Twitch-like Contractions Research Articles

Influence of magnesium on the beta-inhibition of catecholamines in the uterine circular muscle of estrogen-treated rats.

The effects of Mg and isoprenaline on the uterine circular muscle of ovariectomized rats which were injected with estradiol for 2--6 days were investigated. The muscle strip exhibited an action potential that was composed of a spike and a plateau, and also exhibited a twitch-like contraction due to electrical stimulation. The duration of the action potential became longer when the treatment with estrogen was applied until the maximal effect was built up in 4--5 days. Both the duration of the action potential and the amplitude of contraction were reduced when isoprenaline was applied. When Mg (0.5 mM) was added to the bathing solution these beta-effects were progressively potentiated and the maximal effect was obtained in about 1 hr. An increase in membrane conductance was induced by the application of isoprenaline in a control Locke solution and it was further accelerated by exposure to Mg-containing solution. The possibility that the beta-action is potentiated by Mg ions which penetrated into the membrane is concluded.

Antagonistic effect of compound 48/80 on the inhibitory actions of morphine and methionine-enkephalin on electrically-induced contractions of the guinea-pig ileum.

1 The effect of compound 48/80 was studied on the twitch-like contractions of the longitudinal muscle of guinea-pig ileum induced by electrical stimulation of intramural cholinergic nerves. 2 Compound 48/80 alone, at concentrations up to 30 microgram/ml, had no effect on the twitch contractions. The contraction to exogenously applied acetylcholine was slightly depressed by the compound. 3 At 100 microgram/ml, compound 48/80 caused a weak but long-lasting increase in tone and irregular contractile activity in the ileum, part of which was reduced but not completely abolished by pretreatment with chlorpheniramine (1 muM) or by repeated applications of compound 48/80. 4 The inhibitory effects of morphine and methionine-enkephalin on the twitches were antagonized by the presence of compound 48/80 (3 to 30 microgram/ml), possibly in a competitive manner. The antagonism was not affected by pretreatment with the antihistaminics, chlorpheniramine and/or metiamide. 5 The inhibitory effects of noradrenaline and adrenaline on the twitches were slightly but significantly increased by the presence of compound 48/80 (10 or 30 microgram/ml), whereas that of ATP was not modified. 6 Thesese results indicate that compound 48/80 acts as a selective and competitive antagonist at opiate receptors located in the intramural cholinergic nerves of guinea-pig ileum.

Average muscle tension triggered by stimulation of gamma fusimotor fibers

Average soleus muscle tension of the anesthetized cat, triggered by repetitive stimulation to a fine ventral root filament of L 7, was isometrically recorded by use of blocking methods of the alpine decending impulses and it showed a twitch-like contraction with very rapid time course. Its mean time to peak and half decay time of relaxation in 12 observations was 21.7 and 26.8 msec, respectively. It was concluded that such a rapid concentration corresponds to a twitch of nuclear chain fibers.

Do potassium ions released from nerve modulate the sensitivity to transmitter in ‘close’ neuro-effector junctions of the vas deferens?

In guinea-pig isolated vas deferens, in which the neural stores of noradrenaline had been labelled by preincubation with [ 3H](−)-noradrenaline, the secretion of tracer noradrenaline induced by electrical stimulation of the extramural nerves with trains of 300 shocks at 10 Hz was markedly depressed when the K + concentration of the medium was raised stepwise from 2.7 mM. Although at 21.6 m m K + the secretion of noradrenaline was reduced to less than 25% of the control level, the strength of the early twitch contraction in response to the same stimulation was almost doubled. Thus, noradrenaline could be the transmitter for the twitch only if the rise in [K +] in the medium had increased the responsiveness of the twitch generating mechanism to noradrenaline by about 10-fold. This turned out to be the case. Cumulative addition of noradrenaline to the bathing medium caused a rise in basal tone and, superimposed on this, twitch-like contractions of the vas. The threshold for the twitches was drastically lowered as [K +] in the medium was raised, while their strength increased. An eight-fold rise in [K +] caused the responsiveness of the twitch generating mechanism to noradrenaline to increase fifty-fold. These findings could be of physiological relevance if a rise in [K +] restricted to the cleft of neuro-effector junctions similarly lowers the threshold of the postjunctional membrane to the neurotransmitter secreted from the prejunctional terminals. In tissues where the transmitter does not primarily promote K + efflux from the postjunctional cells, the additional K + would initially have to be derived from nerve. Judging from evidence in the literature concerning K + release from mammalian C-fibres, activation of the motor nerves to the close junctions in the vas deferens may well result in a local and transient rise in [K +] to levels sufficient to cause considerable focal depolarization of the subsynaptic part of the muscle membrane, and thus possibly to facilitate neuro-effector transmission. The sensitizing effect of high [K +] in the medium was not restricted to noradrenaline but could be demonstrated for acetylcholine as well; this suggests that variation in the local concentration of K + may possibly exert similar modulating effects in other close junctions, regulating the sensitivity of the postjunctional membrane to the transmitter specific for the individual junction.

Investigation of occurrence of tolerance to bronchodilator drugs in chronically pretreated guinea-pigs.

1 The actions of sympathomimetic amines on isolated preparations of atria, trachea and ileum were studied in vitro in guinea-pigs, which had been pretreated for 5 or 12 days, by subcutaneous injection, with adrenaline (5 mug/kg), salbutamol (0.5 mug/kg), salbutamol (0.5 mug/kg), methoxamine (250 mug/kg) or saline (0.9% w/V NaCl solution). 2 In the trachea, a decrease in sensitivity (tolerance) to the relaxant effect of adrenaline was induced by pretreatment, for 12 but not for 5 days, with adrenaline. In these animals, cross-tolerance to isoprenaline or salbutamol was not observed. Tolerance to the relaxant actions of adrenaline isoprenaline or salbutamol was not observed after pretreatment with salbutamol. 3 In the trachea, pretreatment with methoxamine or adrenaline for 12 days did not change the sensitivity to the alpha-adrenoceptor-mediated contractor action of methoxamine. 4 In the atria from those guinea-pigs pretreated with adrenaline or salbutamol, there was no reduced sensitivity to the beta-adrenoceptor agonist actions of adrenaline, isoprenaline or salbutamol. In animals pretreated with methoxamine or adrenaline, there was no observable tolerance or cross tolerance to methoxamine with respect to its alpha-adrenoceptor-mediated positive inotropic action in the atria and no unequivocal evidence of a reduced sensitivity to that action of adrenaline. 5 It was confirmed that the twitch-like contractions of the longitudinal muscle of the electrically stimulated ileum were inhibited by sympatomimetic amines acting on alpha- and beta-adrenoceptors. There was no reduced sensitivity to the inhibitory actions of noradrenaline or isoprenaline on the twitch of ileum isolated from animals pretreated with adrenaline, salbutamol or methoxamine for 5 or 12 days. 6 From our results on these three preparations from the same animals, it is concluded that generalizations regarding changes in sensitivity to sympathomimetic amines following their prolonged administration should not be made in any one species.

Invertebrate nerve-muscle systems

Abstract Recent studies of invertebrate motor nerves and muscles are summarized, with particular attention given to insects and crustaceans. Skeletal muscles, generally, are innervated by both excitatory and inhibitory motoneurons. Excitatory motoneurons may fire in short bursts and elicit rapid, twitch-like contractions (phasic motoneurons) or they may fire in long bursts and produce slow, graded contractions (tonic motoneurons). Inhibitory motoneurons form axo-axonal synapses with excitatory motoneurons (presynaptic inhibition) and neuromuscular synapses (postsynaptic inhibition). Only a few neurons innervate each muscle, but each forms thousands of synapses. Skeletal muscle fibers may contract rapidly (few msec) or slowly (few sec). The former (phasic fibers) have short sarcomeres (2–4 μm); the latter (tonic fibers) have long sarcomeres (6–14 μm). Inhibitory motoneurons release γ-aminobutyric acid (GABA) which increases chloride ion conductance. Excitatory motoneurons in arthropods may release glutamic acid, while those of annelids and molluscs release acetylcholine; both transmitters increase sodium and potassium ion conductance. Most motoneurons respond to injury with axonal degeneration followed by regeneration which restores neuronal integrity. However, crustacean motor axons do not degenerate after isolation from their somata and when stimulated directly, release transmitter in a normal fashion. Certain molluscan muscles (catch muscles) maintain tension long after the end of neural activation. Relaxation is achieved through relaxing nerves which release 5-hydroxytryptamine.

Graded activation in rabbit mesotubarium smooth muscle.

The outer margin of the mesotubarium superius, an accessory ligament from the femal rabbit reproductive system, contains a long and slender bundle of smooth muscle fibers well aligned with the long axis of the tissue. Very little connective tissue is present. Massive alternating-current electrical field stimuli (variable in frequency, amplitude, and duration) applied to isolated mesotubaria from mature, nonpregnant animals produced contractions in which isometric force (P) and its first derivative (dP/dt) could be continuously graded; an optimum existed for stimulus frequency and amplitude, but not for its duration. Twitchlike contractions could not be produced. Isometric contractions in which P was plotted against dP/dt to generate a phase-plane trajectory were used to predict the ultimate (time = infinity) force (Po) in graded contractions; this value agreed with the Po derived from isotonic force-velocity curves fitted to the Hill equation. Quick stretches applied during the rise of contractile force revealed a slow onset of the ability to bear the Po force.

Neuromuscular physiology of the longitudinal muscle of the earthworm, Lumbricus terrestris. II. Patterns of innervation.

ABSTRACT Patterns of innervation of the longitudinal muscle of the earthworm, Lumbricus terrestris, were examined electrophysiologically. The longitudinal musculature of a segment is innervated by relatively few axons, a fast and slow axon being present in segmental nerve I and in the double nerve, segmental nerve II–III. Single-pulse stimulation of the fast axon produces large external muscle potentials and small twitch-like contractions, which with repetitive stimulation are antifacilitating. Repetitive stimulation of the slow axon produces large, slowly developing and sustained mechanical responses, with electrical and mechanical responses showing summation and facilitation. The amplitude and time course of slow mechanical responses are related to the frequency of stimulation. Individual longitudinal muscle fibres are innervated by either the fast or slow axon in a segmental nerve, or by both fast and slow axons. No evidence was found for peripheral inhibitory innervation of the longitudinal muscle.

The antagonistic actions of calcium and magnesium on the superfused ventricle of the snail Helix pomatia.

1. The isolated, superfused half-ventricle of the snail (Helix pomatia) maintains a degree of tonic activity even when not beating, since the membrane is depolarized beyond the tension threshold. Beating may be initiated by lateral stretch of the ventricle and by 5-hydroxytryptamine.2. Ca and Mn each have a hyperpolarizing action, while K and to a lesser extent Na cause depolarization: the tonic activity of the ventricle is affected accordingly.3. An increase in the extracellular concentration of Mg also causes the tension to fall, but without change in membrane potential. It can also initiate beating.4. Contractures induced by 30 mM-K are steadily maintained, but start with a more-or-less distinct twitch-like contraction. A contracture induced by a concentration of K above about 50 mM is poorly sustained and is followed by a further brief contracture when the K concentration is reduced to normal.5. Relations between contracture tension and the concentrations of Ca and Mg accord with the hypothesis that the two ions compete for attachment to a binding site on the cell surface and that tension is proportional to the amount of bound Ca. On this hypothesis, the apparent dissociation constant of the Mg complex is 11.4 mM and that of the Ca complex 0.15 mM or less.6. This effect of Mg is like that of Na on frog ventricle and some of the differences in the behaviour of snail and frog ventricles are abolished by appropriate adjustment of the extracellular concentrations of these ions.

Spindle secondary ending responses elicited by stimulation of static fusimotor axons

1. Indirect evidence on the time course of intrafusal events associated with stimulation of a static fusimotor axon was obtained by analysis of the responses from muscle spindle secondary endings of the cat.2. In many instances, a single impulse in a single static fusimotor axon to a muscle spindle evoked a transient increase in discharge of the secondary ending the duration of which ranged from 30 to 60 msec with a rising phase of 6-20 msec. Increasing the length of the muscle increased the slope of the rising phase and augmented the magnitude of the response.3. Different static fusimotor axons produced different effects upon secondary endings. A single impulse in some fusimotor axons did not produce an effect while repetitive stimulation produced a smooth increase in discharge frequency. When efficacious, a single fusimotor impulse evoked approximately a similar duration of increased discharge from a secondary ending and a primary ending of the same muscle spindle. The relative magnitude of effects of a given fusimotor axon on primary and secondary endings of a spindle varied.4. During partial curarization the changes in the discharges of a secondary ending evoked by stimulation of a single static fusimotor axon occurred in a step-like fashion.5. These results are interpreted as supporting the idea that static fusimotor axons induce twitch-like contractions of intrafusal elements (nuclear chain) through the intermediary of propagated action potentials.

Nervous control and spontaneous activity of the chromatophores of a cephalopod, Loligo opalescens

1. 1. The radial muscle fibers of the chromatophores receive only a motor innervation. In those of ventral skin, one motor fiber innervates the muscles of several chromatophores. In the dorsal skin, each motor fiber innervates only part of the muscle complement of a number of chromatophores; a bundle of motor axons innervate the same skin territory, thus permitting stepwise expansion of chromatophores through tetanic contractions of increasing numbers of their muscle fibers. 2. 2. Single nerve impulses cause twitch-like contractions. Above 2/sec summation of individual contractions occurs with transition to a smooth tetanus at frequencies above 12/sec. 3. 3. ACh causes tonic contractions and “pulsations”. 5-Hydroxytryptamine antagonizes spontaneous and ACh-induced tonic contractions. It does not interfere with nerve-induced contractions but augments the rate of shortening and of relaxation. Tetrodotoxin does not interfere with spontaneous tonic contractions but abolishes the effects of nerve stimulation. Banthine blocks the action of motor fibers and of ACh. 4. 4. Spontaneous and ACh-induced tonic contractions are accompanied by local potentials resembling miniature potentials. Spontaneous contractions are preceded by spikes which arise from generator depolarization. Nerve stimulation results only in local junction potentials. This explains why single muscle fibers can be independently activated by motoneurons even though spontaneous contractions may occur nearly simultaneously in many or all muscle fibers of a chromatophore.

The sensory and motor innervation ofNereis

The motor innervation of the segmental musculature of nereid polychaetes appears to be both polyneuronal and multiterminal, which closely resembles the condition found in the Arthropoda. The cell bodies of the motor axons lie in the cord and terminate in endings of the 'en grappe’ type on the surface of the muscle fibres. These observations provide an anatomical basis for the experiments that showed the muscles were capable of fast twitch-like contractions associated with the escape response and slower graded contractions normally seen during the ambulatory cycle of the parapodia. The sensory equipment of the parapodia includes several prominent neurons which function as rapidly adapting stretch receptors. Some of these are activated by the movements of the parapodia and body wall during locomotion and have inputs to the giant fibres and to an unpolarized network of sensory fibres within the cord. These receptors may serve to integrate the activity of individual segments and ensure co-ordinated flexibility of the movements of the worm as a whole.

The mechanism of dual responsiveness in muscle fibers of the grasshopper Romalea microptera.

Dually innervated Romalea muscle fibers which respond differently to stimulation of their fast and slow axons are excited by intracellularly applied depolarizing stimuli. The responses, though spike-like in appearance, are graded in amplitude depending upon the strength of the stimuli and do not exceed about 30 mv. in height. In other respects, however, these graded responses possess properties that are characteristic of electrically excitable activity: vanishingly brief latency; refractoriness; a post-spike undershoot. They are blocked by hyperpolarizing the fiber membrane; respond repetitively to prolonged depolarization, and are subject to depolarizing inactivation. As graded activity, these responses propagate decrementally. The fast and slow axons of the dually responsive muscle fibers initiate respectively large and small postsynaptic potentials (p.s.p.'s) in the muscle fiber. These responses possess properties that characterize electrically inexcitable depolarizing activity. They are augmented by hyperpolarization and diminished by depolarization. Their latency is independent of the membrane potential. They have no refractory period, thus being capable of summation. The fast p.s.p. evokes a considerable or maximal electrically excitable response. The combination, which resembles a spike, leads to a twitch-like contraction of the muscle fiber. The individual slow p.s.p.'s elicit no or only little electrically excitable responses, and they evoke slower smaller contractile responses. The functional aspects of dual responsiveness and the several aspects of the theoretical importance of the gradedly responsive, electrically excitable component are discussed.

The Double Innervation of Muscles in the Clam (Mya Arenaria)

ABSTRACT It is a matter of common observation that lamellibranch molluscs may close the valves of their shell by a fairly rapid twitch-like contraction of the adductor muscles, and may thereafter in certain circumstances relax the adductors, immediately allowing the valves to gape, while in other circumstances these muscles may be maintained for a considerable time in the shortened state even when large tensions are applied to the valves.