THE RESPIRATORY RATE OF THE SCIATIC NERVE OF THE FROG IN REST AND ACTIVITY

1. With the indicator method of Haas, the rates of carbon dioxide production have been measured in the case of the sciatic nerve, various parts of the brain, and the sartorius muscle of the frog. The rate of respiration of the sciatic nerve is from 10 to 30 per cent of that of the other tissues, varying somewhat with the individual. 2. Stimulation of the sciatic nerve with induction shocks sufficient to induce tetanus of the muscle does not increase the output of carbon dioxide from the sciatic nerve, even if continued as long as 30 minutes. Sartorius muscle used as a control showed a marked increase in carbon dioxide production upon relaxation after contraction resulting from such stimulation. 3. These facts indicate that the nerve impulse does not depend upon processes leading to the production of carbon dioxide.     

An investigation of arterial insufficiency in the rat hindlimb. Correlation of skeletal muscle bloodflow and glucose utilization in vivo.

Muscle bloodflow and the rate of glucose uptake and phosphorylation were measured in vivo in rats 7 days after unilateral femoral artery ligation and section. Bloodflow was determined by using radiolabelled microspheres. At rest, bloodflow to the gastrocnemius, plantaris and soleus muscles of the ligated limb was similar to their respective mean contralateral control values; however, bilateral sciatic nerve stimulation at 1 Hz caused a less pronounced hyperaemic response in the muscles of the ligated limb, being 59, 63 and 49% of their mean control values in the gastrocnemius, plantaris and soleus muscles respectively. The rate of glucose utilization was determined by using the 2-deoxy[3H]glucose method [Ferré, Leturque, Burnol, Penicaud & Girard (1985) Biochem. J. 228, 103-110]. At rest, the rate of glucose uptake and phosphorylation was statistically significantly increased in the gastrocnemius and soleus muscles of the ligated limb, being 126 and 140% of the mean control values respectively. Bilateral sciatic nerve stimulation at 1 Hz caused a 3-5-fold increase in the rate of glucose utilization by the ligated and contralateral control limbs; furthermore, the rate of glucose utilization was significantly increased in the muscles of the ligated limb, being 140, 129 and 207% of their mean control values respectively. For the range of bloodflow to normally perfused skeletal muscle at rest or during isometric contraction determined in the present study, a linear correlation between the rate of glucose utilization and bloodflow can be demonstrated. Applying similar methods of regression analysis to glucose utilization and bloodflow to muscles of the ligated limb reveals a similar linear correlation. However, the rate of glucose utilization at a given bloodflow is increased in muscles of the ligated limb, indicating an adaptation of skeletal muscle to hypoperfusion.     

Tetanic fatigue and proximate post-tetanic recuperation in sartorius and flexor carpi radialis muscles of the male frog. Effects of iodoacetic acid (author's transl)]

The time-course of the isometric tension output, at 20 degrees C, during a long tetanus and after a short period of rest, was investigated in two isolated frog muscles : the sartorius and flexor carpi radialis muscles. To prevent aerobie and glycolytic recovery processes, some muscles were poisoned with 0,4 mM iodoacetic acid (IAA) and nitrogen, for 20 or 40 min. 1. For the unpoisoned sartorius muscle, tetanic tension declined quickly, but after a 0,8 sec period of rest, the muscle was able to develop high tension. Poisoning with IAA-N2 increased fatigue without suppressing the property of a proximate post-tetanic recuperation. 2. In the flexor carpi radialis muscle resistance to fatigue was very large before poisoning and diminished after poisoning. Proximate recuperation was very weak. 3. The results show that the recovery processes are not a primary factor of the development of the short-term fatigue ; they enhance the hypothesis that a failure of the electromechanical coupling can explain the rate of the tension fall in tetanized sartorius muscles.     

Anaerobic energy provision in aged skeletal muscle during tetanic stimulation

The effect of age on skeletal muscle anaerobic energy metabolism was investigated in adult (11 mo) and aged (25 mo) Fischer 344 rats. Hindlimb skeletal muscles innervated by the sciatic nerve were stimulated to contract with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1 Hz for 60 s, with an occluded circulation. Soleus, plantaris, and red and white gastrocnemius (WG) were sampled from control and stimulated limbs. All muscle masses were reduced with age (9-13%). Peak isometric tensions, normalized per gram of wet muscle, were lower throughout the stimulation in the aged animals (28%). The potential for anaerobic ATP provision was unaltered with age in all muscles, because resting high-energy phosphates and glycogen contents were similar to adult values. Anaerobic ATP provision during stimulation was unaltered by aging in soleus, plantaris, and red gastrocnemius muscles. In the WG, containing mainly fast glycolytic (FG) fibers, ATP and phosphocreatine contents were depleted less in aged muscle. In situ glycogenolysis and glycolysis were 90.0 +/- 4.8 and 69.3 +/- 2.6 mumol/g dry muscle (dm) in adult WG and reduced to 62.3 +/- 6.9 and 51.5 +/- 5.5 mumol/g dm, respectively, in aged WG. Consequently, total anaerobic ATP provision was lower in aged WG (224.5 +/- 20.9 mumol/g dm) vs. adult (292.6 +/- 7.6 mumol/g dm) WG muscle. In summary, the decreased tetanic tension production in aged animals was associated with a decreased anaerobic energy production in FG fibers. Reduced high-energy phosphate use and a greater energy charge potential after stimulation suggested that the energy demand was reduced in aged FG fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of step changes in pH on isometric tetanic tension of toad sartorius muscle

The effect of a rapid change in pHe (pH of bathing solution) on the isometric tetanic tension developed by sartorius muscles of toads acclimated to 5 and 25 degrees C was measured at 5 and 25 degrees C. The pH was altered by changing the carbon dioxide concentration of a bicarbonate buffered physiological solution. Acclimation temperature did not modify the response to a rapid change in pH, but test temperature did. Following a pH decrease from 9.0 to 6.0, tetanic tension decreased at a faster rate at 5 degrees C than at 25 degrees C. A new steady state was reached in 15 min at 5 degrees C but in 40 min at 25 degrees C. Following a pH increase from 6.0 to 8.5, tetanic tension increased at a faster rate at 25 degrees C than at 5 degrees C. A new steady state was reached in 60 min at 5 degrees C but in 10 min at 25 degrees C. We conclude that the rate of carbon dioxide diffusion through the sartorius muscle is only one factor that determines how rapidly tetanic tension changes following the step change in pH, and that muscle resists pH change more effectively at higher temperatures.     

The effect of activity during early postnatal development on motor unit size

At early stages of neuromuscular development, motor unit territory is expanded, with each muscle fibre being supplied by several axons. During postnatal development, some synapses are eliminated, motor unit size decreases, and the adult distribution of motor unit sizes emerges. This process depends on activity, since it proceeds more rapidly when the nerve is activated and is slower when activity is reduced. Here we studied whether, in addition to influencing the rate of retraction of motor unit territory, activity during the critical period of development affects the final outcome of the distribution of motor unit sizes. The sciatic nerve of 8- to 12-day-old rats was stimulated daily. One week later the tension of the extensor digitorum longus muscle and that of its individual motor units was recorded. The sizes of individual motor units were calculated and compared with those from animals that received no stimulation. The distribution of motor unit sizes from stimulated muscles was not significantly different from those from control muscles. Therefore, we conclude that although activity increases the rate at which motor units attain their adult size, it does not influence the final outcome of motor unit size distribution.     

Increase in the amount of adenylate cyclase in rat gastrocnemius muscle after denervation

After section of the sciatic nerve, the basal adenylate cyclase (AC) activity in rat gastrocnemius muscle increased 6-7 times per membrane protein and about 2 times per whole muscle in the following 30 or 40 days. The AC activity in the muscle 30 days after denervation was increased about 4 times by forskolin. Calcitonin gene-related peptide (CGRP) also increased the adenylate cyclase activity in the denervated muscle. The binding of [3H]-forskolin (10nM) to cells isolated from gastrocnemius muscle was examined to determine the amount of AC molecules. Inhibition of [3H]-forskolin binding by increasing amounts of unlabeled forskolin gave a sigmoid curve with a IC50 value of 3 x 10(-7) M. Results showed that the number of [3H]-forskolin binding sites per cell was higher on the denervated side than on the control side, like the basal AC activity. The IC50 values for inhibition by unlabeled forskolin of binding of [3H]-forskolin were similar to muscles on the control and denervated sides. These results suggest that an increase in the AC activity induced by denervation was due to an increase in the numbers of AC molecules in the muscle.     

Is the change in intracellular pH during fatigue large enough to be the main cause of fatigue?

The intracellular pH of frog sartorius muscles exposed to an extracellular pH 8.0 (25 mM HCO3-, 1% CO2) was 6.9-7.1. Following a fatiguing stimulation period (one tetanic contraction per second for 3 min), the intracellular pH was 6.5-6.7. When similar experiments were repeated with frog sartorius muscles exposed to pH 6.4 (2mM HCO3-, 1% CO2), the intracellular pH was 6.8-6.9 at rest and 6.3-6.4 following fatigue. So, in both experiments the intracellular pH decreased by 0.4-0.5 pH unit during fatigue. When the CO2 concentration of the bathing solution was increased from 1 to 30%, the intracellular pH of resting muscles decreased from 7.0 to 6.2-6.3. Although the effect of CO2 on the intracellular pH was greater than the fatigue effect, the decrease in tetanic force with CO2 was less than 40%, while during fatigue the tetanic force decreased by at least 70%. Therefore in frog sartorius muscle the decrease in tetanic force during fatigue exceeds the decrease that is expected from just a change in intracellular pH.     

THE LOCUS OF THE ACTION OF VERATRIN

1. In Mnemiopsis veratrin shows two stages of veratrin poisoning. First, inhibition of the beats of the plates which disappears on cutting them away either singly or in small groups. Second, after half an hour mechanical stimulation excites the beat of the plates in the intact veratrinized animal. It is concluded that veratrin acts on nervous tissue and not on the substance of the swimming plates. 2. In Lumbricus, veratrin acts on the ventral nerve cord alone, and not on the muscles and peripheral nerves. 3. In Musca, veratrin first causes opisthotonos, then spasms and extreme flexion of the legs. Decapitation causes these effects to disappear hence veratrin acts on the cerebral ganglia of the fly. 4. Veratrin applied to the sciatic nerve of the frog causes, after a latent period of 20 minutes, irregular contractions of the gastrocnemius which persist for an hour or more. Veratrin is thus a neurophil alkaloid of the first class as well as second and in this way resembles tetraethyl ammonium chloride. 5. If the end of a sciatic nerve is dipped into veratrin solution, then direct stimulation of the gastrocnemius muscle results in contraction with delayed relaxation, although the muscle itself is not subject to the action of veratrin. 6. By means of preparations of the sartorius muscle of the frog it is shown that veratrin acts not on the muscle cells directly but on the nerve fibers. Hence veratrin produces the characteristic muscle curve showing delayed relaxation by its action on the nervous elements.     

Effect of hyperglycaemia on muscle glycogen mobilization during muscle contractions in the rat

In the rat, muscle glycogen is mobilized during the first stage of exercise, despite normoglycaemia. The aim of the present study was to examine if this process could be prevented or reduced by hyperglycaemia. Three experiments were carried out: in the first, rats were forced to run on a treadmill; in the second the gastrocnemius muscle group was made to contract by stimulation of the sciatic nerve and in the third adrenaline was administered subcutaneously. Each group was divided into two subgroups: control and enriched with glucose (hyperglycaemic). It was shown that hyperglycaemia has no effect on running-induced glycogen mobilization in hind-limb muscles of different fibre composition but prevented it totally in diaphragm muscle. Hyperglycaemia also did not affect the glycogen mobilization induced by stimulation of the sciatic nerve. However, it delayed and reduced markedly the glycogenolytic effect of adrenaline. It is concluded that increased glycogenolysis in muscles at the beginning of exercise may be a consequence of a delay in the activation of glucose transporting mechanisms in muscle cells.     

Exercise pressor reflex in decerebrate and anesthetized rats

I investigated whether muscular contraction evokes cardiorespiratory increases (exercise pressor reflex) in alpha-chloralose- and chloral hydrate-anesthetized and precollicular, midcollicular, and postcollicular decerebrated rats. Mean arterial pressure (MAP), heart rate (HR), and minute ventilation (Ve) were recorded before and during 1-min sciatic nerve stimulation, which induced static contraction of the triceps surae muscles, and during 1-min stretch of the calcaneal tendon, which selectively stimulated mechanosensitive receptors in the muscles. Anesthetized rats showed various patterns of MAP response to both stimuli, i.e., biphasic, depressor, pressor, and no response. Sciatic nerve stimulation to muscle in precollicular decerebrated rats always evoked spontaneous running, so the exercise pressor reflex was not determined from these preparations. None of the postcollicular decerebrated rats showed a MAP response or spontaneous running. Midcollicular decerebrated rats consistently showed biphasic blood pressure response to both stimulations. The increases in MAP, HR, and Ve were related to the tension developed. The static contractions in midcollicular decerebrated rats (381 +/- 65 g developed tension) significantly increased MAP, HR, and Ve from 103 +/- 12 to 119 +/- 24 mmHg, from 386 +/- 30 to 406 +/- 83 beats/min, and from 122 +/- 7 to 133 +/- 25 ml/min, respectively. After paralysis, sciatic nerve stimulation had no effect on MAP, HR, or Ve. These results indicate that the midcollicular decerebrated rat can be a model for the study of the exercise pressor reflex.     

Changes in the mechanical properties of human and amphibian muscle after eccentric exercise

Following a series of eccentric contractions, that is stretching of the muscle while generating active tension, the length-tension relationship of isolated amphibian muscle has been shown to shift towards longer muscle lengths (Katz 1939; Wood et al. 1993). Here we report observations of electrically stimulated ankle extensor muscles of nine human subjects, demonstrating a similar shift in optimum angle for torque generation [3.9 (1.5)°] following exercise on an inclined treadmill that involved eccentric contractions in one leg. (All values are means with the SEMs in parentheses.) The shift in the unexercised, control leg was significantly less [mean 0.4 (0.7)°P < 0.05]. Correlated with this shift was a drop in torque [25.1 (5.6)% for the experimental leg; 1.6 (0.7)% for the control leg, P < 0.002]. Optimum angles returned to pre-exercise values by 2 days post-exercise, while torque took a week to recover. A similar shift in optimum length [12 (1.3)% of rest length] was obtained for five toad (Bufo marinus) sartorius muscles subjected to 25 eccentric contractions. Isometrically contracted control muscles showed a smaller shift [3.5 (1.6)%, n = 5]. Accompanying the shift was a drop in tension of 46 (3)% after the eccentric contractions [control isometric, 23 (6)%, P < 0.0001]. By 5 h after the eccentric contractions the shift had returned to control values, while tension had not recovered. When viewed with an electron microscope, sartorius muscles fixed immediately after the eccentric contractions exhibited many small, and a few larger, regions of myofilament disruption. In muscles fixed 5 h after the contractions, no small regions of disruption were visible, and the number of large regions was no greater than in those muscles fixed immediately after the eccentric contractions. These disruptions are interpreted as the cause of the shift in length-tension relationship. Accepted: 9 January 1997     

The peripheral nerve allograft in the primate immunosuppressed with Cyclosporin A: II. Functional evaluation of reinnervated muscle.

Isometric contractile function was evaluated in primates receiving peripheral nerve allografts and autografts. Twelve adult male cynomolgus monkeys received both sural nerve allografts and autografts to the ulnar nerve in opposite forearms. Half the animals received Cyclosporin A (CsA) immunosuppression (25 mg/kg per day); the remaining animals received placebo. One year following nerve engraftment, isometric contractile muscle function was evaluated in reinnervated abductor digiti quinti and intact abductor pollicis brevis muscles. Maximal twitch tension (Pt), tetanic tension (P(o)), time to peak tension (tpt), rate of rise of twitch tension (DP/dt), and muscle fatigue were evaluated at optimal muscle length (L(o)). All reinnervated muscles distal to nerve autografts and allografts in both Cyclosporin A-immunosuppressed and placebo-treated animals generated equivalent maximal twitch tension, tetanic tension, and time to peak tension, with no significant difference between groups (p > 0.05 by ANOVA). There was a tendency toward increased muscle fatiguability in Cyclosporin A-treated animals (p > 0.05). However, the rate of rise of twitch tension was significantly faster in the reinnervated and intact muscles of Cyclosporin A-treated primates (p < 0.05). Evidence of excellent functional reinnervation across nerve allografts and autografts similar to that seen in histologic and electrophysiologic studies was noted. Cyclosporin A immunosuppression did not significantly enhance recovery of muscle function distal to nerve allografts in this model.     

Electrophysiological Motor Unit Number Estimation (MUNE) Measuring Compound Muscle Action Potential (CMAP) in Mouse Hindlimb Muscles

Compound muscle action potential (CMAP) and motor unit number estimation (MUNE) are electrophysiological techniques that can be used to monitor the functional status of a motor unit pool in vivo. These measures can provide insight into the normal development and degeneration of the neuromuscular system. These measures have clear translational potential because they are routinely applied in diagnostic and clinical human studies. We present electrophysiological techniques similar to those employed in humans to allow recordings of mouse sciatic nerve function. The CMAP response represents the electrophysiological output from a muscle or group of muscles following supramaximal stimulation of a peripheral nerve. MUNE is an electrophysiological technique that is based on modifications of the CMAP response. MUNE is a calculated value that represents the estimated number of motor neurons or axons (motor control input) supplying the muscle or group of muscles being tested. We present methods for recording CMAP responses from the proximal leg muscles using surface recording electrodes following the stimulation of the sciatic nerve in mice. An incremental MUNE technique is described using submaximal stimuli to determine the average single motor unit potential (SMUP) size. MUNE is calculated by dividing the CMAP amplitude (peak-to-peak) by the SMUP amplitude (peak-to-peak). These electrophysiological techniques allow repeated measures in both neonatal and adult mice in such a manner that facilitates rapid analysis and data collection while reducing the number of animals required for experimental testing. Furthermore, these measures are similar to those recorded in human studies allowing more direct comparisons.     

Muscle plasticity: comparison of a 30-Hz burst with 10-Hz continuous stimulation

The changes in the contractile properties induced by a 30-Hz phasic stimulation paradigm were measured and compared with the changes induced by a 10-Hz continuous stimulation paradigm. The study was performed on the tibialis anterior muscles of cats with one paradigm applied to one hindlimb muscle and the other to the contralateral limb. Both hindlimb muscles received the same number of stimuli in a day, making the average stimulation frequency 10 Hz. Two periods of daily stimulation were studied, 8 and 24 h/day. Muscles stimulated at 30 Hz produced greater overall tetanic tension and, during a prolonged stimulation test, exerted a greater mean tension than muscles stimulated at 10 Hz (50 and 32% increase for animals stimulated for 8 and 24 h/day, respectively). Muscle mass was least reduced and fewer pathological abnormalities were observed in the muscles stimulated at 30 Hz. There were no apparent differences in the histochemistry or biochemistry between muscles stimulated at 10 and 30 Hz, which could account for these differences in muscle properties. These results indicate the 30-Hz paradigm may be better suited than 10 Hz continuous stimulation for applications requiring sustained muscle tension such as correction of scoliosis or muscle conditioning for motor prostheses.     

Influence of overload on recovery of rat plantaris from partial denervation

A functional index of neural adaptability is the capacity of motoneurons to extend and establish supernumerary connections with neighboring denervated muscle fibers. The purpose of this study was to guage this response in rat plantaris muscles subjected to increased levels of activity resulting from the surgical removal of the synergistic gastrocnemius and soleus muscles. Thirty-seven days of overload increased plantaris absolute (69%) and relative (82%) weight, whole muscle (35%) and individual fiber (37%) mean cross-sectional area, half-relaxation time (1/2RT; 25%), and maximum tetanic tension (P0; 21%). In a separate group of animals that had undergone 30 days of overload, three-quarters of the plantaris muscle fibers were denervated by sectioning radicular nerve L4. At 7 days postlesion, contractile responses were obtained from sprouting motor units remaining in radicular nerve L5, and the results compared to a nonoverloaded group that had undergone this same procedure. Twitch time to peak tension and 1/2RT were prolonged in normal partially denervated (PD) and overloaded partially denervated (OPD) muscles, and this response was significantly greater in the overloaded muscles. Both PD and OPD muscles increased twitch tension (38%) and peak tension developed at 25 Hz (34%) to a similar extent, during recovery from partial denervation. These increases, attributable to sprouting of L5 motor axon collaterals, were matched in PD muscles with a corresponding increase in P0, a response which did not occur in OPD muscles. Additionally, a more extensive decrease in P0 occurred as a result of partial denervation in OPD muscles compared with whole muscle P0 of nondenervated muscle (L4 plus L5 stimulation).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of disuse atrophy of rat skeletal muscle based on muscle energy metabolism assessed by 31P-MRS

The purpose of this study was to evaluate disuse atrophy of skeletal muscle using a hind-limb suspension model, with special reference to energy metabolism. Twenty-four Sprague-Dawley rats were divided into four groups: control group (C), hind-limb suspended for 3 days (HS-3), for 7 days (HS-7) and for 14 days (HS-14). The gastrocnemius-plantaris-soleus (GPS) muscles in each group were subjected to the following measurements. After a 2-min rest, contraction of the GPS muscles was induced by electrical stimulation of the sciatic nerve at 0.25 Hz for 10 min, then the frequency was increased to 0.5 and 1.0 Hz every 10 min. During the stimulation, twitch forces were recorded by a strain gauge, and 31P-MRS was performed simultaneously. Maximum tension was measured at the muscle contraction induced at 0.25 Hz; the wet weight of the whole and each muscle in the GPS muscles was also measured. From the 31P-MR spectra during muscle contraction, the oxidative capacity was calculated and compared among the groups. The weights of the whole GPS muscles in C, HS-3, HS-7 and HS-14, were 2.66 +/- 0.09, 2.39 +/- 0.21, 2.34 +/- 0.21 and 2.18 +/- 0.14 (g) respectively. Thus, the muscle mass significantly decreased with time (p < 0.05). Among the GPS muscles, the decrease in weight of the soleus muscle was especially remarkable; in the HS-14 group its weight decreased to 60% of that in the C group. We evaluated maximum tension and oxidative capacity as the muscle function. The maximum tensions in C, HS-3, HS-7 and HS-14 were 519 +/- 43, 446 +/- 66, 450 +/- 23 and 465 +/- 29 (g), respectively. This was significantly greater in the C group than in any other groups, however there were no significant differences among the three HS groups. The oxidative capacity during muscle contraction in the C group was higher than in any HS group and it did not further decrease even if the suspension of the limbs was prolonged beyond 3 days. The present study showed that in disuse atrophy, muscle mass and muscle function did not change simultaneously. Thus, it is necessary to develop countermeasures to prevent muscle atrophy and muscle function deterioration independently.     

Gold sodium thiomalate improves membrane potential impaired by high-frequency stimulation

Effects of gold sodium thiomalate (GSTM) on membrane potential and tetanus tension were examined to elucidate whether the gold compound improves mechanical and electrical muscle dysfunction produced by continuous repeated stimulation of frog skeletal muscles. Continuous stimulation (50 Hz for 2 min, 0.05 ms pulse duration) to the sartorius muscle depolarized the membrane, decreased action potential amplitude, and prolonged action potential duration. GSTM (0.1 mM), unlike thiomalic acid (0.1 mM), markedly decreased impairment of these electrical parameters produced during the stimulation period. In the presence of 500 units/mL of catalase, fatigue stimulation still lengthened by 1.5-fold the half-duration of the action potential after a 5-min rest. The prolongation was, however, smaller than that in controls (no catalase). Application of both catalase and GSTM led to no further changes in action potential compared with the application of catalase alone. GSTM did not affect resting tension of single toe muscle fibers though it suppressed the maximum tension after continuous stimulation. These findings suggest that GSTM can inhibit excitable dysfunction of skeletal muscles subjected to continuous stimulation and that such protective effects of GSTM may be partially mediated by H2O2.     

Nerve stimulation with a multi-contact cuff electrode: validation of model predictions

The recruitment characteristics of muscle selective nerve stimulation by a multi-contact nerve cuff electrode, as predicted by computer modeling, have been investigated in acute experiments on rabbits. A nerve cuff containing five or six dot electrodes was placed around the sciatic nerve in five rabbits. M-waves were recorded with wire electrodes from the lateral gastrocnemius, soleus, tibialis anterior, and extensor digitorum longus muscles. The muscle recruitment performances of three contact configurations (monopole, transverse bipole, transverse tripole) were compared. The selectivity was quantified by the recruitment of two muscles (one extensor and one flexor) in response to a particular stimulus. The results showed that only in a few cases, transverse bi- and tripolar stimulation provided a better selectivity than monopolar stimulation. Neither of the two extensors, nor of the two flexors could be stimulated separately. In accordance with the results of the modeling studies, bi- and tripolar stimulation required higher stimulus currents than monopolar stimulation, whereas maximum recruitment and slopes of recruitment curves were lower. The rabbit sciatic nerve appears to be a less suitable preparation for reproducible selectivity experiments, due to the variability in the number and size of the fascicles and their position in this nerve.     

Neuromuscular organization of feline anterior sartorius: I. Asymmetric distribution of motor units.

The neuromuscular organization of feline anterior sartorius was examined using three experimental approaches. First, the branching pattern of the nerve supplying anterior sartorius was inspected in muscles taken from a large number of feline cadavers. All muscles were found to be supplied by two major nerve branches, one directed proximally and the other directed distally, and most muscles (42/51) had a third distinct branch that entered the muscle centrally. Second, the motoneuronal populations supplying the three nerve branches were investigated by electrophysiological techniques. Motoneurons that supplied axons to the distally-directed branch did not appear to have collaterals in more proximally-located branches. In contrast, other motoneurons supplying the proximally-directed branch also appeared to supply axon collaterals to the centrally-directed branch. This result suggested that the motoneuronal population of the distally-directed branch was largely separate from that supplying the proximally- and centrally-directed branches. Third, the motor unit territories supplied by different nerve branches were mapped using glycogen-depletion methods. Muscle fibers supplied by the distally-directed nerve branch were mostly distributed to the medial portion of anterior sartorius, whereas the fibers supplied by the other two branches were generally found more anteriorly. Further, the muscle fibers supplied by an individual nerve branch were present in greater numbers at the end of the muscle closest to the entry point of that branch. Thus, the motor units supplied by discrete nerve branches were found to be distributed asymmetrically within anterior sartorius, but were arranged neither strictly in-parallel nor strictly in-series.