Selective stimulation of peripheral nerve fibers using dual intrafascicular electrodes

We have studied activation of nerve fibers by pairs of Pt-Ir wire electrodes implanted within single fascicles of the nerve innervating the gastrocnemius muscle in cats. The purpose of this study was to determine if these intrafascicular electrodes can activate nerve fibers in different fascicles independently of each other and if they can also be used to activate separate subsets of axonal populations within a single fascicle. The average overlap of activated nerve fiber populations was 5.5% between fascicles and 27% within a fascicle, indicating that such selective activation is possible with these electrodes.

H reflexes in close muscles: cross-talk or genuine responses?

At rest, common peroneal nerve stimulation induced weak H responses in the peroneus longus (PL) and/or in the tibialis anterior (TA) muscles in half the subjects. The genuine H response of each muscle had to be proved since cross-talk could be suspected. PL and TA H responses were identified since (i) the reflex was often observed in only one muscle; (ii) H and M recruitment curves of each muscle were not systematically correlated; (iii) weak voluntary contractions induced opposite reflex changes in the antagonist PL and TA; (iv) direct stimulation of one of the muscles did not induce any cross-talk in the other provided that the EMG was of low voltage. It is concluded that H reflexes of close muscles can be studied independently.

Differential projection of the sural nerve to early and late recruited human tibialis anterior motor units: change of recruitment gain

The effect of a stimulation of the cutaneous sural nerve [three shocks, 2.5 x perception threshold (PT)] was studied on the tibialis anterior (TA) H-reflex and single voluntarily activated TA motor units using post-stimulus time histograms (PSTH). In both cases, when studying only the first recruited motor units, an inhibition with a delay of 10 ms, in relation to the monosynaptic latency of Ia afferents in the common peroneal nerve, was observed. This inhibition had a duration of 10-20 ms. The inhibition was evoked by low-threshold cutaneous fibres, since it could be seen at a stimulation strength close to the perception threshold. The central delay of the inhibition was calculated in two subjects to be 1.8 ms and 1.2 ms respectively. The TA motor units were characterized by their recruitment threshold and minimal firing frequency and the effect of the sural nerve stimulation was subsequently investigated. Early recruited low frequency motor units were found to be inhibited, whereas later recruited motor units with a higher minimal firing frequency were facilitated. Similarly small TA H-reflexes were inhibited, whereas large reflexes were facilitated. This difference in the effect of the sural nerve stimulation was not caused by a difference in the descending command, since the same early recruited motor unit was still inhibited when firing at a high frequency and at a high torque level. Stimulation of the femoral nerve was found to produce a monosynaptic facilitation of the TA H-reflex and a heteronymous monosynaptic peak in the PSTH of single motor units. A stimulation of the sural nerve increased the size of the reflex facilitation, but had no effect on the size of the monosynaptic peak in the PSTH of the single motor units. It is concluded that the effect of the sural nerve stimulation on human TA motor units is similar to observations in the cat and that a similar interneuronal system may be responsible. It is furthermore suggested that the sural nerve stimulation increases the recruitment gain of the TA motoneuronal pool.

Recruitment of motor units in human flexor carpi ulnaris

In 1949, Denny-Brown reported that the motoneuron pool of the human flexor carpi ulnaris (FCU) muscle was fractionated into subpopulations, each subpopulation being activated during a different voluntary motor task. The following report presents data on motor unit recruitment in the human FCU muscle for the tasks of isometric flexion and ulnar deviation, cocontraction of the forearm muscles and non-isometric flexion of the wrist. These observations show that every FCU motor unit tested reliably, contributed to all four tasks, that is, no separate subpopulations were observed for any of the contractions tested. Furthermore, the order of recruitment was the same for all four tasks.

Maximum shortening speed of motor units of various types in cat lumbrical muscles

1. Isotonic shortening of cat superficial lumbrical muscles was studied during maximal tetanic contractions of single motor units of identified types. For each motor unit, the maximal speed of contraction, Vmax, was determined by extrapolating to zero the hyperbolic relation between applied tension and speed of shortening. 2. The maximal speeds of shortening of motor units formed a continuum with the highest velocities observed for the fast fatigable motor units and the lowest for the slow motor units. 3. On average, the maximum speed of shortening increased with the tetanic tension developed by the motor units. 4. In motor units with isometric twitch contraction times less than 35 ms, these times showed a significant inverse correlation with Vmax. Progressively longer contraction times were associated with rather small changes in Vmax. 5. The implications of these findings on the speed of muscle shortening during motor-unit recruitment are discussed.

Variation in firing order of human soleus motoneurons during voluntary and reflex activation

The activation of motoneurons in a muscle pool is said to proceed as an ordered array in response to both dorsal root stimulation and voluntary activation, with small motoneurons being recruited before larger ones. We have examined 19 voluntarily recruited soleus motor units in 5 normal subjects and found that in 18 cases, the lowest threshold motor unit recruited by slowly increasing 'tonic' voluntary activity was different from the lowest threshold unit recruited by electrical stimulation of the tibial nerve in the popliteal fossa (phasic Hoffman reflex). The initially recruited voluntary units were, however, part of the pool influenced by the stimulated afferents because, during tonic activation, the timing of their discharge could be shown to be altered by electrical stimulation at a lower intensity than that required for H recruitment at rest. These findings suggest that the pool of soleus motoneurons responding to the voluntary command "tonically plantar flex your ankle" differs somewhat, in order of activation, from the pool responding to phasic stimulation of the largest diameter fibers in the tibial nerve, perhaps because of inhomogeneities in the distribution of descending or segmental inputs to the soleus motoneuron pool. Whether this partitioning is functional or a reflection of minor, random variations in synaptic density remains to be determined.

Motor unit recruitment in human medial gastrocnemius muscle during combined knee flexion and plantarflexion isometric contractions

Previous work on multifunctional muscle has suggested that motor unit recruitment during a combined force task is the result of an interactive effect of weighted inputs acting simultaneously on the motoneuron pool. The present study shows that a similar effect describes motor unit activation in a two-joint muscle as forces are combined at both proximal and distal attachments. The recruitment thresholds of single motor units in medial gastrocnemius muscle were determined during combined knee flexion and plantarflexion isometric contractions. Slow isometric ramp contractions in knee flexion were produced while maintaining various background levels of plantarflexion force. The combination of knee flexion and plantarflexion forces at which a motor unit initially discharged was used to characterize recruitment as represented by the slope of the regression line fit to the individual data points. Each subject completed two experiments; one at each of two knee joint angles, with the ankle joint fixed at 90 degrees. The effect of knee angle was assessed by comparing the slopes of the regression lines that characterized motor unit recruitment at each knee angle. Motor units in medial gastrocnemius were recruited when the linear sum of the forces exerted in plantarflexion and knee flexion exceeded a certain threshold of combined force. Specifically, the apparent force threshold of recruitment in knee flexion decreased as the level of force maintained in plantarflexion increased. Further, evidence is provided indicating that the linear relationship describing recruitment in two-joint muscle is dependent upon joint angle.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered patterns of reflex excitability subsequent to contusion injury of the rat spinal cord

1. The present study investigated regulation of reflex excitability after experimental contusion injury of the spinal cord. 2. Four measures of H-reflex excitability were evaluated in normal rats and at 6, 28, and 60 days after contusion injury at the T8 level: 1) reflex thresholds, 2) slope of the reflex recruitment curves, 3) maximal plantar H-reflex/maximal plantar M-response (Hmax/Mmax) ratios, and 4) rate-sensitive depression (i.e., the decrease in reflex magnitude relative to repetition rate). 3. Tested as a function of the afferent volley magnitude, the thresholds for reflex initiation fell progressively subsequent to contusion injury. No change was observed at 6 days postinjury, and the decrease at 28 days was not significant. However, by 60 days postinjury, the threshold had decreased by 23% of the maximal afferent volley, and this decrease was significant, [analysis of variance (ANOVA, P < or = 0.01)]. 4. Hmax/Mmax ratios elicited in postcontusion animals at 0.3 Hz were not significantly different from those recorded in normal animals. 5. The slopes of the recruitment curves were markedly reduced subsequent to contusion injury. The decrease was greatest at 6 days postinjury. Although some recovery toward normal occurred at 28 and 60 days postinjury, the slopes of recruitment curves in postcontusion animals remained significantly decreased. 6. H-reflexes elicited at 1-5 Hz were less sensitive to rate depression in postcontusion animals than in normal animals at the same respective frequencies. The decrease was progressive in onset, becoming significant by 28 days postinjury, and of an enduring nature, i.e., still significantly different from normal in the reflexes tested 60 days postinjury. 7. Rate sensitivity of the tibial nerve monosynaptic reflex (MSR) was also compared in normal and postcontusion animals. Rate sensitivity of the tibial MSRs was significantly reduced at 28 and 60 days post-contusion, compared with normal animals. 8. These data indicate that significant changes in lumbar reflex excitability result from midthoracic contusion injury of the spinal cord. These changes include reflex threshold, slope of recruitment, and rate-sensitive depression. Although recruitment slope was most altered in the shortest postinjury interval tested, followed by some recovery, the other changes were progressive in onset and enduring in duration.

Effect of velocity and mechanical history on the forces of motor units in the cat medial gastrocnemius muscle

1. Two fundamental aspects of the dynamic behavior of motor units of the cat medial gastrocnemius (MG) muscle were measured. Force-velocity (FV) relationships were measured with the use of constant velocity shortening and lengthening movements. Effects of mechanical history were assessed via comparisons of forces immediately after or during slow movements with standard isometric forces. Isometric force-length (FL) relations were also measured, and the effect of different stimulation rates on both FV and FL data was assessed. 2. Prior or concurrent movement greatly potentiated motor-unit force, but this movement potentiation was highly dependent on the amplitude of the unit's force. The smallest twitch forces of type S units (< 10 mN) were potentiated more than threefold, but no potentiation occurred for unit forces > 200 mN. It was tentatively concluded that movement potentiation may play little role in normal movements because it does not occur at forces > 1% of maximal isometric force of the MG. 3. During shortening, the normalized FV relations of type S units were relatively steeper than those of type FR or FF units. For lengthening, there was no evident relation between FV steepness and motor-unit type. 4. Stimulation rate affected both the FV and FL relationships of the motor units. The peak of the FL relationship (Lo) clearly shifted to shorter muscle lengths as stimulation rate was increased. The steepness of the FV relationship for shortening was decreased by increasing stimulation rate, but this effect was modest. 5. The shift in motor-unit Lo and the differences in motor-unit FV relationships were hypothesized to play significant roles during normal motor behavior. Realistic computer simulations of FL and FV functions for a population of motor units undergoing normal steady-state recruitment and rate modulation supported these hypotheses. As the level of simulated neural drive increased, the population Lo shifted to considerably shorter lengths, and the normalized FV function became much less steep. The significance of these results for models of muscle are discussed.

Discharge patterns of phrenic motoneurons during fictive coughing and vomiting in decerebrate cats

In decerebrate, paralyzed, and ventilated cats, we recorded the activity of 100 spontaneously active phrenic motor axons during the increased phrenic discharges characteristic of fictive vomiting (FV) and coughing (FC). During control respiratory cycles, approximately one-half the neurons were recruited in the first decile of inspiration; recruitment continued throughout inspiration. During FV, the duration of phrenic discharge was halved; 20 of 26 motoneurons studied were recruited in the first decile of the burst. During FC, recruitment times did not change compared with control, although the duration of the phrenic burst doubled. Discharge frequencies increased and recruitment order of phrenic motoneurons was virtually unaffected during FC and FV. Limited recruitment of previously inactive neurons in the filaments from which we recorded was found during FV and FC. During FV, 1 previously inactive motoneuron was recruited in 16 filaments containing 25 spontaneously active motor axons. During FC, 3 new motoneurons were recruited in addition to the 64 already active in 35 filaments. Recruitment during FV and FC was absent even when recording from filaments known, on the basis of antidromic activation, to contain inactive motor axons. During FV, 10 of 26 motoneurons began their discharges with doublets (interspike interval < 10 ms); doublets occurred in only 4 of 67 motoneurons during FC. Already active phrenic motoneurons contributed to the intense phrenic activity associated with both respiratory (coughing) and nonrespiratory (vomiting) behavior by increases in discharge frequency, earlier recruitment, and doublets; the contribution of previously quiescent motoneurons remains uncertain.

Differences between steady-state and transient post-synaptic potentials elicited by stimulation of the sural nerve

In cat medial gastrocnemius motoneurons, single stimuli to the cutaneous sural nerve evoke a post-synaptic potential with a mixture of depolarization and hyperpolarization, depolarization being dominant in type F cells and hyperpolarization in type S cells. This pattern is consistent with previous reports showing that activation of the sural nerve can sometimes reverse the normal order of motor unit recruitment by inhibiting S motor units while simultaneously exciting F motor units. However, during repetitive stimulation for 1-2 s, we found that the hyperpolarizing component of the sural input to medial gastrocnemius motoneurons was not persistent, but instead gave way to depolarization after the first 30 ms. The net steady-state response after 0.5-1.0 s of stimulation was depolarization in all cells, regardless of motor unit type. This suggests that tonic sural input may be incapable of producing prolonged recruitment reversals.

Motoneuron activity and muscle fibre type composition in hemiparesis

The firing of single motor units (MUs) in musculus tibialis anterior (TA) was studied during maximal voluntary effort and maximal speed of walking in 10 patients with severe chronic hemiparesis and the findings compared to normal data. As shown in a previous study, the paralysed TA exhibited an increase in proportion of type 2 fibres as compared with normal muscle. Thus, 57% of the muscle fibres were type 1 and 43% type 2, while the normal percentages were 80% and 20% respectively. The present findings indicate that in the paralysed muscles a little less than half of the fibres, i.e. roughly the equivalent of the type 2 population, was not tonically active either during sustained voluntary contraction or during locomotion. Normally high threshold MUs reached high rates during both modes of activation. The findings paralysed muscles also indicate that a little more than half of the fibres, i.e. roughly the equivalent of the type 1 population, could be brought into tonic firing during voluntary contraction as well as during walking. Their maximal firing rate was, however, no more than two thirds of that of normal low threshold MUs.

Extraocular motor units: type classification and motoneuron stimulation frequency-muscle unit force relationships

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat. Using a combination of two motor unit properties, the fusion frequency and an index of fatigability, the population of twitch motor units could be separated into 4 subgroups: fast fatigable (FF), fast fatigue resistant (FR), slow fatigable (SF) and slow fatigue resistant (S). Nontwitch motor units, a fifth subgroup (NT), formed 10% of the total studied population. The twitch tension and the maximum tetanic tension of the FF motor unit type were significantly stronger than all other motor unit types. The use of frequency varying stimulation patterns did not further differentiate the motor unit types. The relation between a series of single motoneuron stimulation frequencies and the resultant single muscle unit forces generated a slope defined as a motor unit's kt value. Motor units with low kt values had higher twitch tensions, higher maximum tetanic tensions, higher fusion frequencies and lower fatigue indices than motor units with high kt values. Motoneuron recruitment was tested by electrical stimulation of the medial rectus subdivision of the contralateral oculomotor nucleus. No correlations were seen between recruitment order and the mechanical parameters of the single abducens motor units.

Repetitive doublets of human motoneurones: analysis of interspike intervals and recruitment pattern

In order to study the probable mechanisms of repetitive doublets in human motoneurones, the firing patterns of single motor units (MUs) of the trapezius were analysed during a weak voluntary muscle contraction. The mean frequencies of MUs were 9.4-21.7 imp/sec (the mean interspike interval ranged from 46.0 to 106.7 msec). Repetitive doublets (up to 28 in succession) were recorded in 21 out of 120 MUs, mostly at the onset of a slow recruitment. These were followed by single discharges. Intradoublet intervals ranged between 2.5 and 20.0 msec. A significant difference between single spike firing and doublet firing was revealed by plotting interspike interval histograms, showing that two distinct mechanisms were involved. The analysis of interspike interval successions belonging to several MUs firing simultaneously showed that one of the MUs could start with doublets while the others went on firing single spikes with the regular mean frequency and interspike interval scatter. The results lead us to suggest that the intrinsic properties of motoneurones can be regarded as the main factor in the origin of repetitive doublets. It seems that a descending synaptic drive also contributes to the control of double firing since in a number of cases no doublets were produced at the beginning of MU activity. The findings are discussed with regard to the problems of regulating repetitive firing of human motoneurones by after-potentials. Steady delayed depolarization is assumed to be a possible mechanism of repetitive doublets.

Recruitment of motor units in human forearm extensors

1. Experiments were conducted on single motor units of two forearm muscles, extensor carpi radialis (ECR) and extensor digitorum communis (EDC) of human subjects. Our interest was whether or not task groups could be identified in these forearm muscles, and, if so, was there orderly recruitment within each task group. 2. To test for the presence of separate task groups within ECR, motor-unit recruitment was examined for two isometric contractions:wrist extension and radial deviation. Each of the ECR motor units tested repeatedly discharged during contractions in both directions, indicating the absence of separate task groups in ECR for contractions in these two directions. 3. Recruitment order between pairs of ECR motor-unit action potentials was examined for wrist extension and radial deviation. For 58 paired comparisons, the order of recruitment was the same in both directions. In terms of force output, plots of twitch torque versus recruitment threshold of ECR motor units showed a positive correlation for both directions, wrist extension and radial deviation, demonstrating size-ordered recruitment of ECR motoneurons for both contractions. 4. The EDC motoneuron pool exhibited two partially overlapping subpopulations of motoneurons on the basis of task, one subpopulation recruited for middle finger extension and the second one for ring finger extension. Contractions involving the index and little fingers were not examined. It is concluded that motor-unit task groups do exist within EDC motoneuron pool. Plots of twitch torque versus recruitment threshold showed positive correlations for each of these two task groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Relation between the motor units recruitment threshold and their potentials propagation velocity at isometric activity

The relationship between the propagation velocity of the excitation along the muscle fibers of the motor units (MUs) and their threshold of recruitment at different level of isometric voluntary contraction was investigated. The threshold of recruitment was measured by the value of the muscle force, expressed in percents from the maximal voluntary contraction (MVC) at which the first impulse of the MU appeared. A wire subcutaneous branched electrode was used to select the potentials from a single MU. The selected in this way MU impulses were used as a trigger to average two electromyographic (EMG) signals picked up by means of two monopolar surface electrodes with small leading-off areas mounted on a common plate at a distance of 10 mm from one another. The propagation velocity of the extraterritorial potentials of the MUs increased non-linearly with the increase of the recruitment threshold. The relationship was fitted as V = square root of a+b.theta, where v is the propagation velocity, theta is the threshold of recruitment and a and b are constants. The consideration of the velocity of propagation as a "size principle parameter" was discussed and the limitations of the latter are pointed out.

Surface electromyographic estimates of recruitment/rate coding of masseteric motor units

Surface electromyographic (EMG) recordings were obtained from the masseter muscles in healthy subjects performing brisk maximum voluntary teeth clenching (MVC) for about 1 s. During the onset (0-600 ms) of ballistic MVC activity, the peak amplitude of the EMG interference patterns showed a consistent and significant increase, on examination for 0-200 ms, 200-400 ms, and 400-600 ms of MVC activity. The peak (maximum) and median (centroid) frequencies of power spectrum density functions of the raw surface EMGs (interference patterns) showed an absence of consistent and significant changes during 0-600 ms of ballistic MVC activity. However, the estimated total energy contents (peak amplitude x peak frequency) of the surface interference patterns showed a consistent and significant increase from 0 to 600 ms of ballistic MVC activity, and this was interpreted as global recruitment/rate coding of masseteric motor units.

[Trace rhythm recruitment by the neurons of the rabbit sensorimotor cortex in old age]

Spectral analysis (ACG and gSP) of the impulse activity of the neurones of the old rabbits sensorimotor cortex allowed to reveal a trace recruitment of the rhythm--CR analogue to time--in after-action f rhythmic stimulation. Connection was established between the number of presented series of periodic electrocutaneous stimulation and expressiveness of the trace rhythm recruitment depending on the animals age. Trace rhythm recruitment took place slower in old animals (54-56 months) than in young ones (up to 1 year), chiefly in 2-3 experimental days after 2-4 series of rhythmic stimulation and was preserved in a small percent of cases the next day after stimulation. In the background activity of a number of neurones an initial periodicity was discovered, which was intensified under the influence of stimulation by another frequency, or the initial rhythm was extinguished, and stimulation rhythm was reproduced. Periodical stimulation in very old animals (66-85 months) practically did not evoke plastic reconstructions of the cortical neurones. Under the influence of the stimulation a non-specific trace increase of the frequency of neurones background activity of the old animals was observed. The revealed characteristics of plastic neurones properties may testify to projected disturbances of mnestic processes at definite age stages of normal aging.

Recruitment within the groups of gamma 1, alpha 2 and alpha 3-motoneurons in dogs and humans following bladder and anal catheter pulling

Conduction velocity frequency distribution histograms were constructed from lower sacral root recordings of single intrafusal (gamma) and extrafusal (alpha) motoneurons. The velocity distributions of occasional and stimulated activity before and following bladder and anal catheter pulling were almost identical for dogs and humans. The limits of the velocity ranges of alpha 1(FF), alpha 2(FS) and alpha 3(S) and gamma beta(?)-motoneurons were determined from the broadness of the single peaks. The borders of the partly fused peaks of gamma 1 and gamma 21-motoneurons were estimated from their different functional properties. Activity levels of the alpha 1, alpha 2, alpha 3, gamma beta, gamma 1 and gamma 21-motoneurons were too complex to allow safe conclusions from the dog measurements, probably because of the representation of leg and tail in addition to sphincter functions in the lower sacral root. In the human dorsal S4 root, in which mainly efferent functions of sphincters only were contained, the gamma 21, gamma 1, alpha 2 and alpha 3-motoneurons showed simple behaviour. Distribution changes of conduction velocities in each group of alpha and gamma-motoneurons were used for recruitment analysis. Following stimulation in dogs and humans, within the groups of gamma 1, alpha 3 and alpha 2-motoneurons, slowly conducting fibres were activated before the faster conducting ones. The alpha 3-motoneurons were recruited later than the alpha 2-motoneurons. In the dog, low gamma 1 and alpha 2-motoneuron velocities occurred preferentially 0 to 0.2 s following strong bladder catheter pulling, probably in the mono- and oligosynaptic pathways. Low conduction velocities of alpha 3-motoneurons occurred more often 1 to 1.2 s following stimulation. At 2 to 2.2 s following stimulation, the high gamma 1 and alpha 2-motoneuron velocities were more activated. At 4 to 4.2 s following stimulation, low gamma 1 and alpha 2-motoneuron velocities were recruited again. Following strong bladder catheter pulling, co-recruitment of the gamma 1 and alpha 2-motoneurons seemed to occur. Following anal catheter pulling in the dog there was no co-recruitment of gamma 1 and alpha 2-motoneurons. In the case of gamma 1-alpha 2 co-recruitment, the gamma 1-motoneurons were recruited additionally once in between the co-recruitment. The higher frequency of recruitment of the gamma 1-motoneurons, and the separate recruitment of the gamma 1 and alpha 2-systems, indicate that the activation of gamma 1 and alpha 2-motoneurons are not strongly linked.(ABSTRACT TRUNCATED AT 400 WORDS)

Complementary roles of central command and muscular reflex in the regulation of heart rate during submaximal isometric contraction

During submaximal isometric contraction, the heart rate (HR) and the electromyographic activity (EMG) increase continuously. Although activation of the muscle and the cardiovascular center is placed partly under the common control of the central command, the nature of the relationship that may exist between HR and the integrated electromyogram (iEMG) is seldom studied. Seventeen healthy men, 22.4 +/- 0.5 years of age (M +/- SE), performed isometric contractions with the right elbow flexors. Forces of 25, 40, 50 and 65% of the maximum voluntary contraction (MVC) were used, and the contractions were sustained until (isotonic isometric contraction: IIC) and beyond exhaustion (anisotonic isometric contraction: AIC). During IIC, a linear relationship exists between HR and iEMG; the slope of this relationship is independent of the relative force developed, which is in favor of a predominant role played by the central command in HR increase. The increase in the ratio iEMG/HR at the approach of local muscular exhaustion would indicate that at the end of IIC there is an increase in the relative part furnished by the information of peripheral origin in HR regulation. During AIC, the force (F) decreases in an exponential manner and stabilizes at around 25% MVC from tAIC = 70 s on. The iEMG and HR change independently: iEMG decreases like F such that iEMG/F remains constant; HR continues to increase in the first phase corresponding to the rapid decrease in F and iEMG, then in a second phase, it decreases linearly with respect to time. Our results suggest that the action of the central command is dominant during stage 1 of AIC, while during stage 2 the relative part furnished by the muscle reflexes increases. Beyond tAIC = 70 s, there seems to be a certain degree of central fatigue.

Voluntary activation of human quadriceps during and after isokinetic exercise

The extent of voluntary activation in fresh and fatigued quadriceps muscles was investigated during isometric and isokinetic voluntary contractions at 20 and 150 degrees/s in 23 normal human subjects. The muscles were fatigued by a total of 4 min of maximal knee extension at an angular velocity of 85 degrees/s. Voluntary activation was determined by the superimposition of tetanic electrical stimulation at 100 Hz for 250 ms, initiated at a constant knee angle. The relationship between voluntary and stimulated force was similar to that found with the established twitch superimposition technique used on isometric contractions. In fresh muscle all the subjects showed full voluntary activation during isometric contractions. Some activation failure was seen in five subjects at 20 degrees/s [2.0 +/- 0.9 degrees (SE)] and in two subjects at 150 degrees/s (0.7 +/- 0.5). After fatigue all subjects showed some activation failure at 0 and 20 degrees/s (36.4 +/- 3.1 and 28.8 +/- 4.1 degrees, respectively), but only two showed any at 150 degrees/s (1.4 +/- 5.7). We conclude that brief high-intensity dynamic exercise can cause a considerable failure of voluntary activation. This failure was most marked during isometric and the lower-velocity isokinetic contractions. Thus a failure of voluntary activation may have greater functional significance than previous studies of isometric contractions have indicated.

Feedback control methods for task regulation by electrical stimulation of muscles

Three feedback control algorithms of varying complexity were compared for controlling three different tasks during electrical stimulation of muscles. Two controllers use stimulus pulse width (or recruitment) modulation to grade muscle force (the fixed parameter, first-order PW controller and the adaptive controller). The third controller varies both stimulus pulse width and period simultaneously for muscle force modulation (the PW/SP controller described in the comparison paper). The three tasks tested were isometric torque control, unloaded position tracking, and control of transitions between isometric and unloaded conditions. The first task involved the muscle recruitment nonlinearity. The second task added the effects of muscle length-tension and force-velocity nonlinearities. The third task included a sudden changes in external loading conditions. The comparative evaluation was carried out in an intact cat ankle joint with stimulation of tibialis anterior and medial gastrocnemius muscles. The simplest PW controller demonstrated robust control for all tasks. The PW/SP controller improved the performance of the PW controller significantly for control of isometric torque and load transition, but only slightly for control of unloaded joint position. However, the adaptive controller did not consistently achieve a significant improvement in performance compared with the PW controller for any task. Results suggest that muscle length-tension and force-velocity nonlinearities affect the performance of these controllers similarly within the tested ranges of movement amplitudes and speeds. Abrupt changes in the system, such as those due to recruitment nonlinearity and external loading transitions, tend to limit the performance of the adaptive controller. The study provides guidelines for choosing control algorithms for neural prostheses.

Selectivity of intraneural prosthetic interfaces for muscular control

Intraneural stimulation with multi-electrodes in principle offers the best possibilities to reach selectivity at motor unit level and to improve recruitment order. The selectivity of stimulation in the peroneal nerve of the rat is explored in the paper by calculations and measurements, using a linear 12-electrode array and a newly devised selectivity test method. With analytical models for potential field distributions, areas of excitation can be calculated for arbitrary electrode configurations. It is demonstrated, using tripolar electrode combinations, how selectivity can be further enhanced and recruitment order improved.

Effect of resistive loads on pattern of respiratory muscle recruitment during exercise

In healthy subjects, we compared the effects of an expiratory (ERL) and an inspiratory (IRL) resistive load (6 cmH2O.l-1.s) with no added resistive load on the pattern of respiratory muscle recruitment during exercise. Fifteen male subjects performed three exercise tests at 40% of maximum O2 uptake: 1) with no-added-resistive load (control), 2) with ERL, and 3) with IRL. In all subjects, we measured breathing pattern and mouth occlusion pressure (P0.1) from the 3rd min of exercise, in 10 subjects O2 uptake (VO2), CO2 output (VCO2), and respiratory exchange ratio (R), and in 5 subjects we measured gastric (Pga), pleural (Ppl), and transdiaphragmatic (Pdi) pressures. Both ERL and IRL induced a high increase of P0.1 and a decrease of minute ventilation. ERL induced a prolongation of expiratory time with a reduction of inspiratory time (TI), mean expiratory flow, and ratio of inspiratory to total time of the respiratory cycle (TI/TT). IRL induced a prolongation of TI with a decrease of mean inspiratory flow and an increase of tidal volume and TI/TT. With ERL, in two subjects, Pga increased and Ppl decreased more during inspiration than during control suggesting that the diaphragm was the most active muscle. In one subject, the increases of Ppl and Pga were weak; thus Pdi increased very little. In the two other subjects, Ppl decreased more during inspiration but Pga also decreased, leading to a decrease of Pdi. This suggests a recruitment of abdominal muscles during expiration and of accessory and intercostal muscles during inspiration. With IRL, in all subjects, Ppl again decreased more, Pga began to decrease until 40% of TI and then increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor-unit recruitment in the decerebrate cat: several unit properties are equally good predictors of order

1. Recruitment order was studied in pairs of motor units of the medial gastrocnemius (MG) muscle of decerebrate cats with the use of dual microelectrode recording from intact ventral root filaments. Excitation was provided by stretch of MG, stretch of synergists [lateral gastrocnemius (LG), plantaris (PL), and soleus (SOL) muscles] or electrical stimulation of the caudal cutaneous sural (CCS) nerve. Motor units were characterized by axonal conduction velocity (CV), tetanic tension (Pmax), twitch contraction time (CT), and fatigue index (FI). 2. Consistent with the recruitment pattern described by others, most often in relation to either CV or Pmax, the first unit of a pair to be recruited by MG stretch was typically the one with the lower CV and Pmax, and the higher FI and CT. The proportion of pairs that agreed in rank order of each property and recruitment order was as follows: for CT, 94%; for CV, 87%; for Pmax, 84%; and for FI, 75%. With a single marginal exception (CT vs. FI), no motor-unit property proved to be significantly better than the others at predicting recruitment (G test; P greater than 0.05). 3. In all 11 tested pairs containing one slow (type S) and one fast (type F) unit, the S was more easily recruited by stretch. Type F units divided into groups with high (type FR), low (type FF), and intermediate (type FInt) values for FI were recruited in order from FR to FInt to FF in 8/11 pairs. Thus our findings were similar to earlier demonstrations that recruitment proceeds in order by type. 4. Stretch of MG synergists usually recruited units in the same order as MG stretch. In two S-S pairs, recruitment order was switched with synergist stretch. 5. Stimulation of the CCS nerve was generally excitatory to the MG units sampled. Most unit pairs were recruited by CCS stimulation in the same order as by MG stretch, but, for 6 of 39 pairs, CCS stimulation switched the order produced by stretch. Thus, whereas sural afferent input can preferentially excite some units over others as suggested by Kanda et al., that effect is not widespread or selective for unit type under these conditions. 6. Assuming that all MG motor units cooperate as a single functional pool in homonymous stretch reflexes, we support others in concluding that a motoneuron's recruitment threshold is not strictly determined by its size. However, our data do not distinguish other schemes that predict recruitment order more accurately than the size principle.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of percutaneous stimulation on motor unit firing behavior in man

Motor unit firing behavior in human first dorsal interosseous (FDI) muscle was studied during controlled constant force isometric contractions. The threshold at which motor units were recruited and the mean firing rate at 50% of maximal voluntary contraction (MVC) were evaluated following stimulation of the skin area over the second digit. Stimulation of cutaneous receptors tended to increase the recruitment threshold of most of the motor units recruited under 20% MVC, while high-threshold motor units (those recruited over 30% MVC) generally exhibited a decrease in recruitment threshold. Less dramatic changes in motor unit firing rates were observed, but those motor units recruited over 30% MVC exhibited some increase in firing rate. The relationship between the change in recruitment threshold and change in motor unit firing rate is not rigid and seems to be susceptible to considerable synaptic noise.

Muscle recruitment with intrafascicular electrodes

We have studied muscle recruitment with Teflon-insulated, 25 microns diameter, Pt-Ir intrafasicular electrodes implanted in nerves innervating the gastrocnemius and soleus muscles of cats. The purpose of this study was to measure the performance of these bipolar electrodes, which had been designed to optimize their ability to record unit activity from peripheral nerves, as stimulating electrodes. Recruitment curves identified the optimal stimulus configuration as a biphasic rectangular pulse, with an interphase separation of about 500 microseconds and a duration of about 50 microseconds. The current required for a half-maximal twitch contraction was on the order of 50 microA. Current and charge densities needed for stimulation were well below levels believed to be safe for the tissue and electrode materials involved. When the spinal reflex pathway was interrupted by crushing the nerve, the force produced by a given stimulus changed in some cases, but not in others, implying that the spinal reflex contribution was not the same in all the implants. We conclude that intrafascicular recording electrodes are also a potentially valuable technology for functional neuromuscular stimulation, and warrant further development.

Activity-dependent recruitment of silent synapses

At the crayfish neuromuscular junction, a long-lasting enhancement of synaptic transmission can be induced by tetanic stimulation of 10-20 Hz for several minutes. The long-lasting enhancement is presynaptic in origin, because quantal content increases but not quantal size, and is not dependent upon broadening or enlargement of the presynaptic action potential. The enhancement can be selectively blocked by presynaptic injection of agents that inhibit adenylate cyclase or the cyclic AMP-dependent protein kinase. Entry of calcium may not be sufficient in itself to produce the enhancement. Analyses of quantal events using both a simple binomial statistical method, and a more refined method that takes into account the possibility of unequal probabilities of responding units, have shown that the number of responding units increases during the long-lasting enhancement. In addition, there is an increase in the probability of transmitter release at preexisting units. In contrast, during short-term facilitation accompanying repetitive stimulation, response probability increases greatly whereas the number of responding units increases only moderately with frequencies of activation up to 20 Hz, which increase quantal output severalfold. These results indicate that responding units, hypothesized to be transmitting synapses, can be recruited to active transmission from an unresponsive pool by tetanic activity, and that protein phosphorylation is required for long-lasting changes to occur. The existence of an excess of synapses on crustacean nerve terminals is indicated by ultrastructural studies, which invariably show many synapses on the terminals. The number of morphologically defined synapses is always greater than the number of responding units seen in statistical analyses of quantal release for the same recording location.

Persistence of the Hering-Breuer reflex beyond the neonatal period

There is conflicting evidence regarding the persistence of the Hering-Breuer reflex (HBR) beyond the 1st wk of life. This study was designed to assess the influence of postnatal age on the HBR. The airway occlusion technique was used to assess changes in respiratory timing during stimulation of the HBR in healthy full-term unsedated infants measured shortly after birth and at 6-8 wk of life. The strength of the HBR was assessed from the relative change in expiratory time (TE) after end-inspiratory occlusion compared with resting TE during spontaneous breathing. Paired studies were performed in 31 infants at approximately 2 days and 6 wk of age. There was a significant increase in TE during each occlusion in every infant irrespective of age at measurement. No maturational changes were observed. The increase in TE after end-inspiratory occlusion was 91.9 +/- 31.6% (SD) (range 38-158%) at approximately 2 days and 89.8 +/- 30.7% (range 44-175%) at approximately 6 wk. We conclude that the activity of the HBR during tidal breathing persists beyond the neonatal period and that there is no statistically significant change in its strength during the first 2 mo life in healthy infants during natural sleep.

Respiratory-related recruitment of the masseter: response to hypercapnia and loading

To test the hypothesis that a muscle that closes the jaw, the masseter, can be recruited by ventilatory stimuli, we studied the electromyographic activation of the masseter and genioglossus in seven normal awake males who were exposed in random order to progressive hyperoxic hypercapnia, inspiratory threshold loading (-40 cmH2O), and combined hypercapnia and loading. With hypercapnia, the masseter was generally recruited after the genioglossus had been activated. Once recruited, activation of both muscles increased linearly with increasing CO2. Combined hypercapnia and loading produced more activation than either stimulus alone. These data indicate that the masseter is activated by ventilatory stimuli that activate the genioglossus. Earlier recruitment of the genioglossus suggests that activation of the masseter serves to stabilize the mandible and allow the genioglossus to function as a more efficient dilator of the upper airway.

EMG patterns of rat ankle extensors and flexors during treadmill locomotion and swimming

Intramuscular electromyography (EMG) was used to determine and compare the recruitment patterns of the rat soleus (Sol), tibialis anterior (TA), and a deep and a superficial portion of the medial gastrocnemius (MG) during treadmill locomotion at various speeds and inclines and during swimming. Raw EMG signals for 10-20 step or stroke cycles were rectified, averaged, and processed to determine cycle period (EMG onset of one cycle to EMG onset of the next cycle), EMG burst duration, and integrated area of the rectified burst (IEMG). Mean EMG per burst was calculated as IEMG/burst duration. IEMG/min was calculated as IEMG times the number of bursts (cycles) per minute. Cycle period and burst duration of the extensors decreased hyperbolically, while the TA burst duration was unchanged, with increased treadmill speed. With increased treadmill speed, IEMG was decreased in the Sol and unchanged in the MG and TA, whereas IEMG/min decreased in the Sol and increased in the MG and TA. An elevation in treadmill incline resulted in an increase in the activation levels of the MG but not in the Sol or TA. These data indicate that the additional power required at increased speeds and/or inclines of treadmill locomotion is derived from the recruitment of the fast extensors, e.g., the MG. The mean cycle period during swimming was similar to that observed during the fastest treadmill locomotion. EMG burst durations and amplitudes, however, were higher in the TA, relatively similar in the MG, and lower in the Sol during swimming than treadmill locomotion.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal changes in the distribution of succinate dehydrogenase activities among diaphragm muscle fibers

The distributions of oxidative capacities among type-identified muscle fibers in the developing cat diaphragm were examined by quantifying succinate dehydrogenase (SDH) activity using a microdensitometric procedure. Animals were studied during the first six weeks of postnatal development and compared to adults. Muscle fiber SDH activities were initially low during the first 2 postnatal wk, then increased to their highest values between 3 and 6 wk. Thereafter, fiber SDH activities declined to adult values. At each age, the distributions of SDH activities for both type I and II fibers were unimodal. Thus, no objective basis exists for subclassifying type II fibers based on differences in oxidative capacity. Fibers could be subclassified as type IIA, IIB, or IIC based on the acid pH lability of ATPase staining. In neonates, approximately 90% of all fibers were classified as type IIC. Thereafter, the proportion of IIC fibers decreased while the proportions of type I, IIA, and IIB increased. Adult fiber type proportions were reached by 6 wk of age. The SDH activity of type I fibers was generally higher than that of type II fibers at all ages, although there was considerable overlap in the distributions of SDH activities among type I and II fibers. The SDH activity of type IIC fibers was also higher than that of either type IIA or IIB during development. Only in the adult diaphragm was the SDH activity of type IIA fibers higher than that of type IIB. At no age could type IIA, IIB, or IIC fibers be discriminated based solely on differences in oxidative capacity.

Blood flow distribution within the rib cage muscles

We used 15-microns radiolabeled microspheres to study the regional distribution of blood flow (Q) among parasternal (PS), transversus thoracis, and external (EI) and internal intercostal muscles (II) in nine anesthetized supine mongrel dogs. We measured Q (ml.min-1.100 g-1) in each intercostal space (ICS) during spontaneous breathing, inspiratory resistive loading, and mechanical ventilation following paralysis. At necropsy the EI, II, and PS were excised and sampled separately for each ICS. During paralysis there was no consistent gradient in Q among the PS, II, and EI muscles. During spontaneous breathing, Q to PS increased linearly by 125% between the first and fourth to sixth ICS, Q to EI decreased progressively from the first/second ICS to the fifth/sixth ICS, whereas Q to the II was uniform. During inspiratory resistive loading, in which mouth pressures of -16 +/- 4 cmH2O were generated, the PS gradient was similar to that during spontaneous breathing. Also, Q to the EI increased in the cranial interspaces (P less than 0.02), whereas Q to the II of the seventh/eighth ICS was greater than that of the first/second ICS (P less than 0.001). Furthermore, with loading, ventrodorsal gradients in Q appeared within both EI and II interspaces. There was no consistent gradient in Q within the transversus thoracis muscle during any of the interventions. Our results demonstrate nonuniform Q within PS, EI, and II during both spontaneous and inspiratory resistive loaded breathing. On the assumption that changes in Q reflect changes in activation, our results suggest systematic topographical patterns of recruitment of rib cage respiratory muscles.

Changes in central delay of soleus H reflex after facilitatory or inhibitory conditioning in humans

1. Central delay (CD) changes after facilitatory or inhibitory conditioning of the soleus H reflex have been investigated in a group of normal subjects as a function of the conditioning and test stimulus intensities and also as a function of the Hmax/Mmax ratio. Both facilitation and inhibition of the reflex response have been obtained by conditioning stimulation of the ipsilateral tibial nerve at suitable conditioning-test stimulus intervals. CD changes have been extrapolated from the variations of the time interval between afferent and efferent neural volleys underlying the H reflex, directly recorded from the sciatic nerve. 2. The CD was significantly decreased by facilitatory and increased by inhibitory conditioning. Facilitatory CD changes were positively related to test stimulus strength (at a given conditioning stimulus intensity) and negatively related to conditioning stimulus strength (at a given test stimulus intensity). Both trends were reversed after inhibitory conditioning. The effectiveness of facilitatory conditioning was positively related to the individual Hmax/Mmax ratio whereas a negative relationship could be observed after inhibitory conditioning. 3. Also, the "conditioning threshold" (the minimal conditioning stimulus strength affecting the reflex size) and the "maximal conditioning effect" (the conditioning stimulus intensities leading to either the saturation of the facilitatory effect or the suppression of the reflex response) were significantly related to the Hmax/Mmax ratio. 4. We suggest that temporal changes in the H reflex pathway after facilitatory or inhibitory conditioning stimuli depend both on the size of the motoneuronal pool underlying the reflex response, as determined by the test stimulus intensity, and on the individual excitability of the motoneuronal pool, as defined by the Hmax/Mmax ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Recruitment of crab gastric mill neurons into the pyloric motor pattern by mechanosensory afferent stimulation

1. The gastropyloric receptor (GPR) cells are stretch-sensitive muscle receptors in the crab stomatogastric nervous system that use both 5-hydroxytryptamine (serotonin) and acetylcholine as cotransmitters. Brief stimulation of these afferent neurons causes two gastric mill neurons to be recruited into the pyloric motor pattern. 2. The GPR cells evoke complex synaptic potentials in the lateral gastric (LG) and medial gastric (MG) motor neurons, two component neurons of the gastric mill central pattern generator. When the gastric mill is quiescent (as often happens in vivo), GPR stimulation transiently inhibits LG and MG. After this transient inhibition, these cells undergo a prolonged excitation during which they fire bursts of action potentials at a constant phase relation to the pyloric motor pattern. 3. To determine the causes for this effect, we examined the effects of GPR stimulation on these two cells and on the inferior cardiac motor neuron, which is electrically coupled to them. When GPR is stimulated, all three cells receive rapid biphasic synaptic potentials that are blocked by nicotinic antagonists, followed by a slow, prolonged depolarizing potential. 4. The slow, prolonged depolarizing potential is not blocked by nicotinic or muscarinic cholinergic antagonists but is mimicked and occluded by exogenously applied serotonin. 5. The prolonged excitation, mediated at least in part by serotonin, may be responsible for the recruitment of the gastric mill neurons into the pyloric motor pattern. Thus sensory input can directly exert prolonged modulatory effects that change the functional cellular composition of pattern-generating circuits.

Recursive parameter identification of constrained systems: an application to electrically stimulated muscle

In the application of real-time identification methods for diagnosis or adaptive control of biomedical systems, there is often known model information that is ignored. Constraints on the allowable values of parameters, which may be based on physical considerations, are often neglected because the information does "fit" easily into commonly used parameter-identification algorithms. In this paper a method of incorporating constraints on model parameters is developed. This method is applicable to most recursive parameter-identification algorithms. It enforces linear equality constraints on identified parameters. The use of this method for the real-time identification of autoregressive moving-average-type time series models, subject to parameter constraints, is described in detail. These constraints may be time varying. At each time step, the parameter estimate obtained by a recursive least squares estimator is orthogonally projected onto the constraint surface. This simple idea, when appropriately executed, enhances the output prediction accuracy of estimated parameters. Using constraint information in this way is important when we do not wish to destroy a "natural" parameterization of the model (by an initial projection to incorporate equality constraints), or when we cannot use a single initial model simplification (because the constraints are time varying or involve inputs and outputs). Because it improves output prediction at future times, this method is advantageous for use in predictive adaptive controllers. The use of this algorithm is demonstrated in the identification of electrically stimulated quadriceps muscles in paraplegic human subjects, using percutaneous intramuscular electrodes. The nonlinear steady-state force versus pulsewidth recruitment characteristic of the electrode-muscle system is identified simultaneously with the input-output muscle response dynamics, using a Hammerstein-type model. Knowledge of the recruitment curve's shape is translated into constraints on the identified parameters. This information improves the experimental predictive quality of the identified model.

A recruitment theory of force-time relations in the production of brief force pulses: the parallel force unit model

A theory, the parallel force unit model, is advanced in which the buildup and decline of force in rapid responses of short duration are assumed to reflect variability in timing of several parallel force units. Response force is conceived of as being a summation of a large number of force units, each acting independently of one another. Force is controlled by either the number of recruited force units or the duration each unit contributes its force. Several predictions are derived on the basis of this theory and are shown to be in qualitative agreement with empirical findings about both the mean and variability of brief force impulses. The model also has consequences for the temporal properties of a response. For example, under certain circumstances, it predicts a reciprocal relation between reaction time and response force. Although the theory is proposed as a psychological account, relations between the assumptions and basic principles in neurophysiology are considered. Possible future applications and generalizations of the theory are discussed.

Contribution of electromyography to the diagnosis, treatment and prognosis of cervical disc disease: a study of 114 patients

The aim of this study is to verify whether EMG has the same diagnostic and prognostic capacity in cervical root syndrome as it does in lumbo-sacral syndromes. Our population includes 114 patients affected by a cervico-brachialgia with EMG denervation provoked by cervical disc disease. 26 patients were submitted to a myelography confirming the discal origin of the syndrome: there were 3 cases of disc protrusion and 23 cases of cervical spondylosis. The agreement between EMG and myelographical data was significant in 20 cases (76.9%). There were no significant differences of evolution between operated and non-operated cases. EMG failed to provide prognostic criteria as it does in lumbo-sacral syndromes: it must hence be concluded that all patients presenting cervico-brachialgia and EMG signs of denervation should in any case be admitted to the hospital for exploratory examination. In this context, the particular role of EMG is to establish the degree of the muscular impairment, the number of roots involved, the duration of the syndrome and--recently--the identification of the intervertebral space to be explored by spinal CT.

Changes in hip position modulate soleus H-reflex excitability in man

The effects of hip flexion and extension on the ipsilateral soleus Hoffmann (H) reflex recruitment curve were studied in 11 healthy subjects. Hip flexion (50 degrees), but not hip extension (15-20 degrees), produced changes in the H-reflex. A maintained facilitation, peaking at intensities of stimulation producing a maximal H-reflex (Hmax), was observed in 6/18 sessions. Inhibition, peaking at intensities submaximal for Hmax, was seen in 7/18 sessions. In some of the latter experiments, there was also a facilitation at high intensities of stimulation (greater than Hmax). The remaining experiments were classified as showing no effect: 3 were unmodulated but 2 showed a facilitation at high intensities of stimulation (greater than Hmax). Since the knee was extended in the test position, a second series of experiments (n = 7) were carried out to determine the possible influence of stretch of the biarticular hamstrings muscle group on the soleus H-reflex by comparing the effects of hip flexion with the knee extended with those obtained when the knee was flexed, thereby relaxing the hamstrings. The results provided no evidence that the variability could be explained by differences in the relative degree of stretch on the hamstrings muscle group. There were, however, systematic variations in the shape of the corresponding control H-reflex recruitment curves between subjects: the mean slope of the rising limb of the recruitment curve was highest in those experiments showing an inhibition, intermediate in the ineffective experiments and lowest in those showing a maintained facilitation. These observations indicate that the reflex output studied was different in the three groups, possibly reflecting differences in the relative proportions of slow- and fast-twitch motor units contributing to the reflex response.

Emergence and propagation of interictal spikes in the subcortically denervated hippocampus

Spontaneous and evoked field potentials and cellular discharges of the subcortically denervated dorsal hippocampus were studied by multisite recordings in the freely behaving rat. Characteristic short-duration (< 100 ms), large-amplitude (up to 10 mV) transients, termed interictal spikes (IIS), were seen after fimbria-fornix (FF) lesion. Both pyramidal cells and putative interneurons fired maximally during IIS, with some interneurons sustaining long bursts (up to 400 ms) of high-frequency discharges (400-600 Hz) after the IIS. The speed of propagation of IIS along the longitudinal axis of the hippocampus varied from 0.2 m/s to > 3 m/s. The majority of IIS (type 1) could be accounted for by an enhanced activity of the intrahippocampal associational systems; a second class of IIS (type 2) had positive polarities in the stratum radiatum of CA1 and CA3 and propagated very rapidly (> 1.5 m/s). The authors propose that type 2 IIS reflect somatic depolarization and discharge of pyramidal neurons due to nonsynaptic (probably ephaptic) effects. Ephaptic interactions may also explain the longitudinal propagation of IIS at speeds higher than the conduction velocities (0.5 m/s) of hippocampal fiber systems. IIS emerged during the first 3 weeks after fimbria-fornix lesion, their incidence reaching a plateau of 2/min thereafter. During the same time period, paired-pulse suppression increased in the dentate gyrus. The amplitude of test responses to angular bundle stimulation was potentiated by small-amplitude IIS but suppressed by large-amplitude IIS. The incidence of IIS was significantly suppressed during walking relative to standing still. Tetanic stimulation of the angular bundle or handling-induced stress resulted in a 10- to 20-fold increase in the incidence of IIS that lasted for about 30 minutes. There was a negative correlation between evoked field PSP slope and population spike amplitude in the dentate gyrus of FF-lesioned rats; this correlation was positive in intact rats. The authors attribute the above pathophysiological changes to sprouting of both excitatory and inhibitory GABAergic pathways as a result of denervation of the intrahippocampal circuitry. They hypothesize that the majority of the observed physiological alterations can be traced to a weakening of feedforward inhibition coupled with an enhancement of feedback inhibition and excitation.

Abdominal muscle activity during CO2 rebreathing in sleeping neonates

Comparison of the abdominal muscle response to CO2 rebreathing in rapid-eye-movement (REM) and non-REM (NREM) sleep was performed in healthy premature infants near full term. Eight subjects were studied at a postconceptional age of 40 +/- 1.6 (SD) wk (range 38-43 wk) during spontaneous sleep. Sleep stages were defined on the basis of electrophysiological and behavioral criteria, and diaphragmatic and abdominal muscle electromyographic activity was recorded by cutaneous electrodes. The responses to CO2 were measured by a modified Read rebreathing technique. The minute ventilation and diaphragmatic and abdominal muscle electromyographic activities were calculated and plotted against end-tidal CO2 partial pressure. Both the ventilatory and diaphragmatic muscle responses to CO2 decreased from NREM to REM sleep (P less than 0.05). Abdominal muscles were forcefully recruited in response to CO2 rebreathing during NREM sleep. In REM sleep, abdominal muscle response to CO2 was virtually absent or decreased compared with NREM sleep (P less than 0.05). We conclude that 1) the abdominal muscles are recruited during NREM sleep in response to CO2 rebreathing in healthy premature infants near full term and 2) the abdominal muscle recruitment is inhibited during REM sleep compared with NREM sleep, and this REM sleep-related inhibition probably contributes to the decrease in the ventilatory response to CO2 rebreathing in REM sleep.

Selective activation of small motor axons by quasi-trapezoidal current pulses

We have found a method to activate electrically smaller nerve fibers without activating larger fibers in the same nerve trunk. The method takes advantage of the fact that action potentials are blocked with less membrane hyperpolarization in larger fibers than in smaller fibers. In our nerve stimulation system, quasitrapezoidal-shaped current pulses were delivered through a tripolar cuff electrode to effect differential block by membrane hyperpolarization. The quasitrapezoidal-shaped pulses with a square leading edge, a 350 microsecond(s) plateau, and an exponential trailing phase ensured the block of propagating action potentials and prevented the occurrence of anodal break excitation. The tripolar cuff electrode design restricted current flow inside the cuff and thus eliminated the undesired nerve stimulation due to a "virtual cathode." Experiments were performed on 13 cats. The cuff electrode was placed on the medial gastrocnemius nerve. Both compound and single fiber action potentials were recorded from L7 ventral root filaments. The results demonstrated that larger alpha motor axons could be blocked at lower current levels than smaller alpha motor axons, and that all alpha fibers could be blocked at lower current levels than gamma fibers. A statistical analysis indicated that the blocking threshold was correlated with the axonal conduction velocity or fiber diameter. This method could be used in physiological experiments and neural prostheses to achieve a small-to-large recruitment order in motor or sensory systems.

A method to effect physiological recruitment order in electrically activated muscle

A new stimulation method has been utilized to achieve physiological recruitment order of small-to-large motor units in electrically activated muscles. The use of quasitrapezoidal-shaped pulses and a tripolar cuff electrode made selective activation of small motor axons possible, thus recruiting slow-twitch, fatigue-resistant muscle units before fast-twitch, fatigable units in a heterogeneous muscle. Isometric contraction force from the medial gastrocnemius muscle was measured in five cats. The physiological recruitment order was evidenced by larger twitch widths at lower force levels and smaller twitch widths at higher force levels in the muscles tested. In addition, force modulation process was more gradual and fused contractions were obtained at lower stimulation frequencies when the new stimulation method was employed. Furthermore, muscles activated by the new method were more fatigue-resistant under repetitive activation at low force levels. This stimulation method is simpler to implement and has fewer adverse effects on the neuromuscular system than previous blocking methods. Therefore, it may have applications in future functional neuromuscular stimulation systems.

Sensitivity and selectivity of intraneural stimulation using a silicon electrode array

Artificial electrical stimulation of peripheral nerves needs the development of multielectrode devices which stimulate individual fibers or small groups in a selective and sensitive way. To this end, a multielectrode array in silicon technology has been developed, as well as experimental paradigms and model calculations for sensitivity and selectivity measures. The array consists of twelve platinum electrode sites (10 x 50 microns at 50 microns interdistance) on a 45 microns thick tip-shaped silicon substrate and a Si3N4 insulating glass cover layer. The tip is inserted in the peroneal nerve of the rat during acute experiments to stimulate alpha motor fibers of the extensor digitorum longus muscle. Sensitivity calculations and experiments show a cubic dependence of the number of stimulated motor units on current amplitude of the stimulatory pulse (recruitment curves), starting at single motor level. Selectivity was tested by a method based on the refractory properties of neurons. At the lowest stimulus levels (for one motor unit) selectivity is maximal when two electrodes are separated by 200-250 microns, which was estimated also on theoretical grounds. The study provides clues for future designs of two- and three-dimensional devices.

The distribution of corticocortical, thalamocortical, and callosal inputs on identified motor cortex output neurons: mechanisms for their selective recruitment

Motor cortex neurons were identified antidromically in anesthetized cats by their axonal projections to one of six targets: (1) somatosensory cortex, (2) opposite motor cortex, (3) red nucleus, (4) lateral reticular nucleus, (5) spinal cord, and (6) ventrolateral thalamus. Three inputs to motor cortex were tested for their influences on the identified cortical efferent neurons. The tested inputs originated from ipsilateral somatosensory cortex, opposite motor cortex, and ventral thalamus. Subthreshold effects of input pathways were detected by monitoring latency variations of antidromic responses. The three afferent sources, when activated by electrical stimulation, were not equally effective on motor cortex neurons. Ipsilateral corticocortical and thalamocortical excitation were found for the majority of neurons; the influenced proportions ranged from 55% to 100%, according to the target of the output neurons. Effects from the opposite hemisphere were found for only 5% to 30% of the neurons in the same projection classes. Many neurons (36 of 81, or 44%) were excited from more than one source, but few (5 of 37, or 14%) were influenced by all three possible sources of input, even in small regions of cortex innervated by all three of the inputs. Among 19 electrode tracks where all three inputs were present, there were only 2 tracks where all the neurons shared the same combination of inputs. Even for neurons in closest anatomical proximity ("clusters"), it was unusual (only 7 of 25 clusters) for all the neurons to have the same input pattern. Among the seven clusters where all the neurons shared the same input pattern, five of the clusters projected to the same target. These variable combinations of inputs to motor cortex neurons support the conclusion that efferent neurons could be recruited selectively from separate cortical layers or from within clusters of nearby neurons, according to the target of their axonal projection.

How do geometric factors influence epidural spinal cord stimulation? A quantitative analysis by computer modeling

Effects of both anatomic and electrode geometry on the recruitment of rostrocaudal fibers in the spinal cord were investigated by computer simulation of epidural spinal cord stimulation. A three-dimensional model was used, representing the geometry and electrical conductivity of the spinal cord and surrounding tissues, in combination with a model representing the electrical properties of a myelinated nerve fiber. Recruitment contours in the dorsal columns were calculated at various spinal geometries as a function of electrode position, combination and area. Cathodal position appeared to be most significant. Recruitment areas resulting from different contact combinations of a mediodorsal array were almost identical. It was shown that perception threshold largely depends on both dorsal cerebrospinal fluid width and fiber size. The usual bipolar contact separation appeared to approximate the theoretically optimal value, resulting in maximum fiber recruitment at minimum stimulus.

Activity-dependent recruitment of endogenous glutamate for exocytosis

The Ca(2+)-dependent release of the neurotransmitter glutamate from purified nerve terminals (synaptosomes) of the rat hippocampus was studied in a rapid perfusion apparatus. The response of the terminals was investigated with respect to the kinetics and duration of the release of endogenous glutamate upon brief and sustained stimulation and upon repetitive stimulation. The synaptosomes were stimulated by sustained chemical depolarization (0.5-3 min 30 mM K+). The cellular levels of glutamate, free Ca2+ and ATP in the nerve terminals were measured. The Ca(2+)-dependent release of glutamate showed an immediate elevation upon K(+)-depolarization. When the stimulation was maintained, a prolonged phase of glutamate release was observed. After 3 min, the Ca(2+)-dependent release stopped, although K(+)-depolarization was still effective. When synaptosomes were stimulated again after a relatively short stimulation period (30 s), the second response was similar to the previous one. After a longer stimulation period, maintained until termination of release, the second response did not show the immediate initial elevation of Ca(2+)-dependent glutamate release. Only 30 s after stimulation the release developed with a time profile comparable to the first response. This initial lack of response was not due to low cytosolic levels of glutamate or ATP or to changes in cellular Ca(2+)-buffering. It can be concluded that the capacity to release glutamate after brief depolarizations is fully restored during the repolarization period. However, if stimulation periods are of long duration (until termination of release), this capacity is no longer fully restored, especially with respect to a fast component of release. New glutamate is recruited only during the subsequent depolarization and with a delay.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in coordination of elbow flexor muscles in force tasks and in movement tasks

Motor-unit activity in m. biceps brachii, m. brachialis and m. brachioradialis during isometric contractions has been compared with motor-unit activity during slow voluntary (extension and flexion) movements made against external loads. During these slow movements the recruitment threshold of m. biceps motor units is considerably lower than it is during isometric contractions but the recruitment threshold of both m. brachialis and m. brachioradialis motor units is considerably higher. For all three elbow flexor muscles the motor-unit firing frequency seems to depend on the direction of movement: the firing frequency is higher during flexion movements (3 deg/s) and lower during extension movements (-3 deg/s) than during isometric contractions. The relative contribution of the biceps to the total exerted flexion torque during slow voluntary movements is estimated to increase from 36% to about 48% and that of the brachialis/brachioradialis is estimated to decrease from 57% to about 45% compared to the relative contribution of these muscles during isometric contractions. This difference in the relative contribution of the three major elbow flexor muscles is shown to be caused by differences in the central activation in force tasks and movement tasks.

Reductions in recruitment force thresholds in human single motor units by successive voluntary contractions

Recruitment force thresholds of biceps brachii single motor units were studied in 4 male subjects before and after an isometric muscle contraction, passive muscle stretch, or following successive muscle contractions, muscle stretches or during alternations between muscle stretches and muscle contractions. Isometric muscle contractions of 5 s duration decreased subsequent single motor unit force thresholds. These force thresholds could usually be reset at or near precontraction force threshold values by passive muscle stretch induced by elbow extension. Single motor units showing reduced force thresholds following contraction were momentarily derecruited during and/or after muscle stretch. Successive muscle stretches alone did not significantly alter single motor unit force thresholds. In contrast, single motor unit recruitment force thresholds during successive weaker contractions were progressively lowered. Intercontraction muscle stretches maintained the single motor unit force thresholds at or near the initial force threshold level. The mechanism(s) underlying a muscle contraction-induced lowering of single motor unit force thresholds may reside in stretch reflex pathways.