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At least 523 records · Page 29Linked to original sources
PubMed1983

Muscle activity and the loss of electrical coupling between striated muscle cells in Xenopus embryos.

The gap junctions between embryonic striated muscle cells are lost during development. The time course of their elimination has been examined with electrophysiological techniques in myotomes of Xenopus laevis embryos. Gap junctions were detected by the passage of electronic current or the fluorescent dye, Lucifer Yellow, from one muscle cell to another. These tracers only spread to neighboring cells when injected intracellularly. All the muscle cells are electrically coupled at stage 24 when neuromuscular transmission begins, but normally many cells become uncoupled during the next 48 hr. In contrast, the muscle cells remain electrically coupled if neuromuscular transmission is blocked during that period with tricaine or alpha-bungarotoxin. When muscle activity recovers, the loss of coupling resumes. Once the coupling has disappeared, neuromuscular blockade does not restore it. Muscle contraction is blocked during development in a mutant of Xenopus, even though the muscle cells remain electrically excitable. After stage 32 in these immobile embryos, the muscle cells are stimulated repeatedly by regular bursts of neural activity. Although they never contract, the mutant muscle cells become uncoupled at the same time as the muscle cells in normal embryos. The results suggest that some consequence of repeated cholinergic activation, other than contraction, stimulates the loss of gap junctions between striated muscle cells during development. The elimination of gap junctions may be required for neural control of subsequent muscle differentiation.

Aminobenzoates↗
PubMedDate not supplied

Muscle fibre orientation of abdominal muscles and suggested surface EMG electrode positions.

The ability of surface electrodes to accurately detect the activity of a particular muscle relies not only on their being placed over the muscle but also on their position in relation to muscle fibre orientation. For optimal pick-up of electromyographic (EMG) signals, surface electrodes are best aligned in parallel with the fibre orientation of the underlying muscle. This study aimed to measure muscle fibre orientation and other parameters of muscle morphology of the abdominal muscles in relation to palpable bony landmarks. Thirty-seven embalmed cadavers (19 males and 18 females) were examined. Results showed that the fibres of obliquus externus abdominis were about 4 degrees more vertical than the lower edge of the eighth rib. Below the rib cage, the muscle fibres of obliquus externus abdominis were approximately 5 degrees closer to vertical than a reference line between the most inferior point of the costal margin and the contralateral pubic tubercle. In the anterolateral abdominal wall area below the anterior superior iliac spine (ASIS), the obliquus internus abdominis was superficial being covered only by the aponeurosis of obliquus externus abdominis. At the level of ASIS, the muscle fibres of obliquus internus abdominis were almost horizontally orientated but at 2 cm below ASIS were aligned about 6 degrees inferomedially to the horizontal. The muscle fibres of upper rectus abdominis were 2 degrees inferolateral to the midline while the lower rectus abdominis muscle fibres deviated inferomedially from the midline by about 8 degrees. The appropriate surface electrode placements which follows the muscle fibre orientation of the obliquus externus abdominis, obliquus internus abdominis and rectus abdominis have been suggested.

Abdominal Muscles↗
PubMed2003

Improvement of muscle healing through enhancement of muscle regeneration and prevention of fibrosis.

Skeletal muscle is able to repair itself through regeneration. However, an injured muscle often does not fully recover its strength because complete muscle regeneration is hindered by the development of fibrosis. Biological approaches to improve muscle healing by enhancing muscle regeneration and reducing the formation of fibrosis are being investigated. Previously, we have determined that insulin-like growth factor-1 (IGF-1) can improve muscle regeneration in injured muscle. We also have investigated the use of an antifibrotic agent, decorin, to reduce muscle fibrosis following injury. The aim of this study was to combine these two therapeutic methods in an attempt to develop a new biological approach to promote efficient healing and recovery of strength after muscle injuries. Our findings indicate that further improvement in the healing of muscle lacerations is attained histologically by the combined administration of IGF-1 to enhance muscle regeneration and decorin to reduce the formation of fibrosis. This improvement was not associated with improved responses to physiological testing, at least at the time-points tested in this study.

Animals↗
PubMed2001

Differences in myosin heavy-chain composition between human jaw-closing muscles and supra- and infrahyoid muscles.

Jaw-closing muscles have architectural features suited to force production; supra- and infrahyoid muscles are better adapted to produce velocity and displacement. It was hypothesized that this difference in function would be reflected in myosin heavy-chain (MyHC) composition (equivalent to contraction velocity) and fibre-type cross-sectional area (equivalent to force). MyHC composition was determined in muscles obtained from eight human cadavers, using monoclonal antibodies against MyHC isoforms. Jaw closers contained 4.2 times fewer type IIA fibres and 5.2 times more hybrid fibres than suprahyoid muscles, and 3.9 times fewer type IIA fibres and 3.2 times more hybrid fibres than the infrahyoid muscles. In the jaw closers, MyHC-I was expressed in approx. 70% of all fibres (pure+hybrid), in the suprahyoid muscles in approx. 40%, and in the infrahyoid muscles in approx. 46%. In the jaw closers, type I fibres were 40% larger in diameter than in the supra- and infrahyoid muscles. It can be concluded that the jaw closers have characteristics of slow muscles, and that the supra-/infrahyoid muscles have characteristics of fast muscles.

Aged↗
PubMed1999

Denervation of rabbit gastrocnemius and soleus muscles: effect on muscle-specific enolase.

We report here, for the first time, the expression of the muscle-specific isoform of the glycolytic enzyme, enolase (EC 4.2.1. 11) (beta enolase), in rabbit skeletal muscles. We have analysed the fast-twitch gastrocnemius and the slow-twitch soleus muscles during normal postnatal development and following denervation. We show that, in rabbit, as already described in rodents, beta enolase gene expression behaves as a good marker of the fast-twitch fibers. In soleus muscle, the beta enolase transcript level is 10-20% of that found in gastrocnemius. Denervation, performed at 8 postnatal days, induces an important drop of beta enolase transcript levels in both developing soleus and gastrocnemius muscles, with a 80% decrease observed 1 week after denervation in the operated muscles, as compared to the corresponding contralateral muscles. Thereafter, the beta enolase transcript level continues to decrease in the fast-twitch muscle, with the beta enolase subunit being detectable only in the atrophic fast-twitch fibers. In contrast, the beta transcript level tends to increase in the denervated slow-twitch muscle, reaching about 50% of that in contralateral soleus, at 7 weeks after surgery. The level of beta enolase transcripts still expressed after denervation seems to stabilize at the same low level in both types of inactive muscles. This suggests that the small fraction of beta enolase expression which is not controlled by the nerve, or by the contractile activity imposed by it, is independent of the muscle phenotype.

Animals↗
PubMed2005

Measurement of peripheral muscle resistance in rats with chronic ischemia-induced paraplegia or morphine-induced rigidity using a semi-automated computer-controlled muscle resistance meter.

In experimental and clinical studies, an objective assessment of peripheral muscle resistance represents one of the key elements in determining the efficacy of therapeutic manipulations (e.g. pharmacological, surgical) aimed to ameliorate clinical signs of spasticity and/or rigidity. In the present study, we characterize a newly developed limb flexion resistance meter which permits a semi-automated, computer-controlled measurement of peripheral muscle resistance (PMR) in the lower extremities during a forced flexion of the ankle in the awake rat. Ischemic paraplegia was induced in Sprague-Dawley rats by transient aortic occlusion (10 min) in combination with systemic hypotension (40 mm Hg). After ischemia the presence of spasticity component was determined by the presence of an exaggerated EMG activity recorded from gastrocnemius muscle after nociceptive or proprioceptive afferent activation and by velocity-dependent increase in muscle resistance. Rigidity was induced by high dose (30 mg/kg, i.p.) of morphine. Animals with defined ischemic spasticity or morphine-induced rigidity were then placed into a plastic restrainer and a hind paw attached by a tape to a metal plate driven by a computer-controlled stepping motor equipped with a resistance transducer. The resistance of the ankle to rotation was measured under several testing paradigms: (i) variable degree of ankle flexion (40 degrees, 50 degrees, and 60 degrees), (ii) variable speed/rate of ankle flexion (2, 3, and 4 sec), (iii) the effect of inhalation anesthesia, (iv) the effect of intrathecal baclofen, (v) the effect of dorsal L2-L5 rhizotomy, or (vi) systemic naloxone treatment. In animals with ischemic paraplegia an increased EMG response after peripheral nociceptive or proprioceptive activation was measured. In control animals average muscle resistance was 78 mN and was significantly increased in animals with ischemic spasticity (981-7900 mN). In ischemic-spastic animals a significant increase in measured muscle resistance was seen after increased velocity (4 > 3 > 2 sec) and the angle (40 degrees > 50 degrees > 60 degrees) of the ankle rotation. In spastic animals, deep halothane anesthesia, intrathecal baclofen or dorsal rhizotomy decreased muscle resistance to 39-80% of pretreatment values. Systemic treatment with morphine induced muscle rigidity and corresponding increase in muscle resistance. Morphine-induced increase in muscle resistance was independent on the velocity of the ankle rotation and was reversed by naloxone. These data show that by using this system it is possible to objectively measure the degree of peripheral muscle resistance. The use of this system may represent a simple and effective experimental tool in screening new pharmacological compounds and/or surgical manipulations targeted to modulate spasticity and/or rigidity after a variety of neurological disorders such as spinal cord traumatic or ischemic injury, multiple sclerosis, cerebral palsy, or Parkinson's disease.

Anesthetics, Inhalation↗
PubMed2001

Mutually exclusive muscle designs: the power output of the locomotory and sonic muscles of the oyster toadfish (Opsanus tau).

Animals perform a vast array of motor activities. Although it has generally been accepted that muscles are well suited to the function that they must perform, specialization for performing one function may compromise their ability for carrying out another. We examined this principle in the toadfish muscular system: slow-twitch red and fast-twitch white myotomal muscles are used for powering swimming at relatively low frequencies, while the superfast swimbladder muscle powers mating calls by contracting at 100 Hz. We measured muscle power output over a wide range of frequencies. The red and white locomotory muscles could not generate power over ca. 2.2 and 12 Hz, respectively and, hence, could not power sound production. In contrast, the swimbladder muscle has many specializations that permit it to generate power at frequencies in excess of 100 Hz. However, these specializations drastically reduce its power output at low frequencies: the swimbladder muscle generated only one-twentieth of the power of the red muscle and one-seventh of the power of the white muscle at the frequencies used during swimming. To generate the same total power needed for swimming would require unfeasibly large amounts of swimbladder muscle that could not fit into the fish. Hence, the designs of the swimbladder and locomotory muscles are mutually exclusive.

Action Potentials↗
PubMed1998

Incomplete rematching of nerve and muscle properties in motor units after extensive nerve injuries in cat hindlimb muscle.

1. Motor units were characterized in partially denervated or completely denervated and reinnervated cat medial gastrocnemius (MG) muscles where the number of innervating motor axons was severely reduced to determine (1) to what extent the nerve and muscle properties are rematched in enlarged motor units, (2) whether the normal size relationships between axon size, unit tetanic force and contractile speed are re-established, and (3) whether the type of nerve injury and/or repair affects the re-establishment of nerve and muscle properties. 2. Single MG units were sampled in (1) partially denervated muscles and in reinnervated muscles after either (2) crushing or (3) transecting the nerve and suturing its proximal end to either the distal nerve stump (N-N), or (4) directly to the muscle fascia (N-M). 3. The majority (75-88 %) of motor units in all muscles were classified as S (slow), FR (fast fatigue resistant), FI (fast fatigue intermediate) and FF (fast fatigable). However, there was an increased number of FI and unclassifiable motor units compared to normal. These results suggest that motor unit properties are not entirely regulated by the reinnervating motoneurone. 4. Despite more overlap in the range of unit force between different motor unit types the tetanic force of each type increased in all muscles when reinnervated by few (< 50 %) motor axons. This increase in unit force was due to an expansion in motor unit innervation ratio. 5. The normal relationships between axon size, unit tetanic force, and contractile speed were re-established in all muscles except when reinnervated by < 50 % of their normal complement of motor units after N-M suture. This lack of correlation was due to the reduced fast glycolytic (FG) fibre size and the proportionately greater increase in force of the S units. 6. After reinnervation the ranges in fibre cross-sectional area within single FF units were very similar to those found within the entire FG fibre population. 7. These results show that when few axons make functional connections in partially denervated or reinnervated muscles the normal relationships between axon size and motor unit contractile properties are re-established provided the nerves regenerate within the distal nerve sheath. This rematching of motoneurone size and motor unit contractile properties occurs primarily because the size of the motor axon governs the number of muscle fibres it supplies.

Animals↗
PubMed1997

Fast-to-slow conversion following chronic low-frequency activation of medial gastrocnemius muscle in cats. I. Muscle and motor unit properties.

This study of cat medial gastrocnemius (MG) muscle and motor unit (MU) properties tests the hypothesis that the normal ranges of MU contractile force, endurance, and speed are directly associated with the amount of neuromuscular activity normally experienced by each MU. We synchronously activated all MUs in the MG muscle with the same activity (20 Hz in a 50% duty cycle) and asked whether conversion of whole muscle contractile properties is associated with loss of the normal heterogeneity in MU properties. Chronically implanted cuff electrodes on the nerve to MG muscle were used for 24-h/day stimulation and for monitoring progressive changes in contractile force, endurance, and speed by periodic recording of maximal isometric twitch and tetanic contractions under halothane anesthesia. Chronic low-frequency stimulation slowed muscle contractions and made them weaker, and increased muscle endurance. The most rapid and least variable response to stimulation was a decline in force output of the muscle and constituent MUs. Fatigue resistance increased more slowly, whereas the increase in time to peak force varied most widely between animals and occurred with a longer time course than either force or endurance. Changes in contractile force, endurance, and speed of the whole MG muscle accurately reflected changes in the properties of the constituent MUs both in extent and time course. Normally there is a 100-fold range in tetanic force and a 10-fold range in fatigue indexes and twitch time to peak force. After chronic stimulation, the range in these properties was significantly reduced and, even in MU samples from single animals, the range was shown to correspond with the slow (type S) MUs of the normal MG. In no case was the range reduced to less than the type S range. The same results were obtained when the same chronic stimulation pattern of 20 Hz/50% duty cycle was imposed on paralyzed muscles after hemisection and unilateral deafferentation. The findings that the properties of MUs still varied within the normal range of type S MUs and were still heterogeneous despite a decline in the variance in any one property indicate that the neuromuscular activity can account only in part for the wide range of muscle properties. It is concluded that the normal range of properties within MU types reflects an intrinsic regulation of properties in the multinucleated muscle fibers.

Afferent Pathways↗
PubMed1998

Nerve-muscle interactions during flight muscle development in Drosophila.

During Drosophila pupal metamorphosis, the motoneurons and muscles differentiate synchronously, providing an opportunity for extensive intercellular regulation during synapse formation. We examined the existence of such interactions by developmentally delaying or permanently eliminating synaptic partners during the formation of indirect flight muscles. When we experimentally delayed muscle development, we found that although adult-specific primary motoneuron branching still occurred, the higher order (synaptic) branching was suspended until the delayed muscle fibers reached a favourable developmental state. In reciprocal experiments we found that denervation caused a decrease in the myoblast pool. Furthermore, the formation of certain muscle fibers (dorsoventral muscles) was specifically blocked. Exceptions were the adult muscles that use larval muscle fibers as myoblast fusion targets (dorsal longitudinal muscles). However, when these muscles were experimentally compelled to develop without their larval precursors, they showed an absolute dependence on the motoneurons for their formation. These data show that the size of the myoblast pool and early events in fiber formation depend on the presence of the nerve, and that, conversely, peripheral arbor development and synaptogenesis is closely synchronized with the developmental state of the muscle.

Animals↗