Biventer Cervicis Muscle Research Articles

Stonustoxin: effects on neuromuscular function in vitro and in vivo

K. S. Y. Low, M. C. E. Gwee, R. Yuen, P. Gopalakrishnakone and H. E. Khoo. Stonustoxin: effects on neuromuscular function in vitro and in vivo. Toxicon 32, 573–581, 1994.—Stonustoxin (8–50 μg/ml) produced a rapid and concentration-dependent rise in tension (contracture) of the electrically stimulated mouse hemidiaphragm followed by a gradual waning of tension from the peak to the baseline; the nerve-evoked and the directly (muscle)-evoked twitches of the hemidiaphragm were also progressively and irreversibly blocked in a time- and concentration-dependent manner. Stonustoxin (22 and 44 μg/ml) produced a similar rapid rise in tension of the chick biventer cervicis muscle as well as irreversible and concentration-dependent blockade of nerve-evoked twitches and contractures produced by acetylcholine (200 μM), carbachol (8 μM) and KCl (40 mM). The muscle contracture produced by stonustoxin was blocked by dantrolene sodium (6 μM) but not by tubocurarine (15 μM). Moreover, stonustoxin (40 μg/ml) did not inhibit nerve conduction in the toad sciatic nerve and stonustoxin (60 μg/ml) did not exhibit any anticholinesterase activity. The inhibition of neuromuscular function by stonustoxin in the mouse hemidiaphragm and chick biventer cervicis muscle can therefore be attributed to some irreversible myotoxic action(s) of the toxin, whereas the stonustoxin-induced muscle contractures could have been mediated via depolarization of muscle fibres.

Screening of snake venoms for neurotoxic and myotoxic effects using simple in vitro preparations from rodents and chicks

A. L. Harvey, A. Barfaraz, E. Thomson, A. Faiz, S. Preston and J. B. Harris. Screening of snake venoms for neurotoxic and myotoxic effects using simple in vitro preparations from rodents and chicks. Toxicon 32, 257–265, 1994.—Eight snake venoms designated by the WHO as International Reference Venoms, and one additional venom were assessed for neurotoxic and myotoxic effects in vitro using the chick biventer cervicis and the rat and mouse phrenic nerve-diaphragm preparations. The objective was to determine whether any of the preparations could be used to detect evidence of neurotoxic or myotoxic activity prior to a more detailed examination. Bungarus multicinctus venom at concentrations above 1 μg ml ′1 selectively blocked neuromuscular transmission, with no direct effect on muscle fibres. Naja naja kaouthia and Notechis scutatus venoms selectively blocked neuromuscular transmission at low concentrations, but at higher concentrations both venoms caused direct effects on skeletal muscle resulting in contractures, loss of tension following direct stimulation and a loss in sensitivity to elevated [K +] 0, Vipera russelli (Thailand) venom also blocked neuromuscular transmission but it was less potent than the venoms of B. multicinctus, N. n. kaouthia and N. scutatus. It also caused contractures in the chick biventer cervicis muscle. The venoms of Echis carinatus (Iran and Mali), Crotalus atrox, Bothrops atrox asper and Trimeresurus flavoviridis had limited neuromuscular blocking activity, and most of these venoms blocked [K +] 0 and cholinoceptor stimulation in the chick muscle. Although both chick and rodent muscles allowed the assessment of neurotoxic and myotoxic activity, the chick biventer cervicis was simpler and more robust in use than either of the rodent phrenic nerve-diaphragm preparations. We propose that the chick biventer cervicis muscle could be used as a standard preparation for the screening of snake venoms for neurotoxic and myotoxic effects, and that it may be possible to use this preparation as a means to check that antivenoms can neutralize neurotoxic and direct myotoxic actions of venoms.

Pharmacological effects and pathological changes induced by the venom of Pseudechis australis in isolated skeletal muscle preparations

S. Z. J. Chen, P. Gopalakrishnakone and M. C. E. Gwee. Pharmacological effects and pathological changes induced by the venom of Pseudechis australis in isolated skeletal muscle preparations. Toxicon 32, 303–315, 1994.—The venom from the Australian king brown snake Pseudechis australis produced a concentration-dependent and progressive inhibition of directly and indirectly evoked twitches of the mouse hemidiaphragm: 2, 5 and 10 μg/ml of venom produced complete blockade of directly evoked twitches after 87 ± 26 min, 66 ± 28 min and 56 ± 13 min, respectively, whereas twitches elicited by indirect stimulation were blocked after 43 ± 4 min, 34 ± 2 min and 21 ± 2 min, respectively. Two, 5 and 10 μg/ml of the venom also progressively inhibited the twitch responses of the chick biventer cervicis muscle evoked by nerve stimulation: the times (min) to 90% blockade of the twitches were 127 ± 12, 41 ± 8 and 26 ± 6, respectively. Light and electron microscopic studies confirmed that the venom caused severe muscle damage and necrosis in the chick and mouse skeletal muscle. The degree of muscle damage was proportional to the concentration of venom used. The muscle degeneration was characterized by disorganized myofibrils, dilatation of the sarcoplasmic reticulum and swollen or broken-down mitochondria. Depletion and a decrease in population of storage vesicles in the neuromuscular junction were also observed. Thus, the inhibition of neuromuscular function by the venom of P. australis was a consequence of the extensive morphological damage to muscle fibres, motor nerve terminals and cytoplasmic organelles which could be attributed to a direct myotoxic action presumably mediated by the enzymatic action of phospholipase A 2 present in the venom of P. australis.

Met-8 of the β1-bungarotoxin phospholipase A2 subunit is essential for the phospholipase A2-independent neurotoxic effect

beta 1-Bungarotoxin consists of a phospholipase A2 subunit and a non-phospholipase A2 subunit. The toxin was oxidized with a 100-fold molar excess of chloramine T with respect to the methionine content of the protein in 0.1 M Tris/HCl at pH 8.5 and at room temperature. Reactivities of the two methionine (Met-6 and Met-8 of the phospholipase A2 subunit), five histidine, 14 tyrosine and one tryptophan residues of one toxin molecule with chloramine T were assessed from the change in intrinsic fluorescence and amino acid composition of the protein. Met-8 and one tyrosine on the phospholipase A2 subunit and less than one histidine were oxidized, while Met-6 remained intact after 30 min of reaction. One histidine and approx. two tyrosine residues were oxidized when both methionine residues were oxidized after 90 min of reaction. The sole tryptophan was oxidized slightly throughout the reaction. The chloramine T oxidation did not destroy the two Ca(2+)-binding domains, though it modified the toxin to become less effective at binding Ca2+. The modified toxin obtained after 30 or 90 min reaction time retained 65% or 40% of the phospholipase A2 activity of the parent toxin, but both were not lethal to mice and showed a very weak ability to induce the indirectly evoked contraction of chick biventer cervicis muscle. It is suggested that Met-8 may play an important role in the phospholipase A2-independent interaction with the nerve terminal membrane during the neurotoxic effect of beta 1-bungarotoxin.

Effect of Bothrops insularis venom on the mouse and chick nerve-muscle preparation

J. C. Cogo, J. Prado-Franceschi, M. A. Cruz-Hofling, A. P. Corrado and L. Rodrigues-Simioni. Effect of Bothrops insularis venom on the mouse and chick nerve-muscle preparation. Toxicon 31, 1237–1247, 1993.—The effects of Bothrops insularis venom were examined in vivo in mice and chicks and in vitro using the mouse phrenic nerve diaphragm and chick biventer cervicis muscle preparations. Incubation of the indirectly or directly stimulated mouse preparation with B. insularis venom (20–80 μg/ml) produced an initial increase in twitch tension followed by irreversible blockade. With direct stimulation in the presence of d-tubocurarine, no increase in twitch tension was observed prior to the onset of blockade. A venom-induced effect on presynaptic activity was suggested by the marked increase in the frequency of the mepps recorded in vitro 5–15 min after venom addition. A direct muscular effect was shown by the dose- and time-dependent reduction in the resting membrane potential of the diaphragm. Chick preparations were more sensitive than those of the mouse. In the isolated chick biventer cervicis muscle preparation, B. insularis venom induced a contracture and a dose-dependent block of responses to indirect stimulation. At low venom concentrations (1–5 μg/ml), no significant release of creatine kinase (CK) was observed from this preparation. However, a dose-dependent release of CK was detected at higher doses (10–80 μg/ml). For morphological studies, B. insularis venom was injected into the chick left pectoralis muscle. At low doses (0.4 μg), only an inflammatory reaction was present, while at high doses (20–80 μg) increasing numbers of necrotic fibers were observed as well as occlusive thrombosis and hemorrhage. The muscular effect, also observed on the incubated muscle, points to a direct myolytic action of the whole venom.

Descending pathways mediating disynaptic excitation of dorsal neck motoneurones in the cat: brain stem relay

The location of intercalated neurones mediating disynaptic excitation from tectum, tegmentum and pyramids to dorsal neck motoneurones has been investigated by: (a) recording field potentials in the lower brain stem evoked from the above systems, (b) systematic stimulation in the brain stem during intracellular recording from motoneurones innervating the splenius, biventer cervicis and complexus muscles, and (c) comparing the effects of lesions of the brain stem with kainic acid on the disynaptic EPSPs elicited from the above three systems. Electrical stimulation of the contralateral superior colliculus evoked monosynaptic field potentials which were largest in the caudal pontine reticular formation rostral to the abducens nucleus and in the rostral part of the medullary reticular formation caudal to the abducens nucleus. Likewise, stimulation of the ipsilateral tegmentum (the cuneiform and subcuneiform nucleus) evoked field potentials which were large in the caudal medulla and small in the pons. In contrast, stimulation of the contralateral tegmentum was ineffective in evoking field potentials. Stimulation of the pyramid 2–3 mm rostral to the obex elicited monosynaptic field potentials in the reticular formation of the lower brain stem that were only about 25% of those from the superior colliculus. In contrast to the field potentials from the superior colliculus, the pyramidal ones were large in the medulla and small in the pons. Lesions of the reticular formation in the lower brain stem by unilateral kainic acid injection caused disappearance of disynaptic EPSPs in motoneurones from the above three systems. These results strongly suggest that the intercalated neurones mediating pyramidal, tectal and tegmental EPSPs are reticulospinal neurones in the lower brain stem. Systematic stimulation in various locations of the lower brain stem showed that monosynaptic EPSPs were evoked from the regions of the reticular formation which received projection from the above three descending systems. The effective regions for evoking the EPSPs in splenius (SPL) were located somewhat more dorsally than for biventer cervicis and complexus (BCC) motoneurones. The descending axons of presumed reticulospinal neurones were stimulated with electrodes placed in medial, middle and lateral positions at the spinomedullary junction. Monosynaptic EPSPs in SPL and BCC motoneurones were evoked from the medial and middle electrodes but not from the lateral electrode.

Effects induced by bothropstoxin, a component from Bothrops jararacussu snake venom, on mouse and chick muscle preparations

Bothropstoxin, a 13,700 mol. wt myotoxic phospholipase homologue isolated from the venom of Bothrops jararacussu and devoid of PLA 2, proteolytic or hemolytic activities, inhibited muscle twitch tension, evoked either directly or indirectly through stimulation of the motor nerve in the mouse phrenic-diaphragm preparations. The compound action potential of the muscle was also abolished with a similar time course. In addition, the toxin (0.7mM) evoked membrane depolarization which was inhibited in the presence of 10mM Ca 2+. In chick biventer cervicis muscle, the toxin (2 mM) induced a contracture that reached its maximum amplitude in 44.8±15.6 min ( n = 6) and was not blocked by either d-tubocurarine or tetrodotoxin. The time to maximum amplitude was reduced to 5.5±1.0 min ( n = 4) in nominally Ca 2+-free Krebs solution and was completely abolished in Ca 2+-free Krebs solution containing 1 mM EGTA.

Studies on curare-like action of 2,2',2''-tripyridine in the mouse phrenic nerve-diaphragm.

1. The curare-like action of 2,2',2''-tripyridine (a synthetic by-product of the herbicide, paraquat) was studied in mouse phrenic nerve-diaphragm preparation. The inhibition by 2,2',2''-tripyridine of nerve-evoked twitches was dependent on the concentration, ranging from 1 to 100 microM, which had no significant effect on the twitch amplitudes evoked by direct muscle stimulation. 2. The twitch inhibition by 2,2',2''-tripyridine was reversible and could be antagonized by anticholinesterase agents such as neostigmine, physostigmine as well as ecothiophate. 3. Pretreatment with either 0.7 microM (+)-tubocurarine or 2.2 microM succinylcholine shifted the concentration-inhibition curve of 2,2',2''-tripyridine to the left. 4. 2,2'2''-Tripyridine inhibited not only acetylcholine-induced contracture of the denervated mouse diaphragm but also that of the chick biventer cervicis muscle. Like (+)-tubocurarine, 2,2',2''-tripyridine protected the twitches from the inhibition by alpha-bungarotoxin and also specifically inhibited the binding of [125I]-alpha-bungarotoxin to the mouse diaphragm. All of these findings indicate that 2,2',2''-tripyridine possesses curare-like action and inhibits the muscle contractions through binding to postsynaptic acetylcholine receptors. 5. The postsynaptic inhibition exhibited by 2,2',2''-tripyridine was also implicated in the tetanic fade, a decrease in the amplitude of miniature endplate potential (m.e.p.p.) and endplate potential (e.p.p.). 6. The clinical implication of these findings is that 2,2',2''-tripyridine may be involved in the cause of respiratory failure in paraquat-intoxicated workers since 2,2',2''-tripyridine is a by-product of paraquat synthesis.

Electromyographic studies of neck muscles in the intact cat. II. Reflexes evoked by muscle nerve stimulation.

Short-latency reflexes were studied in the neck muscles of four alert cats following electrical stimulation of nerves supplying biventer cervicis (BC), splenius (SP) or rectus capitis posterior (RCP). Reflexes were assessed by comparing levels of EMG activity of muscles before and after each stimulus, as the cats lapped milk, licked their paws or walked on a treadmill. When BC or SP nerves were stimulated at 1.5-4 times threshold (T) for their motor axons, no short-latency heteronymous reflexes could be identified in most neck muscles. However, stimulation of RCP nerves produced inhibitory effects as early as 3-4 ms in the ipsilateral BC, CM, and SP muscles and 6 ms in contralateral BC. At stimulus strengths above 4xT, a more complex pattern of inhibitory or excitatory effects was observed in CM, SP and the intervertebral muscle spinalis dorsi. The reflex effects were attenuated or abolished by partial or complete C1 dorsal rhizotomy (2 cats). Cervicocollic reflex data may need to be reevaluated to consider the possible effects of disinhibition rather than excitation in short-latency reflex pathways.

PATHOGENESIS OF SUXAMETHONIUM-INDUCED MUSCLE DAMAGE IN THE BIVENTER CERVICIS MUSCLE IN THE CHICK

Muscle damage induced by suxamethonium, and the influence of halothane on it, has been examined by measuring the efflux of creatine kinase (CK) in the biventer cervicis muscle of the chick. Whereas halothane and suxamethonium alone did not increase the enzyme efflux significantly, the combination of the two was associated with significant increase in the concentration of CK in the bathing medium by 59-157%. The increase in CK was prevented by adding chlorpromazine 100 mumol litre-1 to the medium, suggesting the involvement of phospholipases in the pathogenesis of suxamethonium-induced muscle damage.

Differential sensitivities of avian and mammalian neuromuscular junctions to inhibition of cholinergic transmission by ω-conotoxin GVIA

Nerve stimulation-induced contractions of the chick biventer cervicis muscle were slowly reduced by ω-conotoxin. However, ω-conotoxin had no effect on skeletal muscle function after i.v. injection in mice or on nerve stimulation-induced contractions of focally innervated muscle of the rat diaphragm or the rabbit proximal oesophagus, or the multiply innervated extra-ocular rectus muscle from rabbit. The lack of effect of ω-conotoxin on mammalian neuromuscular junctions was not due to the high safety factor in transmission or to a high local concentration of Ca 2+ originating from the muscle, and could not be accounted for in terms of the operation of facilitatory or inhibitory feedback modulation of transmitter release from motoneurone terminals. It is concluded that the Ca 2+ channels of mammalian motoneurone terminals differ from those of avian motoneurone terminals and other ω-conotoxin-sensitive nerve terminals.

Use of the biventer cervicis nerve-muscle preparation to detect early changes following exposure to organophosphates inducing delayed neuropathy

Indices of organophosphorus (OP)-induced delayed neuropathy (OPIDN) in the hen model have traditionally been restricted to the early inhibition of neuropathy target esterase (NTE) and ataxia with associated pathological changes in hind limb peripheral nerve which occur more than 7 days after OP exposure. The biventer cervicis nerve-muscle preparation was used to evaluate OPIDN in adult hens at various time periods after treatment with either the protoxicant tri- o-tolyl phosphate (TOTP), 360 mg/kg po, or the active congener phenyl saligenin phosphate (PSP), 2.5 mg/kg im. NTE activity was 21 and 48% of control for TOTP and PSP, respectively, 4 days after administration. Clinical signs were notable by 10 days and progressed in severity to paralysis by 21 days. Partial clinical recovery was evident at 37 days. Denervation hypersensitivity of biventer cervicis muscle to acetylcholine (ACh) was evident as early as 4 days following TOTP or PSP treatment. The sensitivity to ACh was greatest 21 days after OP administration, with partial recovery at 37 days. Strength-duration curves (SDC) of preparations from OP-treated hens showed an increase in excitability thresholds and elevated rheobase with shorter chronaxie than did preparations from controls as early as 4 days following treatment with either compound. SDC at 37 days indicated partial reinnervation. Peripheral nerve myelinated fiber degeneration and regeneration consistent with these physiological changes was seen on histopathological examination. This study suggests that the biventer cervicis nerve-muscle preparation may prove useful for detection of functional and morphological changes that occur during the interval between NTE inhibition and appearance of clinical deficits.

Neostigmine's effect on metoclopramide-induced aldosterone secretion in the presence of atropine or pirenzepine

K. S. Y. Low, M. C. E. Gwee, R. Yuen, P. Gopalakrishnakone and H. E. Khoo. Stonustoxin: effects on neuromuscular function in vitro and in vivo. Toxicon 32, 573–581, 1994.—Stonustoxin (8–50 μg/ml) produced a rapid and concentration-dependent rise in tension (contracture) of the electrically stimulated mouse hemidiaphragm followed by a gradual waning of tension from the peak to the baseline; the nerve-evoked and the directly (muscle)-evoked twitches of the hemidiaphragm were also progressively and irreversibly blocked in a time- and concentration-dependent manner. Stonustoxin (22 and 44 μg/ml) produced a similar rapid rise in tension of the chick biventer cervicis muscle as well as irreversible and concentration-dependent blockade of nerve-evoked twitches and contractures produced by acetylcholine (200 μM), carbachol (8 μM) and KCl (40 mM). The muscle contracture produced by stonustoxin was blocked by dantrolene sodium (6 μM) but not by tubocurarine (15 μM). Moreover, stonustoxin (40 μg/ml) did not inhibit nerve conduction in the toad sciatic nerve and stonustoxin (60 μg/ml) did not exhibit any anticholinesterase activity. The inhibition of neuromuscular function by stonustoxin in the mouse hemidiaphragm and chick biventer cervicis muscle can therefore be attributed to some irreversible myotoxic action(s) of the toxin, whereas the stonustoxin-induced muscle contractures could have been mediated via depolarization of muscle fibres.

Monosynaptic excitatory connexions of reticulospinal neurones in the nucleus reticularis pontis caudalis with dorsal neck motoneurones in the cat

1. Projections of reticulospinal neurones (RSNs) in the nucleus reticularis pontis caudalis (N.r.p.c.) to dorsal neck motoneurones supplying splenius (SPL, lateral head flexor) and biventer cervicis and complexus (BCC, head elevator) muscles were studied in the cat anaesthetized with pentobarbiturate or alpha-chloralose. 2. Threshold mapping for evoking antidromic spikes revealed that most of RSNs tested projecting down to brachial segments but not to lumbar segments (C-RSNs) gave off collaterals to the gray matter of the upper spinal cord in C2-C3 segments. 3. Spike triggered averaging showed that negative field potentials were evoked after firing of a single C-RSN (single fibre focal synaptic potentials, FSPs) in the region of C2-C3 where large antidromic field potentials from nerves supplying SPL or BCC muscles were evoked. The single fibre FSPs ranged between 1 and 10 microV in amplitude and had latencies between 0.7 and 1.2 ms from the onset of the triggering spike. In most cases, a presynaptic spike preceded the negative potential by 0.3 ms. These results indicated that C-RSNs project to the SPL or BCC motor nucleus. 4. Spike triggered averaging of postsynaptic potentials revealed EPSPs (single fibre EPSPs) in 36 dorsal neck motoneurones, predominantly in SPL (25) and less in BCC (11) motoneurones, evoked from 15 C-RSNs. The amplitude of the single fibre EPSPs ranged from 5 to 310 microV, and had latencies of 0.8-2.0 ms from the onset of the triggering spikes of C-RSNs, or 0.3-0.5 ms from the presynaptic spike when recorded. The results indicated monosynaptic excitatory connexions of C-RSNs to dorsal neck motoneurones. 5. Single fibre EPSPs from a C-RSN were usually recorded from either BCC or SPL motoneurones but not from both types of motoneurones, when tested in many motoneurones. This showed that connexions of C-RSNs with dorsal neck motoneurones were muscle specific. 6. RSNs projecting down to the lumbar segment (L-RSN) also showed branching in C2-C3 segments. Excitatory monosynaptic connexion of L-RSNs with neck motoneurones were demonstrated by recording single fibre postsynaptic population potentials (p.s.p.p.s.) from the C2 ventral root perfused with sucrose. The probability of evoking monosynaptic single fibre p.s.p.p.s. was less (19%) than for C-RSNs (59%).

Nicardipine inhibits axon conduction but causes dual changes of acetylcholine release in the mouse motor nerve

The effects of nicardipine, a dihydropyridine Ca2(+)-channel antagonist, on neuromuscular transmission and impulse-evoked release of acetylcholine were compared with those of nifedipine. In the isolated mouse phrenic nerve diaphragm, nicardipine (50 microM), but not nifedipine (100 microM), induced neuromuscular block, fade of tetanic contraction, and dropout or all-or-none block of end-plate potentials. Nicardipine had no significant effect on the resting membrane potential and the amplitude of miniature end-plate potentials but increased the frequency and caused the appearance of large size miniature potentials. The quantal contents of evoked end-plate potentials were increased. In the presence of tubocurarine, however, nicardipine depressed the amplitude of end-plate potentials. The compound nerve action potential was also decreased. It is concluded that nicardipine blocks neuromuscular transmission by acting on Na+ channels and inhibits axonal conduction. Nicardipine appeared to affect the evoked release of acetylcholine by dual mechanisms, i.e., an enhancement presumably by an agonist action on Ca2+ channels, like Bay K 8644 and nifedipine, and inhibition by an effect on Na+ channels, like verapamil and diltiazem. In contrast with its inactivity on the amplitude of miniature end-plate potentials, depolarization of the end plate in response to succinylcholine was greatly depressed. The contractile response of baby chick biventer cervicis muscle to exogenous acetylcholine was noncompetitively antagonized by nicardipine (10 microM), but was unaffected by nifedipine (30 microM). These results may implicate that nicardipine blocks the postsynaptic acetylcholine receptor channel by enhancing receptor desensitization or by a use-dependent effect.

Pharmacological actions of the gut active principles in Croton penduliflorus hutch. Seed on isolated smooth and skeletal muscles and on blood pressure

AbstractThe effects of Croton penduliflorus seed crystals (CP crystals) were investigated on isolated guinea‐pig ileum, chick biventer cervicis muscle and frog rectus abdominis muscle. The effect of the crystals on flood pressure of normotensive Sprague Dawley rats was also investigated. CP crystals induced concentrations‐dependent contractions in the guinea‐pig ileum with EC50 of 2.39 × 106 g/mL. Contractions induced by CP crystals in the guinea‐pig ileum were not inhibited by atropine (2 × 10−7‐1.2 × 10−6 g/mL), tetrodotoxin (3 × 10−8‐2 × 10−7 g/mL), hexamethonium (7 × 10−6‐2.8 × 10−5 g/mL), mepyramine (2 × 10−7 g/mL) and noradrenaline (2 × 10−5 g/mL) but were completely (100%) inhibited by indomethacin (7.2 × 10−6 g/mL). CP crystals (1.0 × 10−6‐5.0 × 10−4 g/mL) could not elicit contractions in either chick biventer cervicis or frog rectus abdominis muscle. The mean blood pressure of normotensive Sprague Dawley rats was significantly reduced by CP crystals (1.6 × 10−4‐2.5 × 10−3 g/mL) while the heart rate was significantly reduced by an intravenous infusion of the crystals (2.5 × 10−3 g/kg) after 2min. The methods employed and the significance of the results obtained are discussed.

Chapter 15 Interaction between vestibulocollic and cervicocollic reflexes: automatic compensation of reflex gain by muscle afferents

Recent experiments on the sagittal VCR in the dece-rebrate cat have examined the possible contribution from otolith afferents to this reflex, and the interaction of the sagittal VCR with afferent inputs from muscle stretch receptors in the neck muscles. The role of otolith afferents has been investigated by comparing, during very slow sinusoidal movements of the head, the response of the sagittal VCR with that known for the horizontal (yaw) VCR. Unlike the horizontal VCR whose ability to follow low frequencies of head movement is limited by the time constant of integration of horizontal semicircular canal inputs, the sagittal VCR shows relatively constant gain and phase at frequencies between 0.015 and 0.1 Hz. This indicates a functionally significant role at low frequencies for an input signal related to actual head position, presumably from otolith afferents whose activity is modulated by head movement in the sagittal (pitch) plane but not by horizontal (yaw) movements. Separate experiments in which the biventer cervicis muscles were systematically stretched or shortened during 1 Hz vestibular stimulation have shown that the sagittal VCR interacts powerfully with afferent inputs from stretch receptors (presumably muscle spindles) in these muscles. The reflex response to a given head movement is amplified severalfold if the muscles are lengthening during vestibular stimulation, or attenuated and even abolished if they are shortening. The combined reflex response to head movement and muscle stretching is estimated reasonably well by a linear addition between the VCR and the CCR evoked by sinusoidal stretching alone.

The role of acetylcholine in tetraethylammonium induced contractures of the chick biventer cervicis muscle in the presence of lidocaine

1. 1. Lidocaine (0.92 mM) potentiated tetraethylammonium (TEA, 0.6–6 mM) induced contractures of the chick biventer cervicis muscle (BVC) in vitro. The dose ratio for TEA (EC 50 in lidocaine/EC 50 control) was 1.16 × 10 −3. 2. 2. Lidocaine (0.92 mM) blocked nerve and muscle action potentials in the BVC preparation, blocked slow fibre MEPPS, and blocked indirectly elicited muscle contraction. 3. 3. Lidocaine (0.92 mM) non-competitively blocked acetylcholine (ACh) induced contractures of the chick BVC. 4. 4. Gallamine competitively blocked ACh contractures (p A 2 6.53) but produced only a relatively weak non-competitive block of TEA induced contractures in lidocaine. 5. 5. TEA induced contractures of the chick BVC in the presence of lidocaine probably do not involve ACh.

A reappraisal of the evidence for a direct action of tetraethylammonium on the chick biventer cervicis muscle equilibrated with lidocaine

1. 1. Contractures of the chick biventer cervicis muscle were recorded in vitro in response to exogenous acetylcholine (ACh) and to tetraethylammonium (TEA) and concentration response curves (CRC) produced for these drugs. 2. 2. In BVC muscles equilibrated with lidocaine (1.07 mM), physostigmine (3.63 μM) blocked TEA induced contractures in a reversible non-competitive manner. It had a similar action on ACh induced contractures. 3. 3. Lidocaine (1.07 mM) had no action on the ACh CRC except that it reduced the response to the highest concentration of ACh tested. 4. 4. The data of this and of previous experiments was analyzed and a theory proposed to account for TEA contractures. 5. 5. It suggested that background extravesicular ACh release results in the passage of K + into the transverse tubular system (TT). 6. 6. In the presence of TEA blocking the K + channels accumulation of K + in the TT no longer occurs. 7. 7. Na +/K + exchange is depressed in favour of Na +/Ca 2+ exchange and the resulting rise in intracellular Ca 2+ acting on the sarcoplasmic reticulum leads to contraction. 8. 8. Lidocaine potentiates the contraction by blocking Ca 2+ uptake into the sarcoplasmic reticulum.

A potent factor in extracts of the skin of the Australian frog, PSeudophryne coriacea—III : Potentiation of contractions elicited in avian and mammalian isolated skeletal muscle preparations by direct and indirect electrical stimulation

Extracts of the skin of the Australian frog Pseudophryne coriacea displayed a striking potentiating effect on contractions evoked in isolated skeletal muscle preparations of mammals (phrenic nerve diaphragm) and birds (chick biventer cervicis and semispinalis muscles) by indirect and direct electrical stimulation. There was both a conspicuous increase in the amplitude of the twitch, up to 10-fold, and a remarkable prolongation of the duration of the twitch. The effect was dose- and frequency-dependent. In the presence of the extract, fusion of twitches after tetanic stimulation occurred earlier. No tachyphylaxis upon repeated stimulation by the extract was observed and the response to large doses persisted, declining slowly, for hours. These effects must be ascribed to an alkaloid related in structure to pumiliotoxin B. Response to the extract of Pseudophryne coriacea by indirectly-stimulated preparations was potentiated by physostigmine and blocked by tubocurarine and α-bungarotoxin, demonstrating that in these preparations the extract acted pre-synaptically to facilitate the release of acetylcholine from motor nerve endings. However, the extract of Pseudophryne coriacea displayed equally potent effects in directly stimulated preparations, insensitive to physostigmine and to blockers of nicotinic acetylcholine receptors, indicating a direct action on the skeletal muscle. It is suggested that, like pumiliotoxin B, the Pseudophryne coriacea alkaloid may interfere in the regulation of calcium channels in both nerve and muscle fibres.