Cat Soleus Research Articles

Peripheral muscle effects of levodopa in the anesthetized cat.

1. Levodopa, dopamine, noraderanaline and adrenaline (in increasing order of potency) depressed the tension and degree of fusion of incomplete tetanic contractions of the slow-contracting soleus muscle in chloralose-anaesthetized cats. 2. The effects of all compounds were antagonized by propranolol (50-20 microgram/kg), but not practolol (1.0-5.0 mg/kg). This indicates that effects are mediated by beta2-adrenoceptor stimulation. 3. The effect of levodopa, but not of the catecholamines, was antagonized by prior administration of the dopa decarboxylase inhibitior benserazide. This indicates that levodopa itself is inactive, whereas its decarbodylated metabolites are active. 4. The depressant action of beta-adrenoceptor agonists on incomplete tetanic contractions of the cat soleus muscle, which are exerted directly on the muscle fibres, is a model for effects exerted on slow-contracting units of human muscles; the latter effects probably underlie the tremor observed after beta-adrenoceptor agonist administration. 5. These results therefore suggest that levodopa, via its decarboxylated metabolites, dopamine, noradrenaline and adrenaline, may produce or exacerabate tremor in man. Thus in Parkinsonian patients any centrally induced relief of tremor that levodopa may produce may be masked by tremorogenic effects of such metabolites exerted in the periphery.

The role of muscle spindle afferents in stretch and vibration reflexes of the soleus muscle of the decerebrate cat

The role of muscle spindle afferents in stretch and vibration reflexes of the soleus muscle of the decerebrate cat

Neuronal mechanisms underlying physiological tremor.

1. Tremor force was recorded during stationary isometric contractions of intrinsic hand muscles of normal subjects. Subjects maintained a steady force level between their thumb and forefinger for 30 s. The force level varied from weak (0.2 kg) to strong contractions (7 kg). These experimental conditions were the same as those in two preceding studies, where single motor-unit activity (14) and the correlation between the discharges of two simultaneously recorded motor units and physiological tremor (11) have been investigated. 2. Two alterations of the power spectra were observed at successively stronger contractions: increase of tremor amplitude and changes in the shape of the power spectrum. At all force levels, the power spectra of tremor force show the well-known decay of tremor amplitude from the lower to the higher frequencies with a local peak at 6--10 Hz. This peak does not show a significant change with respect to frequency when the force level is varied. It is shifted toward lower frequencies in a pathological condition (Parkinsonism) where the recruitment firing rates of the motor units are significantly lower than in the normal. 3. Higher frequencies (greater than 20 Hz) are barely present in the power spectrum during the very weak contractions. They become significant as the contractions become stronger. 4. The steep decay of the power spectrum toward higher frequencies has a similar slope (--43 dB/decade) as the reduction in amplitude of the unfused part of the muscle contractions with increasing stimulus rates (--38 dB/decade). The cutoff of the power spectrum above 25 Hz parallels the achievement of total fusion of muscle twitches above this rate. 5. The results are consistent with the hypothesis that the power spectrum over the range of 6--25 Hz is mainly caused by the unfused parts of the twitch contractions of motor units firing between recruitment (6--8/s) and total fusion of the twitches (25--30/s). The decline of the power spectrum toward higher frequencies can be explained by mechanical damping, which results from increasing fusion of the twitch contractions. The low-frequency part of the power spectrum is assumed to be the result of the slow force deviations produced by changes in the net output of the motoneuron pool. 6. These assumptions were supported by additional animal experiments where the number and rate of force-producing elements could be controlled. Bundles of ventral root filaments innervating cat soleus and gastrocnemius muscles were stimulated synchronously and asynchronously at a number of different rates. The force output of the strain gauge was recorded, filtered, and analyzed in the same way as the human force records. 7. Stimualtion of one nerve bundle at one fixed frequency led to a sharp peak in the power spectrum at that frequency plus peaks of decreasing height representing the harmonics of the stimulation frequency. The height of the peaks decreased at --37 dB/decade. 8...

A comparison of the neuromuscular and autonomic blocking activities of (+)-tubocurarine and its N-methyl and O,O,N-trimethyl analogues

(+)-Tubocurarine was compared to its methyl analogues (+)-N-methyltubocurarine and (+)-O,O,N-trimethyltubocurarine for differences in neuromuscular and autonomic blocking activities. On the cat soleus muscle O,O,N-trimethyltubocurarine had the highest neuromuscular blocking activity, tubocurarine the lowest and N-methyltubocurarine was intermediate. The ganglion blocking activities of tubocurarine and N-methyltubocurarine were equal, but O,O,N-trimethyltubocurarine was less active. Vagolytic activity was determined in the cat and all three compounds were equipotent, but only O,O,N-trimethyltubocurarine blocked the bradycardial response of acetyl-β-methylcholine. It is suggested that tubocurarine and N-methyltubocurarine block parasympathetic vagal ganglia and only O,O,N-trimethyltubocurarine blocks cardiac muscarinic receptors. The neuromuscular blocking activities were also compared by determination of pA 2 values, against carbachol, on the isolated chick biventer cervicis muscle. These results indicated competitive antagonism by all three antagonists and showed that activities were different in different species. It is concluded that previous workers overestimated the increased neuromuscular blocking activity due to O-methylation of tubocurarine.

A 'late supernormal period' in the recovery of excitability following an action potential in muscle spindle and tendon organ receptors.

1. Discharge patterns have been recorded from five types of stretch receptor; frog muscle spindles, lizard tendon organs, cat soleus tendon organs and primary and secondary endings of cat soleus muscle spindles.2. The fully adapted discharge of each type of receptor is irregular, especially for frog spindles and primary endings of cat spindles as compared with the other three types (the ;regularly firing' receptors). Frog spindles and some cat spindle primary endings would maintain a discharge at very low mean rates (1/sec or less) while the remaining receptors would stop suddenly, as soon as their rate of discharge fell below a critical value characteristic for each individual ending.3. This pattern of discharge suggests that there is a peak in the excitability of ;regularly firing' receptors at a time following a preceding impulse, which corresponds to the intervals between impulses at each particular receptor's slowest rate of maintained firing, and that the excitability subsequently falls again. Primary endings of cat muscle spindles also showed some evidence of such a ;late supernormal period', but frog spindles did not.4. Direct evidence for the ;late supernormal period' was obtained from experiments in which a maintained discharge was restarted by an antidromic action potential in a receptor which had stopped firing, and to which had been applied a stretch just too small to restart the discharge.5. It is shown in an Appendix that a model receptor in which the recovery of excitability following an impulse has a hyperbolic time course, and in which Gaussian distributed noise is superimposed on the generator potential, can have a discharge pattern very closely resembling that of a frog spindle (cf. Buller, 1965).6. After addition of a late supernormal period to the model, its discharge pattern could mimic closely that of a lizard or cat tendon organ, or of a secondary ending of a cat spindle.

The response of Golgi tendon organs to single motor unit contractions.

1. Cross-correlation analysis has been used to quantify the responses of cat soleus tendon organs to repetitive twitch contractions of: (a) different motor units within the muscle, (b) single motor units at different muscle lengths, and (c) single motor units when the pulse-train pattern of stimulation delivered to the motor unit axon was altered. 2. Ib afferents were observed which responded to each of several hundred successive motor unit twitches with identical numbers of spikes and with relatively invariant latencies. 3. The present results show that tendon organs are sensitive to subtle alterations in motor unit twitch wave form and amplitude, and that this sensitivity is reflected in the precise timings of their afferent discharge. 4. Examination of these tendon organ responses indicates that the forces produced by single motor units couples to the receptor capsule are well above threshold. Calculations based on these results, and earlier soleus motor unit and muscle fibre data, suggest that the absolute force threshold for tendon organs may be as little as 4 mg, which is less than the estimated minimum twitch force generated by individual soleus muscle fibres. 5. Considering the number of tendon organs in a muscle, and the likelihood that every motor unit is connected with at least one receptor, the sensitivity of tendon organs ensures that every twitch of every motor unit will be reflected in the population of afferent signals projecting to the spinal cord.

SUBACUTE NEUROPATHIC EFFECTS OF DIISOPROPYLFLUOROPHOSPHATE AT THE CAT SOLEUS NEUROMUSCULAR JUNCTION

Subacute neuropathic effects of diisopropylfluorophosphate at the cat soleus neuromuscular junctionDiisopropylfluorophosphate (DFP), an organophosphate cholinesterase inhibitor, causes a neuromuscular dysfunction 1–3 days after exposure (subacute) as well as a delayed (21 day post‐drug) peripheral neuropathy. The effects of DFP were studied in cats given 2 mg/kg DFP into one femoral artery. The contractile strength of the soleus muscle evoked by indirect, single stimulation was reduced 4 h later and fell to 16% of normal after 3 days; by 7 days post‐drug the contractile strength recovered to 76% of control values. A pre‐junctional neuromuscular defect was investigated by assessing a motor nerve terminal function at 4 h and 1, 3, 5 and 7 days after DFP injection. In untreated control animals 83% of alpha soleus motor axons generated post‐tetanic repetition (PTR) but after DFP the incidence of PTR was 40% at 4 h, 7% at 1 day, 8% at 3 days, 27% at 5 days and 72% at 7 days. Electron micrographs of neuromuscular junctions 3 days after DFP showed severe disruption of end‐plates and a conspicuous reduction in the number of motor nerve terminals. The remaining nerve terminals were retracted from the end‐plate region and were enveloped by Schwann cell processes. Internodal sprouting of soleus axons and post‐junctional alterations were still present at 7 days. The time course of the pre‐junctional defect coincided with, and contributed to, the reported post‐functional necrosis.

Effects of Phenytoin on the Cyclic Nucleotide System in the Motor Nerve Terminal

The effects of phenytoin on the motor nerve terminal were evaluated on the in vivo cat soleus nerve muscle preparation. Phenytoin, 10 mg/kg, reduced the repetitive aftercharges in motor nerve endings due to tetanic conditioning. It also reduced the repetitive activity due to adenylate cyclase activation with NaF, or to exogeneous dibutyryl cyclic AMP. These effects of phenytoin could be reversed by administering theophylline, a phosphodiesterase inhibitor, or by increasing the extracellular concentration of calcium. The effects of phenytoin could also be reversed by 3-aminopyridine, but not by tetraethylammonium chloride. Verapamil, a calcium current antagonist, produced effects that were identical to phenytoin. It is concluded that phenytoin blocks a cyclic nucleotide-mediated calcium influx that is associated with transmitter release. This calcium flux also appears to control a slow potassium current that is responsible for post-tetanic hyperpolarization.

Histochemical and physiological profile of a skeletofusimotor (β) unit in cat soleus muscle

Histochemical and physiological profile of a skeletofusimotor (β) unit in cat soleus muscle

Oxygen consumption and contractile force during vibrations of cat soleus muscle.

The influence of longitudinal vibrations (50 Hz, 0.4 mm) on isometric twitch force development (4 Hz), blood flow and oxygen consumption was studied in the acutely denervated soleus muscle of the anesthetized cat. It was found that the sinusoidal vibrations reduced the twitch amplitude by 60 per cent whereas oxygen consumption and blood flow were lowered by 15 per cent only. Similar reduction in twitch force was also obtained by lowering the nerve stimulation intensity (4 Hz). This was associated with a diminution in oxygen consumption, the degree of which was linearly related to the attenuation of active force, i.e. the number of activated motor units. The results are in agreement with previous observations as to the mechanical effect of vibrations on active force in smooth and striated muscle. They demonstrate that vibrations prevent the contractile response with maintained high oxygen consumption which adds further support to the hypothesis forwarded by Joyce et al. (1969) that vibrations cause increased rate of detachment of actin-myosin cross-links. In addition it appears possible that vibrations to some extent prevent formation of such cross-links.

Evidence for a cyclic nucleotide-mediated calcium flux in motor nerve terminals

WE recently suggested that a cyclic nucleotide system might participate in neuromuscular transmission by regulating the flux of calcium and/or sodium into the nerve ending1–3. The purpose of the present investigation was to refine this postulate by studying the interactions between the cyclic nucleotide system and reagents that affect calcium and sodium flux in nerve terminals differentially. Recording from single motor axons of cat soleus nerves in vivo, we found that the calcium antagonists verapamil (iproveratril)4,5 and lanthanum6 blocked stimulus-bound repetitive action potentials (SBR) produced by the administration of dibutyryl (db) cyclic AMP and sodium fluoride, whereas tetrodotoxin (TTX) did not alter the nerve or muscle response to these reagents. The results suggest that an intraterminal cyclic nucleotide system may regulate calcium flux into the motor nerve ending.

Static and dynamic fusimotor action on the response of Ia fibres to low frequency sinusoidal stretching of widely ranging amplitude.

1. Single fusimotor fibres were stimulated repetitively to test their action on the responsiveness of muscle spindle primary endings in the cat soleus to sinusoidal stretching of both large and small amplitude. Frequencies of 0.06-4 Hz were used at amplitudes from 10 mum to 3 mm.2. The response was assessed by fitting a sinusoid to the cycle histogram of the afferent firing throughout the course of the cycle; this linear approximation measures the fundamental of the response and ignores any harmonics. The sine was allowed to project to negative values and any empty bins in the histogram were ignored when fitting.3. With small amplitudes of stretching the histograms were reasonably sinusoidal, but with large amplitudes they showed appreciable distortion of the wave form for the passive ending and during dynamic fusimotor stimulation. Non-linearity of response manifested itself also, with increasing amplitude of stretching, by an increase in the phase advance of the response, by increasing r.m.s. deviation of the histogram points from the fitted sine and (for dynamic stimulation) by an increase in the mean value of the fitted sine.4. With increasing amplitude the response modulation ceased to increase proportionately with the stimulus, so that the sensitivity of the ending to a large stretch (defined as afferent modulation/stretch amplitude) was appreciably less than for a small stretch. This effect was most pronounced for the passive ending.5. Whatever the amplitude of movement the modulation during static stimulation was less than that for the passive or during dynamic stimulation. For small amplitudes the response during dynamic stimulation was less than that of the passive, but for large amplitudes the response during dynamic stimulation was always the greater. At some intermediate cross-over amplitude the two responses were the same size, though still differing slightly in other respects. The value of the cross-over amplitude was usually about 200 mum at 1 Hz, and increased on lowering the frequency. Thus dynamic fusimotor action does not uniformly produce either an increase or a decrease in the sensitivity of the ending in relation to the passive.6. Bode plots, for each amplitude, of sensitivity and phase against frequency suggested that(a) under all conditions the ending is relatively insensitive to frequency in the range studied, for the slope of the log-log sensitivity lines was only 0.15-0.2 (3.5-6 db/decade);(b) the mechanism which makes for non-linearity is not particularly frequency sensitive;(c) static fusimotor stimulation does not change the frequency sensitivity of the ending;(d) dynamic fusimotor stimulation very slightly increases the frequency sensitivity of the ending for large amplitudes.In reaching these conclusions more attention was paid to the slope of the sensitivity lines than to the values of phase.7. It appears that the major effect of fusimotor action, whether static or dynamic, is to regulate the sensitivity of the primary ending to stretching for all amplitudes of movement (i.e. gain) rather than to control the relative values of its sensitivity to length and to velocity (i.e. crudely, the damping in a feed-back loop).

Effects of a Single Methylprednisolone Dose on a Facilitatory Response of Mammalian Motor Nerve

Long-term glucocorticoid dosing directly enhances a facilitatory function of cat soleus motor nerve terminals. Posttetanic potentiation (PTP) of soleus contraction is a manifestation of this prejunctional facilitation. The present study demonstrates that the same enhancement of facilitation is produced with a single large intravenous dose methylprednisolone. The single dosing method, however, showed an initial suppression of facilitation that neared recovery in four hours. Thereafter, the characteristic augmentation of prejunctional facilitation emerged, peaking in 24 hours. Return to control required four days. Knowledge of this time course enabled centrally disconnected motor nerve endings to be identified as the site of both phases of the steroid action. Since the neuromuscular facilitation studied is equivalent to that triggered by neostigmine-like drugs, the results infer that the antimyasthenic effect of glucocorticoids may involve a direct action on motor nerve endings.

Effects of combining static and dynamic fusimotor stimulation on the response of the muscle spindle primary ending to sinusoidal stretching.

1. A pair of fusimotor fibres, one static and the other dynamic, were stimulated simultaneously to test their combined action on the response of muscle spindle primary endings in the cat soleus to sinusoidal stretching. A frequency of 1 Hz was chiefly used, with a wide range of amplitudes (10 micronm-2 mm). The response of the ending was assessed from the parameters of the sine fitted to its firing averaged throughout the course of the cycle; this was felt useful even though the responses to the larger stretches showed certain non-linear features. 2. With small stretches (up to about 50 micronm amplitude) static action dominated, and the modulation of firing during conbined stimulation was little or no larger than that found during the static stimulation on its own, and much smaller than that found during the static stimulation on its own, and much smaller than that found during the dynamic stimulation. The phase of the response was, however, much the same for all three conditions. 3. With larger stretches the modulation with combined stimulation was intermediate between the values found on stimulating either fusimotor fibre on its own; the dynamic contribution increased progressively with the amplitude of stretching. 4. With larger stretches the phase of the response during combined stimulation was appreciably closer to that for static action than to that for dynamic action. But the differences between the various conditions were small (below 20 degrees) and seem attributable to various distortions of the response wave from away from a true sinusoid, rather than betokening a difference in the ratio of velocity to length sensitivity under the various conditions. This view was supported by the effects on phase of grading the rate of stimulation of one fusimotor fibre while holding that of the other constant. 5. Detailed comparison of the cycle histograms obtained under different conditions showed an interestingly asymmetrical pattern of summation and occlusion of the effects of the two kinds of fusimotor fibre. At the peak of the response to a large stretch static action summed with dynamic action, which was here the stronger, so that at this phase of the cycle the firing was greater with the combined stimulation than with either fibre on its own. But, in the trough of the response to the same stretch static action occluded any dynamic action, which was now the weaker, so that at this phase of the cycle the firing with combined stimulation was virtually the same as that with static stimulation on its own. With a small stretch, static action normally occluded dynamic action throughout the cycle; this is in line with the firing during static action now usually being greater than that during dynamic action for all phases of the cycle.

EMG of slow and fast ankle extensors of cat during posture, locomotion, and jumping.

EMG of slow and fast ankle extensors of cat during posture, locomotion, and jumping.

A model of a human neuromuscular system for small isometric tensions.

A model is constructed of the motor units in the human first dorsal interosseus (FDI) muscle. Each motorneuron is simulated using a pseudo-steady-state model that omits the membrane capacity and the events underlying the action potential. Properties of individual twitches in the corresponding muscle units are based on the data of Milner-Brown et al. for the FDI, while the transduction between steady firing rate and percentage of maximum tension in a muscle unit is based on the work of Rack and Westbury on the cat soleus muscle. Since we are concerned only with small isometric tensions, we ignore effects due to muscle spindles and to recurrent inhibition. The model allows one to to determine, by simulation, the tension-time functions produced by different "programs" of input to an entire pool of 120 motorneurons. Thus, for example, in order to produce tension rising linearly with time, it suffices to deliver to each neuron in the pool a non-linearly rising conductance; the conductance can be the same for all neurons in the pool, but can NOT be scaled in proportion to the surface area of the respective neurons. The input may be delivered to any part of the neuron's dendritic tree, as long as the electrotonic distribution of input is the same for all the neurons. For a linearly rising force produced in this way, most of the motorneurons yield similar slopes for their frequency-force curves, as observed by Milner-Brown et al. To produce tensions greater than about 1 kg, mechanisms not included in this model must come into play, i.e. perhaps introduction of phasic motorneurons. The most important data needed to improve this model are sets of isometric frequency-force curves for muscle units of different twitch tensions.

Separation of active and passive components of short-range stiffness of muscle.

The short-range stiffness of smoothly but submaximally contracting isometric soleus muscles of anesthetised cats was measured by applying small fast stretches. The ratio of isometric tension to stiffness was plotted against tension over a wide range of muscle lengths and stimulus rates. The results fitted a straight line well, as predicted from crossbridge theory, showing the stiffness to be a function of tension only, independent of the combination of length and stimulus rate used to generate the tension. The major deviation from this line was attributed to incomplete fusion at low frequencies of stimulation. Values believed to be tendon compliance and crossbridge tension per unit of stiffness were found from the graph, and the tendon compliance correlated with the maximum muscle tension. Shortening the tendon by attaching nearer to the muscle changed the results in a manner consistent with the theory, provided that appropriate precautions were taken against slippage.

Azathioprine

The neuromuscular effects of azathioprine were examined in the in-vivo cat soleus muscle preparation. In concentrations ranging from 10 to 1,000 mug/kg, administered intra-arterially, the agent caused motor axons to fire repetitively and produced a dose-related increase in the force of contraction. The drug reversed neuromuscular blockage produced by d-tubocurarine and potentiated the neuromuscular blockade produced by succinylcholine. The effects of theophylline, a phosphodiesterase inhibitor, on neuromuscular transmission were identical to those produced by azathioprine. Using an in-vitro assay preparation, azathioprine was found to produce 50 per cent inhibition (IC50) of phosphodiesterase at a concentration of 2 X 10(-5) M. In the same preparation, theophylline had an IC50 of 1 X 10(-4) M. Neither agent in concentrations to 10(-2) M affected cholinesterase activity measured in vitro. It is concluded that the effects of azathioprine on neuromuscular transmission are due to inhibition of phosphodiesterase in the motor nerve terminal.

Evidence for a prejunctional role of cyclic nucleotides in neuromuscular transmission

THERE is evidence for1–3 and against4,5 the involvement of cyclic nucleotides in the release of neurotransmitters. We have investigated this question using a method more suited to the detection of prejunctional drug actions than those used previously. Instead of trying to infer the action of nucleotides on nerve endings by recording endplate potentials or muscle contractions in vitro, we recorded from single motor axons of cat soleus nerves in vivo. We found that dibutryl (db) cyclic AMP initiated activity in unstimulated motor axons and produced stimulus-bound AMP, db cyclic GMP, sodium butyrate and 5′ AMP had no repetitive activity (SBR) in stimulated axons, whereas cyclic effect. NaF and theophylline also initiated activity in unstimulated axons and produced SBR in stimulated axons. Pretreatment with the theophylline potentiated the effects of db cyclic AMP or NaF. The results suggest that cyclic AMP is involved in the excitation–secretion sequence of mammalian motor nerve endings.

Effect of thyrotoxicosis on skeletal muscle proprioceptor activity

The effect of prolonged administration of thyroid hormones on the discharges from muscle spindles of the cat soleus was investigated. In animals with thyrotoxicosis the response of the primary spindle endings to a constantly acting and to a sudden rapid stretching increased considerably in both the dynamic and the static phase. The discharge frequency of the secondary endings remained substantially unchanged. It is postulated that the observed changes in activity of the primary endings are connected with disturbances of metabolism in the muscle and also with its atrophy.