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D G Stuart

Publications and source records attributed to D G Stuart.

At least 37 records · Page 2Linked to original sources
PubMed1989

Fatigability of rat hindlimb muscle: associations between electromyogram and force during a fatigue test.

1. An experimental protocol designed to assess fatigability in motor units (Burke, Levine, Tsairis & Zajac, 1973) has been applied to the whole muscles of anaesthetized adult rats, and the association between the electromyogram (EMG) and force was monitored over the course of the test. 2. Both test muscles (soleus and extensor digitorum longus) exhibited a wide range of fatigability, which was defined as the decline in isometric peak force at 6 min, such that the data could be separated into five levels of fatigability. Fatigue indices for each test muscle were distributed across three levels. 3. The EMG was quantified with four measures of amplitude, four of duration, and one interaction term (area). Correlation analyses indicated that the EMG was adequately represented by one measure of amplitude (absolute amplitude), one of duration (peak-to-peak duration) and area. The best single measure was area. 4. The EMG-force associations for soleus varied markedly among its three fatigability groups. In contrast, over the course of the test, all three extensor digitorum longus groups displayed qualitatively similar EMG-force associations. 5. Multiple regression analyses indicated that the EMG parameters were able to predict peak force better for extensor digitorum longus than for soleus. Furthermore, for both test muscle, the prediction was best for the most fatigable group. 6. The associations between EMG and force exhibited three patterns for the two test muscles and three levels of fatigability. These differences suggested variation in the mechanisms, related to both fibre-type composition and susceptibility to fatigue, that dictate the performance elicited by this particular stimulus regimen. The mechanisms seem to include both intracellular and transmission processes.

Action Potentials↗
PubMed1988

Effects of chloralose-urethane anesthesia on single-axon reciprocal Ia IPSPs in the cat.

Reciprocal Ia inhibitory postsynaptic potentials (IPSPs) generated by single afferents have been recorded with signal averaging in unanesthetized ischemic-decapitate cats for comparison with measurements previously obtained from preparations anesthetized with a mixture of chloralose and urethane. The results are similar to those which we obtained recently for single-axon recurrent IPSPs. Together, the studies show that chloralose-urethane anesthesia has a depressant effect on two widely studied circuits in the mammalian spinal cord.

Anesthesia↗
PubMed1988

Association between biochemical and physiological properties in single motor units.

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.

3-Hydroxyacyl CoA Dehydrogenases↗
PubMed1988

Fatigue-related changes in neuromuscular excitability of rat hindlimb muscles.

Brief repetitive trains of supramaximal nerve stimulation produce intermittent muscle activation and, in time, a progressive decline in force (i.e., neuromuscular fatigue) and depression of the electromyogram (EMG). These changes may include within-train reductions in EMG due to a failure of neuromuscular propagation. The aim of the present study was to investigate changes in EMG during a 360-second stimulus regimen designed to fatigue soleus and extensor digitorum longus muscles of anesthetized rats by activating the muscle with repetitive trains of 40 Hz stimuli. Measurements included peak force for each tetanus, variation of the within-train EMG (coefficient of variation for area), and magnitude of the first EMG waveform (area) of each train. Fatigue was characterized as the relative decline in force over the course of the test. The responses of the test muscles were categorized, based on an absolute scale of fatigability, into five groups: potentiated, nonfatigable, low fatigability, intermediate fatigability, and high fatigability. Fatigable muscles (low, intermediate, and high fatigability groups) demonstrated a decreased EMG magnitude and an increased EMG-area variation with repetitive activation. This increased variation, however, was nonmonotonically related to fatigability such that the least and most fatigable muscles had the smallest within-train EMG variation. We suggest that these data can be explained by considering the EMG (compound muscle action potential) as a stochastic process that represents a composite of single-fiber events (axonal to sarcolemmal transmission) with variable probabilities.

Animals↗
PubMed1988

Coexistence of twitch potentiation and tetanic force decline in rat hindlimb muscle.

An experimental protocol designed to assess fatigability in motor units has been applied to two hindlimb muscles of anesthetized adult rats to study the effects of whole-muscle fatigue on the isometric twitch. Both soleus and extensor digitorum longus exhibited a linear relationship between fatigability (i.e., force decline after a 360-s fatigue test) and the magnitude of the twitch force following the fatigue test. Twitch force after the fatigue test was potentiated (i.e., greater than the value before the fatigue test) in many muscles, despite the development of considerable fatigue. This coexistence of fatigue and twitch potentiation was observed in 7% (5/70) of soleus and 48% (31/64) of extensor digitorum longus muscles. The coexistence was exhibited only by the least fatigable muscles of the fast-contracting extensor digitorum longus. The extensor digitorum longus muscles that did not exhibit twitch potentiation probably experienced a higher proportion of muscle-fiber inactivation, such as due to failure of neuromuscular propagation, that was induced by the fatigue regimen.

Animals↗
PubMed1988

Activation of muscle fibers in individual motor units revealed by 2-deoxyglucose-6-phosphate.

Motor units of the cat tibialis posterior muscle were selectively activated by prolonged electrical stimulation of functionally isolated motor axons in situ. During the activation, the glucose analog 2-deoxyglucose (DG) was administered systemically. Single muscle fibers were subsequently examined for accumulation of the metabolite 2-deoxyglucose-6-phosphate (DG6P) by an analytical assay and for depletion of glycogen by a PAS glycogen-specific staining reaction (periodic acid Schiff; PAS). In general, levels of DG6P were 20 times greater in unstained (PAS-negative) fibers compared with stained (PAS-positive) fibers. However, some glycogen-depleted fibers, particularly in putative ischemic fascicles of the muscle, did not have elevated DG6P, suggesting that depletion of glycogen is not always a reliable indicator of fiber activation. Furthermore, the PAS-staining reaction was not necessarily indicative of quantitative glycogen levels in single fibers. Thus, this report shows that DG6P accumulation enhances the identification of motor-unit fibers selectively activated via their common motor-nerve axon. Evidence is also presented for differential glucose uptake in muscle fibers of different phenotype, thereby indicating that the DG6P measurement in muscle has broad applicability to the investigation of cellular glucose utilization.

Animals↗
PubMed1987

Measurement of axonal conduction velocity in single mammalian motor axons.

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.

Action Potentials↗
PubMed1987

The measurement of single motor-axon recurrent inhibitory post-synaptic potentials in the cat.

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)

Action Potentials↗
PubMed1987

Distribution of single-axon recurrent inhibitory post-synaptic potentials in a single spinal motor nucleus in the cat.

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.

Action Potentials↗
PubMed1986

Amplitude reduction of motor unit twitches during repetitive activation is accompanied by relative increase of hyperpolarizing membrane potential trajectories in homonymous alpha-motoneurons.

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.

Animals↗
PubMed1986

Partitioning of monosynaptic Ia excitatory postsynaptic potentials in the motor nucleus of the cat lateral gastrocnemius muscle.

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.

Afferent Pathways↗
PubMed1986

Uniformity of metabolic enzymes within individual motor units.

Individual muscle fibers of 10 motor units from the tibialis posterior muscle of cat were identified by glycogen depletion techniques, characterized for histochemical type, diameter, and intramuscular locations, and analyzed by quantitative biochemical methods. Four enzymes, representing different energy-yielding pathways, were quantitatively assayed in muscle fibers belonging to motor units selected from each of the three major physiological types. All four enzymes demonstrated identical activities among fibers within a motor unit, while showing up to 11-fold differences among fibers belonging to different motor units. Moreover, fibers within a single motor unit, but of substantially different diameters, were nevertheless homogeneous in specific enzyme activities.

3-Hydroxyacyl CoA Dehydrogenases↗
PubMed1985

Stimulus-related correlations between medial gastrocnemius muscle tension and homonymous motoneuron membrane potential result from non-linearities.

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.

Animals↗
PubMed1985

Distribution of sensory receptors in the flexor carpi radialis muscle of the cat.

The structures and distribution of encapsulated muscle receptors were examined in serial transverse sections of flexor carpi radialis in the adult cat. Four types of receptors (muscle spindles, Golgi tendon organs, paciniform, and Pacinian corpuscles) were identified. Their structures resembled those encountered in other limb muscles. Pacinian corpuscles were rare and occurred only in the external fascial coat of the muscle near its origin. The other three receptor types were distributed in an uneven but consistent pattern throughout the muscle. As noted previously (Gonyea and Ericson, '77), spindles were largely confined to a deep muscle region comprising less than 20% of the muscle volume, located directly between the long tendon of origin and the tendon of insertion. This region contains the largest proportion of type SO muscle fibers (Gonyea and Ericson, '77). Tendon organs and paciniform corpuscles were concentrated along the tendons that lined the spindle-rich muscle region. This region appeared to be composed of extrafusal fibers that were shorter and of more oblique pinnation than those in other regions. The localization of muscle receptors to the "oxidative" core of the muscle in its direct line of pull may have functional implications for afferent input to the spinal cord which are discussed. In addition, the possibility is raised that there are more paciniform corpuscles in flexor carpi radialis (and possibly other muscles) than previously thought.

Animals↗
PubMed1985

Detection of synchrony in the discharge of a population of neurons. I. Development of a synchronization index.

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.

Afferent Pathways↗
PubMed1985

Detection of synchrony in the discharge of a population of neurons. II. Implementation and sensitivity of a synchronization index.

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.

Afferent Pathways↗