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Biomedical subjects

D G Stuart

Publications and source records attributed to D G Stuart.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1985

Synchronous afferent discharge from a passive muscle of the cat: significance for interpreting spike-triggered averages.

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.

Action Potentials↗
PubMed1985

Partitioning of monosynaptic Ia excitatory post-synaptic potentials in the motor nucleus of the cat semimembranosus muscle.

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.

Action Potentials↗
PubMed1985

The contribution of neuroscience to exercise studies.

Recent advances in neuroscience have resulted in theoretical and technological developments and a substantial data base that provide new opportunities for those involved in exercise studies. The scope of opportunities available as a result of these advances is explored in brief reviews of four areas of inquiry: phase- and state-dependent reflexes; the effects of motor activity on neuronal responses to injury; the orderly recruitment of motor units; and fatigue. Within the framework of a tripartite neural control model, an examination of the extent of our knowledge on fatigue revealed two features. First, although numerous failure sites have been identified as potential contributors to the force reduction of fatigue, little is known about the relative significance of these failure sites in everyday activity. Second, technical developments have made it feasible to address the central nervous system aspects of fatigue on a cellular basis.

Action Potentials↗
PubMed1979

The regularity of muscle spindle discharge in man.

1. The variability of discharge of thirty-nine muscle spindle afferents from the pretibial muscles of normal human subjects was determined for spike train sequences recorded with the ankle joint fixed in 25 degrees plantar flexion, during further stretch and during graded voluntary contractions of the receptor-bearing muscle. 2. In non-contracting muscles with the ankle joint in 25 degrees plantar flexion, a sustained discharge was maintained by twenty-four of the thirty-nine endings. The mean discharge frequency for the active endings was 11.1 Hz (range 4.8--22.1 Hz), the mean coefficient of variation 0.073 (range 0.021--0.183). With further stretch, the discharge of endings maintaining frequencies below 10--12 Hz became more regular. For endings maintaining higher frequencies, changes in the coefficient of variation were small and occurred in either direction. All secondary endings maintained a highly regular discharge, but, at these frequencies, there was no statistically significant difference in the variability of primary and secondary endings. 3. It is considered that these findings are comparable to those of Matthews & Stein (1969) for de-efferented feline spindle endings, and support the view that there is no functionally effective background fusimotor drive to non-contracting muscles of normal human subjects. 4. A voluntary contraction sufficient to accelerate a spindle ending invariably decreased the regularity of its afferent discharge. During voluntary contractions, coefficients of variation up to 0.345 were recorded. However, coefficients as low as 0.1 were not uncommon, and thus the absence of fusimotor drive cannot necessarily be inferred from a regular afferent discharge pattern. 5. With contractions of different strength, the increase in the coefficient of variation did not parallel the increase in discharge frequency. It is concluded that not all fusimotor influences acting on a spindle ending are translated into variability, and that measurements of the variability of discharge do not accurately reflect the level of fusimotor drive. 6. The discharge frequency of some spindle endings decreased slightly in some contractions and this was accompanied by an increase in the variability of discharge. It is suggested that contracting extrafusal muscle fibres can modulate the discharge pattern of spindle endings and contribute to the variability of discharge during a voluntary contraction. 7. In contracting muscles the irregular fusimotor-driven spindle discharge contained a 'hidden' periodicity, but this was not as extensive as has been reported for the cat. No such periodicity could be demonstrated for spindle endings in non-contracting human muscles.

Action Potentials↗
PubMed1978

Speed-force relations in the motor units of the cat tibialis posterior muscle.

The neuro-mechanical properties and speed-force relations of 81 cat tibialis posterior motor units have been studied. Statistically significant correlations were found between alpha-axonal conduction velocity (CV) and average twitch tension (log10 TwT; r = 0.459, p less than 0.001), between CV and twitch contraction time (CT; r = -0.395, p less than 0.001) and between CT and log10 TwT (r = -0.277, p less than 0.02). The present correlations for the intermediate-sized tibialis posterior are stronger than those previously reported for large muscles such as soleus, medial gastrocnemius, plantaris and flexor hallucis longus. However, they are considerably weaker than those reported for the much smaller lumbrical muscles of the cat's foot. These findings support the contention that the spinal mechanisms governing an orderly recruitment of motor units according to the size of their muscle units must be more complex for large than for small muscles, at least in the cat hindlimb.

Animals↗