Biomedical subjects

D Kernell

Publications and source records attributed to D Kernell.

At least 37 records · Page 2Linked to original sources

PubMed1998

The final common pathway in postural control--developmental perspective.

A brief review is given concerning postural specialisations among mammalian muscle fibres and motor units. Most skeletal muscles contain a mixture of fibres with different characteristics, and their slow-twitch (S) units are well-known to possess properties suitable for postural tasks: they are highly fatigue-resistant, well equipped for oxidative metabolism, and their slowness makes them energetically cheap in (semi-)isometric contractions. These features are adequately employed in motor behaviour owing to characteristics of the associated motoneurones. In adult mammals, the way in which a muscle is used can influence its proportion of S units. This adjustment occurs within a restricted 'adaptive range' which differs between muscles and animal species, presumably being preset at an early age. In the course of early foetal development, part of the slow vs. fast differentiation of muscle fibre properties can take place independently of innervation. Once innervation has taken place, however, motoneurones influence the differentiation in various ways. On the whole, a well coordinated timing seems to exist between the early differentiation of central motor mechanisms and of the peripheral machinery, largely causing the neuromuscular system to be/become ready for use when the brain needs it.

Animals↗

PubMed1998

Use and fibre type composition in limb muscles of cats.

As a background for studies concerning the effects of training on the properties and fibre type composition of skeletal muscle, information is needed concerning the normal duration of muscle use per day. Data of this kind were collected from adult cats, using implanted electrodes for electromyographic recording from hindlimb muscles acting across the ankle joint: extensor digitorum longus (EDL), tibialis anterior (TA), peroneus longus (PL), lateral gastrocnemius (LG) and soleus (SOL). The accumulated duration of any recorded activity was expressed, for each electrode site, as a percentage of total sampling time ("duty time"). As measured intermittently across 24 h periods (4 min sampling per 30 min), these duty times were markedly and significantly different among the various muscles, averages varying from 1.9% for EDL up to 9.5% for posterior PL and 13.9% for SOL. The distribution of activity across the various muscles was markedly different between highly active periods (mid-day) and periods of rest (mid-night). The 24 h duty times were strongly and significantly correlated to duty times obtained for only mid-day activity but not to those for only mid-night activity. Following the end of the physiological measurements, the animals were sacrificed and the muscles were analyzed with regard to fibre type composition (histochemistry). There was a significant positive correlation between the 24 h duty time and the percentage of type I fibres ("slow"). In the Discussion, the present results from cats are briefly compared to previously published data for humans.

Activity Cycles↗

PubMed1998

Muscle regionalization.

In this review, the term muscle fibre regionalization signifies the presence of regional intramuscular differences in fibre type composition. As is well known, highly regionalized muscles commonly have greater concentrations of slow fibres deep than superficially. However, the degree of regionalization varies markedly between muscles and is not confined to deep vs. superficial locations. Fibres of the same myosin type may show regionalized differences in their metabolic enzyme activity, even within single motor units (Larsson, 1992). Regionalization of fibre type composition occurs also within single neuromuscular partitions. The intraspinal position of motoneurones is often coarsely related to the intramuscular sites of their muscle units. Muscles with a marked fibre type regionalization tend to show a corresponding regionalization of activity; in several muscles, however, the activity regionalization may vary depending on the motor task. During early development, fibre type regionalization emerges even under aneural conditions. The mechanisms are still unknown; relevant aspects of early development are briefly reviewed.

Animals↗

PubMed1998

Relation between fibre composition and daily duration of spontaneous activity in ankle muscles of the cat.

UNLABELLED: This study concerns the relation between use and fibre type composition among limb muscles. The histochemical properties were investigated for ankle muscles from cats that had previously been studied in 24 hr electromyographic (EMG) recordings of daily spontaneous activity. We then reported average daily "duty times" (i.e. the percentage of total sampling time filled with EMG activity) of 1.9% for extensor digitorum longus (EDL), 2.1 and 4.0% for anterior and posterior sites of tibialis anterior (TA), 6.6 and 9.5% for anterior and posterior sites of peroneus longus (PL), and 13.9% for soleus (SOL). In the present experiments, muscles from which these data had been obtained were sectioned in a cryostat and stained for myofibrillar ATPase. Fibres were classified as type I (presumably slow) or II (presumably fast), the latter fibres being further categorized as IIA, IIB and a minor portion of transitional IIAB fibres. As expected, SOL was 100% type I. Among the muscles of mixed fibre-type composition ("mixed muscles"), a statistically significant difference in the mean percentages of type I fibres was found between TA or EDL (2.9-6.0%) vs. PL (11.8-14.6%). For TA the percentage of type I fibres was higher in posterior (deep) than in anterior (superficial) sampling regions; for PL no clear antero-posterior difference was found. A significant correlation was obtained between the percentage of type I fibres and the total duration of daily activity recorded from corresponding mixed muscle sites (5 different recording sites in 4 cats, totally 15 cases of successfully combined physiological and histochemical measurements, r = 0.76, P < 0.001). Similarly, within TA the total duration of daily activity was higher for sites with high (posteriorly) than for those with low (anteriorly) percentages of type I fibres. IN CONCLUSION: a "coarse-grain" relationship was found between fibre type composition and the duration of daily activity among mixed muscles. Possible mechanisms underlying this relationship are discussed.

Animals↗

PubMed1997

Daily durations of spontaneous activity in cat's ankle muscles.

For an understanding of how various degrees of altered use (training, disuse) affect the properties of skeletal muscles, it is important to know how much they are used normally. The main aim of the present project was to produce such background knowledge for hindlimb muscles of the cat. In four adult female cats, each one being studied in several experimental sessions, ankle muscles were chronically implanted with electrodes for electromyographic (EMG) recording. The muscles recorded from were: extensor digitorum longus (EDL), peroneus longus (PL), tibialis anterior (TA), lateral gastrocnemius (LG) and soleus (SOL). For PL, TA and LG, there were anterior as well as posterior recording sites. During 24-h experimental sessions, the studied animal stayed, together with another cat, in a box large enough for playing and walking around. Using telemetric techniques, samples of EMG signals were recorded on tape for 4 min every 30 min. In an off-line analysis, measurements were made of the total accumulated duration of activity from each one of the studied muscle regions. These "duty times" were expressed as a percentage of total sampling duration. When averaged over the whole 24-h experimental period, the mean duty times per muscle region varied from 1.9% for EDL up to about 13.9% for SOL. Also, among predominantly fast muscles of mixed-fibre composition (i.e. all studied muscles except SOL), marked and statistically significant differences in duty time were found, mean values varying fivefold from 1.9% (EDL) to 9.5% (PL, posterior site). For all three muscles with simultaneous recordings from different sites, consistent and statistically significant differences in daily duty time were found between anterior and posterior regions (anterior less than posterior for TA and PL; anterior more than posterior for LG). We also measured the extent to which each 4-min sampling period was filled with activity (if any). As compared to muscles with a low mean 24-h duty time, those with high duty times were not active during more sampling periods per day, but, whenever being used, their activity lasted relatively longer. Such results were consistent with the view that differences in mean 24-h duty time might largely reflect differences in the extent to which the various muscles and muscle regions were used for long-lasting stabilizing contractions.

Animals↗

PubMed1996

Calreticulin expression in spinal motoneurons of the rat.

We have examined the expression of calreticulin in rat spinal motoneurons in order to reveal the occurrence and distribution of Ca2(+)-storage organelles in these neurons. Calreticulin, the non-muscle equivalent of calsequestrin, is the low-affinity, high-capacity calcium-binding protein responsible for intracompartmental Ca2(+)-storage in a number of different cell types. The results of the present immunohistochemical study show that all spinal motoneurons express calreticulin at approximately the same level; no significant differences in cytoplasmic immunostaining intensity were observed between different motoneuron pools or between small and large spinal motoneurons. Immunoelectron microscopy revealed that the intracellular localization of calreticulin within spinal motoneurons was confined to the endoplasmic reticulum and to spherical or pleiomorphic, frequently 'coated' vesicles with a diameter ranging between 120 and 150 nm. Some of these vesicles may represent the so-called calciosomes, the intracellular Ca2(+)-storage vesicles described in liver cells and in cerebellar Purkinje cells. The molecular components responsible for the uptake and release of Ca2+ from the Ca2(+)-storage organelles in spinal motoneurons still remain to be identified.

Animals↗

PubMed1995

Relative degree of stimulation-evoked glycogen degradation in muscle fibres of different type in rat gastrocnemius.

1. The relative degree of glycogen degradation, caused in different fibre types by supramaximal electrical activation of the muscle nerve, was investigated in m. gastrocnemius medialis of young adult rats under general pentobarbitone anaesthesia. Four different protocols of intermittent maximal tetanic activation were used, each lasting 6 s (33% duty cycle; fast and slow isovelocity concentric (shortening) contractions, brief- and long-burst isometric contractions; 6 rats per group). All contractions were evoked under ischaemic conditions. 2. Work output finally dropped to 29% of the initial value for the fast concentric and to 87% for the slow concentric contractions. In isometric protocols evoked by the same stimulation patterns, the force x time area rose to 110% for brief-burst contractions and dropped to 95% for the long-burst contractions. 3. Following the physiological procedures, the experimental muscle and its contralateral control were removed and prepared for histochemical analysis. Serial sections were stained for glycogen (periodic acid-Schiff (PAS) method) and myofibrillar ATPase (mATPase), the latter reactions being used for classifying the fibres as types I, IIA, IIBd and IIBm. 4. For deep 'red' regions of non-stimulated contralateral control muscles the optical density of PAS staining was ranked between fibre types such that I < IIA < IIBd < IIBm. In superficial 'white' regions of the same muscles, no significant difference in PAS staining density was found between IIBd and IIBm fibres (types I and IIA not present). 5. All contractile protocols produced a significant glycogen degradation in IIBm fibres, and the fast concentric activation procedure was associated with a significant decline of PAS staining in all fibre types. For all activation protocols, the relative degree of glycogen degradation within a given region was ranked such that IIBm > IIBd > IIA > I. For IIBm vs. IIBd fibres, the differences in relative degradation were greater and more consistently significant for superficial white regions than in the deeper red muscle portions. 6. The results are discussed in relation to glycogen degradation measurements in studies of motor unit recruitment. Furthermore, the results from red vs. white muscle regions underline that fibres of seemingly the same mATPase type may differ considerably in other properties.

Adenosine Triphosphatases↗

PubMed1995

Spatial differences in fatigue-associated electromyographic behaviour of the human first dorsal interosseus muscle.

1. Fatigue-associated electromyographic (EMG) reactions of intrinsic hand muscles were studied during maintained isometric voluntary contractions of normal subjects. Most measurements concerned actions of the first dorsal interosseus (FDI). In a smaller number of subjects, complementary measurements were obtained for adductor pollicis (AP). 2. Measurements were made of isometric force (thumb adduction, index finger abduction and flexion) and of surface EMG amplitudes (AP and FDI) after rectification and smoothing (rsEMG). 3. In the analysis of fatigue, the subjects were required to maintain a steady isometric force (index finger abduction or thumb adduction) of half their maximum voluntary contraction (1/2MVC test) for as long as possible. Average endurance times were 88 +/- 19 s (mean +/- S.D.) for FDI and 119 +/- 29 s for AP (Student's t test, P < 0.02). 4. Pronounced differences in fatigue-associated EMG behaviour were observed between AP and FDI. In AP the reaction was as expected: a rise of EMG during maintained force (mean rsEMG at end of fatigue test/mean rsEMG at start of test (rsEMG-FI): 181 +/- 64%). In FDI this reaction was seen in half of the recorded cases, the remainder displaying bidirectional changes or a more or less marked decrease of EMG during the endurance task (mean for all cases together: rsEMG-FI, 103 +/- 15%; difference between AP vs. FDI significant, P < 0.01). 5. The unexpected EMG variability of the FDI reactions was further analysed with multiple bipolar recordings of surface EMG. For all the four thoroughly studied subjects, recordings were obtained which showed simultaneously occurring EMG changes in opposite directions (decrease and increase) at different sites of FDI while force was kept constant at 50% of the maximum voluntary contraction (MVC). 6. Further observations on FDI showed that EMGs simultaneously obtained from different recording sites could show dramatic differences in their responses depending on 'synergistic context' (e.g. in relation to changes in index finger extension force during maintained abduction at 50% MVC). Evidence for 'task switching' (shift in rsEMG distribution, shift in hand muscle synergy) was frequently observed during the performance of the 1/2MVC test. 7. The results indicate that FDI is not handled in a topographically homogeneous manner during the execution of an isometric constant force endurance test. Furthermore, the results suggest that this seemingly simple motor performance can be executed in several alternative manners associated with the activation of different muscle synergies and with different distributions of activity within the FDI.

Adult↗

PubMed1995

Neuromuscular frequency-coding and fatigue.

In daily life, muscle fatigue often becomes noticeable as an apparent decline in the efficiency of force production by central commands, making it necessary to increase drive (or "effort") to produce a constant motor output. Such aspects of fatigue may be caused by changes in the way in which synaptic messages arriving at the motoneurons are translated into forces by the muscle fibers. Therefore, an understanding of these neuromuscular gradation mechanisms is essential for any analysis of motor fatigue. A brief general review is given of 1) how muscle fibers transduce motoneuronal discharge rates into force; 2) how synaptic currents are transduced into motoneuronal discharge rates; 3) how activity-dependent changes in the neuromuscular transduction mechanisms contribute to neuromuscular fatigue; and 4) how the matching between the transduction mechanisms of motoneurons and those of their muscle fibers may help to optimize neuromuscular gradation efficiency and decrease the severity of fatigue.

Animals↗

PubMed1995

Measures of "fastness": force profiles of twitches and partly fused contractions in rat medial gastrocnemius and tibialis anterior muscle units.

Recordings of isometric force were obtained for twitches and (sub)maximal tetani of gastrocnemius medialis (MG) and tibialis anterior (TA) muscle units in female Wistar rats. We assessed the relationships between unit properties that have all been associated with "speed": (1) the relative degree of peak force attained during repetitive activation at 40 Hz (P40/Pmax), (2) the relative degree of final twitch fusion during the same test burst (Fus-end), and (3) various measures of the time-course of single twitches, including twitch time-to-peak and a parameter referred to as "initial fusion ratio" (Fus-in; relative decline from peak force at 25 ms from twitch onset). The various measures of twitch time-course were significantly correlated to each other with correlation coefficients varying over a fairly wide range (0.35-0.64 for MG; 0.50-0.80 for TA). Twitch time-course was also significantly correlated with Fus-end during the 40-Hz repetitive activation; the highest correlation coefficient (0.69 for MG, 0.80 for TA) was obtained for Fus-in, which was also numerically similar to Fus-end. Thus, the degree of fusion indeed seemed to be largely dependent upon aspects of twitch time-course. However, the relative degree of force mobilization obtained in the same contractions elicited by stimulation at 40 Hz was not consistently better correlated with Fus-end than with measures of single twitch time-course. Furthermore, in fast-twitch units having the same twitch time-to-peak, the force mobilization elicited by stimulation at 40 Hz (P40/Pmax) was the same for MG and TA, while the degree of fusion was significantly smaller for TA than for MG units. The results demonstrate the complexity of the concept of isometric "speed" and underline the need for using several speed indicators in parallel in studies concerning the differentiation of muscle (unit) properties.

Animals↗

PubMed1994

Fatigue associated EMG behavior of the first dorsal interosseous and adductor pollicis muscles in different groups of subjects.

We have studied the fatigue-associated behavior of surface EMG in two histochemically different muscles of the hand: first dorsal interosseous (FDI) and adductor pollicis (AP; relatively more type I fibers in AP than in FDI). During a fatigue test evoked by electrical stimulation of the ulnar nerve, the mean amplitudes of compound muscle action potentials (M-waves) exhibited the same overall pattern for both muscles: a rapid phase of potentiation followed by a gradual decline. However, if the group of subjects was subdivided on the basis of hand length, significant differences emerged in the reactions of AP: in large hands, no fatigue-associated M-wave decline was seen, whereas in small hands a distinct decline was observed. A possible explanation for this phenomenon might be the presence of a greater amount of EMG contamination from other muscles in smaller hands. In the supposedly "cleaner" recordings from larger hands, significant differences between FDI and AP were observed with regard to their fatigue-associated EMG reactions (M-wave depression in FDI but not in AP). The direction of these differences was in accordance with expectations on the basis of known differences in histochemical fiber type composition.

Adult↗

PubMed1994

Threshold-spacing in motoneurone pools of rat and cat: possible relevance for manner of force gradation.

In the context of an analysis concerning factors of importance for the relative contributions of recruitment and rate gradation of muscle force, the distribution of electrical excitability was analyzed for medial gastrocnemius (MG) motoneurones of rat and cat. The experimental data came from previously collected intracellular measurements in animals anaesthetized with pentobarbitone. Electrical excitability was measured as the threshold (nanoamperes) for single spike generation (rheobase) in rat and for maintained repetitive firing (rhythmic threshold) in cat. Furthermore, the data included measurements of axonal conduction velocity and of contractile properties of the muscle units innervated by the studied motoneurones. The units were categorized into types S (slow-twitch, fatigue-resistant), FR (fast-twitch, fatigue-resistant) and FF (fast-twitch, fatiguable) on the basis of the combined criteria of twitch-speed and sensitivity to fatigue. We confirmed that, in spite of the presence of normal-looking symmetrical distributions of axonal conduction velocity, there was a positive skew in the distribution of electrical excitability (relatively high numbers of cells with low thresholds, few with high ones). Within each unit category (S, FR, FF), we ranked the motoneurones according to their relative electrical excitability and calculated the threshold difference between consecutive cells ("threshold spacing"). In accordance with the skewed distribution of electrical excitability, we found that the mean threshold spacing was ranked in the same way as the mean thresholds, i.e. S < FR < FF; the statistical analysis showed that, for cats as well as rats, small threshold spacing steps were significantly more common for S than for FF motoneurones.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Index finger position and force of the human first dorsal interosseus and its ulnar nerve antagonist.

In normal subjects, maximum voluntary contraction (MVC) and electrical ulnar nerve stimulation (UNS; 30-Hz bursts of 0.33 s) were systematically compared with regard to the forces generated in different directions (abduction/adduction and flexion) and at different degrees of index finger abduction. With a "resting" hand position in which there was no index finger abduction, UNS produced about one-half of the abduction force elicited by an MVC (mean ratio 51%). Qualitatively, such a discrepancy would be expected, because UNS activates two index finger muscles with opposing actions in the abduction/adduction plane of torques: the first dorsal interosseus (FDI) and the first palmar interosseus (FPI). The abduction forces produced by MVC and UNS were very sensitive to index finger abduction angle: at a maximum degree of abduction, the UNS-generated force even reversed its direction of action to adduction (with FPI dominating) and the abduction MVC declined to 37% of that in the resting hand position. Inasmuch as these declines in MVC- and UNS-generated abduction force could not be explained by a change in moment arm, the main alternative seemed to be abduction-associated alterations in FDI fiber length (analysis by previously published biomechanical data). The FDI and FPI were further compared by application of a UNS-generated fatigue test (5-min burst stimulation), with the index finger kept at a "neutral" angle, i.e., the abduction angle at which, in the unfatigued state, the forces of the FDI and FPI were in balance (zero net UNS-generated abduction/adduction force).(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗

PubMed1993

Matching between motoneurone and muscle unit properties in rat medial gastrocnemius.

1. Electrical and contractile (isometric) properties were studied for sixty-six motoneurone-muscle unit combinations from rat medial gastrocnemius (MG). The animals were anaesthetized with pentobarbitone. 2. The muscle units were classified into S (slow) and F (fast) on the basis of measurements of speed and fatigue resistance: the 'slow' category comprised units with an initial twitch contraction time exceeding those found among fatigue-sensitive units (border value 20 ms). 3. Twitch speed was assessed by three different measures: (i) contraction time (time to peak, range 11.4-28.0 ms), (ii) half-relaxation time (8.4-56.5 ms), and (iii) total twitch duration (34-116 ms). All three measures were mutually highly correlated and their respective values showed a continuous and unimodal distribution across the unit population. 4. The motoneurones were investigated with regard to their time course and amplitude of post-spike after-hyperpolarization (AHP; range of total durations 30-116 ms, amplitudes 0.9-8.0 mV), rheobase (0.8-17.1 nA), input resistance (0.8-5.1 M omega) and axonal conduction velocity (33-85 m/s). 5. Motoneurones of slow-twitch muscle units (type S) had, on average, a significantly slower time course of AHP, a smaller rheobase, a higher input resistance and more slowly conducting axons than those innervating fast-twitch muscle units. 6. Across the whole neuronal sample, input conductance (reciprocal of input resistance) correlated well with rheobase (r = 0.74). However, the differences in rheobase did not seem to be caused exclusively by the associated differences in input conductance. 7. Throughout the sampled population, the relative slowness of AHP showed a continuous and highly significant correlation with the relative slowness of the corresponding unit twitch. The absolute duration of AHP was close to that of the twitch. In the Discussion it is argued that this 'speed match' between motoneurone and muscle unit would help ensure that barely recruited motoneurones start firing at a frequency that is optimally suited for the subsequent rate gradation of force. 8. AHP amplitude was, on average, significantly smaller for fast-twitch than for slow-twitch motoneurones. Calculations indicated that these differences were almost completely caused by the associated differences in input resistance; the computed value for the conductance change underlying the AHP was nearly the same for fast- and slow-twitch motoneurones. 9. A simple neurone model was used to calculate the consequences of the differences in AHP amplitude and duration for repetitive discharge properties of fast and slow cell categories.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Average but not continuous speed match between motoneurons and muscle units of rat tibialis anterior.

1. Properties of single motoneuron/muscle-unit combinations were determined for tibialis anterior (TA) in rats anesthetized with pentobarbital. The TA observations were systematically compared with those obtained earlier by the use of the same techniques from rat medial gastrocnemius (MG). 2. TA motoneurons were investigated with regard to afterhyperpolarization (AHP; total duration 32-74 ms, amplitude 0.39-4.96 mV) and axonal conduction velocity (41-79 m/s). TA muscle-unit measurements included the time course of the isometric twitch (time-to-peak force 10.8-18.0 ms; total duration 42-92 ms), the maximum tetanic force (22-217 mN), and a measure of fatigue sensitivity (fatigue index 5-100%). The range of twitch and AHP durations ("speed range") was markedly smaller in the present TA material than for MG. 3. The mean duration of the TA motoneuronal AHP (49 +/- 8 ms, mean +/- SD) was close to that of its muscle-unit twitch (56 +/- 12 ms). Thus an "average" speed match existed between TA motoneurons and their muscle fibers. 4. For TA there was no correlation between the time courses of AHP and twitch. Thus there was for TA no "continuous" speed match between the motoneurons and their muscle fibers. 5. For TA twitches or AHPs studied separately, there was a significant correlation between different time course measures. Furthermore, compared with TA units having relatively fast twitches, those with slower twitches tended to show 1) a smaller maximum tetanic force and 2) a greater AHP amplitude. Fatigue-resistant units tended to have slower twitches than fatigue-sensitive ones.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Task-related differences in distribution of electromyographic activity within peroneus longus muscle of spontaneously moving cats.

In the hindlimb muscle peroneus longus (PerL) of cats, electromyographic activity (EMG) was recorded from anterior and posterior regions during voluntary motor behaviour. In spite of the fact that this muscle is composed of units that all exert their forces in the same direction, the intra-muscular EMG distribution differed in a marked and reproducible way between different types of motor behaviour. Anterior as well as posterior regions were both strongly active in relation to the swing-phase of stepping. In comparison to this stepping-activity, there was a marked predominance of posterior PerL activity during hindlimb standing (or take-off for a jump) and an equally pronounced predominance of anterior PerL activity when the cat was preparing to land from being lifted (or at the end of a jump). It is suggested that these task-associated differences in EMG distribution reflect topographical aspects of the intraspinal organization of motor tasks.

Animals↗

PubMed1992

Organized variability in the neuromuscular system: a survey of task-related adaptations.

This survey concerns the physiology of the neuromuscular system, as studied at the level of the single mammalian limb muscle and its motoneurones (MNs). Particular attention is devoted to the ways in which the properties and the organization of spinal MNs are adapted for the control of muscle (unit) force. These questions are discussed in relation to: a) The general and basic task of the system: providing a smooth and finely gradeable force by the mechanisms of rate- and recruitment-modulation of MN activity. b) Gradation problems in relation to specific peripheral requirements in connection with: (i) tasks of different duty-time (i.e. problems related to fatigue and endurance); (ii) tasks of different speed; (iii) task using different muscle lengths; (iv) tasks requiring different adjustment-gains. c) The adaptational properties of the neuromuscular system as it is subjected to long-term changes in its motor tasks. These matters are largely discussed in relation to experiments for studying the responses of the neuromuscular system to different patterns of chronic electrical stimulation. d) The manner in which the neuromuscular system, at the level of a single unidirectional muscle, is used for different motor programs. Evidence is summarized which shows the presence of task-related variations in MN recruitment patterns, and it is pointed out that such variations may be related to the intraspinal topography of the respective MNs. It is suggested that these task-related variations in MN recruitment behaviour might largely reflect topographic (and other) differences in the organization of spinal interneuronal systems responsible for the execution of different motor programs.

Adaptation, Physiological↗

PubMed1991

External recording of twitch time course in cat ankle muscles.

In chronic experiments concerning the activity-dependent plasticity of muscle properties, a simple and noninvasive method was used for monitoring changes of twitch speed in conscious adult cats. The animals had been provided with implanted electrodes for nerve stimulation, and a hand-held force transducer was pressed against the fully extended ankle joint while single test pulses were delivered to the common peroneal nerve. In the present report, this technique for the recording of ankle twitches is subjected to critical analysis and evaluation. The measurements were highly reproducible with respect to contraction time (time-to-peak) but less so for half-relaxation time and twitch amplitude; other methods should be used for the long-term monitoring of contractile force. The total force (torque) of the ankle twitch was mainly produced by tibialis anterior (about 45%), peroneus longus (PerL; 27%) and extensor digitorum longus (23%). The ankle twitch produced by PerL alone had about the same contraction time as that of all the muscles together. Among muscles that had become changed as a result of long-term electrical stimulation there was, in general, a good correspondence between the contraction times from simple external recordings of ankle twitches and those separately measured for PerL under general anesthesia (force transducer then directly connected to PerL tendon).

Animals↗