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PubMedDate not supplied

Sensible manual muscle strength testing to evaluate and monitor strength of the intrinsic muscles of the hand: a commentary.

Hand therapists often assess, evaluate, and monitor the status of, and changes in the strength of, the intrinsic muscles of the hand. Some common indications are peripheral and central neuropathies and nerve lacerations and repairs. The therapist will often use a muscle chart that lists all the muscles innervated by the ulnar and median nerves, and all muscles will be tested. Not all muscles in the hand can be sufficiently isolated to grade their strength, nor is it always necessary to test all muscles innervated by a particular nerve to evaluate the presence or extent of motor function impairment or monitor changes. This paper discusses the tests by which changes in strength of the ulnar and median innervated intrinsic muscles can be assessed and the reasons that certain muscles cannot or need not be tested. Information about the reliability of muscle testing is also given.

Exercise Test↗

PubMed2003

Muscle activation patterns of selected lower extremity muscles during stepping and cutting tasks.

Lower extremity muscle activations during crossover and side step cut tasks are hypothesized to play an important role in controlling knee motion, and therefore, impact the design of knee injury prevention and rehabilitation programs. However, the contribution of lower extremity muscles to frontal and transverse plane moments during cutting tasks is unclear. The purpose of this study was to compare the muscle activation patterns of selected lower extremity muscles (vastus lateralis, medial/lateral hamstrings and medial/lateral gastrocnemius) of subjects performing a stepping down and side step cut, a stepping down and crossover cut and an equivalent straight ahead task. Ground reaction force was used to determine the cut angle, stance time and compare the lower limb loading during each task. Electromyography data during all tasks were normalized to the average activation during the straight ahead tasks to determine relative changes in muscle activation between the straight ahead and different cut styles (crossover and side step). There were no differences in the pattern of muscle activation of the vastus lateralis, or lateral hamstring muscles when comparing the cutting tasks to the equivalent straight ahead task. However, the crossover cut task resulted in significantly higher muscle activation of the medial hamstrings and lateral gastrocnemius muscles relative to both the side step cut and straight ahead tasks. These results suggest the medial/lateral hamstrings and medial/lateral gastrocnemius play a role in transverse and frontal plane control during cut tasks.

Adult↗

PubMed2002

Horse lumbrical muscle: possible structural and functional reorganization in regressive muscle.

An anatomical study of horse lumbrical muscle (Lm) was carried out by light and electron microscopy in combination with immunochemical and cytochemical methods. Paraffin sections were subjected to haematoxylin and eosin (H & E) and Masson's trichrome staining for morphometric analysis. Paraffin sections were also used for immunostaining by anti-PGP 9.5 for reaction with nerve-protein associated-structures, anti-heat-shock protein 70 (hsp 70) for detection of gene expression changes, anti-fast myosin for the determination of muscle fibre types, and for detection of apoptotic gene expression of muscle fibres by the TUNEL method. The distribution of muscle fibre types on frozen sections was also examined by assaying ATPase (pH 4.2). We found that the proximal end of the tendon of the unipennate-shaped Lm binds to the deep digital flexor tendon, and the distal end of the Lm tendon connects to the medial surface of the palmar annular ligament. The Lm was not always present, but when found it varied in length greatly, up to 8 cm (muscle part alone), and weighed less than 1 g. The Lm was white, pale, or reddish in colour depending on the ratio of muscle fibre and connective tissue contents. The semi-tendinized regressive Lm was composed of rich vasculature, peripheral nerves, and nerve-like organs similar to the neuromuscular spindle (NMS). The extrafusal muscle fibres (e-lm) that surround the NMS were replaced with a thick outer capsule of connective tissues (CT) in the Lm nerve-like organ, which we named the neurotendinous capsule (NTC) organ. NTC organs exist alone or as multiple structures (up to eight) surrounded by a common outer capsule at the outermost CT ring. The NTC possesses several intrafusal muscle fibres (ifm) just as the NMS does. That the ifm was associated with nerve endings was confirmed by anti-PGP 9.5 and electron microscopic observation. Some muscle fibres in ifm and e-lm reacted with anti-fast twitch myosin and with anti-hsp 70. The e-lm exhibited at least two fibre types, determined by ATPase (pH 4.2) assay. The ifm exhibited mainly type I (slow twitch) fibres. No apoptotic gene expression was detected in either ifm or e-lm, suggesting the Lm is a vital organ. The degenerating fibres observed in ifm and e-lm indicate that the turnover rate of cytoplasmic components is accelerated. We attribute this phenomenon to the necessity for adaptation to new environmental demands. The surprising finding of tubular aggregates (TAs) in ifm of the NTC organ suggests that the Lm is continuously adapting. Some results related to variation in diameter of the collagen fibrils, isolation of the NTC organ and the myofibrillar protein constituents are also discussed. In conclusion, the so-called regressive Lm has rich vasculature, many peripheral nerves, and newly described NTC organs. The induction of heat-shock protein, lack of apoptotic gene expression in ifm and e-lm fibres, and TA formation in ifm suggest that horse Lm responds to environmental stress through reorganization and/or remodelling of cell constituents. We hypothesize that the horse Lm has lost its original role as a contractile element and changed to another function, likely as a vital nerve organ.

Animals↗

PubMed1984

Development of human fetal muscles: a comparative histochemical analysis of the psoas and the quadriceps muscles.

The development of human muscle can be divided into three distinct phases on the basis of the enzyme histochemistry (Dubowitz 1965). On the other hand, muscles can be classified into trunk muscles innervated by the motoneurons in the anteromedial nucleus of the anterior horn and limb muscles innervated by those of the lateral nucleus. We compared histochemically the development of the psoas muscle as a trunk muscle with the quadriceps. Four legally aborted fetuses and sixteen autopsied preterm or full-term infants with conceptional ages ranging from sixteen to fourty-two weeks, served as subjects. Serial crossly cut frozen sections of the psoas and the quadriceps were stained with HE, modified Gomori-trichrome, NADH-TR, myofibrillar ATPase (routine, preincubation at pH 4.6, 4.3), and analyzed quantitatively. In the psoas and the quadriceps, all muscles consisted of undifferentiated type IIC fibers before the eighteenth week of gestation. Type I fiber began to appear around the eighteenth week in both muscles. Before the thirtieth week, both muscles comprised a great number of smaller type IIC fibers and scattered large type I fibers. The psoas had already a little more type I fibers than the quadriceps in this phase. After the thirtieth week, the percentage of type I fibers increased greatly to about 50% at term in the psoas, gradually to 20-23% at term in the quadriceps. Type IIA and IIB fibers appeared suddenly around the thirty-seventh week. Total percentage of these two type fibers increased at term to about 50% and 70% in the psoas and the quadriceps, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗

PubMed2002

Age effects on urethral striated muscle. II. Anatomic location of muscle loss.

OBJECTIVE: The purpose of the study was to measure the thickness and cross-sectional area of urethral muscle layers to identify localized striated muscle loss. STUDY DESIGN: The urethra and surrounding tissues from 25 female cadavers (mean age, 52 +/- 18 [SD] years; range, 15-80 years) were used for this study. Axial and median sagittal histologic sections were prepared. Median sagittal muscle layer thickness was measured every 10% of urethral length (each decile) in the dorsal wall (adjacent to the vagina) and ventral wall, beginning at the caudal margin of the detrusor muscle (0%) and ending at the caudal margin of the striated muscle (100%). In the midurethral cross-section, the thickness of each layer was measured along radial lines placed every 45 degrees with 0 degrees at the ventral midline and 180 degrees at the dorsal midline. RESULTS: In the median sagittal sections, striated muscle layers of urethras were thinner at the vesical neck in older women. In the ventral wall, it decreased by a mean of 18 to 23 microm (3.4%-4.3%; P <.001) per year at 10% to 30% of urethral wall length. Dorsal striated muscle layers were thinner at every decile by 11 to 16 microm (3.2%-4.3%; P <.05); their total cross-sectional areas decreased by 0.19 mm2) (3.8%) per year short ( P <.001). In the midurethral cross-sections, the muscle was thinner by 16 to 25 microm (1.5% and 4.6%; P <.05) at 90, 135, and 180 degrees. CONCLUSION: Striated muscle was lost at the bladder neck and along the dorsal wall of the urethra as women aged.

Adolescent↗

PubMed1996

The role of transcription enhancer factor-1 (TEF-1) related proteins in the formation of M-CAT binding complexes in muscle and non-muscle tissues.

M-CAT sites are required for the activity of many promoters in cardiac and skeletal muscle. M-CAT binding activity is muscle-enriched, but is found in many tissues and is immunologically related to the HeLa transcription enhancer factor-1 (TEF-1). TEF-1-related cDNAs (RTEF-1) have been cloned from chick heart. RTEF-1 mRNA is muscle-enriched, consistent with a role for RTEF-1 in the regulation of muscle-specific gene expression. Here, we have examined the tissue distribution of TEF-1-related proteins and of M-CAT binding activity by Western analysis and mobility shift polyacrylamide gel electrophoresis. TEF-1-related proteins of 57, 54 and 52 kDa were found in most tissues with the highest levels in muscle tissues. All of these TEF-1-related proteins bound M-CAT DNA and the 57- and 54-kDa TEF-1-related polypeptides were phosphorylated. Proteolytic digestion mapping showed that the 54-kDa TEF-1-related polypeptide is encoded by a different gene than the 52- and 57-kDa TEF-1-related polypeptides. A comparison of the migration and proteolytic digestion of the 54-kDa TEF-1-related polypeptide with proteins encoded by the cloned RTEF-1 cDNAs showed that the 54-kDa TEF-1-related polypeptide is encoded by RTEF-1A. High resolution mobility shift polyacrylamide gel electrophoresis showed multiple M-CAT binding activities in tissues. All of these activities contained TEF-1-related proteins. One protein-M-CAT DNA complex was muscle-enriched and was up-regulated upon differentiation of a skeletal muscle cell line. This complex contained the 54-kDa TEF-1-related polypeptide. Therefore, RTEF1-A protein is a component of a muscle-enriched transcription complex that forms on M-CAT sites and may play a key role in the regulation of transcription in muscle.

Amino Acid Sequence↗

PubMed1998

Comparison of eccentric knee extensor muscle actions at two muscle lengths on indices of damage and angle-specific force production in humans.

In this study, we investigated the effects of knee extensor length during eccentric exercise on indices of muscle damage and adaptation. Subjects (n = 7) performed two bouts of 75 maximal voluntary eccentric muscle actions at a knee joint angular velocity of 1.57 rad s(-1). One bout was performed at a short muscle length (bout S) with a knee joint range of motion of 2.79 to 1.40 radians (160 degrees to 80 degrees), and a second with the contralateral knee extensors at a long muscle length (bout L) with a range of motion of 2.01 to 0.7 radians (120 degrees to 40 degrees). The maximum voluntary contractile force (MVC) was measured before and 5 min after exercise, and again on days 3, 5, 7, 10 and 12, at knee angles of 160 degrees, 120 degrees and 80 degrees. Muscle soreness was measured before exercise and on each day after exercise. Serum creatine kinase activity was measured before exercise and on days 3, 5, 7, 10 and 12 post-exercise. The MVC declined after each bout (P < 0.01), with a greater decline after bout L (P < 0.05). Muscle soreness was higher relative to bout S on days 1, 2, 3, 5 and 6 (P < 0.05). Although serum creatine kinase activity was elevated after both exercise bouts (P < 0.01), there was no difference between bouts. Functional muscle damage markers and muscle soreness suggest greater damage after bout L. Post-exercise angle-specific force decrements suggest a transient increase in muscle length after bout L but not bout S.

Adaptation, Physiological↗

PubMed1993

Trichinella spiralis infected skeletal muscle cells arrest in G2/M and cease muscle gene expression.

Infection by Trichinella spiralis causes a variety of changes in skeletal muscle cells including the hypertrophy of nuclei and decreased expression of muscle specific proteins. Potential cellular processes leading to these changes were investigated. In synchronized muscle infections, [3H]thymidine was incorporated into infected cell nuclei from 2-5 days post infection. Labeled nuclei were stably integrated into the infected cell up to 60 days post infection and appear to originate from differentiated skeletal muscle nuclei present at the time of infection. These nuclei were further shown to contain a mean DNA content of approximately 4N, indicating that the [3H]thymidine uptake reflects DNA synthesis and subsequent long-term suspension of the infected cell in the cell cycle at G2/M. Associated with these changes, muscle specific gene transcripts were reduced to < 1- < 0.1% in the infected cell compared to normal muscle. Transcript levels of the muscle transcriptional regulatory factors myogenin, MyoD1, and Id were reduced to < 10, < 1, and increased approximately 250%, respectively, in the infected cell compared to normal muscle, indicating transcriptional inactivation of muscle genes. DNA synthesis in the infected cell may represent the initiation event which leads to expression of this infected cell phenotype.

Animals↗

PubMed2000

Clonal isolation of muscle-derived cells capable of enhancing muscle regeneration and bone healing.

Several recent studies suggest the isolation of stem cells in skeletal muscle, but the functional properties of these muscle-derived stem cells is still unclear. In the present study, we report the purification of muscle-derived stem cells from the mdx mouse, an animal model for Duchenne muscular dystrophy. We show that enrichment of desmin(+) cells using the preplate technique from mouse primary muscle cell culture also enriches a cell population expressing CD34 and Bcl-2. The CD34(+) cells and Bcl-2(+) cells were found to reside within the basal lamina, where satellite cells are normally found. Clonal isolation and characterization from this CD34(+)Bcl-2(+) enriched population yielded a putative muscle-derived stem cell, mc13, that is capable of differentiating into both myogenic and osteogenic lineage in vitro and in vivo. The mc13 cells are c-kit and CD45 negative and express: desmin, c-met and MNF, three markers expressed in early myogenic progenitors; Flk-1, a mouse homologue of KDR recently identified in humans as a key marker in hematopoietic cells with stem cell-like characteristics; and Sca-1, a marker for both skeletal muscle and hematopoietic stem cells. Intramuscular, and more importantly, intravenous injection of mc13 cells result in muscle regeneration and partial restoration of dystrophin in mdx mice. Transplantation of mc13 cells engineered to secrete osteogenic protein differentiate in osteogenic lineage and accelerate healing of a skull defect in SCID mice. Taken together, these results suggest the isolation of a population of muscle-derived stem cells capable of improving both muscle regeneration and bone healing.

Alkaline Phosphatase↗

PubMed1984

Components of Torpedo electric organ and muscle that cause aggregation of acetylcholine receptors on cultured muscle cells.

The synaptic portion of a muscle fiber's basal lamina sheath has molecules tightly bound to it that cause aggregation of acetylcholine receptors (AChRs) on regenerating myofibers. Since basal lamina and other extracellular matrix constituents are insoluble in isotonic saline and detergent solutions, insoluble detergent-extracted fractions of tissues receiving cholinergic input may provide an enriched source of the AChR-aggregating molecules for detailed characterization. Here we demonstrate that such an insoluble fraction from Torpedo electric organ, a tissue with a high concentration of cholinergic synapses, causes AChRs on cultured chick muscle cells to aggregate. We have partially characterized the insoluble fraction, examined the response of muscle cells to it, and devised ways of extracting the active components with a view toward purifying them and learning whether they are similar to those in the basal lamina at the neuromuscular junction. The insoluble fraction from the electric organ was rich in extracellular matrix constituents; it contained structures resembling basal lamina sheaths and had a high density of collagen fibrils. It caused a 3- to 20-fold increase in the number of AChR clusters on cultured myotubes without significantly affecting the number or size of the myotubes. The increase was first seen 2-4 h after the fraction was added to cultures and it was maximal by 24 h. The AChR-aggregating effect was dose dependent and was due, at least in part, to lateral migration of AChRs present in the muscle cell plasma membrane at the time the fraction was applied. Activity was destroyed by heat and by trypsin. The active component(s) was extracted from the insoluble fraction with high ionic strength or pH 5.5 buffers. The extracts increased the number of AChR clusters on cultured myotubes without affecting the number or degradation rate of surface AChRs. Antiserum against the solubilized material blocked its effect on AChR distribution and bound to the active component. Insoluble fractions of Torpedo muscle and liver did not cause AChR aggregation on cultured myotubes. However a low level of activity was detected in pH 5.5 extracts from the muscle fraction. The active component(s) in the muscle extract was immunoprecipitated by the antiserum against the material extracted from the electric organ insoluble fraction. This antiserum also bound to extracellular matrix in frog muscles, including the myofiber basal lamina sheath. Thus the insoluble fraction of Torpedo electric organ is rich in AChR-aggregating molecules that are also found in muscle and has components antigenically similar to those in myofiber basal lamina.

Animals↗

PubMed1973

The structure of Mytilus smooth muscle and the electrical constants of the resting muscle.

The individual muscle fibers of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. are uninucleate, 1.2-1.8 mm in length, 5 microm in diameter, and organized into bundles 100-200 microm in diameter, surrounded by connective tissue. Some bundles run the length of the whole muscle. Adjacent muscle cell membranes are interconnected by nexuses at frequent intervals. Specialized attachments exist between muscle fibers and connective tissue. Electrical constants of the resting muscle membrane were measured with intracellular recording electrodes and both extracellular and intracellular current-passing electrodes. With an intracellular current-passing electrode, the time constant tau, was 4.3 +/- 1.5 ms. With current delivered via an extracellular electrode tau was 68.3 +/- 15 ms. The space constant, lambda, was 1.8 mm +/- 0.4. The membrane input resistance, R(eff), ranged from 23 to 51 MOmega. The observations that values of tau depend on the method of passing current, and that the value of lambda is large relative to fiber length and diameter are considered evidence that the individual muscle fibers are electrically interconnected within bundles in a three-dimensional network. Estimations are made of the membrane resistance, R(m), to compare the values to fast and slow striated muscle fibers and mammalian smooth muscles. The implications of this study in reinterpreting previous mechanical and electrical studies are discussed.

Animals↗

PubMed2006

Repeatability of a computerized muscle tonometer and the effect of tissue thickness on the estimation of muscle tone.

The aim of the present study was to assess the intrarater repeatability of the computerized muscle tonometer (CMT), which is a computer-driven device designed to measure and register the tone of connective tissues. The second aim was to evaluate the effect of muscle thickness on results and repeatability as well as sensitivity, which was done by comparison to a mechanical hand-held tonometer (HHT). Participants were ten healthy male volunteers. The thickness of soft tissues was measured with ultrasound and the CMT was used to evaluate the muscle tone of thigh muscles 10, 15 and 20 cm and the HHT 15 cm above the patella on both sides. Great individual variability in muscle thickness and tone was noted. Muscle thickness increased proximally on the thigh and thus the mean distance travelled by the indenter of the CMT also increased from distal to proximal from 22 to 26 mm and the work done increased from 116 to 161 mJ. Intraclass correlation (ICC) for travel and work ranged from 0.85 to 0.92 and 0.94 to 0.98, respectively. Repeatability coefficients increased somewhat at proximal measurement sites. ICC for the HHT was 0.78. The CMT also showed a considerably higher range of results, indicating greater resolution power. Despite the twofold pressure used with the HHT the distance measured was on average 60% lower compared to the CMT. In conclusion, the repeatability of the CMT was found to be good. Muscle thickness greatly affected the results and should therefore be taken into consideration when measuring muscle tone. Work was a more reliable unit of measurement than travel, which has been commonly used to describe muscle tone.

Adult↗

PubMed1987

Technetium pyrophosphate muscle scans in inflammatory muscle disease.

Technetium-99M pyrophosphate (TcPYP) nuclear scans of extremities were performed on 15 patients at 10 minutes and 2 hours after isotope injection. Scans were carried out both to confirm the diagnosis of myositis and to direct subsequent muscle biopsy. Five of six patients with clinical features strongly suggestive of inflammatory muscle disease had positive scans. All muscle biopsies performed at areas of increased isotope uptake showed inflammatory muscle disease. All nine patients not suspected of active inflammatory muscle disease had negative scans. Two of these underwent muscle biopsy with negative results. Our observations suggest that TcPYP muscle scans may be useful both to confirm the clinical suspicion of inflammatory muscle disease and in directing the choice of site for muscle biopsy.

Biopsy↗

PubMed2003

Muscle-sparing latissimus dorsi myocutaneous flap with maintenance of muscle innervation, function, and aesthetic appearance of the donor site.

In this report, the authors describe the application of a muscle-sparing technique to harvest a myocutaneous latissimus dorsi muscle flap, including only a tiny lateral muscle segment but carrying a large skin paddle, with the advantage of leaving intact innervation and function of the remaining latissimus dorsi muscle. According to the experiences and complications associated with the pure thoracodorsal artery perforator harvest at the authors' institution, the necessity of increasing the reliability of the vascular pedicle demands that a small muscle strip be left embedding the perforator vessels attached to the skin paddle. This procedure was applied in eight cases with only one minor complication, which was a distal flap tip necrosis in the largest flap used. The muscle function and aesthetic contour of the posterior axillary fold were preserved in every case. Harvesting a large skin paddle flap that is carried by a diminutive longitudinal segment of latissimus dorsi muscle circumvents thoracodorsal nerve damage and maintains muscle function. In contrast to a thoracodorsal artery perforator flap without muscle, the harvesting of which is a delicate procedure, this procedure is regarded as easier and safer.

Adolescent↗

PubMed1991

A comparison of static and dynamic characteristics between rectus eye muscle and linear muscle model predictions.

The characteristics of a muscle model are analyzed using rectus eye muscle parameter values and compared to rectus eye muscle data. The muscle is modeled as a viscoelastic parallel combination connected to a parallel combination of active state tension generator, viscosity element, and length tension elastic element. Each of the elements is linear and their existence is supported with physiological evidence. The static and dynamic properties of the muscle model are compared to rectus eye muscle data. The length-tension characteristics of the model are in good agreement with the data within the operating region of the muscle. With the muscle model incorporated into a lever system to match the isotonic experiment paradigm, simulation results for this linear system yield a nonlinear force-velocity curve. Moreover, the family of force-velocity curves generated with different stimulus rates reported in the literature match the predictions of the model without parametric changes. The results of this paper are important in studies involving the oculomotor plant and oculomotor neural networks. Additionally, these results may be applicable to other muscles.

Isometric Contraction↗

PubMed1990

Changes in expression and organization of smooth-muscle-specific alpha-actin during fibronectin-mediated modulation of arterial smooth muscle cell phenotype.

The spreading of freshly isolated arterial smooth muscle cells on a substrate of fibronectin is mediated by an integrin receptor on the cell surface. It is associated with organization of actin filaments in stress fibers and marked changes in cell morphology and function, collectively referred to as a transition from a contractile to a synthetic phenotype. To study further how extracellular matrix components affect smooth muscle phenotype, we have analyzed the expression and organization of smooth-muscle-specific alpha-actin in freshly isolated rat aortic smooth muscle cells cultured on a substrate of fibronectin under serum-free conditions. Northern-blot analysis showed that the expression of mRNA for smooth muscle alpha-actin, but not for nonmuscle actin, was strongly repressed during primary culture. On the other hand, the cellular content of alpha-actin was only moderately changed during the same period. Indirect immunofluorescence staining revealed that nonmuscle actin was rapidly organized in stress fibers, which did not stain with a monoclonal antibody against smooth muscle alpha-actin. Filament bundles containing alpha-actin were most prominent in the central parts of the cytoplasm and gradually disappeared as the spreading of the cells progressed. In contrast to the situation with nonmuscle actin, there was no apparent overlap in the staining for alpha-actin and the fibronectin receptor (alpha 5 beta 1), indicating that this receptor interacted with nonmuscle actin during the initial spreading process. Taken together, the results show that the expression and organization of smooth muscle alpha-actin are changed during interaction of the cells with fibronectin early in primary culture. They support the notion that integrin-mediated interactions between extracellular matrix components and arterial smooth muscle cells take part in the control of smooth muscle phenotype.

Actins↗

PubMed1994

The effect of denervation on myosin isoform synthesis in rabbit slow-type and fast-type muscles during terminal differentiation. Denervation induces differentiation into slow-type muscles.

The soleus and gastrocnemius medialis of eight-day-old rabbits were denervated and the effects were examined after fifty-two days by biochemical, cytochemical and mechanical methods. The contralateral soleus exhibited the properties of slow-type muscle, namely a predominance of slow-type myosin isoforms and slow-type oxidative fibers, slow twitch and low maximal velocity for shortening. The contralateral gastrocnemius exhibited the properties of fast-type muscle, namely a predominance of fast-type myosin isoforms and fast-type non-oxidative fibers, fast twitch and high maximal velocity of shortening. Denervation of muscles caused the differentiation of the two muscles towards slow-type muscles. Both denervated soleus and gastrocnemius muscles exhibited a predominance of slow-type myosins (either the normal type, made up of slow heavy and light chains, or the hybrid type, made up of slow heavy and regulatory light chains and fast essential light chains), a predominance of slow-type fibers, and slow mechanical properties. Thus, innervation in rabbit appears to be a determining factor for differentiation into fast-type muscle, but it is not necessary for differentiation into slow-type muscle. This conclusion contradicts the findings of previous studies in rat and thus raises new questions concerning the role of nerves in controlling the expression of myosin isoforms and the differentiation of muscle fibers.

Animals↗

PubMed1996

The effect of muscle length on intracellular calcium and force in single fibres from mouse skeletal muscle.

1. The effect of muscle length on the myoplasmic free Ca2+ concentration ([Ca2+]i) and tetanic force production of intact single muscle fibres was investigated in the mouse flexor brevis muscle. Muscle fibres were analysed at 100 microns intervals from 100 microns shorter than the optimum length (L(zero)) to 300 microns longer than L(zero). The fluorescent Ca2+ indicator indo-1 was used to measure [Ca2+]i. 2. Changes in muscle length did not significantly affect tetanic or resting [Ca2+]i. However, at lower stimulus frequencies (30-50 Hz) the force-length relation was shifted to longer muscle lengths. 3. Force-tetanic [Ca2+]i curves were constructed at each muscle length. A comparison of these curves revealed that the Ca2+ sensitivity was length dependent. Ca2+ sensitivity increased over the ascending limb and plateau phase of the force-length curve, but over the descending limb Ca2+ sensitivity reached a plateau and then started to decrease once more. 4. These results show that length-dependent changes in Ca2+ sensitivity of single muscle fibres cannot simply be related to variations in sarcomere length. It is proposed that other factors, such as cross-bridge attachment and/or developed force, have a role in determining the Ca2+ sensitivity at a particular muscle length.

Animals↗