Autogenetic reflex action in tibialis anterior compared with that in soleus muscle in the decerebrate cat

Summary The regulatory actions of autogenetic reflex pathways on the mechanical properties of an ankle flexor (tibialis anterior) and an extensor (soleus) in the premammillary decerebrate cat were studied. The two muscle were isolated in the same cat and each was stretched during its separate activation. The yield in stiffness shown by areflexive muscles during stretch was largely compensated for in tibialis anterior as well as in soleus by reflex action. Resultant (total) stiffness varied by less than a factor of two over a wide range of contractile forces in the two muscles. Further, resultant stiffness increased as stretch amplitude decreased in both muscles, but the variation was less for TA. In most preparations, the resultant stiffness in soleus was significantly larger than the resultant stiffness of tibialis anterior. It is concluded that autogenetic reflexes govern the mechanical properties of both flexors and extensors. In addition, the extensor bias in the decerebrate preparation is due not only to greater activation in extensors but to a greater resultant stiffness as well.     

Mechanical analysis of heterogenic inhibition between soleus muscle and the pretibial flexors in the cat.

1. The role of proprioceptive pathways linking the direct antagonists soleus (S) and tibialis anterior (TA) muscles in governing the mechanical properties of the ankle joint were studied in the decerebrate cat. Actions of these heterogenic pathways were compared with those between S and extensor digitorum longus (EDL), a muscle that also acts at the metatarsophalangeal joint. These neurally mediated interactions between S and either TA or EDL were studied by applying controlled length changes to the isolated tendons of pairs of these muscles and recording the resulting changes in force. The muscles were activated with the use of electrically evoked crossed-extension reflexes, flexion reflexes, and brain stem stimulation. 2. Heterogenic inhibition from TA or EDL onto S was well developed whether S was initially quiescent or activated by a crossed-extension reflex. The inhibition persisted for the duration of the stretch of TA or EDL. During a crossed-extension reflex, TA did not generate background force, but brief stretch reflexes could be obtained. During flexion reflexes, stretch reflexes in S were usually abolished, and heterogenic inhibition from S to TA was weak or absent. 3. The strength of the heterogenic inhibition onto S was dependent on the initial length and activation level of TA and EDL. Changes in flexor length or activation level per se did not alter the background force or strength of the stretch reflex in S. Even taking into account the variation of strength of inhibition with the initial state of the muscle of origin, the strength of the inhibition was stronger from TA to S than the other way around. 4. The contributions of heterogenic inhibition from TA and EDL to S were independent in the sense that these components summed linearly with each other and with the autogenic reflex in S. In addition, the magnitude of the inhibition from TA to S was proportional to the amplitude of stretch for low to intermediate levels of initial force in S. The inhibition appeared to affect the mechanical responses of S essentially as rapidly as the stretch reflex in this muscle. 5. The heterogenic inhibition from TA to S was reduced or abolished by intravenous injections of strychnine but unaffected by injections of picrotoxin or bicuculline. These results, together with the observation that the inhibition sums linearly with the stretch reflex, suggest that the mechanism of this heterogenic inhibition is glycinergic and postsynaptic and, therefore, may include Ia-disynaptic reciprocal inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)     

Segmental reflex action in normal and decerebrate cats

1. The objective of this study was to evaluate the action of the stretch reflex on the ankle extensor muscles of normal and decerebrate cats. 2. Experiments were performed on nine freely standing, unrestrained cats and repeated after decerebration at the premammillary level. The length, force, and electromyograph (EMG) of the soleus (SOL) and lateral gastrocnemius (LG) muscles were recorded with the use of implanted transducers and electrodes. 3. The left ankle joint was unexpectedly and reproducibly dorsiflexed by briefly stimulating the common peroneal (CP) nerve with electrodes within an implanted nerve cuff. The ensuing twitch contractions of the ankle dorsiflexor muscles stretched the ankle extensor muscles by 0.3-2.0 mm. Lidocaine was infused into another nerve cuff proximal to the stimulation site, to reversibly block the central propagation of evoked volleys in the CP nerve. 4. Reflex action before and after decerebration was measured from the responses to perturbations of similar amplitude and duration delivered at approximately matched background values of muscle length and force. In most cats the temperature of the hindlimb was monitored with an implanted thermistor and was restored to normal values with radiant heat after decerebration. 5. A stretch imposed on the tonically active ankle extensor muscles immediately caused a considerable rise in the force recorded from the triceps tendon. Within 30-40 ms the triceps force peaked, reaching a value 10-20 N greater than background, and then rapidly declined while the extensor muscles were still lengthening. The initial rise in force preceded any change in triceps EMG. It was attributed to the intrinsic viscoelasticity of the stretched muscles and tendons. After decerebration the magnitude and timing of the initial force peak did not change. 6. A short-latency reflex EMG burst was typically recorded from both the SOL and LG muscles, starting 11-17 ms after stimulus onset. After decerebration the area of the reflex EMG burst increased in all nine cats, typically by a factor of 2 or 3. 7. After decerebration a second, smaller increase in force was typically observed starting 60-80 ms after onset of stretch. This later force rise, interpreted to be of reflex origin, was rarely apparent in normal cats. 8. Decerebration introduced consistent modifications in postural behavior that were revealed by pushing down on the back of quietly standing cats. In normal cats, after brief pushes that stretched the ankle extensor muscles by 1-2 mm, the EMG, force, and length quickly stabilized near their initial values.(ABSTRACT TRUNCATED AT 400 WORDS)     

Intrinsic properties and reflex compensation in reinnervated triceps surae muscles of the cat: effect of activation level

The manner in which activation levels influence intrinsic muscular properties and contributions of the stretch reflex were studied in homogeneous soleus (SOL) and heterogeneous gastrocnemius (G) muscles in the decerebrate cat. Intrinsic mechanical properties were represented by the initial stiffness of the muscle, measured prior to reflex action, and by the tendency of the muscle to yield during stretch in the absence of the stretch reflex. Stiffness regulation by the stretch reflex was evaluated by measuring the extent to which reflex action reduces yielding and the extent to which stiffness depends on background force. Intrinsic mechanical properties were measured in muscles deprived of effective autogenic reflexes using the method of muscular reinnervation. Reinnervated muscles were recruited to force levels comparable to those achieved during natural locomotion. As force declined during crossed-extension reflexes in reinnervated and intact muscles, initial stiffness declined according to similar convex trajectories. The data did not support the hypothesis that, for a given force level, initial stiffness is greatest in populations of predominantly type I motor units. Incremental stiffness (Deltaf/Deltal) of both G and SOL increased in the presence of the stretch reflex. Yielding of SOL (ratio of incremental to initial stiffness) substantially decreased in the presence of the stretch reflex over the full range of forces. In reflexive G, yielding significantly decreased for low to intermediate forces, whereas at higher forces, yielding was similar irrespective of the presence or absence of the stretch reflex. The stretch reflex regulates stiffness in both homogeneous and heterogeneous muscles.     

Factors determining segmental reflex action in normal and decerebrate cats

1. In the companion paper the gain of the stretch reflex in the ankle extensor muscles of normal cats was shown to increase after decerebration. The objectives of this study were 1) to identify the origin of the increased reflex and 2) to evaluate the contribution from afferents other than ankle extensor muscle afferents to the short-latency reflex. 2. Six cats were trained to stand unaided on four pedestals. Three cats were also trained to control the force exerted with the left hindlimb. The left soleus (SOL) and lateral gastrocnemius (LG) electromyogram (EMG), length, force, and temperature were recorded by chronically implanted electrodes and transducers. Measurements were taken before and after decerebration at the premammillary level. After decerebration limb temperature was returned to its normal range by the use of radiant heat. 3. Reproducible ramp-and-hold stretches and releases of the ankle extensor muscles were produced by a servo-controlled motor that rotated the left rear pedestal about the ankle joint. The length of the ankle extensor muscles changed by 2-3 mm within 30-35 ms after the onset of a ramp perturbation. Reflex responses before and after decerebration were compared at matched background values of muscle length and force. 4. In both the SOL and LG muscles, a short-latency EMG burst appeared 8-12 ms after stretch onset and lasted approximately 20 ms. After decerebration the onset of the rectified and smoothed EMG burst remained unchanged, but its area was increased by 36-89%. 5. The lateral gastrocnemius-soleus (LG-S) electroneurogram (ENG) was chronically recorded in two cats with a nerve cuff recording electrode implanted on the LG-S nerve. LG-S ENG activity started to increase soon after stretch onset and remained high during the entire ramp phase. The stretch-evoked LG-S ENG burst started approximately 8 ms earlier than the short-latency SOL and LG EMG bursts. It was interpreted to reflect mainly an increase in the activity of Group Ia and Ib muscle afferents, caused by increases in both muscle length and muscle force during the stretch. After the cats were decerebrated, for matched postural conditions, the area of the stretch-evoked LG-S ENG burst was increased by 29-35%. Because the length and force changes sensed by the muscle receptors before and after decerebration were similar, this suggests that the sensitivity of muscle spindles was increased as a consequence of altered activity in fusimotor neurons after decerebration.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential synaptic effects on physiological flexor hindlimb motoneurons from cutaneous nerve inputs in spinal cat.

1. We previously demonstrated in the spinal cat that superficial peroneal cutaneous nerve stimulation produced strong reflex contraction in tibialis anterior (TA) and semitendinosus (St) muscles but unexpectedly produced mixed effects in another physiological flexor muscle, extensor digitorum longus (EDL). The goal of the present study was to further characterize the organization of ipsilateral cutaneous reflexes by examining the postsynaptic potentials (PSPs) produced in St, TA, and EDL motoneurons by superficial peroneal and saphenous nerve stimulation in decerebrate, spinal cats. 2. In TA and St motoneurons, low-intensity cutaneous nerve stimulation that activated only large (A alpha) fibers [i.e., approximately 2-3 times threshold (T)], typically produced biphasic PSPs consisting of an initial excitatory phase and subsequent inhibitory phase (EPSP, IPSP). Increasing the stimulus intensity to activate both large (A alpha) and small (A delta) myelinated cutaneous fibers supramaximally (15-45 T) tended to enhance later excitatory components in TA and St motoneurons. 3. In EDL motoneurons, 2-3 T stimulation of the superficial peroneal nerve evoked initial inhibition (of variable magnitude) in 7/10 EDL motoneurons tested, with either excitation (n = 2) or mixed effects (n = 1) observed in the remaining EDL motoneurons. Saphenous nerve stimuli produced excitation either alone, or preceded by an inhibitory phase in EDL. Increasing the stimulus intensity enhanced later inhibitory influences from superficial peroneal and excitatory influences both from superficial peroneal and saphenous nerve inputs in EDL motoneurons. 4. Short-latency (less than 1.8 ms) EPSPs were observed in a few motoneurons in all reflex pathways examined, except for EPSPs in EDL motoneurons evoked by saphenous stimulation. IPSPs with central latencies less than 1.8 ms were also produced by both saphenous (TA, n = 1; EDL, n = 2) and superficial peroneal (EDL, n = 4) nerve stimulation. 5. The results, in comparison with other reports employing spinal and nonspinal preparations, suggest that removal of influences from higher centers reveals inhibitory circuits from the superficial peroneal and saphenous nerves to EDL motoneurons in the spinal preparation. The inhibitory inputs observed are thought to reflect the activation of "specialized" reflex pathways. Additionally, the demonstration of short-latency EPSPs and IPSPs suggest that the minimal linkage in both the excitatory and inhibitory cutaneous reflex pathways examined is disynaptic. The results are discussed in relation to previous studies on classically conditioned flexion reflex facilitation in spinal cat.     

Intermuscular interaction between synergists in rat originates from both intermuscular and extramuscular myofascial force transmission

The purpose of the present study is to investigate the origin of mechanical interactions between the rat extensor digitorum longus (EDL) muscle and the grouped tibialis anterior and extensor hallucis longus muscles (TA+EHL). The proximal and distal tendons of EDL as well as the tied distal tendons of TA+EHL were transected and connected to force transducers. Connective tissues at the muscle bellies of the anterior crural compartment were left intact. Supramaximal stimulation of the common peroneal nerve activated all muscles maximally and simultaneously. Length-isometric force characteristics of distal TA+EHL were assessed. Simultaneously, forces exerted at the proximal and distal tendons of EDL, kept at constant muscle-tendon complex length and position, were measured. Intermuscular interaction was tested in two conditions: (a) after full longitudinal compartmental fasciotomy, and (b) after blunt dissection of the intermuscular connective tissue linkages between EDL and TA+EHL. Note that in the latter condition, intermuscular myofascial pathways were eliminated. In the initial condition, lengthening TA+EHL by 12 mm increased proximal (by 0.14 N, i.e. 9.5%) and decreased distal EDL force (by 0.21 N, i.e. 11.8%), despite the fact that EDL muscle-tendon complex length was kept constant. Blunt dissection decreased TA+EHL and distal EDL forces at low TA+EHL lengths only, while proximal EDL force decreased for all TA+EHL lengths tested. The dissection caused no changes in the TA+EHL length effects on proximal EDL force. In contrast, the amplitude of change in the distal EDL force curve decreased significantly (by 39%) subsequent to blunt dissection. It is concluded that mechanical interaction between synergists originates from both intermuscular as well as extramuscular connective tissues. The highest contribution, however, should be ascribed to the extramuscular pathway.     

Effects of firing frequency on length-dependent myofascial force transmission between antagonistic and synergistic muscle groups

Effects of stimulation frequency on myofascial force transmission between rat peroneal and triceps surae and antagonistic anterior crural muscles, and between extensor digitorum longus (EDL) and tibialis anterior and extensor hallucis longus (TA + EHL) muscles were investigated for lengthening of all anterior crural muscles. Muscles contracted isometrically at firing rates of 10, 20, 30 and 100 Hz. EDL and TA + EHL were distally lengthened. Peroneal and triceps surae muscles attained a constant muscle-tendon complex length. Peroneal and triceps surae distal active force decreased significantly as a function of anterior crural muscle length, also at submaximal activation. The absolute decrease was highest for 100 Hz (peroneal muscles -0.87 N; triceps surae muscles -0.92 N), but the highest normalized decrease occurred at 10 Hz stimulation (peroneal muscles -34%; triceps surae muscles -18%). At all muscle lengths, a negative proximo-distal difference in EDL active force was present which decreased with lower firing frequencies (from -0.4 N at 100 Hz to -0.03 N at 10 Hz). The passive proximo-distal force difference attained positive values. EDL and TA + EHL length-force characteristics agree with effects of firing frequency, except for 10 Hz stimulation, where active force was higher than expected and optimum length shifted to lower muscle lengths. It is concluded that also at submaximal stimulation frequencies, extramuscular myofascial force transmission between peroneal and triceps surae muscles and antagonistic anterior crural muscles is substantial. Although lengthening of submaximally active anterior crural muscles decreases the net myofascially transmitted load on EDL, myofascial force transmission significantly alters effects of firing frequency on length-force characteristics.     

Extramuscular myofascial force transmission within the rat anterior tibial compartment: proximo-distal differences in muscle force.

Intramuscular connective tissues are continuous to extramuscular connective tissues. If force is transmitted there, differences should be present between force at proximal and distal attachments of muscles. Extensor digitorum longus (EDL), tibialis anterior (TA), and extensor hallucis longus muscles (EHL) were excited simultaneously and maximally. Only EDL length was changed, exclusively by moving the position of its proximal tendon. Distal force exerted by TA + EHL complex was not affected significantly. Proximal and distal EDL isometric force were not equal for most EDL lengths: Fprox - Fdist ranged from 0 to approximately +22.7% of Fprox at higher lengths and from 0 to approximately -24.5% at the lowest lengths. It is concluded that extramuscular connections transmit force from muscle. Significant proximo-distal differences of EDL force remained after repeated measurements, regardless of length order, although their length dependence was altered. Measurements of both proximal and distal EDL force were highly reproducible, if EDL did not attain higher lengths than target length. After being active at high lengths, proximal and distal length-force curves were altered at low lengths but not for the highest length range. Extensor digitorum longus length-active force hysteresis was present for proximal as well as distal EDL measurements with increasing and decreasing isometric length order. Further isolating EDL removed the proximo-distal difference for active EDL force. However a decreased difference for passive EDL force remained, which was ascribed to remaining extramuscular connective tissue linkages. It is concluded that extramuscular myofascial force transmission is an important feature of muscle that is not isolated from its surrounding tissues.     

Neuromuscular and biomechanical coupling in human cycling: modulation of cutaneous reflex responses to sural nerve stimulation

This study tested the hypothesis that the modulation of cutaneous reflexes during human cycling would be dependent on muscle biomechanical function and phase of leg movement. The coupling between neuromuscular (electromyographic, EMG), kinetic and kinematic responses to brief innocuous (75% of the pain threshold PnT) and noxious (125% PnT) sural nerve stimulation were studied. Stimuli were delivered pseudorandomly at eight equidistant (45°) positions of the crank cycle. Peak ipsilateral middle latency EMG reflex responses were calculated between 70 and 130 ms post stimulus in Biceps Femoris (BF), Rectus Femoris (RF), Tibialis Anterior (TA) and Soleus (SOL). Peak torque, knee and ankle joint angle changes were calculated between 140 and 220 ms post stimulus to quantify net kinetic and kinematic reflex modulation. Reflex responses were predominately suppressive during early activation of all muscles and facilitatory during BF and TA muscle inactivation. EMG reflex responses in monoarticular lower leg muscles TA and SOL were well correlated with ankle angle in dorsi/plantaflexion, whereas the correlation between reflex modulation in biarticular upper leg muscles (BF and RF) and knee angle changes in flexion/extension was weaker. Stimulation provoked significant ankle eversion over the whole crank cycle for both stimulus intensities, which was correlated with TA and BF EMG reflex responses. Torque modulation followed EMG and kinematic changes in a movement phase-dependent manner. Reflex magnitude was stimulation intensity-dependent. Supplementary nociceptive activation may contribute for this increase. We conclude that sural nerve stimulation during human cycling evokes distinct reflex responses in muscles operating around the knee (BF and RF) and the ankle (TA and SOL). These reflexes are modulated in a phase-dependent manner depending on muscle biomechanical function to generate energy for limb and crank propulsion during a specific region in the cycle. This modulation contributed to a specific adaptation of joint motion and force production in order to maintain task performance.     

Force: velocity pproperties of kitten muscles

1. The characteristics of isometric contractions and force:velocity properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles of neonatal kittens were determined in situ.

2. The mean contraction time is 51 msec for EDL and 70 msec for SOL and the half-relaxation time is 51 msec for EDL and 109 msec for SOL.

3. The average maximum isometric tetanic tension per unit cross-sectional area of muscle is 1·27 kg/cm² for EDL and 1·17 kg/cm² for SOL.

4. The average twitch:tetanus ratio is 0·28 for EDL and 0·119 for SOL; the low value for SOL was found for both indirect and direct stimulation.

5. The average maximum speed of shortening of a sarcomere is 22·8 μ/sec for EDL and 12·7 μ/sec for SOL.

6. These properties of neonatal muscles are compared with those of adult cat muscles and discussed in connexion with differentiation of mammalian muscles into fast and slow types.

     

Extramuscular myofascial force transmission within the rat anterior tibial compartment: proximo‐distal differences in muscle force

Intramuscular connective tissues are continuous to extramuscular connective tissues. If force is transmitted there, differences should be present between force at proximal and distal attachments of muscles. Extensor digitorum longus (EDL), tibialis anterior (TA), and extensor hallucis longus muscles (EHL) were excited simultaneously and maximally. Only EDL length was changed, exclusively by moving the position of its proximal tendon. Distal force exerted by TA + EHL complex was not affected significantly. Proximal and distal EDL isometric force were not equal for most EDL lengths: F_prox – F_dist ranged from 0 to ≈+22.7% of F_prox at higher lengths and from 0 to ≈–24.5% at the lowest lengths. It is concluded that extramuscular connections transmit force from muscle. Significant proximo‐distal differences of EDL force remained after repeated measurements, regardless of length order, although their length dependence was altered. Measurements of both proximal and distal EDL force were highly reproducible, if EDL did not attain higher lengths than target length. After being active at high lengths, proximal and distal length–force curves were altered at low lengths but not for the highest length range. Extensor digitorum longus length–active force hysteresis was present for proximal as well as distal EDL measurements with increasing and decreasing isometric length order. Further isolating EDL removed the proximo‐distal difference for active EDL force. However a decreased difference for passive EDL force remained, which was ascribed to remaining extramuscular connective tissue linkages. It is concluded that extramuscular myofascial force transmission is an important feature of muscle that is not isolated from its surrounding tissues.     

Improvement in linearity and regulation of stiffness that results from actions of stretch reflex.

We studied stretch reflexes of soleus muscles of intercollicularly decerebrated cats using a new technique for estimating the component of a stretch reflex that results from the purely mechanical properties of the active muscle (mechanical response). The difference between a net stretch reflex and its underlying mechanical response provided a direct measure of reflex action. 1. The relative contributions of reflex action and the mechanical response are different for stretch and release. With stretch, reflex action is generally large and the mechanical response small. The opposite is true with release. The property that remains relatively constant, when stretch and release are compared, is the net stiffness in opposition to length change. We concluded that reflex action compensates for variations in the inherent stiffness of the muscle. 2. Compensation is effective over a range of intermediate values of initial force, but it fails whenever the reflex force approaches zero or the maximal force at that length. 3. Reflex action is capable of modifying muscular force within 22 ms of the onset of length change. This indicates that even during a fast gallop, there is sufficient time for reflex action. 4. The mechanical properties of the active soleus muscle are highly nonlinear; e.g., muscular stiffness becomes negative transiently during stretch. In stable decerebrate preparations we found that reflex action resulted in a considerable improvement in linearity. 5. Our results support two complementary hypotheses: a) stiffness may be the regulated property of the stretch reflex, and b) the main function of autogenetic reflexes may be to conpensate for variations in the properties of skeletal muscle rather than to oppose changes in load.     

Rhythmic arm cycling modulates Hoffmann reflex excitability differentially in the ankle flexor and extensor muscles

Rhythmic arm movement significantly suppresses H-reflex amplitude in the soleus (SOL) muscle. This is evidence of neural linkages between the arms and legs which can be exploited during locomotion and have been ascribed to the descending effects of CPGs for arm cycling. However, the generalizability of the effects of arm movement on reflex excitability within the lower leg musculature has not been confirmed, as findings have been limited solely to the ankle extensor group. Here we tested the hypothesis that rhythmic arm movement similarly modulates H-reflex amplitude in both the ankle flexors and extensors by observing responses in the SOL and tibialis anterior (TA) muscles. SOL and TA H-reflex recruitment curves were recorded bilaterally during control and 1 Hz arm cycling conditions. Our results showed significant suppression in H-reflex amplitude (H_max) in the SOL muscle in both the dominant and non-dominant legs during arm movement. However, results also revealed an unpredicted bidirectional (i.e. either suppression or facilitation) modulation of TA reflex amplitude that was not present in the SOL muscle. These findings suggest a differential regulation of ankle flexor and extensor H-reflex responses during rhythmic arm movement. This may be the result of differences in CPG output to the flexors and extensors during rhythmic movement, as well as increased involvement of cortical drive to the flexors relative to the extensors during rhythmic movement. These findings may be pertinent to future investigation of rehabilitative therapies that involve facilitative modulation of ankle flexor motor responses.     

Parallel reflex pathways from flexor muscle afferents evoking resetting and flexion enhancement during fictive locomotion and scratch in the cat

Reflex actions of muscle afferents in hindlimb flexor nerves were examined on ipsilateral motoneurone activity recorded in peripheral nerves during midbrain stimulation-evoked fictive locomotion and during fictive scratch in decerebrate cats. Trains of stimuli (15–30 shocks at 200 Hz) were delivered during the flexion phase at intensities sufficient to activate both group I and II afferents (5 times threshold, T ). In many preparations tibialis anterior (TA) nerve stimulation terminated ongoing flexion and reset the locomotor cycle to extension (19/31 experiments) while extensor digitorum longus (EDL) stimulation increased and prolonged the ongoing flexor phase activity (20/33 preparations). The effects of sartorius, iliopsoas and peroneus longus muscle afferent stimulation were qualitatively similar to those of EDL nerve. Resetting to extension was seen only with higher intensity stimulation (5 T ) while ongoing flexor activity was often enhanced at group I intensity (2 T ) stimulation. The effects of flexor nerve stimulation were qualitatively similar during fictive scratch. Reflex reversals were consistently observed in some fictive locomotor preparations. In those cases, EDL stimulation produced a resetting to extension and TA stimulation prolonged the ongoing flexion phase. Occasionally reflex reversals occurred spontaneously during only one of several stimulus presentations. The variable and opposite actions of flexor afferents on the locomotor step cycle indicate the existence of parallel spinal reflex pathways. A hypothetical organization of reflex pathways from flexor muscle afferents to the spinal pattern generator networks with competing actions of group I and group II afferents on the flexor and extensor portions of this central circuitry is proposed.     

Chronic hypoxia induces prolonged angiogenesis in skeletal muscles of rat

Skeletal muscle capillarity and fibre cross-sectional area were investigated within and between diaphragm (Diaph), extensor digitorum longus (EDL), soleus (SOL) and tibialis anterior (TA) muscles of control and chronic hypoxic (12 % O(2) for 6 weeks) adult male Wistar rats (final body mass approximately 355 g). Cryostat sections were stained for alkaline phosphatase activity to depict all capillaries, and for succinic dehydrogenase to demonstrate regional differences in oxidative capacity within the muscles. Hypoxia-induced angiogenesis occurred in all muscles (P < 0.01), with capillary-to-fibre ratio (C:F) being higher in the more active and oxidative muscles, Diaph (27 %) and SOL (26 %), than phasically active and glycolytic muscles, TA (21 %) and EDL (15 %). Diaph, SOL and EDL maintained fibre size, and hence showed an increased capillary density (CD) and reduced intramuscular diffusion distance (DD), whereas TA showed fibre hypertrophy and maintained CD and DD compared to control muscles. The extent of angiogenesis among different regions of muscle varied so as to suggest that muscle fibre size has an additional influence on capillary growth during chronic systemic hypoxia, which is progressive over an extended period of systemic hypoxia.     

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. Received: 15 July 1996 / Accepted: 29 November 1996     

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.     

Changes in K+, Na+ and calcium contents during in vivo stimulation of rat skeletal muscle

The effects of in vivo stimulation via the sciatic nerve on Na⁺ K⁺ and calcium contents in slow-twitch and fast-twitch muscles were compared. Whereas intermittent stimulation for 24 h at 20 Hz caused only minor changes in soleus (SOL), a considerable loss of K⁺ (around 24%) and gain of Na⁺ (around 84%) was observed in extensor digitorum longus (EDL) and tibialis anterior (TA) muscles. These changes could be detected within 0.5 h and a plateau was maintained from 2 to 24 h. Total calcium content increased progressively, reaching values 245 and 382% above the control level in EDL and TA muscle, respectively, after 24 h of 20 Hz stimulation. Whereas the Na⁺ and K⁺ content recovered within a few hours, calcium content did not return towards control level until after 48 h of rest. In a pilot study performed with continuous stimulation at 10 Hz, the changes in Na⁺ and K⁺ contents in SOL, EDL and TA muscle were comparable to those at 20 Hz. The concentration of the Na⁺-K⁺ pumps was highest in the fast-twitch EDL and TA muscles and was unaffected by 10 Hz stimulation. It is concluded that a stimulation pattern leading to a rise in intracellular Na⁺ and a loss of K⁺ may cause a marked accumulation of calcium. These events seem to be related to insufficient activation of the Na-K⁺ pump rather than to variations in the total Na⁺-K' pump capacity.