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Systems biology has experienced dramatic growth in the number, size, and complexity of computational models. To reproduce simulation results and reuse models, researchers must exchange unambiguous model descriptions. We review the latest edition of the Systems Biology Markup Language (SBML), a format designed for this purpose. A community of modelers and software authors developed SBML Level 3 over the past decade. Its modular form consists of a core suited to representing reaction-based models and packages that extend the core with features suited to other model types including constraint-based models, reaction-diffusion models, logical network models, and rule-based models. The format leverages two decades of SBML and a rich software ecosystem that transformed how systems biologists build and interact with models. More recently, the rise of multiscale models of whole cells and organs, and new data sources such as single-cell measurements and live imaging, has precipitated new ways of integrating data with models. We provide our perspectives on the challenges presented by these developments and how SBML Level 3 provides the foundation needed to support this evolution., (© 2020 California Institute of Technology Published under the terms of the CC BY 4.0 license.)

The majority of studies on the electrical properties of neurons are carried out in rodents, and in particular in mice. However, the minute size of this animal compared with humans potentially limits the relevance of the resulting insights. To be able to extrapolate results obtained in a small animal such as a rodent, one needs to have proper knowledge of the rules governing how electrical properties of neurons scale with the size of the animal. Generally speaking, electrical resistances of neurons increase as cell size decreases, and thus maintenance of equal depolarization across cells of different sizes requires the underlying currents to decrease in proportion to the size decrease. Thus it would generally be expected that voltage-sensitive currents are smaller in smaller animals. In this study, we used in vivo preparations to record electrical properties of spinal motoneurons in deeply anesthetized adult mice and cats. We found that PICs do not scale with size, but instead are constant in their amplitudes across these species. This constancy, coupled with the threefold differences in electrical resistances, means that PICs contribute a threefold larger depolarization in the mouse than in the cat. As a consequence, motoneuronal firing rate sharply increases as animal size decreases. These differences in firing rates are likely essential in allowing different species to control muscles with widely different contraction speeds (smaller animals have faster muscle fibers). Thus from our results we have identified a possible new mechanism for how electrical properties are tuned to match mechanical properties within the motor output system. NEW & NOTEWORTHY The small size of the mouse warrants concern over whether the properties of their neurons are a scaled version of those in larger animals or instead have unique features. Comparison of spinal motoneurons in mice to cats showed unique features. Firing rates in the mouse were much higher, in large part due to relatively larger persistent inward currents. These differences likely reflect adaptations for controlling much faster muscle fibers in mouse than cat., (Copyright © 2017 the American Physiological Society.)

The purpose of this study was to investigate the extent to which correlated neural inputs, quantified as EMG-EMG coherence across intrinsic and extrinsic hand muscles, varied as a function of wrist angle during a constant force precision grip task. Eight adults (5 males; mean age 29 years) participated in the experiment. Subjects held an object using a two-digit precision grip at a constant force at a flexed, neutral, and extended wrist posture, while the EMG activity from intrinsic and extrinsic hand muscles was recorded through intramuscular fine-wire electrodes. The integral of z-transformed coherence computed across muscles pairs was greatest in the flexed wrist posture and significantly greater than EMG-EMG coherence measured in the neutral and extended wrist posture (P < 0.01 and 0.05, respectively). Furthermore, EMG-EMG coherence did not differ statistically between the extrinsic and intrinsic muscle pairs, even though it tended to be greater for the extrinsic muscle pair (P ≥ 0.063). These findings lend support to the notion of a functional role of correlated neural inputs to hand muscles for the task-dependent coordination of hand muscle activity that is likely mediated by somatosensory feedback., (Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.)

The hyperexcitability of alpha-motoneurons and accompanying spasticity following spinal cord injury (SCI) have been attributed to enhanced persistent inward currents (PICs), including L-type calcium and persistent sodium currents. Factors controlling PICs may offer new therapies for managing spasticity. Such factors include calcium-activated potassium (KCa) currents, comprising in motoneurons an after-hyperpolarization-producing current (I KCaN) activated by N/P-type calcium currents, and a second current (I KCaL) activated by L-type calcium currents (Li and Bennett in J neurophysiol 97:767-783, 2007). We hypothesize that these two currents offer differential control of PICs and motoneuron excitability based on their probable somatic and dendritic locations, respectively. We reproduced SCI-induced PIC enhancement in a two-compartment motoneuron model that resulted in persistent dendritic plateau potentials. Removing dendritic I KCaL eliminated primary frequency range discharge and produced an abrupt transition into tertiary range firing without significant changes in the overall frequency gain. However, I KCaN removal mainly increased the gain. Steady-state analyses of dendritic membrane potential showed that I KCaL limits plateau potential magnitude and strongly modulates the somatic injected current thresholds for plateau onset and offset. In contrast, I KCaN had no effect on the plateau magnitude and thresholds. These results suggest that impaired function of I KCaL may be an important intrinsic mechanism underlying PIC-induced motoneuron hyperexcitability following SCI.

Spasticity is commonly observed after chronic spinal cord injury (SCI) and many other central nervous system disorders (e.g., multiple sclerosis, stroke). SCI-induced spasticity has been associated with motoneuron hyperexcitability partly due to enhanced activation of intrinsic persistent inward currents (PICs). Disrupted spinal inhibitory mechanisms also have been implicated. Altered inhibition can result from complex changes in the strength, kinetics, and reversal potential (E(Cl(-))) of γ-aminobutyric acid A (GABA(A)) and glycine receptor currents. Development of optimal therapeutic strategies requires an understanding of the impact of these interacting factors on motoneuron excitability. We employed computational methods to study the effects of conductance, kinetics, and E(Cl(-)) of a dendritic inhibition on PIC activation and motoneuron discharge. A two-compartment motoneuron with enhanced PICs characteristic of SCI and receiving recurrent inhibition from Renshaw cells was utilized in these simulations. This dendritic inhibition regulated PIC onset and offset and exerted its strongest effects at motoneuron recruitment and in the secondary range of the current-frequency relationship during PIC activation. Increasing inhibitory conductance compensated for moderate depolarizing shifts in E(Cl(-)) by limiting PIC activation and self-sustained firing. Furthermore, GABA(A) currents exerted greater control on PIC activation than glycinergic currents, an effect attributable to their slower kinetics. These results suggest that modulation of the strength and kinetics of GABA(A) currents could provide treatment strategies for uncontrollable spasms.

Fingertip force control requires fine coordination of multiple hand muscles within and across the digits. While the modulation of neural drive to hand muscles as a function of force has been extensively studied, much less is known about the effects of fatigue on the coordination of simultaneously active hand muscles. We asked eight subjects to perform a fatiguing contraction by gripping a manipulandum with thumb, index, and middle fingers while matching an isometric target force (40% maximal voluntary force) for as long as possible. The coordination of 12 hand muscles was quantified as electromyographic (EMG) muscle activation pattern (MAP) vector and EMG-EMG coherence. We hypothesized that muscle fatigue would cause uniform changes in EMG amplitude across all muscles and an increase in EMG-EMG coherence in the higher frequency bands but with an invariant heterogeneous distribution across muscles. Muscle fatigue caused a 12.5% drop in the maximum voluntary contraction force (P < 0.05) at task failure and an increase in the SD of force (P < 0.01). Although EMG amplitude of all muscles increased during the fatiguing contraction (P < 0.001), the MAP vector orientation did not change, indicating that a similar muscle coordination pattern was used throughout the fatiguing contraction. Last, EMG-EMG coherence (0-35 Hz) was significantly greater at the end than at the beginning of the fatiguing contraction (P < 0.01) but was heterogeneously distributed across hand muscles. These findings suggest that similar mechanisms are involved for modulating and sustaining digit forces in nonfatiguing and fatiguing contractions, respectively.

Coherence between electromyographic (EMG) signals has been used to identify correlated neural inputs to motor units (MUs) innervating different muscles. Simulations using a motor-unit model (Fuglevand et al. 1992) were performed to determine the ability of coherence between two multi-unit EMGs (mEMG) to detect correlated MU activity and the range of correlation strengths in which mEMG coherence can be usefully employed. Coherence between motor-unit and mEMG activities in two muscles was determined as we varied the strength of a 30-Hz periodic common input, the number of correlated MU pairs and variability of MU discharge relative to the common input. Pooled and mEMG coherence amplitudes positively and negatively accelerated, respectively, toward the strongest and most widespread correlating inputs. Furthermore, the relation between pooled and mEMG coherence was also nonlinear and was essentially the same whether correlation strength varied by changing common input strength or its distribution. However, the most important finding is that while the mEMG coherence saturates at the strongest common input strengths, this occurs at common input strengths greater than found in most physiological studies. Thus, we conclude that mEMG coherence would be a useful measure in many experimental conditions and our simulation results suggest further guidelines for using and interpreting coherence between mEMG signals.

The discharge properties of hindlimb motoneurons in ketamine-xylazine anesthetized rats were measured to assess contributions of persistent intrinsic currents to these characteristics and to determine their distribution in motoneuron pools. Most motoneurons (30/37) responded to ramp current injections with adapting patterns of discharge and the frequency-current (f-I) relations of nearly all motoneurons included a steep subprimary range of discharge. Despite the prevalence of adapting f-I relations, responses included indications that persistent inward currents (PICs) were activated, including increased membrane noise and prepotentials before discharge, as well as counterclockwise hysteresis and secondary ranges in f-I relations. Examination of spike thresholds and afterhyperpolarization (AHP) trajectories during repetitive discharge revealed systematic changes in threshold and trajectory within the subprimary, primary, and secondary f-I ranges. These changes in the primary and secondary ranges were qualitatively similar to those described previously for cat motoneurons. Within the subprimary range, AHP trajectories often included shallow approaches to threshold following recruitment and slope of the AHP ramp consistently increased until the subprimary range was reached. We suggest that PICs activated near recruitment contributed to these slope changes and formation of the subprimary range. Discharge characteristics were strongly correlated with motoneuron size, using input conductance as an indicator of size. Discharge adaptation, recruitment current, and frequency increased with input conductance, whereas both subprimary and primary f-I gains decreased. These results are discussed with respect to potential mechanisms and their functional implications.

We report here the first direct measurements of persistent inward currents (PICs) in rat hindlimb motoneurons, obtained from ketamine-xylazine anesthetized rats during slow voltage ramps performed by single-electrode somatic voltage clamp. Most motoneurons expressed PICs and current-voltage (I-V) relations often contained a negative-slope region (NSR; 13/19 cells). PICs activated at -52.7 ± 3.89 mV, 9 mV negative to spike threshold. NSR onset was -44.2 ± 4.1 mV. PIC amplitudes were assessed by maximum inward currents measured relative to extrapolated leak current and to NSR-onset current. PIC conductance at potentials just positive to activation was assessed by the relative change in slope conductance (g(in)/g(leak)). PIC amplitudes varied widely; some exceeded 5 and 10 nA relative to current at NSR onset or leak current, respectively. PIC amplitudes did not vary significantly with input conductance, but PIC amplitudes normalized by recruitment current decreased with increasing input conductance. Similarly, g(in)/g(leak) decreased with increasing input conductance. Currents near resting potential on descending limbs of I-V relations were often outward, relative to ascending-limb currents. This residual outward current was correlated with increases in leak conductance on the descending limb and with input conductance. Excluding responses with accommodation, residual outward currents matched differences between recruitment and derecruitment currents, suggesting a role for residual outward current in frequency adaptation. Comparison of potentials for PIC activation and NSR onset with interspike trajectories during discharge demonstrated correspondence between PIC activation and frequency-current (f-I) range boundaries. Contributions of persistent inward and outward currents to motoneuron discharge characteristics are discussed.

The ability to modulate digit forces during grasping relies on the coordination of multiple hand muscles. Because many muscles innervate each digit, the CNS can potentially choose from a large number of muscle coordination patterns to generate a given digit force. Studies of single-digit force production tasks have revealed that the electromyographic (EMG) activity scales uniformly across all muscles as a function of digit force. However, the extent to which this finding applies to the coordination of forces across multiple digits is unknown. We addressed this question by asking subjects (n = 8) to exert isometric forces using a three-digit grip (thumb, index, and middle fingers) that allowed for the quantification of hand muscle coordination within and across digits as a function of grasp force (5, 20, 40, 60, and 80% maximal voluntary force). We recorded EMG from 12 muscles (6 extrinsic and 6 intrinsic) of the three digits. Hand muscle coordination patterns were quantified in the amplitude and frequency domains (EMG-EMG coherence). EMG amplitude scaled uniformly across all hand muscles as a function of grasp force (muscle x force interaction: P = 0.997; cosines of angle between muscle activation pattern vector pairs: 0.897-0.997). Similarly, EMG-EMG coherence was not significantly affected by force (P = 0.324). However, coherence was stronger across extrinsic than that across intrinsic muscle pairs (P = 0.0039). These findings indicate that the distribution of neural drive to multiple hand muscles is force independent and may reflect the anatomical properties or functional roles of hand muscle groups.

Background: Polyhydroxyalkanoates (PHAs) can be degraded by many microorganisms using intra- or extracellular PHA depolymerases. PHA depolymerases are very diverse in sequence and substrate specificity, but share a common alpha/beta-hydrolase fold and a catalytic triad, which is also found in other alpha/beta-hydrolases., Results: The PHA Depolymerase Engineering Database (DED, http://www.ded.uni-stuttgart.de) has been established as a tool for systematic analysis of this enzyme family. The DED contains sequence entries of 587 PHA depolymerases, which were assigned to 8 superfamilies and 38 homologous families based on their sequence similarity. For each family, multiple sequence alignments and profile hidden Markov models are provided, and functionally relevant residues are annotated., Conclusion: The DED is a valuable tool which can be applied to identify new PHA depolymerase sequences from complete genomes in silico, to classify PHA depolymerases, to predict their biochemical properties, and to design enzyme variants with improved properties.

The relation between impedance change and the location and magnitude of a tonic synaptic conductance was examined in compartmental motoneuron models based on previously published data. The dependency of motoneuron impedance on system time constant (tau), electrotonic length (L), and dendritic-to-somatic conductance ratio (rho) was examined, showing that the relation between impedance phase and rho differed markedly between models with uniform and nonuniform membrane resistivity. Dendritic synaptic conductances decreased impedance magnitude at low frequencies; at higher frequencies, impedance magnitude increased. The frequency at which the change in impedance magnitude reversed from a decrease to an increase-the reversal frequency, F(r)-was a good estimator of electrotonic synaptic location. A measure of the average normalized impedance change at frequencies less than F(r), cuDeltaZ, estimated relative synaptic conductance. F(r) and cuDeltaZ provided useful estimates of synaptic location and conductance in models with nonuniform (step, sigmoidal) and uniform membrane resistivity. F(r) also provided good estimates of spatial synaptic location on the equivalent cable in both step and sigmoidal models. Variability in relations between F(r), cuDeltaZ, and conductance location and magnitude between neurons was reduced by normalization with rho and tau. The effects on F(r) and cuDeltaZ of noise in experimental recordings, different synaptic distributions, and voltage-dependent conductances were also assessed. This study indicates that location and conductance of tonic dendritic conductances can be estimated from F(r), cuDeltaZ, and basic electrotonic motoneuron parameters with the exercise of suitable precautions.

Electrical parameters of spinal motoneurons were estimated by optimizing the parameters of motoneuron models to match experimentally determined impedance functions with those of the models. The model was described by soma area, somatic and dendritic membrane resistivities, and the diameter of an equivalent dendritic cable having a standard profile. The impedance functions of motoneurons and optimized models usually differed (rms error) by <2% of input resistance. Consistent estimates for most parameters were obtained from repeated impedance determinations in individual motoneurons; estimates of dendritic resistivity were most variable. The few cells that could not be fit well had reduced impedance phase lag consistent with dendritic penetrations. Most fits were improved by inclusion of a voltage-dependent conductance G(V) with time constant tau(V). A uniformly distributed G(V) with tau(V) >5 ms provided a better fit for most cells. The magnitude of this conductance decreased with depolarization. Impedance functions of other cells were adequately fit by a passive model or by a model with a somatic G(V) and tau(V) <5 ms. Most of these neurons (7/8) had resting potentials positive to -60 mV. The electrotonic parameters rho, tau, and L, estimated from model parameters, were consistent with published distributions. Most motoneuron parameters obtained in somatic shunt and sigmoidal models were well correlated, and parameters were moderately affected by changes in dendritic profile. These results demonstrate the utility and limitations of impedance measurements for estimating motoneuron parameters and suggest that voltage-dependent conductances are a substantial component of resting electrical properties.

The mean location of Renshaw synapses on spinal motoneurons and their synaptic conductance were estimated from changes in impedance magnitude produced by sustained recurrent inhibition. Motoneuron impedance was determined by injecting quasi-white noise current into lumbosacral motoneurons of pentobarbital-anesthetized cats. Synaptic location and conductance were estimated by comparing observed impedance changes to simulation results obtained using standard motoneuron models and compartmental models fit to each impedance function. Estimated synaptic locations ranged from 0.10 to 0.41lambda, with a mean of 0.19 or 0.24lambda, depending on the estimation method. Average dendritic path length was 262 microm. Average synaptic conductance was 23 to 27 nS (range: 6.7 to 57.9 nS), corresponding to conductance changes of 78 to 88% of resting membrane conductance. Estimated accuracy was supported by consistency using different estimation methods, agreement with Fyffe's 1991 morphological data, and comparisons of observed and simulated recurrent IPSP amplitudes. Synaptic location, but not synaptic conductance, was correlated with rheobase, a measure of motoneuron excitability. Synaptic conductance did not depend on synaptic location. A regression analysis demonstrated that synaptic conductance and cell impedance were the principal factors determining recurrent IPSP amplitude. Simulations using the observed values and locations of Renshaw conductance demonstrate that recurrent inhibition can require as much as an additional 14 to 18% sustained excitatory synaptic conductance to depolarize motoneurons sufficiently to activate somatic or dendritic inward currents and recruit motoneurons or amplify excitatory synaptic currents.

The incidence of recurrent inhibition and facilitation in motor nuclei innervating the dorsiflexors of the ankle and digits was examined in spinalized, decerebrate cats. Motoneurons innervating the anterior and posterior portions of the tibialis anterior (TAa and TAp, respectively) received strong recurrent inhibition following stimulation of either of the homonymous muscle nerves. Both motoneuron species received substantial recurrent inhibition from the semitendinosus (St), but stimulation of the nerve to the extensor digitorum longus (EDL), an ankle flexor synergist, evoked smaller recurrent IPSPs. TA motoneurons received mainly facilitation from hindlimb extensors of the hip and ankle. Motoneurons of the EDL and extensor digitorum brevis (EDB), synergists which share mechanical action at the metatarsophalangeal joint and the digits, received little recurrent inhibition in response to stimulation of the nerve to either muscle. Overall, stimulation of heteronymous flexor nerves (including TAa, TAp, and St) failed to evoke responses in most of the EDB and EDL neurons tested (50-83%), and the amplitude of recurrent inhibitory responses was small. Recurrent facilitation from the extensors was more common in these motor nuclei. Most responses recorded in EDB motoneurons following either flexor or extensor nerve stimulation were recurrent facilitations. The sensitivity of this facilitation in EDB motoneurons to injection of polarizing current and its central latency indicate that it is mediated by a disinhibitory, trisynaptic pathway. Stimulation of the nerve to EDB produced recurrent IPSPs in some flexor motoneurons, but these potentials were infrequent and their amplitude was usually small. Based on a comparison of the distribution of recurrent inhibition to published reports of the activities of TAa, TAp, EDL, and EDB during different forms of locomotion, we conclude that recurrent inhibition is large for motor nuclei that exhibit stereotypical activity, while motor nuclei that are activated independently receive and produce little recurrent inhibition. Despite the absence of recurrent inhibition in some motor nuclei, recurrent circuits may still participate in their control through disinhibitory, facilitatory mechanisms.

The patterns of correlation found between motoneuron pools during fictive locomotion are the same whether the coherence functions used to detect the correlations are determined using pairs of rectified ENGs or motoneuron LDPs and rectified ENGs. This finding suggests that the higher frequencies in rectified ENGs (and, perhaps, EMGs) contain information about the synaptic input to motoneurons. Nevertheless, differences between the coherence functions of rectified ENG pairs and those of LDPs and rectified ENGs suggests that this information is distorted by harmonics introduced by rectification. The activities of many motoneuron pools are correlated during the flexor or extensor phase of fictive locomotion, indicating that they receive common synaptic input from branched presynaptic axons or from pools of interneurons whose activities are synchronized. Similar findings were reported by Bayev (1978), based on temporal correlations. Our results indicate that the investigated motor nuclei, which innervate muscles with actions at the hip, knee and ankle, are subject to a set of common locomotor commands. These commands are also received by inhibitory interneurons that project to the motor nuclei of antagonists, as indicated by the correlations between the hyperpolarizing phase of LDPs and activity in the rectified ENGs of antagonists. This last result is consistent with a modular organization for the spinal locomotor generator, in which one set of interneurons drives a motor pool and the inhibitory interneurons that project to the motor pool's antagonist (Jordan, 1991). However, these results also suggest that the spinal modules for locomotion may not be separable into independent unit-burst generators that produce commands for control of each joint as Grillner (1981) has suggested. Our results are more consistent with a model in which a generator distributes flexor and extensor commands to many motor pools (like the half-center model) with as yet unidentified spinal mechanisms that determine differences in the initiation and termination of activity of individual motor nuclei. Alternatively, the correlations between motor pools that we have observed could be explained by spinal mechanisms that synchronize the activity of unit-burst type generators. Despite the distribution of common locomotor commands to many functionally diverse motor nuclei, the spinal locomotor pattern generator is differentiated to the extent that some motor nuclei, like EDL and FDL, receive separate locomotor commands. This conclusion is consistent with other observations. EDL and FDL display distinctive, individualized patterns of locomotor activity that may vary in a facultative manner or in different forms of locomotion (O'Donovan et al., 1980; Trank et al., 1996). A recent study has shown that during fictive locomotion EDL and FDL motoneurons receive input from different sets of last-order interneurons than those which project to other motor pools (Degtyarenko et al., 1998). These results suggest that spinal locomotor generators are differentiated for the individualized control of some digit muscles, like FDL and EDL.

Red nucleus stimulation inhibits within the inferior olive. J. Neurophysiol. 80: 3127-3136, 1998. In the anesthetized cat, electrical stimulation of the magnocellular red nucleus (RNm) inhibits responses of rostral dorsal accessory olive (rDAO) neurons to cutaneous stimulation. We tested the hypothesis that RNm-mediated inhibition occurs within the inferior olive by using stimulation of the ventral funiculus (VF) of the spinal cord in place of cutaneous stimulation of the hindlimb. Fibers in the VF terminate on hindlimb rDAO neurons, so inhibition of this input would have to occur within the olive. rDAO responses elicited by VF stimulation were inhibited by prior stimulation of the RNm, indicating that inhibition occurs within the olive. In contrast, evoked potentials recorded from the VF or dorsal columns following hindlimb stimulation were not affected by prior stimulation of RNm, indicating that stimulation of the RNm does not inhibit olivary afferents at spinal levels. RNm stimulation that inhibited rDAO responses had little effect on evoked somatosensory responses in thalamus, indicating that inhibition generated by activity in RNm may be specific to rDAO. To test limb specificity of RNm-mediated inhibition, conditioning stimulation was applied to the dorsolateral funiculus at thoracic levels, which selectively activates RNm neurons projecting to the lumbar cord. Stimulation at thoracic levels inhibited evoked responses from hindlimb but not forelimb regions of rDAO, suggesting that inhibitory effects of RNm activity are limb specific. Several studies have reported that olivary neurons have reduced sensitivity to peripheral stimulation during movement; it is likely that RNm-mediated inhibition occurring within the olive contributes to this reduction of sensitivity. Inhibition of rDAO responses by descending motor pathways appears to be a salient feature of olivary function.

The distribution of recurrent inhibition and facilitation to motor nuclei of muscles that act at the cat ankle joint was compared with the locomotor activity and mechanical action of those muscles described in published studies. Emphasis was placed on motor nuclei whose muscles have a principal action about the abduction-adduction axis and the pretibial flexors: tibialis posterior (TP), peroneus longus (PerL), peroneus brevis (PerB), the anterior part of tibialis anterior (TA) and extensor digitorum longus (EDL). Most intracellular recordings in spinalized, unanesthetized decerebrate cats showed only inhibitory or excitatory responses to antidromic stimulation of peripheral nerves, but mixed effects were also seen. Recurrent effects among motor nuclei of ankle abductors and adductors were not distributed uniformly. TP motoneurons received recurrent inhibition from most other nuclei active in stance and stimulation of the TP nerve inhibited these motor nuclei. Although PerB motoneurons are also active during stance, they received primarily facilitation from most motor nuclei. PerL received mixtures of inhibition and facilitation from all sources. Stimulation of the nerves to PerL, PerB, and peroneus tertius (PerT) produced weak recurrent inhibition and facilitation, even in homonymous motoneurons and motoneurons of Ia synergists. The ankle flexors TA and EDL displayed different patterns of recurrent inhibition and facilitation. TA motoneurons received prominent homonymous inhibition and inhibition from semitendinosus (St). EDL, whose activity profile differs from TA and which also acts at the digits, did not receive strong recurrent inhibition from either TA or St, nor did stimulation of the EDL nerve produce much inhibition. The distribution of recurrent inhibition and facilitation is correlated with the pattern of locomotor activity, but with exceptions that suggest an influence of mechanical action, particularly in the antagonistic interactions between TP and PerB. The extended pattern of recurrent inhibition, the reduction or absence of inhibition produced by motor nuclei with individualized functions or digit function and the prevalence of facilitation suggest that the recurrent Renshaw system is organized into inhibitory and disinhibitory projections that participate in the control of sets of motor nuclei engaged in rhythmic and stereotyped movements.

1. The temporal features and strength of recurrent facilitatory potentials were examined in pairs of lumbosacral motoneurons that were separated by a known distance and were identified by antidromic stimulation of muscle nerves. One motoneuron was stimulated by injecting depolarizing current pulses, and responses were recorded in the second motoneuron. The distance between motoneurons in pairs was also measured to assess the spatial distribution in strength of recurrent facilitation in motor pools. All motoneurons in these pairs innervated muscles that act as hip or ankle extensors. 2. Recurrent facilitatory potentials were found frequently among motoneurons innervating the hindlimb extensor muscles examined. Several categories of recurrent facilitatory responses were identified. One category was composed of facilitation responses that followed an inhibition response. A second category was composed of facilitation responses that were not preceded by a significant inhibition and consisted of a monophasic response. There was a considerable range of latencies in this category. 3. Responses in which recurrent facilitatory potentials were preceded by recurrent inhibitory postsynaptic potentials (RIPSPs) among close motoneuron pairs demonstrated an inverse correlation between the durations of the facilitatory and the inhibitory phases. In addition, the duration of inhibition responses without facilitation was longer on average, than the duration of inhibitory responses that were followed by facilitation. It was suggested that recurrent facilitation may restrict the time course of RIPSPs. 4. In contrast to the topographic distribution of RIPSPs described in the previous report, amplitudes of monophasic facilitations were directly correlated with the distance separating motoneurons in pairs, rather than inversely correlated as was the case for RIPSP amplitudes.(ABSTRACT TRUNCATED AT 250 WORDS)

1. The amplitude of recurrent inhibitory postsynaptic potentials (RIPSPs) was examined in pairs of lumbosacral motoneurons that were separated by a known distance and were identified by antidromic stimulation of muscle nerves. One motoneuron was stimulated by injecting depolarizing current pulses, and postsynaptic responses were recorded and averaged in the second motoneuron. Input resistance, rheobase, and conduction velocity were determined for many motoneurons. Most motoneurons innervated extensor muscles. 2. RIPSP values as large as -283 microV were recorded, but most were between -10 and -40 microV. RIPSPs from individual motoneurons of a pool are distributed to several heteronymous motor nuclei and have a range of amplitudes comparable with homonymous RIPSPs. 3. A specific spatial distribution of RIPSP amplitudes was found whereby the largest RIPSP amplitudes (> 40 microV) occurred in motoneurons located within +/- 1.4 mm of the stimulated motoneuron. A significant correlation was found between RIPSP amplitude and the distance between motoneurons for all motoneuron pairs. This correlation was also found within individual groups of motoneuron pairs that innervate the lateral gastrocnemius, medial gastrocnemius, anterior-middle biceps femoris, or soleus muscles. 4. The dependency of RIPSP amplitude on the motoneuron species, which is the particular muscle a motoneuron innervates, is less distinct than the dependency of RIPSP amplitude on topography. Pooling all motoneuron species of close motoneuron pairs indicated that RIPSPs measured in homonymous motoneuron pairs were greater in amplitude than RIPSPs measured in heteronymous pairs. In addition, homonymous RIPSPs of anterior middle biceps femoris or lateral gastrocnemius motoneurons were greater than heteronymous RIPSPs of those motoneurons in all heteronymous combinations. However, homonymous and heteronymous RIPSPs were not significantly different when heteronymous pairs were restricted to individual combinations of species. These findings indicate that RIPSP amplitudes within a set of motor nuclei interconnected by recurrent inhibition are dependent in some cases on the species of motoneurons, but this effect is less important than the effect of topography on RIPSP amplitude. 5. These results indicate that recurrent inhibition in motoneuron pools that innervate hindlimb extensor muscles has a strong topographic organization, such that the strongest recurrent inhibition is produced by each motoneuron in a restricted rostrocaudal zone that includes both homonymous and heteronymous motor nuclei. This suggests that recurrent inhibition is organized for the control of several motor nuclei engaged in common motor activity as well as regulation of activity within individual motor pools.

1. Recurrent collaterals of motoneurons innervating muscles that have a role in control of the hindlimb digits were studied with neuroanatomic tracing methods to determine whether these motoneurons have simple recurrent collateral arbors in comparison with those of hip, knee, and ankle muscles. 2. Motoneurons innervating the hindlimb muscles plantaris (Pln), flexor hallucis longus (FHL), or flexor digitorum longus (FDL) were injected with 10% horseradish peroxidase. Recurrent collaterals were reconstructed from serial transverse sections. 3. No recurrent collaterals were observed in a sample of 10 FDL motoneurons. 4. FHL motoneurons had simple recurrent collateral arbors as assessed by number of first-order collaterals, number of collateral swellings, number of end branches, and the highest-order branch of individual collateral trees. Recurrent collateral arbors of Pln motoneurons were more complex than those of FHL motoneurons. Pln and FHL recurrent collateral arbors were less complex than those described for gastrocnemius-soleus, anterior tibial, and posterior biceps motoneurons. 5. These anatomic findings correspond well with electrophysiological results indicating that the recurrent inhibition produced by FHL motoneurons is weak and that FDL motoneurons do not produce recurrent inhibition. In addition, Pln motoneurons are reported to produce stronger recurrent inhibition than FHL motoneurons in many motor pools. 6. Consideration of these results with respect to the mechanical actions and patterns of motor activity observed in FDL, FHL, and Pln suggests that the complexity of recurrent collaterals of a motoneuron pool and the extent of its contribution to recurrent inhibition diminish with its involvement in the individualized control of the digits.

1. Recurrent inhibitory postsynaptic potentials (IPSPs) to and from motoneurons innervating the flexor digitorum longus (FDL) and flexor hallucis longus (FHL) muscles of the cat were investigated to determine whether recurrent inhibitory projections involving these motoneurons are similar--as would be consistent with the Ia and anatomic synergism of FDL and FHL--or are dissimilar, as are the activities of these muscles during locomotion (O'Donovan et al. 1982). 2. Composite recurrent IPSPs were recorded in several species of motoneurons innervating hindlimb muscles in response to stimulation of a number of muscle nerves in cats allowed to become unanesthetized after ischemic decapitation. 3. No recurrent IPSPs from stimulation of the FDL nerve were observed in motoneurons innervating FDL, FHL, lateral gastrocnemius-soleus (LG-S), medial gastrocnemius (MG), plantaris (Pl), tibialis anterior (TA), or extensor digitorum longus (EDL). 4. The recurrent IPSPs produced by stimulation of FHL were larger and found more frequently in LG-S than in FDL motoneurons. Recurrent inhibition from FHL was also greater in Pl than in FDL motoneurons. 5. The recurrent IPSPs produced by stimulation of LG-S, PL, and MG were larger in FHL than in FDL motoneurons, and those from LG-S and MG were found more frequently in FHL than in FDL motoneurons. 6. Stimulation of the TA nerve produces recurrent IPSPs in FDL but not in FHL motoneurons. A few FDL and FHL cells (6 of 23 and 9 of 34, respectively) received small (less than 0.5 mV) recurrent IPSPs from stimulation of the EDL nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

This report describes the use of a synchronization index (Is; Hamm et al., 1985a) and its sensitivity to various forms and degrees of synchrony between spike trains. The dependence of the Is on signal-to-noise ratio, the number of synchronized spike trains and their degree of synchrony is shown in analog and digital simulations. These simulations and a comparison with peristimulus time histograms under conditions of induced synchrony reveal that the Is is a sensitive measure of synchronization in a population of spike trains.

A test for synchronization among the spike trains of muscle afferents or motor units is described which utilizes averages of neurograms and rectified neurograms. Synchronization is quantified by the increase of a synchronization index Is above a theoretical value for asynchrony. The dependence of the Is on signal amplitude and certain experimental conditions and a method of estimating confidence limits for the test are presented.

Evidence is presented for the existence of a localization of monosynaptic Ia excitatory post-synaptic potentials (e.p.s.p.s) in the motor nucleus of a cat hind limb muscle. Intracellular recordings from biceps femoris motoneurones were made in anaesthetized low spinal cats of the effects of stimuli to the nerve branches supplying the anterior, middle, and posterior portions of the biceps femoris muscle. Recordings were also made during stimulation of nerves to semimembranosus and semitendinosus in order to provide a means of categorizing middle biceps cells as 'extensors' (middle biceps-extensor; i.e. like anterior biceps cells) or as 'flexors' (middle biceps-flexor; like posterior biceps). Homonymous nerve-branch (i.e. from anterior, middle or posterior biceps) monosynaptic Ia e.p.s.p.s were compared within unifunctional (flexor or extensor) groups of motoneurones. In three of four comparisons (anterior biceps nerve branch onto anterior and middle biceps-extensor cells, middle biceps onto middle biceps-flexor and posterior biceps, posterior biceps onto middle biceps-flexor and posterior biceps) the anterior, middle and posterior biceps nerve branches contributed larger e.p.s.p.s to their 'own' motoneurones than to motoneurones supplying other 'compartments' of the muscle. In the fourth case, middle biceps's input appeared to have similar effects onto anterior biceps and middle biceps-extensor cells. A normalization was performed to eliminate the possibility that the differences in e.p.s.p. sizes were due to differences in cell type within the four cell groupings (i.e. differences in the number of cells supplying FF, F(int.), FR and S muscle units). This normalization confirmed that the localization in the first three comparisons was not a consequence of differences in motoneurone type and, in addition, suggested that middle biceps may indeed have greater effects on middle biceps-extensor than anterior biceps cells. In addition to the asymmetrical effects of anterior and middle biceps nerve branches onto anterior biceps and middle biceps-extensor motoneurones, it was shown that while semitendinosus and posterior biceps contributed larger e.p.s.p.s to middle biceps-flexor than to middle biceps-extensor cells, the anterior biceps nerve branch and semimembranosus nerve contributed equally to the two middle biceps groups. Analysis of cell location in the spinal cord and rostro-caudal differences in group I volley sizes gave evidence of a topographic organization of the biceps femoris motor nucleus which could contribute to the observed localization. However, localization was also evident when comparing e.p.s.p. amplitudes in pairs of neighbouring cells of different category, indicating a role for neuronal recognition factors.

1. Signal averaging was used in forty experiments on low-spinal cats to measure and characterize the oligosynaptic responses of seventy-six motoneurons supplying the medial gastrocnemius muscle to the single impulses of antidromically stimulated single motor axons supplying the same muscle. 2. In thirteen experiments on chloralose-urethane anaesthetized preparations, twelve (43%) of the tested twenty-eight motoneurones exhibited a single-axon recurrent inhibitory post-synaptic potential (recurrent i.p.s.p.), as compared to sixty-four (62%) of the 103 motoneurones tested in twenty-seven animals in the absence of anaesthetic after ischaemic decapitation. 3. Single-axon recurrent i.p.s.p.s most often consisted of a single, long-lasting hyperpolarization. Ten of the recurrent i.p.s.p.s contained a second late peak of hyperpolarization. In another eight of the i.p.s.p.s, a small late depolarization was evident. 4. The distinct profiles of the recurrent i.p.s.p.s were readily distinguished from the relatively flat profiles with low noise levels in the averages of the fifty-five 'no-response' cells. The transmembrane and post-synaptic nature of the i.p.s.p.s was confirmed by extracellular control recordings taken immediately outside seven of the cells with positive responses. In addition, ten cells with positive responses were subjected to current passage during the averaging procedure. In all cases, depolarization increased and hyperpolarization reduced the amplitude of their single-axon recurrent i.p.s.p.s. 5. The mean amplitude of the responses was 12.0 microV in chloralose-urethane preparations as compared to a peak-to-peak noise level less than 6.0 microV in the no-response averages. Corresponding values in ischaemic-decapitate preparations were 46.2 microV and less than 7.5 microV, respectively. 6. Latency, rise-time and half-width (i.e. duration at half-amplitude) values of the i.p.s.p.s were similar for chloralose-urethane and ischaemic-decapitate preparations. The average values in both preparations were 2.5, 5.6 and 19.3 ms, respectively. The latency values indicated both disynaptic and, perhaps, longer components in the recurrent i.p.s.p.s. The rise-time and half-width values were relatively similar to those reported or measured from published records for analogous composite recurrent i.p.s.p.s (i.e. responses to antidromic stimulation of the whole muscle nerve rather than single motor axons). A weak, but significant, correlation between rise-time and half-width was observed for the sixty-six single-axon recurrent i.p.s.p.s with a single negative-going peak.(ABSTRACT TRUNCATED AT 400 WORDS)

Experiments were conducted to test the hypothesis that a partitioning of Ia monosynaptic excitatory postsynaptic potentials (Ia EPSPs) is present in motor nuclei supplying muscles with regions capable of different mechanical actions. Intracellular recordings of synaptic potentials were made in lateral gastrocnemius (LG) motoneurons in anesthetized low-spinal cats. The effects were tested of stimuli (group I range) to the four primary nerve branches of the LG nerve supplying muscle compartments LGm, LG1, LG2, and LG3 (terminology of English, Ref. 26) and the nerve to a heteronymous muscle, soleus. Stimulation of a given LG nerve branch produced monosynaptic Ia EPSPs of greater amplitude in "own-branch" motoneurons than "other-branch" cells. A significant partitioning of mean Ia EPSPs was found in three (LG1, LG2, LG3) out of the four homonymous pathways studied. An EPSP normalization (7) was performed to eliminate potential differences in cell type that might affect the amplitudes of the EPSPs between these four cell groups (e.g., differences in the number of cells supplying FF, FR, and S muscle units). This normalization confirmed that the partitioning of monosynaptic Ia inputs upon stimulation of LG1, LG2, and LG3 could not be attributed to differences in cell type. In addition, the effects of LGm stimulation were found to be significantly greater in the LGm motoneurons compared with the other cell groups. Heteronymous input (from soleus) to the LG motor nucleus showed some partitioned effects. Motoneurons innervating compartment LG2 received larger EPSPs from soleus than did the cells supplying compartments LG1, LG3, and LGm. The contributions of location specificity and species specificity (terminology of Scott and Mendell, Ref. 55) in the establishment of these Ia-afferent-motoneuronal connections were examined. Cell location sites within the spinal cord were consistent with location specificity making some contribution to the observed pattern of homonymous Ia connections. A more prominent role for species specificity was indicated by species-dependent differences in EPSP amplitude in pairs of LG motoneurons (e.g., LGm vs. LG2) at similar rostrocaudal locations upon stimulation of a given homonymous or heteronymous nerve/branch.

In anesthetized cats 3 medial gastrocnemius (MG) motor units (MUs) were electrically activated with independent pseudorandom patterns at physiological mean rates. Recordings of isometric MG tension (T) and membrane potential changes (PSPs) in MG alpha-motoneurons (MNs) were averaged with respect to each stimulus train. The resulting T and PSP trajectories displayed the responses of each MN to the 3 average MU twitches. These responses could vary from no response to a sequence of depolarizing and hyperpolarizing waves in a PSP trajectory. The results are consistent with the known behavior of muscle stretch receptors to muscle and MU twitches and the receptors' connections to homonymous MNs.

In deeply anesthetized cats, determinations of motor-axonal conduction velocity (CV) were made using extracellular potentials recorded from single, functionally isolated motor axons innervating the muscle tibialis posterior. Axons were activated by suprathreshold electrical stimulation at the ventral-root level. Action potentials were recorded with 3 bipolar electrodes located on the muscle nerve at the level of the popliteal fossa. The most proximal and distal of the bipolar muscle-nerve electrodes were 16.4-22.0 mm apart. Estimates were made of CV from ventral root to muscle nerve (conventional CV) and between the proximal and the distal pairs of muscle-nerve electrodes (muscle-nerve CV). An evaluation was based on comparison of these CVs, estimates of uncertainties in time and distance measurements and simulations of the effects of recording conditions on CV estimates. The analysis indicated that the uncertainty in the conventional CV measurement of mammalian motor axons is at least +/- 2%. However, variability may be as great as 20% between muscle-nerve CV measurements from different experiments, probably due to such factors as regional variation in CV and differences in recording configuration.

An analysis is made of linear and non-linear summation of the motor unit (MU) tension and motoneuron (MN) membrane potential (PSP) trajectories evoked by stimulation of MUs as previously reported. In most cases, summation of both averaged responses was linear. Seldom, PSPs summed non-linearly, then usually coinciding with non-linear summation of the responsible MU twitches. The results suggest that the afferent pathways mediating the mechanical events in muscle to MNs behave rather linearly, at least in the small-signal range of interest here.

Intracellular recordings form biceps femoris (BF) motoneurons were made in anesthetized low spinal cats during periods of electrical stimulation of the nerve branches supplying the anterior, middle and posterior portions of the BF muscle and the nerves to semimembranosus and semitendinosus. Measurements were made of each cell's composite intrahomonymous and heteronymous monosynaptic Ia-EPSP responses to stimulation of the test nerves (branches). We have found evidence for an intramuscular localization of these monosynaptic Ia reflex effects not only when comparing responses between the two functional components of the BF muscle as is well established [6] but, in addition, when comparing responses between different parts of each functional (hip extensor and knee flexor) component as well. It is argued that both somatotopic and neuronal recognition factors may contribute to the localization of these monosynaptic reflex effects.

Motor units from the cat tibialis posterior muscle were examined for an association between physiological and biochemical properties. Functionally isolated motor units were categorized on the basis of their physiological properties. This was followed by quantitative microbiochemical analysis of single muscle fibers from each unit, identified in cross sections using the glycogen-depletion method. The activities of malate dehydrogenase and beta-hydroxyacyl-CoA dehydrogenase distinguished between fatigable (type FF) and fatigue-resistant (types FR and S) units. The activities of both lactate dehydrogenase and adenylokinase were higher in fast- than in slow-contracting units. Cluster analyses, based on both physiological and biochemical properties or on biochemical properties alone, produced groupings identical to types FF, FR, and S. The association between physiological and biochemical properties substantiates the idea that biochemically distinct groups of motor units correspond to physiologically identifiable groups.

In anaesthetized low-spinal cats, intracellular recordings were made of the Ia excitatory post-synaptic potential (e.p.s.p.) responses of semimembranosus motoneurones to electrical stimulation (Group I range) of nerve branches supplying the anterior and posterior heads of semimembranosus, the anterior and posterior parts of biceps femoris, and the distal part of semitendinosus. Recordings were also made during stimulation of nerves to the gracilis muscle and to the vasti muscle group. Stimulation of the semimembranosus-anterior nerve branch produced Ia e.p.s.p.s. of greater amplitude in semimembranosus-anterior motoneurones than in semimembranosus-posterior cells; likewise, stimulation of the semimembranosus-posterior nerve branch produced larger e.p.s.p.s. in cells which supplied the posterior head than in those which supplied the anterior head. Stimulation of the nerve branches to components of two 'flexor' muscles (Sherrington, 1910), biceps-posterior and semitendinosus-distal, produced larger e.p.s.p.s in semimembranosus-posterior cells than in the anterior motoneurones. A tendency was found for stimulation of the nerve to biceps femoris-anterior (an 'extensor') to produce larger e.p.s.p.s in semimembranosus-anterior than in-posterior motoneurones. However, this effect was of borderline (0.06 greater than P greater than 0.05) significance. The limited monosynaptic input produced by stimulation of the nerves to the gracilis and vasti muscles showed that their Ia axons do not distinguish between the two semimembranosus cell groups. A slight topographic organization of motoneurones within the semimembranosus motor nucleus was found, with anterior cells encountered, on average, at a more rostral level of the spinal cord than posterior cells. A similar topographic arrangement was observed in the rostrocaudal distribution of Group I afferent fibres in the dorsal roots and motor axons from the two sets of motoneurones in the ventral roots. These findings are consistent with 'location specificity' (Scott & Mendell, 1976) being a factor which contributes to the observed pattern of homonymous Ia connexions. A role for 'species specificity' (Scott & Mendell, 1976) in determining the observed pattern of homonymous Ia connexions was indicated by species-dependent differences in e.p.s.p. amplitude in pairs of semimembranosus-anterior and -posterior motoneurones at similar rostrocaudal locations in the spinal cord. The pattern of heteronymous connexions to the semimembranosus motor nucleus also showed evidence for species specificity. However, no clear topographic pattern was evident in these connexions.

Evidence is presented for a lack of localization of monosynaptic Ia excitatory post-synaptic potentials (e.p.s.p.s) in the motor nucleus supplying the atypical cat hind limb muscle semitendinosus, which has anatomically distinct in-series compartments. Recordings were made from dorsal root filaments containing functionally isolated Ia, spindle group II and Ib axons from the proximal and distal compartments of semitendinosus. Twitch of either of these in-series compartments resulted in accelerated discharge of Ia and spindle group II fibres in the other compartment. Ib fibres of either compartment showed an in-series response to twitch of a single compartment which was weaker than twitch of the whole muscle, a finding which was consistent with the diminished force produced by twitch of either compartment alone. In addition, intracellular recordings were made from semitendinosus motoneurones in anaesthetized low-spinal cats during electrical stimulation of the nerve branches to proximal semitendinosus and distal semitendinosus. Comparison of proximal semitendinosus and distal semitendinosus motoneurones failed to reveal any difference between the two cell groups with respect to the average Ia e.p.s.p. amplitude produced by either the proximal or distal semitendinosus nerve branch. However, e.p.s.p.s due to stimulation of distal semitendinosus were approximately 65% larger, on average, than those due to stimulation of proximal semitendinosus in either motoneurone group. Analysis of cell location along the rostro-caudal axis of the spinal cord indicated that the proximal and distal semitendinosus cell groups are largely co-extensive. Recordings of volleys in the proximal and distal semitendinosus nerve branches in response to stimulation of the L6, L7 and S1 dorsal roots showed that group I afferents from the proximal semitendinosus compartment tend to have a more rostral entry point to the spinal cord than do distal semitendinosus afferents. E.p.s.p. amplitude in either cell group due to stimulation of either nerve branch showed little dependence on cell location in the spinal cord. The results are discussed with respect to the relation between muscle function and the distribution of monosynaptic Ia connexions.

Evidence is presented for the existence of synchrony between the spike trains of muscle afferents of the passive cat medial gastrocnemius muscle held at fixed length. Synchrony between the spike trains of a population of muscle afferents was quantified by means of a synchronization index (Is), derived from spike-triggered averages of the muscle-nerve neurogram and the rectified neurogram. A previously used test based solely upon the neurogram average (Watt, Stauffer, Taylor, Reinking & Stuart, 1976) is shown to be invalid. The differences between experimentally derived Is values and theoretical Is values derived for the condition of asynchrony were compared to estimated confidence limits for those differences. This comparison revealed that twenty-two of fifty-three muscle-afferent spike trains whose rectified averages satisfied certain conditions for interpreting the Is were synchronized with the discharge of other afferents. The form of the rectified averages of another eight afferents suggested that these afferents might also have been synchronized. Synchrony in the discharge of muscle afferents was found in three experiments in which the neurogram was recorded from a single nerve branch to medial gastrocnemius, as well as in the data of experiments in which the whole muscle nerve was used. The degree of synchrony was similar for Ia, spindle group II and Ib afferents. The magnitude of the synchrony found in these experiments was judged by comparison to the results of analog simulations and the increase in Is values resulting from the application of small, quick stretches to the medial gastrocnemius muscle. The degree of synchrony found on average was approximately equivalent to that of a single spike occurring once for every four discharges of the reference spike train. Simulations were performed to determine the distortion of monosynaptic excitatory post-synaptic potentials (e.p.s.p.s) obtained by spike-triggered averaging which would be produced by synchrony between the spike trains of Ia and spindle group II afferents of the magnitude found in this study. These simulations indicate that the apparent amplitude would be increased by approximately 4 microV on average. Both the 10-90% rise time and the half-width would increase, the effects being greater for smaller e.p.s.p.s. Consequently, the synchrony found in this study is of most concern in the study of small post-synaptic potentials, such as those due to spindle group II afferents.

In anesthetized cats, medial gastrocnemius motor units (MUs) were electrically stimulated via their ventral-root axons with independent random patterns. Muscle tension and homonymous alpha-motoneuron (alpha-MN) membrane potential fluctuations in response to these stimuli were recorded simultaneously. Cross-correlating these two signals in temporal relation to the stimuli showed the covariance of the two signals around their respective means. With moderate to strong MU-MN couplings indicated by clear and large average membrane potential changes (PSP trajectories), the correlation coefficients tended to be positive at times when the PSP hyperpolarized, and vice versa. These relations are probably caused by the non-linear behavior of MUs, muscle receptors and afferent pathways. Thus, the signal transfer from motor efferents back to homonymous alpha-MNs depended upon the level of background tension, upon which MU twitches were superimposed.

Evidence is presented that marked changes in the electromyographic (EMG) activity of single motor units often occur during a fatigue-test paradigm (12) widely used for the classification of mammalian motor units into fast-fatigable (FF), fast-intermediate-fatigable, fast-fatigue-resistant and slow categories (11), particularly in type FF units. Force output and EMG activity were measured in single motor units of the tibialis posterior muscle of anasthetized cats, while each unit was subjected to a fatigue test consisting of 4 min of motor-axon stimulation, using 1 Hz 330 ms trains of 0.1 ms shocks at 40 Hz. As a measure of the temporal characteristics of the EMG waveform, the reciprocal of the interval between first positive and subsequent negative peak was measured. For parameters of EMG magnitude, peak-to-peak amplitude and area were measured. The fatigue test was shown to produce, on average, significantly greater alterations in the values of the EMG parameters of FF units than of the other unit types. There were no significant EMG alterations among the other unit types. The results are discussed in relation to the interpretation of EMG depression as an indication of excitation failure and the relative fatigability and EMG depression of different motor unit types.

1. The distribution of recurrent Renshaw inhibition was examined in the medial gastrocnemius motor nucleus of the cat by measurement of single-axon recurrent inhibitory post-synaptic potentials (recurrent i.p.s.p.s) in motoneurones supplying medial gastrocnemius produced by single impulses of antidromically stimulated single motor axons supplying the same muscle. 2. Stimulated motor-axon-recorded-motoneurone pairs were classified as 'close' if both contributed their axons to the same portion of the ventral roots (L7 rostral, L7 caudal or S1). Alternatively, they were classified as 'distant' if their axons were in different portions of these ventral roots. 3. Both the incidence and amplitude of 'close-pair' recurrent i.p.s.p.s were shown to be significantly larger than the corresponding values for 'distant-pair' recurrent i.p.s.p.s. 4. The results provided evidence of a strong topographic effect on the distribution of recurrent Renshaw inhibition within a single spinal motor nucleus. This finding is discussed in relation to the roles of topographic and species specificity in determining the organization of recurrent inhibition and to the partitioned distribution of monosynaptic Ia excitation in single motor nuclei.

In anaesthetized cats, medial gastrocnemius motor units (MUs) were electrically stimulated via their ventral-root axons with independent random patterns. Isometric muscle tension and homonymous alpha-motoneuron (MN) membrane-potential fluctuations in response to these stimuli were recorded simultaneously, usually for periods of about 2 min. The tension and membrane potential were averaged with respect to a stimulus train over two disjoint time intervals, one stretching 20-40 s at record beginning, and the other a similar duration at the end of recording. Whereas average MU twitch amplitudes usually decreased between these periods, average membrane potential trajectories did not do so, such that, when normalized to the change in twitch amplitude, the membrane potential trajectories usually increased in size. This suggests that the decline in the mechanical effect of MU activation was accompanied by an increase in the gain of the afferent pathway to homonymous MNs, which was confirmed by gain computations in the frequency domain. This compensation could be a mechanism to maintain the high quality of information about MU contractions transmitted to MNs in the course of MU fatigue.