effectiveness of the common synaptic inputs. The broader peaks of fast-contracting MUs might reflect an additional synchronization of the inputs to fast MUs at high force levels. The areas of the cross-correlogram peaks were similar for both groups and suggest that under our conditions, about three motoneurons would discharge synchronously for a given motoneuron spike. To test whether the amount of MU synchronization could be altered voluntarily, four subjects attempted to increase or decrease synchrony, using as feedback clicks triggered by coincident firings of the recorded MUs. In nine of 15 conditioning sessions, the magnitudes of the synchronization peaks showed significant changes in the intended direction. These results imply that supraspinal centers can control the relative amount of inputs that contribute to the synchronization of motoneuron discharges during voluntary contraction of EDC.

Models of recruitment and rate coding organization in motor-unit pools

1. Isometric muscle force and the surface electromyogram (EMG) were simulated from a model that predicted recruitment and firing times in a pool of 120 motor units under different levels of excitatory drive. The EMG-force relationships that emerged from simulations using various schedules of recruitment and rate coding were compared with those observed experimentally to determine which of the modeled schemes were plausible representations of the actual organization in motor-unit pools. 2. The model was comprised of three elements: a motoneuron model, a motor-unit force model, and a model of the surface EMG. Input to the neuron model was an excitatory drive function representing the net synaptic input to motoneurons during voluntary muscle contractions. Recruitment thresholds were assigned such that many motoneurons had low thresholds and relatively few neurons had high thresholds. Motoneuron firing rate increased as a linear function of excitatory drive between recruitment threshold and peak firing rate levels. The sequence of discharge times for each motoneuron was simulated as a random renewal process. 3. Motor-unit twitch force was estimated as an impulse response of a critically damped, second-order system. Twitch amplitudes were assigned according to rank in the recruitment order, and twitch contraction times were inversely related to twitch amplitude. Nonlinear force-firing rate behavior was simulated by varying motor-unit force gain as a function of the instantaneous firing rate and the contraction time of the unit. The total force exerted by the muscle was computed as the sum of the motor-unit forces. 4. Motor-unit action potentials were simulated on the basis of estimates of the number and location of motor-unit muscle fibers and the propagation velocity of the fiber action potentials. The number of fibers innervated by each unit was assumed to be directly proportional to the twitch force. The area of muscle encompassing unit fibers was proportional to the number of fibers innervated, and the location of motor-unit territories were randomly assigned within the muscle cross section. Action-potential propagation velocities were estimated from an inverse function of contraction time. The train of discharge times predicted from the motoneuron model determined the occurrence of each motor-unit action potential. The surface EMG was synthesized as the sum of all motor-unit action-potential trains. 5. Two recruitment conditions were tested: narrow (limit of recruitment < 50% maximum excitation) and broad recruitment range conditions (limit of recruitment > 70% maximum excitation).(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of neurons in the rat thalamic nucleus submedius to cutaneous, muscle and visceral nociceptive stimuli

The findings of recent studies have suggested that nucleus submedius (Sm) may be an important thalamic relay for nociceptive information. The aim of the present electrophysiological study was to examine in greater detail the activity and response properties of neurons in the rat Sm in order to further evaluate this hypothesis. Single unit extracellular recordings from neurons histologically verified to be in Sm were obtained in urethane/chloralose-anesthetized rats. Noxious but not innocuous mechanical stimulation elicited responses in 75% of the 204 neurons studied. Most (85%) of these neurons were excited, 10% were inhibited and a few neurons (5%) were excited by stimulation at some sites on the body and inhibited from other sites. The receptive fields were usually very large and bilateral. No marked differences were observed in the incidence, response type, or spontaneous activity of neurons located in dorsal, ventral, rostral or caudal parts of Sm. Most of these neurons (99 of 108, 92%) also responded to noxious heating and had a mean threshold of 47 degrees C. The majority of the neurons (19 of 21, 90%) also responded to subcutaneous, intramuscular or intraperitoneal injections of noxious chemicals (formalin or hypertonic saline). The responses elicited by pinching skin or squeezing muscle were frequently facilitated by the subcutaneous or intramuscular injections of formalin. Single electrical stimuli delivered to the cutaneous receptive field rarely produced responses. However, short trains (15-25 msec trains of 200 Hz, 3 msec pulses at 5-10 mA) delivered repetitively elicited responses in 90% (n = 73) of the neurons. These responses appearing after repetitive stimulation frequently resembled the 'wind-up' pattern observed in spinal cord dorsal horn. The conduction velocities of the primary afferents which elicited the Sm neuronal responses as estimated from the latency differences of responses elicited by stimulation at two points along the tail, were indicative of recruitment of A delta and C fibers. These findings provide further support for the proposed role of Sm in thalamic nociceptive mechanisms.

Recruitment of inhibition by enhanced activation of synaptic NMDA responses in the rat cerebral cortex

Intracellular recordings of layer V neurons from rat neocortical slices were obtained to examine the effects of reducing extracellular magnesium on inhibition. Magnesium-free solutions induced interictal and ictal-like events in cortical neurons. Changes in synaptic events underlying epileptogenesis were studied when extracellular calcium was raised (from 2 to 3-7 mM) since this delayed seizure activity. With increasing time of exposure of cells to magnesium-free solutions, there was a significant increase in the size and duration of both the depolarizing and slow synaptic hyperpolarizing responses, but the fast synaptic hyperpolarization significantly declined in amplitude. When cells were recorded with cesium acetate-filled microelectrodes slow hyperpolarizing responses were blocked, but depolarization of cells to 0 mV allowed an isolated fast hyperpolarizing response to be recorded following synaptic stimulation. The amplitude of this response was unchanged after exposure to magnesium-free solutions. Synaptic responses of cells initially bathed in an N-methyl-D-aspartate (NMDA) antagonist (CPP) were unchanged by subsequent exposure to magnesium-free solutions. CPP exposure by itself caused a decrease in depolarization duration, increase in fast hyperpolarizing amplitude, and decrease in slow hyperpolarization amplitude and duration. When the fast hyperpolarization was viewed in isolation (cesium recording electrodes) at 0 mV, the amplitude of this event was unchanged by exposure to CPP. Given these results stimulus-response characteristics of neocortical neurons were reassessed under control conditions. With higher intensity stimuli larger depolarizing and slow hyperpolarizing responses were evoked, but the fast hyperpolarization showed a decremental response. These effects were reversed when CPP was added. When NMDA activity was enhanced by exposure to magnesium-free solutions or electrical stimulation, the amplitude of excitatory events and slow hyperpolarizations increased, but fast inhibitory responses showed limited capacity for incremental recruitment. This suggests fast inhibition is saturated (maximal) at submaximal levels of excitation, and can be overcome by increasing levels of excitation. Such a process is active under physiological conditions, altering the efficacy of inhibition.

Recruitment of motor units in response to transcranial magnetic stimulation in man

1. Short-latency responses of single motor units (SMUs) and surface electromyographic activity (EMG) to transcranial magnetic stimulation (TMS) were examined in five different hand and forearm muscles of human subjects. 2. The response probability, P (number of extra spikes in the response peak above background per stimulus), was, in general, higher at the lower voluntary discharge rate of the motor unit than at the higher rate. 3. Increasing the strength of TMS increased the response probability of a tonically firing motor unit and at the same time recruited new units which discharged phasically during the response peak. This demonstrates rate coding and recruitment of motor units by excitatory inputs resulting from TMS when the motoneurone pool is tonically facilitated by a constant voluntary drive. 4. Next, TMS was delivered without any voluntary facilitation of motoneurones. The order of recruitment for up to four different motor units discharged by TMS was compared to that observed with voluntary input. The threshold of recruitment for each of the two inputs was estimated from the surface EMG value at which the unit was recruited. For these motoneurone pools (eleven sets of observations), the order of recruitment was the same with TMS and voluntary inputs. 5. From these data it is concluded that, despite the complex and phasic nature of the descending corticospinal volleys generated by TMS, it produces orderly recruitment and rate coding of motoneurones similar to that found for voluntary activation.

Motor-unit recruitment in self-reinnervated muscle

1. Recruitment order of motor units in self-reinnervated medial gastrocnemius (MG) muscles was studied in decerebrate cats 16 mo after surgical reunion of the cut MG nerve. Pairs of MG motor units were isolated by dual microelectrode penetration of ventral roots to measure their recruitment sequence during cutaneous reflexes in relation to their physiological properties. 2. Physiological properties of reconstituted motor units appeared normal, as expected. Also normal were the relationships among these properties: twitch and tetanic tension tended to increase with axonal conduction velocity and decrease with twitch contraction time. A small fraction of motor units (10/116) in reinnervated muscles produced either no measurable tension or unusually large amounts of tension compared with controls. This was the only distinct feature of the sample of reconstituted units. 3. In muscles reinnervated after nerve section, stretch was notably ineffective in eliciting reflex contraction of MG muscles or their constituent motor units (only 5/116 units). Incomplete recovery from nerve section was probably the cause of this impairment, because stretch reflexes were readily evoked in adjacent untreated muscles and in one reinnervated MG muscle that was studied 16 mo after nerve crush. In contrast with the ineffectiveness of muscle stretch, sural nerve stimulation succeeded in recruiting 49/116 units, a proportion fairly typical of normal MG muscles. 4. The contractions of the first unit recruited in cutaneous reflexes tended to be slower and less forceful than those of the other unit in a pair. By these measures, recruitment obeyed the size principle. This recruitment order with respect to unit contractile properties was not significantly different (P > 0.05) between untreated and reinnervated muscles but was significantly (P < 0.005) different from random order in both groups. The same recruitment pattern was observed for pairs of motor units sampled from the muscle reinnervated after nerve crush, whether units were recruited by muscle stretch or sural nerve stimulation. 5. The usual tendency for motor units with slower conduction velocity (CV) to be recruited in sural nerve reflexes before those with faster CV was not strong in reinnervated muscles. After nerve section the proportion of units exhibiting the usual recruitment pattern was not significantly different (P > 0.05) from a random pattern for CV. 6. The central finding is that the normal recruitment patterns recover from nerve injury in a muscle that is reinnervated by its original nerve. By contrast, stretch reflexes do not recover well from nerve section, and this deficiency may contribute to motor disability.

The firing rates of human motoneurones voluntarily activated in the absence of muscle afferent feedback

1. To quantify the net influence of muscle afferent feedback on the firing rates of human motoneurones, the discharge frequencies of single motor axons in the common peroneal nerve were recorded during sustained voluntary efforts performed in the absence of feedback from the target muscle. These data were compared with the firing rates of single motor units in the intact tibialis anterior muscle. In five subjects, recordings were made from fifty-two motor axons innervating tibialis anterior during acute deafferentation and paralysis of the dorsiflexor muscles produced by anaesthetic block of the nerve distal to the recording site. 2. Maximal sustainable firing rates were determined for twenty-four motoneurons, twelve of which were classified as relatively low threshold (estimated recruitment level < or = 10% maximal) and six as high threshold. Mean firing rates of the low-threshold motoneurones (21.7 +/- 2.7 Hz; +/- S.E.M.) were significantly higher than those of the high-threshold motoneurones (14.0 +/- 4.4 Hz). The mean firing rate of the twenty-four deafferented motoneurones during maximal efforts to contract the paralysed muscle was 18.6 +/- 1.9 Hz, significantly lower than the maximal firing rates of single motor units recorded from the normally innervated tibialis anterior muscle (28.2 +/- 0.6 Hz). 3. During half-maximal efforts, the mean firing rate of eight deafferented motoneurones (10.8 +/- 1.1 Hz) was significantly lower than that of intact motor units (16.5 +/- 0.2 Hz). A similar finding was apparent during minimal efforts; the mean discharge frequency of seven deafferented motoneurones during weak voluntary efforts was 6.0 +/- 0.9 Hz, compared with 7.3 +/- 0.13 Hz for intact motor units. Overall, the range of motoneurone firing rates (from minimal to maximal levels of voluntary effort) was significantly affected by the acute deafferentation, but was shifted significantly to lower rates. 4. During sustained maximal voluntary efforts of at least 30 s duration the firing rate of deafferented motoneurones decreased over the first 5 s but was then maintained, i.e. there was no progressive decline as occurs with normally innervated motor units during fatiguing contractions. This observation supports a reflex origin for the normal decline in motoneurone discharge. 5. It is concluded that muscle afferents in the common peroneal nerve provide a net facilitation to the tibialis anterior motoneurone pool, reflexly increasing the motor output at all levels of voluntary drive by approximately one-third.

Synchronization of motor-unit firings in several human muscles

1. Synchronization of concurrently active motor-unit firings was studied in six human muscles performing isometric constant-force contractions at 30% of the maximal level. The myoelectric signal was detected with a quadrifilar needle electrode and was decomposed into its constituent motor-unit action-potential trains with the Precision Decomposition technique, whose accuracy has been proven previously. 2. Synchronization was considered as the tendency of two motor units to fire at fixed time intervals with respect to each other more often than would be expected if the motor units fired independently. A rigorous statistical technique was used to measure the presence of peaks in the cross-interval histogram of pairs of motor-unit action-potential trains. The location of the center of peak as well as their width and amplitude were measured. A synch index was developed to measure the percentage of firings that were synchronized. The percentage of concurrently active motor-unit pairs that contained synchronized firings was measured. 3. Synchronization of motor-unit firings was observed to occur in two modalities. The short-term modality was seen as a peak in the cross-interval histogram centered about zero-time delay (0.5 +/- 2.9 ms, mean +/- SD) and with an average width of 4.5 +/- 2.5 ms. The long-term modality was seen as a peak centered at latencies ranging from 8 to 76 ms. On the average, the peaks of the long-term synchronization were 36% lower but had approximately the same width as the peaks for the short-term synchronization. Short-term synchronization was seen in 60% of the motor-unit paris, whereas long-term synchronization was seen in 10% of the pairs. 4. Short-term synchronization occurred in bursts of consecutive firings, ranging in number from 1 to 10, with 91% of all synchronized firing occurring in groups of 1 or 2; and the bursts of discharges appeared at sporadic times during the contraction. 5. The amount of synchronization in motor-unit pairs was found to be low. In the six muscles that were tested, an average of 8.0% of all the firings were short-term synchronized, and an average of 1.0% were long-term synchronized. The synch index was statistically indistinguishable (P = 0.07-0.89) among the different muscles and among 9 of the 11 subjects tested. 6. Sixty percent of concurrently active motor-unit pairs displayed short-term synchronization, 10% of the pairs displayed long-term synchronization, and 8% displayed both modalities.(ABSTRACT TRUNCATED AT 400 WORDS)

H-reflex changes during contractions of the ankle extensors in spastic patients

Soleus H-reflexes during tonic contractions and isometric ramp contractions of the ankle extensors in spastic and healthy subjects were measured. During the tonic contractions, the H-reflex increased with the contraction level. The increase was highest in the patients (p < 0.001). The facilitation of the H-reflex during a ramp contraction is due to a static component resulting from the increased excitation level and a dynamic component resulting from the modulation of the H-reflex. The dynamic H-reflex facilitation during the ramp contraction was decreased in the patients (p < 0.05). The findings suggest that there is a decreased H-reflex control in spastic patients and this could be explained by a decreased presynaptic inhibition or by postsynaptic changes.

Computer simulations of the effects of different synaptic input systems on motor unit recruitment

1. The effects of four different synaptic input systems on the recruitment order within a mammalian motoneuron pool were investigated using computer simulations. The synaptic inputs and motor unit properties in the model were based as closely as possible on the available experimental data for the cat medial gastrocnemius pool and muscle. Monte Carlo techniques were employed to add random variance to the motor unit thresholds and forces and to sample the resulting recruitment orders. 2. The effects of the synaptic inputs on recruitment order depended on how they modified the range of recruitment thresholds established by differences in the intrinsic current thresholds of the motoneurons. Application of a uniform synaptic input to the pool (i.e., distributed equally to all motoneurons) resulted in a recruitment sequence that was quite stable even with the addition of large amounts of random variance. With 50% added random variance, the recruitment reversals did not exceed 8%. 3. The simulated monosynaptic input from homonymous Ia afferent fibers generated a twofold expansion of the range of recruitment thresholds beyond that attributed to the differences in the intrinsic current thresholds. The Ia input generated a small reduction in the number of recruitment reversals due to random variance (6% reversals at 50% random variance). The simulated monosynaptic vestibulospinal input generated a twofold compression of the range of recruitment thresholds that exerted a modest increase in the number of recruitment reversals (12% reversals at 50% random variance). 4. In comparison with the modest effects of the two monosynaptic inputs, the simulated oligosynpatic rubrospinal excitatory input exerted a nine-fold compression in the recruitment threshold range that resulted in a recruitment sequence that was highly sensitive to random variance. With 50% added random variance, the sequence became nearly random (40% reversals). 5. Reciprocal Ia inhibition was simulated by a uniform distribution within the pool, but its effects on recruitment order were highly dependent on the distribution of the excitatory input. Reciprocal inhibition exerted only minor effects on recruitment order when combined with the Ia or vestibulospinal inputs. However, when the excitatory drive was supplied by the rubrospinal input, even small amounts of reciprocal inhibition were sufficient to completely reverse the normal recruitment sequence. 6. The simulated monosynaptic Ia input was highly effective in compensating for the disruptive effects of rubrospinal excitation on recruitment order. Even a small Ia bias combined with the rubrospinal excitation was sufficient to halve the effects of random variance and to restore the normal recruitment sequence in the presence of rather large amounts of reciprocal inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Declining inhibition elicited in cat lumbar motoneurons by repetitive stimulation of group II muscle afferents

1. The aim of the present experiments was to verify whether group II inputs from gastrocnemius medialis (GM) muscle could elicit declining inhibitions similar to those observed during GM contractions in a variety of lumbar motoneurons of the cat spinal cord. Motoneurons were recorded intracellularly in chloralose- or pentobarbitone-anesthetized preparations during electrical stimulation of GM nerve with repetitive trains. 2. With strengths in the group I range, repetitive stimulation evoked the usual Ia excitation in homonymous motoneurons and excitatory postsynaptic potential (EPSP) amplitudes remained constant throughout the stimulation sequence. In synergic plantaris motoneurons lacking an excitatory connection with Ia afferents from GM, the same stimulation, kept at a constant strength throughout the stimulation sequence, elicited rapidly decreasing inhibitory potentials reminiscent of those evoked by GM contractions. 3. In motoneurons of pretibial flexors, quadriceps, and posterior biceps-semitendinosus, the stimulation strength required to observe declining inhibitions resembling those produced by GM contractions was 4-8 times group I threshold, engaging group II in addition to group I fibers. 4. These results show that input from GM group II plus group I afferents can elicit inhibitory effects in a variety of motoneurons. Such observations support the hypothesis that messages from spindle secondary endings and/or nonspecific muscle receptors activated during contraction might contribute to the widespread inhibition caused by GM contractions. 5. Inasmuch as constant input in group II and group I afferents evoked declining inhibitory potentials, the origin of the decline must be central, which suggests that the rapid reduction of contraction-induced inhibitions also depended on a central mechanism.

Declining inhibition in ipsi- and contralateral lumbar motoneurons during contractions of an ankle extensor muscle in the cat

1. Motoneurons of pretibial ankle flexor and knee flexor and extensor muscles were recorded intracellularly in chloralose- or pentobarbitone-anesthetized cats during sustained submaximal contractions of either ipsi- or contralateral gastrocnemius medialis muscle (GM). 2. In a majority of ipsilateral motoneurons, a sustained GM contraction elicited inhibitory potentials that quickly subsided before the end of the contraction. An abrupt increase in contractile force could elicit a new series of inhibitory potentials, which declined again in spite of a maintained force level. 3. Contraction-induced effects were only exceptionally detected in contralateral triceps surae and plantaris motoneurons. In a small number of pretibial flexor and knee flexor and extensor motoneurons, declining inhibitions were observed during sustained contractions of the contralateral GM muscle. 4. At the onset of GM contractions, a variety of motoneurons uniformly receive inhibitory inputs that are quickly filtered out. Although the functional significance of this widespread initial inhibition remains to be elucidated, its rapid decline seems useful to allow subsequent recruitment of motor units as may be required for coordination of posture and movement. 5. Tendon organs are activated during muscle contraction, but it is not certain whether Ib inputs from GM can account for all the effects observed. Contribution of other afferents was considered and tested using a different experimental approach. The companion paper reports observations suggesting that effects elicited by group II afferents may cooperate in the contraction-induced inhibition of motoneurons.

Spinal oscillators in man under normal and pathologic conditions

1. Single-fibre action potentials (APs) were recorded from lower sacral nerve roots of brain-dead humans (HTs) and two paraplegics with thoracical spinal cord lesions. Impulse patterns of single oscillatory firing alpha 2 and alpha 3-motoneurons were identified and analysed. An alpha 2-motoneuron fired typically with 3 AP impulse trains every 160 msec. 2. In stable spinal oscillators of HTs the oscillation period was 70 msec plus 30 msec times the number of APs per impulse train; the respective values in paraplegics were 35 msec plus 40 msec times the number of APs per impulse train. These linear relationships indicate that spinal oscillators consist of at least 2 kinds of nerve cells. 3. Successive interspike intervals (IIs) of the impulse trains increased from about 3.5 msec to over 10 msec in paraplegics similarly as was the case in HTs. 4. The distributions of the first IIs (of impulse trains) and of the oscillation period of unstable continuously oscillatory firing alpha 2-motoneuron showed similar peaks. By relating the peak values of the first II and the oscillation period, a shortest II of 3.5 msec was obtained in a HT and a paraplegic, and a shortest oscillation period of 70 msec was measured in a HT and 40 msec in a paraplegic. The derivation of the shortest II of 3.5 msec from the oscillation itself is taken as an indication for alpha 2-motoneurons being included in the spinal oscillator function, since the shortest soma-dendritic spike intervals of motoneurons is expected to be 3.5 msec. 5. The spinal oscillator of a paraplegic showed more and higher activity changes than that in a HT as if spinal oscillators in paraplegics were insufficiently damped. 6. By interpreting the peaks in oscillation period distributions of unstable oscillators as being different oscillation loop pathways, it is speculated that the oscillators consist of the motoneuron, to which different interneuron pathways are connected. A contrasting of pathways with increasing excitation of the oscillator was observed. 7. The comparison of possible loop pathways of oscillation between paraplegics and HTs (closer to normal) indicates that the loss of descending tracts in paraplegics and the adaptation to it result in an increase of the oscillation pathways for alpha 2-oscillators from 1 to 3 in HTs to up to 6 in paraplegics. 8. Different measured and theoretically predictable spinal alpha 2-oscillators can cover altogether a frequency range between 5 and 10 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Recruitment of motoneurons in the occasional firing mode in paraplegics

1. Single-fibre action potentials were recorded from lower sacral nerve roots of two humans with thoracical spinal cord lesions. Conduction velocities were calculated, distribution histograms of representative conduction velocity frequencies constructed, and recruitment of motoneurons in the different groups was measured. 2. Following anal and bladder catheter pulling, slowly conducting gamma 1, alpha 3, alpha 2 and alpha 1-motoneurons were recruited before the faster conducting motoneurons in each group separately. Motoneurons were recruited repeatedly at approx. every 2.5 sec. 3. The slowly conducting alpha 2-motoneurons (FR) were recruited approx. 1 sec following stimulation, the slowly conducting alpha 3-motoneurons (S) were mostly recruited following stimulation. 4. Upon no additional stimulation, slowly and fast conducting motoneurons were recruited repeatedly within the groups of alpha 2 and alpha 3-motoneurons every 2.5 sec. 5. The faster recruitment of alpha 3-motoneurons in comparison to alpha 2-motoneurons differed from that observed during measurements in brain-dead humans and in rats and dogs, and is discussed with respect to the loss of interneurons in the spinal cord.

Motor unit recruitment during prolonged isometric contractions

Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P < 0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed "motor unit rotation" and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.

Cutaneous stimulation fails to alter motor unit recruitment in the decerebrate cat

1. An attempt was made to repeat the observation that cutaneous input to the cat medial gastrocnemius (MG) muscle sometimes had the differential effect of inhibiting motoneurons with slow axonal conduction velocity while simultaneously exciting others with fast conduction velocity. Dual microelectrode recording from intact ventral root filaments was used to study the effects of cutaneous inputs on recruitment order and on firing frequency of physiologically characterized MG motor units in decerebrate cats. Motor responses to pinch of the skin over the lateral surface of the ankle as well as electrical stimulation of the caudal cutaneous sural (CCS) nerve were contrasted with the responses to static muscle stretch as well as muscle vibration. 2. In contrast to the prediction, recruitment order in pairwise tests was the same for skin pinch or CCS stimulation as it was for MG stretch or vibration in all 32 tested pairs of motor units. This sample included seven pairs comprising one slow-twitch (S) and one fast-twitch motor unit, where the predicted reversal of recruitment should have been most apparent. Regardless of the source of excitation, recruitment of motor units of the MG was consistent with Henneman's size principle in approximately 90% of trials. 3. Skin pinch increased the firing rate of 30 of 32 individual motor units previously activated by stretch or vibration, including 7 slow-twitch units. In the remaining two units, skin pinch transiently (100-400 ms) slowed the firing of an S unit in 11 of 13 vibration + pinch trials. The other unit (type unknown) showed one or two retarded spikes in each of four vibration + pinch trials. In three S units, including the lone inhibitable unit and two others that were only excited by skin pinch, there was a significant positive rank correlation between change in unit firing frequency and change in soleus integrated electromyographic activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Recruitment of alpha and gamma-motoneurons in rats, dogs and humans

1. Single fibre action potentials were recorded with 2 pairs of wire electrodes from human and dog lower sacral nerve roots and the rat nervus suralis. From the widths of single peaks of alpha 1 (FF), alpha 2 (FR), alpha 3 (S) (extrafusal) and gamma 1 and gamma 21-motoneurons (intrafusal) conduction velocity frequency distribution histograms were constructed and the limits of the velocity ranges determined. Distribution changes of conduction velocities in each group of alpha and gamma-motoneurons were used for recruitment analysis in the occasional firing mode. 2. In the dog, the rat and the human, the slower conducting fibres were recruited before the faster conducting ones in each group of motoneurons. In the dog, the slowly conducting gamma 1 and alpha 2-motoneurons were recruited directly following bladder catheter pulling, and the slowly conducting alpha 3-motoneurons were recruited 1 sec later. In the rat, the slowly conducting alpha 1-motoneurons were recruited directly following pin-pricking of the hindlimb, the slowly conducting alpha 2-motoneurons 0.2 sec later, and the slowly conducting alpha 3-motoneurons 1 sec later. In humans, the slowly conducting gamma 1 and alpha 2-motoneurons were recruited 2 sec following bladder catheter pulling, the slowly conducting alpha 3-motoneurons 4 sec following pulling. 3. In the dog, with no additional stimulation slowly (and fast) conducting gamma 21 and alpha 3-motoneurons showed repeated activation 3 to 4 sec later. A subgroup of the alpha 1-motoneurons showed repeated activation every 2 sec. In the rat, without stimulation, a slower conducting subgroup of the alpha 1-motoneurons showed preferential activation of the low and high conduction velocities every 2 sec. In the human, with no additional stimulation, the slowly and fast conducting alpha 2 and alpha 3-motoneurons were recruited repeatedly every 2 sec.

The distal hindlimb musculature of the cat: interanimal variability of locomotor activity and cutaneous reflexes

During stereotyped behaviors such as locomotion, patterns of muscle recruitment are usually quite consistent from animal to animal, even in the face of many surgical and pharmacological reductions. However, as studies of musculoskeletal structure, neuromuscular architecture, and sensorimotor circuitry become more detailed, it is important to ask whether there is some level of organization at which individual differences begin to dominate. This study concentrated on the small muscles of the foot and ankle, using standardized methods that consistently record stereotypical electromyographic activity from prime mover muscles and that permit well-calibrated stimulation of cutaneous nerves to elicit reflexes during treadmill locomotion. Some muscles (particularly the main ankle extensors, triceps surae, and plantaris) had stereotyped activity during both unperturbed locomotion and reflex responses. Others had stereotyped activity during locomotion but variable reflex patterns among animals (tibialis anterior, extensor digitorum longus, flexor hallucis longus, and peroneus brevis). Still others had variable locomotor activity but reflexes that were consistent (flexor digitorum longus) or variable for only peroneal nerve stimulation (peroneus longus), only plantar nerve stimulation (peroneus tertius), or the two (flexor digitorum brevis). Among muscles with interanimal variability, there seemed to be no particular correlation between locomotor and reflexive recruitment in a given animal. This functional heterogeneity is discussed in terms of the development of locomotor and reflex programs and in the context of structural heterogeneity of some of these muscles that is described in the companion paper.

Neural recruitment explains "Weber's law" of spatial position

We ask whether the well known Weber's law between spatial localization and element separation for high contrast, spectrally broad-band stimuli is a consequence of the organization of the early visual filters, or a fundamental constraint on the computation of spatial position by more central mechanisms. We address this question by identifying the individual contributions of mechanisms tuned to different ranges of spatial frequencies and contrast. We measure spatial-alignment and bisection error as a function of element separation at each of a number of spatial scales, using spectrally narrow-band stimuli of fixed supra-threshold contrast. We show that stimuli which minimize the extent of neural recruitment across different spatial channels before the site of extraction of the local contrast energy (and to a lesser extent across different contrast channels) do not exhibit Weber's law for either alignment or bisection. We present evidence that Weber's law for localization with increasing separation, found for stimuli of high contrast and broad-band spatial frequency content, is a consequence of the successive disengagement of unitary neural mechanisms, each of which has different spatial and contrast properties, and none of which individually exhibits Weber's law for spatial position.

Comparison of fluctuations of motor unit recruitment and de-recruitment thresholds in man

Recruitment and de-recruitment thresholds of motor units in the wrist extensor muscles can undergo important random fluctuations, even when they are measured during stereotyped contractions and relaxations. These fluctuations were statistically quantified and compared. The statistical analysis indicated that recruitment and de-recruitment thresholds display the same kind of fluctuations, and that the successive measurements are randomly distributed following a quasi-normal law. We suggest that the notion of force threshold for motor unit recruitment and de-recruitment might be oversimplified and that a motor unit seems to have a range of force in which it can be recruited or de-recruited. Comparison of the mean values of recruitment and de-recruitment thresholds of the motor units in the extensor carpi radialis muscles showed that de-recruitment thresholds were significantly lower than recruitment thresholds. This difference in the thresholds, together with the difference in the motor unit discharge frequency during a contraction and a relaxation, suggests a differential control of the motoneurone activity during contractions and relaxations.

Diaphragm reinnervation by laryngeal motoneurons

Inspiratory activity of the paralyzed diaphragm was restored by reinnervation with brain stem laryngeal motoneurons. In 10 anesthetized cats, the right recurrent laryngeal nerve (RLN) was cut and anastomosed to the distal stump of either one or both roots (C5-C6) of the ipsilateral phrenic nerve. Three to four months later, reinnervation was assessed under deep anesthesia by the reappearance in the paralyzed diaphragm of 1) direct electromyographic (EMG) responses after electrical stimulation of the RLN and 2) spontaneous inspiratory bursts. Serial radiography, performed on five animals, revealed diaphragmatic excursions of comparable amplitude on the normal and reinnervated sides. Six to twelve months after anastomosis, laparotomy (performed under Nembutal anesthesia) allowed inspection and EMG recording of the spontaneous inspiratory contractions of the reinnervated areas and their sustained responses to tetanic RLN stimulation. Inspiratory discharges showed a ramplike recruitment similar to that of the normal diaphragm. Although the RLN contains a number of expiratory axons, multiple-site recordings disclosed expiratory EMG discharges only once. Histological analysis confirmed the substitution of phrenic axons by regenerating RLN fibers.

Soleus H-reflex tests and clinical signs of the upper motor neuron syndrome

Soleus H-reflex tests are used for elucidating pathophysiological mechanisms in motor control. The cumulative vibratory inhibition of the soleus H-reflex, the ratio of the reflex to direct muscle potential (H to M ratio) and the recovery curve of the soleus H-reflex were studied in 38 patients with varying signs of the upper motor neuron syndrome for a possible relation with clinical features. The results were compared with those obtained from a group of healthy volunteers. The magnitude of vibratory inhibition decreased with increase of hypertonia. The H to M ratio increased as the activity of the tendon reflex was enhanced and correlated to a lesser degree with muscle tone. Both the H to M ratio and late facilitation of the soleus H-reflex recovery curve were elevated in clonus. The findings suggest that alterations in the results of soleus H-reflex tests relate to specific clinical features of the upper motor neuron syndrome. Possible pathophysiological implications are discussed.

Change in recruitment order of motor units in human parasternal intercostal muscles with sleep state

Recruitment order of individual motor units in the early part of inspiration in parasternal intercostal muscles was observed in normal human subjects during wakefulness and non-rapid-eye-movement sleep. Electromyograms from bipolar fine wire intramuscular electrodes were recorded while the subjects lay supine in a sleep laboratory, and sleep stage was determined by polysomnography. From wakefulness to sleep there were numerous examples of shifts in order of recruitment among the low threshold units of early inspiration. There were corresponding shifts in the order of derecruitment of these units. Analysis of frequency of firing of units also suggested that the levels of excitatory input to one unit of a pair could be altered relative to the level of input of the other one. The data imply that there are at least minor differences in distribution of excitatory inputs from various sources among motoneurons of this muscle pool.

The roles of spatial recruitment and discharge frequency in spinal cord coding of pain: a combined electrophysiological and imaging investigation

An investigation was conducted to examine both temporal and spatial factors likely to be involved in spinal cord nociceptive coding by wide cord nociceptive neurons. Three separate methodologies were employed. First, the impulse frequency responses of L4 spinal cord wide-dynamic-range (WDR) neurons to gentle mechanical stimulation, vigorous but innocuous brushing, warmth (43 degrees C), and nociceptive thermal stimuli (45-49 degrees C) were electrophysiologically characterized in unanesthetized, spinal cord-transected rats. Second, the spatial distribution of evoked activity in response to the same types of mechanical and thermal stimuli was examined utilizing the 14C-2-deoxyglucose (2-DG) metabolic mapping method in the same type of animal preparation. Finally, the contributions of impulse frequency and numbers of neurons activated to encoding the distinction between painful and non-painful sensations were directly evaluated by electrically stimulating axons within the spinal cord anterolateral quadrant (ALQ) of conscious human subjects. Electrophysiological findings revealed that vigorous but innocuous brushing produced intermediate rates of impulse discharge significantly greater than those produced by 35 and 43 degrees C stimuli, yet indistinguishable from those produced by relatively low nociceptive temperatures (45-47 degrees C). Thus, the discharge frequencies of individual dorsal horn WDR neurons alone do not provide sufficient information to encode the distinction between innocuous and low intensity nociceptive stimuli. Mapping of spinal cord activity by the 2-DG method revealed that nociceptive stimuli activated extensive rostro-caudal regions extending from L1-L5. In contrast, vigorous but innocuous brushing evoked metabolic activity that was confined to a narrow zone within L3. Thus, as predicted from previous studies, the distinction between nociceptive and non-nociceptive sensory events may be encoded, in part, by differences in the spatial distribution, and hence, the relative numbers of spinal cord neurons activated by nociceptive and innocuous stimuli. The responses of conscious human subjects to varying frequencies and intensities of electrical ALQ stimulation clarify the significance of the large numbers of spinal cord neurons activated by nociceptive stimuli. With stimulus frequency held constant at 50 Hz, low stimulus currents, sufficient to activate only small numbers of ALQ axons, produced innocuous sensations. Higher stimulus currents, sufficient to activate larger numbers of neurons, consistently produced painful sensations. Increasing ALQ stimulus frequency at currents subthreshold for pain or increasing stimulus currents at frequencies subthreshold for pain resulted in painful sensations, thus indicating that both discharge frequency and numbers of neurons activated are both important factors in the encoding of pain.(ABSTRACT TRUNCATED AT 400 WORDS)

Recruitment patterns in the cervical paraspinal muscles during cervical forward flexion: evidence of cervical flexion-relaxation

The phenomenon of lumbar paraspinal flexion-relaxation has been well established and its clinical significance to low back pain has been demonstrated. Conversely, cervical paraspinal flexion-relaxation has not been consistently observed. This may be attributable to the inappropriate use of trunk flexion motion used to observe the phenomenon in previous studies. This investigation reports on the observance of cervical flexion-relaxation when flexion is limited to the cervical spine. Results demonstrate the observance of cervical flexion-relaxation in ten of ten asymptomatic subjects when flexion is limited to the cervical region in the seated orthograde position.

Reduced control of motor output in a human hand muscle of elderly subjects during submaximal contractions

1. The effect of age on the motor output of the first dorsal interosseous muscle of 22 (6 female, 16 male) human subjects was investigated. The purpose of the study was to determine the effect of age on the control of muscle force and the associated changes in the discharge behavior and mechanical properties of single motor units. 2. Each subject performed three tasks requiring isometric abduction of the left index finger: a maximum voluntary contraction (MVC), a constant-force task, and a threshold task. The ability to control force was assessed during the constant-force task by quantifying the variation in isometric force about four submaximal target forces (5, 20, 35, and 50% MVC). The threshold task involved sustaining the discharge of the isolated motor unit at a low, steady rate for approximately 3 min. 3. The discharge behavior and the mechanical properties of single motor units were determined during the threshold task by measuring the interimpulse intervals and the peak amplitude and time to peak of the spike-triggered average force. 4. The data indicated that age had an effect on the variation of force about submaximal target forces (range: 5-50% MVC), and that these force variations, when calculated relative to the target force, were greater at lower force levels in the elderly subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired activation pattern in antagonistic elbow muscles of patients with spastic hemiparesis: contribution to movement disorder

Surface electromyographic "EMG" activity in biceps brachii (agonist muscle) and triceps brachii (antagonist muscle) as well as the discharge behaviour of motor units "MUs" (needle recording) in biceps brachii muscle were recorded during slow (0.33 Hz) and fast (0.66 Hz) voluntary elbow flexion movements (auditory matching task) in fifty patients with spastic hemiplegia. In the spastic limbs, a long lasting, small amplitude tonic co-contraction of antagonist muscles was seen during slow flexions (SF) in 29 cases. This effect was strongest during the fast elbow flexion movements (FF). In 33 patients a triphasic pattern of muscle activation was observed on the unaffected side but not on the spastic side. The amplitude of the agonist surface EMG was significantly reduced and the amplitudes of the MU potentials recruited during maximal effort were generally smaller on the spastic side compared to the unaffected side. The agonist-antagonist activation pattern was analysed with respect to three clinically identifiable functional recovery stages of voluntary movements in the spastic limbs, namely synergistic, isolated and useful movements. The MU amplitudes and the amplitude of the surface EMG activity in the agonist muscle recorded during FF movements became significantly larger whereas the amplitude of the antagonist tonic activity became smaller with increasing functional recovery of the limb. It is concluded that impaired recruitment of type II motor units in the agonist muscles and the inability to selectively activate the agonist muscle contribute to the deficit in motor performance in spastic paresis.

A noninvasive strategy for identifying and quantifying innocuous and nociceptive peripheral afferent activity evoked by nerve stimulation

This study evaluated the utility of the compound nerve action potential (CAP) and spinal nociceptive withdrawal (R3) reflex in identifying and quantifying peripheral afferent activity evoked by sural nerve stimulation in humans. The results of this work demonstrate that currents less than or equal to that which elicits a just-maximal CAP can be considered purely innocuous; provide further evidence that the R3 is an objective means of identifying noxious stimulus levels; and suggest that current provides the best noninvasive quantitative estimate of afferent activity throughout the innocuous and noxious range. This work also demonstrates that some of the individual variability in the psychophysical function can be attributed to peripheral factors that affect the amount of current reaching the nerve. It is important, therefore, that these peripheral factors be considered when studying individual differences in the psychophysical function generated by electrical stimulation.

Macrophage response during axonal regeneration in the axolotl central and peripheral nervous system

We have used a monoclonal antibody (5F4) and Griffonia lectin to study the recruitment of macrophages after crushing axolotl central and peripheral axons. In both cases axonal regeneration begins within one to two days and, in the CNS, proceeds at a rate of about 0.05 mm per day. However, in the spinal cord, macrophage entry is restricted to the lesion site whilst in peripheral nerves macrophages rapidly enter the distal nerve stump after injury. These results suggest that the role (if any) played by macrophages during axonal regeneration may differ in these two situations.

Sensory integration by the dorsal spinocerebellar tract circuitry

Monosynaptic connections from sensory receptors to the dorsal spinocerebellar tract are believed to have a significant role in the transmission of sensory information to the cerebellum. However, predominant polysynaptic connections with highly convergent afferent input suggest a functional organization based on integrated sensory representations. We explored this possibility by examining the responses of dorsal spinocerebellar tract neurons to inputs from muscle receptors. We compared results from two sets of experiments designed to activate receptors in the gastrocnemius-soleus muscles. In one set (135 cells) we stimulated muscle receptors by stretching the isolated muscles and in the other set (194 cells) the muscle receptors were activated by passive foot flexion, which concurrently activated cutaneous and joint receptors as well. Population responses of the spinocerebellar neurons were quite different for the two types of stimuli. Foot flexion elicited long-latency excitatory responses in a majority (53%) of the cells, while muscle stretch elicited a large fraction of early peaking excitatory responses (28%) and inhibitory responses (38%). The long-latency responses to flexion could not be accounted for by specific cutaneous inputs or by possible delayed reflex contractions. We concluded that both types of population response resulted from the muscle stretch and therefore the responses of dorsal spinocerebellar tract cells to these stimuli do not simply reflect the activity in specific classes of sensory receptors.

Characteristics of the trace reproduction of rhythm by neurons of the sensorimotor cortex of rabbits in the aftermath of periodic stimulation

The trace recruitment of rhythm (TRR), an analog of the CR to time, which arises in response to prolonged electrodermal stimulation of the forelimb of the awake rabbit at a frequency of 0.5-2 Hz, was studied. The activity of 180 cells of the sensorimotor cortex before (80) and after (100) periodic stimulation for a duration of 10-20 min was recorded. The first series of rhythmical stimulation led to a brief TRR of the stimulation frequency; subsequent series formed a clear TRR which was preserved for several days. The possibility of "retraining" the neurons given a change in the rhythm of stimulation was identified. The capacity of the TRR phenomenon for extinction, prolonged maintenance, and reproduction of traces, as well as "retraining" allies it with processes analogous to the temporary connections.